Energy, Rates, Analysis & Earth Science

Burning natural gas in a Bunsen burner causes the surrounding air to get hotter. What type of reaction is this?

• AA reversible reaction
• BAn endothermic reaction
• CAn exothermic reaction✓
• DA displacement reaction

Answer: Exothermic reactions transfer energy to the surroundings, which is why the air around the flame warms up. Endothermic reactions do the opposite, taking energy in and cooling the surroundings. Reversible and displacement reactions describe how a reaction proceeds or which substances swap places, not the direction of energy transfer.

When solid ammonium nitrate dissolves in water, the temperature of the solution falls. What does this indicate about the process?

• AIt is an endothermic process, taking in energy from the surroundings✓
• BIt is an exothermic process, releasing stored chemical energy out into the surroundings
• CIt is a reversible process occurring exactly at equilibrium
• DIt is a catalysed process, requiring no net transfer of energy in either direction

Answer: A falling temperature shows that energy is being absorbed from the surroundings, which is the definition of an endothermic process. An exothermic process would instead release energy and raise the temperature. Catalysis affects reaction rate rather than energy transfer, and equilibrium describes forward and reverse rates being equal, not a temperature drop.

On a reaction profile diagram, what does the activation energy represent?

• AThe difference in energy between the reactants and products
• BThe average energy of all particles in the mixture
• CThe total energy released by the whole reaction
• DThe minimum energy that colliding particles need to react✓

Answer: Activation energy is the minimum energy that colliding particles must have before a collision can result in a reaction. The overall energy released or absorbed is shown by the height difference between reactants and products, which is a separate feature of the profile. The average particle energy is a property of the temperature, not of the reaction pathway itself.

In the reaction profile of an exothermic reaction, how do the energy levels of the products and reactants compare?

• AThe products have more energy than the reactants
• BThe products have less energy than the reactants✓
• CThe products and reactants store exactly the same energy
• DThe activation energy is always greater than the reactants' energy

Answer: In an exothermic reaction, the products end up with less stored energy than the reactants because energy has been transferred to the surroundings. If products had more energy than reactants, the reaction would be endothermic instead. Equal energy levels would mean no net energy change, and the activation energy is simply the energy barrier to overcome, not a comparison between reactants and products.

In the reaction H2 + Cl2 -> 2HCl, breaking the bonds in the reactants requires 679 kJ/mol and forming the bonds in the product releases 864 kJ/mol. What is the overall energy change, and is the reaction exothermic or endothermic?

• A-185 kJ/mol; the reaction is exothermic because more energy is released forming bonds than is absorbed breaking them✓
• B-185 kJ/mol; the reaction is endothermic because a negative value always and only means that energy is being absorbed
• C+185 kJ/mol; the reaction is endothermic because more energy is absorbed breaking bonds than is released forming them
• D+1543 kJ/mol; the reaction is endothermic because the two given energy values should simply be added together here instead

Answer: The overall energy change is found by subtracting the energy released making bonds from the energy absorbed breaking them: 679 minus 864 gives -185 kJ/mol. A negative value means the surroundings gain energy overall, which makes the reaction exothermic, not endothermic. Adding the two values instead of subtracting them, or assuming a negative sign always means absorption, both lead to the wrong classification.

Which statement about crude oil is correct?

• AIt is a single pure hydrocarbon compound rather than a mixture of many different compounds
• BIt is a finite resource that provides the main feedstock for the petrochemical industry✓
• CIt is a renewable resource that fully regenerates within a single human lifetime, quite easily
• DIt is mainly used as a feedstock for the fertiliser and pesticide industries worldwide

Answer: Crude oil is a finite resource, formed over millions of years, that supplies the raw hydrocarbons used by the petrochemical industry to make fuels and chemicals. It cannot regenerate within a human lifetime, so it is not renewable. It is also a mixture of many different hydrocarbons rather than a single compound, and its main industrial role is as a feedstock for fuels and petrochemicals rather than fertilisers.

Why does increasing the temperature of a reaction mixture increase the rate of reaction?

• AParticles move faster, so collisions are more frequent and more of them have enough energy to react✓
• BParticles become larger and heavier, so collisions are more frequent but noticeably less energetic overall
• CThe activation energy of the reaction itself increases, making collisions more likely to succeed overall
• DParticles move more slowly, giving them more time to collide successfully

Answer: Heating a mixture makes particles move faster, so they collide more often and a greater proportion of collisions have enough energy to overcome the activation energy. Particle size does not change with temperature, and slower-moving particles would actually collide less often, not more. Activation energy is a fixed property of the reaction and does not rise with temperature.

Why does increasing the concentration of a reactant in solution increase the rate of reaction?

• AThere are more reactant particles in a given volume, so collisions between particles happen more frequently✓
• BThe particles themselves move much faster, so each individual collision releases considerably more energy
• CThere are fewer reactant particles packed into a given volume, so successful collisions become noticeably rarer
• DThe activation energy of the reaction is reduced overall, so many more collisions succeed as a result

Answer: A higher concentration packs more reactant particles into the same volume, so they collide with each other more often and the reaction speeds up. Concentration does not change how fast individual particles move or how much energy a single collision releases. It also has no effect on the fixed activation energy needed for a successful collision.

Breaking a large lump of calcium carbonate into smaller pieces before adding it to acid increases the reaction rate. What is the reason for this?

• AThe particles gain extra energy from being broken apart, so collisions succeed noticeably more often
• BA greater surface area is exposed to the acid, so collisions between particles happen more often✓
• CA smaller surface area ends up exposed to the acid, so fewer particles are wasted overall each time
• DThe concentration of the acid gradually increases as the lumps of solid are broken up further

Answer: Breaking a solid into smaller pieces increases its total surface area, exposing more particles to the acid and so increasing the frequency of collisions. It does not reduce the surface area, and simply breaking a solid apart does not give its particles extra energy. The acid's own concentration is unaffected by the size of the solid pieces added to it.

Increasing the pressure of a reaction involving gases usually increases the rate of reaction. Why?

• AThe activation energy needed for a successful collision is permanently lowered by the pressure
• BThe gas particles are pushed closer together, so collisions between them become more frequent✓
• CThe gas particles condense into a liquid, which increases their concentration
• DThe gas particles are pushed much further apart, so each collision becomes noticeably more energetic

Answer: Increasing pressure squeezes gas particles into a smaller space, so they are more crowded and collide with each other more often. Higher pressure pushes particles closer together rather than further apart, and it does not change the activation energy required for a reaction. Gases do not condense into liquids simply because pressure is increased during a typical reaction.

What effect does a catalyst have on a chemical reaction?

• AIt increases the rate of reaction by raising the temperature of the reaction mixture, which speeds up all reactions equally
• BIt increases the rate of reaction by providing a pathway with a lower activation energy, without being used up✓
• CIt increases the overall yield of product by shifting the position of the equilibrium significantly
• DIt increases the rate of reaction by being permanently used up as an extra reactant each and every time

Answer: A catalyst speeds up a reaction by offering an alternative pathway with a lower activation energy, and it is chemically unchanged at the end of the reaction. It does not work by heating the mixture, and it is not consumed like a normal reactant. A catalyst also has no effect on the position of equilibrium, so it does not increase the overall yield of a reversible reaction.

Which statement correctly describes enzymes as biological catalysts?

• AThey speed up any reaction in living organisms and work faster the hotter they get, with no limit
• BThey slow down most metabolic reactions taking place in cells, in order to help conserve energy
• CThey speed up specific reactions in living organisms and can be denatured by excessive heat✓
• DThey are used up completely during the reactions they catalyse, so cells must constantly replace them

Answer: Enzymes are specific to particular reactions, and their active site's shape can be permanently altered, or denatured, if the temperature gets too high. They do not catalyse every possible reaction, and their rate does not keep rising indefinitely with heat. Like other catalysts, enzymes are not used up in the reactions they speed up, and they act to speed up, not slow down, metabolic reactions.

A reversible reaction reaches dynamic equilibrium in a closed system. If the concentration of a reactant is then increased, what happens to the equilibrium position?

• AIt shifts in the direction that tends to reduce the concentration of that reactant, forming more product✓
• BIt shifts in the direction that further increases the concentration of that reactant, forming less product
• CIt stays exactly the same, because an equilibrium position can never change once it has been established
• DIt shifts completely and permanently towards the reactants until the whole reaction eventually stops

Answer: Adding more reactant disturbs the equilibrium, so the system responds by favouring the forward reaction, which uses up some of the extra reactant and forms more product. It would not shift in a direction that increases the reactant concentration even further, and equilibrium positions can and do shift when conditions change. The reaction does not stop or move entirely to one side, since forward and reverse reactions continue at new, equal rates.

In chemistry, what does it mean for a substance to be described as pure?

• AIt is a substance that occurs naturally rather than ever being manufactured artificially
• BIt is a substance that is generally regarded as being safe enough to eat or drink
• CIt consists of a mixture of several different substances, combined together in fixed proportions
• DIt consists of a single element or compound, not mixed with any other substance✓

Answer: In the scientific sense, a pure substance is a single element or compound with nothing else mixed in. A formulation, by contrast, is a mixture of substances combined in fixed proportions for a purpose, which is not the same as being chemically pure. Purity has nothing to do with whether a substance occurs naturally or is safe to consume, since a manufactured or hazardous substance can still be chemically pure.

Filtration is used to separate a mixture of sand and water. What property of the mixture allows this method to work?

• AThe sand is an insoluble solid, so it is trapped by the filter paper while the water passes through✓
• BThe sand is actually a soluble solid, so it dissolves straight through the filter paper while the water is trapped
• CThe sand evaporates far more easily than the water does, leaving it entirely behind on the filter paper
• DThe sand and water happen to share exactly the same boiling point, so gentle heating separates them very cleanly

Answer: Sand does not dissolve in water, so its insoluble particles are too large to pass through the tiny holes in filter paper, while the water simply flows through. Sand does not dissolve at all, so describing it as soluble is incorrect, and filtration relies on particle size rather than boiling point. Sand also does not evaporate, since it remains solid at the temperatures involved in this separation.

Crystallisation is used to obtain solid copper sulfate from a solution. How does this method work?

• AThe solution is evaporated until it becomes saturated, and crystals form as it cools✓
• BThe solution is passed through chromatography paper until the copper sulfate separates out
• CThe solution is filtered, and the copper sulfate is trapped as a solid residue on the paper
• DThe solution is heated until every component boils off at exactly the same temperature

Answer: In crystallisation, some solvent is evaporated so the remaining solution becomes saturated, and solid crystals grow out of solution as it cools. Copper sulfate is dissolved rather than suspended, so filtration cannot trap it, and the components of a solution do not all boil at the same temperature. Chromatography separates substances based on how they travel through a stationary phase, which is a different technique from crystallisation entirely.

Simple distillation is used to obtain pure water from salt water. Which property is exploited to achieve this separation?

• AWater and salt happen to have identical boiling points, so both evaporate together and later fully recombine
• BWater has a much lower boiling point than the dissolved salt, so it evaporates and can be condensed separately✓
• CSalt dissolves far more easily at low temperatures, so simply cooling the mixture down removes it completely
• DSalt actually has a much lower boiling point than water does, so it evaporates first and is collected separately

Answer: Water boils and evaporates at a much lower temperature than the dissolved salt, so it can be driven off as vapour and condensed back into pure liquid, leaving the salt behind. Salt actually has a far higher boiling point than water, not a lower one, and the two substances do not share the same boiling point. Cooling the mixture would not remove the salt, since solubility falling with temperature does not separate a solution by distillation.

On a chromatogram, a dye spot travels 6 cm from the baseline while the solvent front travels 8 cm. What is the Rf value of the dye?

• A0.14
• B1.33
• C0.75✓
• D0.25

Answer: The Rf value is calculated by dividing the distance travelled by the substance by the distance travelled by the solvent front, so 6 divided by 8 gives 0.75. Inverting the fraction to 8 divided by 6 gives 1.33, which is a common calculation error. Using the difference between the two distances, either as a fraction of the solvent distance or on its own, also gives an incorrect result.

A student tests an unknown solid and finds that it melts sharply at exactly 118 degrees C. What does this suggest about the sample?

• AThe sample is likely to be an impure mixture, since mixtures melt sharply at one exact fixed temperature
• BThe sample is likely to be a formulation, since formulations melt at one precise temperature
• CThe sample is likely to be a pure substance, since pure substances melt at a single, specific temperature✓
• DThe sample is likely to be impure, since a pure substance would melt gradually over a range

Answer: A pure substance melts sharply at one specific, reproducible temperature, so a sharp melting point at exactly 118 degrees C points towards purity. Impure substances actually melt gradually over a range of temperatures rather than at one fixed point, and a mixture is impure, so it melts over a range too. Formulations are also mixtures, so they would melt over a range rather than at one precise temperature.

What is the relative formula mass (Mr) of magnesium oxide, MgO? (Ar: Mg = 24, O = 16)

• A40✓
• B56
• C16
• D24

Answer: Relative formula mass is found by adding together the relative atomic masses of every atom in the formula, so 24 plus 16 gives 40 for magnesium oxide. Giving just 16 or just 24 uses only the oxygen or only the magnesium value and forgets to add the other one in. Wrongly assuming two oxygen atoms are present and adding 24 plus 16 plus 16 gives 56, which is also incorrect.

A life-cycle assessment (LCA) is carried out to compare the environmental impact of a plastic bag and a paper bag. What does this assessment consider?

• AThe impact of the product across its whole life, from extracting raw materials to final disposal✓
• BThe impact of the product only while it is being used by the consumer
• CThe cost of the product to the consumer at the point of purchase
• DThe impact of the product only during manufacturing, ignoring raw material extraction and disposal

Answer: A life-cycle assessment tracks the environmental impact of a product across its whole life, covering raw material extraction, manufacture, use and final disposal. Looking only at the manufacturing stage would miss the impact of extracting materials and disposing of the product afterwards. An LCA is also about environmental impact rather than consumer cost, and it does not stop at the point where a product is simply being used.

Recycling aluminium cans rather than extracting new aluminium from ore has a key environmental benefit. What is it?

• AIt significantly increases the total amount of aluminium ore that needs to be freshly mined to meet global demand each year
• BIt completely removes the need for any energy input whatsoever, since recycled aluminium melts without any heating at all
• CIt uses much less energy, since melting existing aluminium requires far less energy than extracting it by electrolysis✓
• DIt uses much more energy overall, since sorting and thoroughly cleaning the used cans requires intensive processing every time

Answer: Recycling aluminium saves energy because melting down existing metal takes far less energy than extracting fresh aluminium from its ore by electrolysis. Sorting and cleaning cans does use some energy, but the overall process still uses much less than extraction, not more. Melting aluminium still requires heat energy, and recycling reduces, rather than increases, the amount of ore that needs to be mined.

Crude oil is separated into fractions by fractional distillation. What property of the hydrocarbons allows this separation?

• AHydrocarbons with different chain lengths have different boiling points, so they condense at different heights in the column✓
• BHydrocarbons with much shorter chains always sink right to the bottom because they are far denser than the longer chains
• CHydrocarbons with noticeably different colours separate out as they rise steadily up through the whole column each time
• DHydrocarbons with different chain lengths actually have identical boiling points, so they condense together at exactly the same height

Answer: Hydrocarbons of different chain lengths have different boiling points, so as vapour rises up the fractionating column it cools and condenses out at different heights, separating the fractions. It is precisely because their boiling points differ, not because they are identical, that separation is possible. Colour plays no part in the separation, and shorter-chain hydrocarbons are actually less dense and rise higher, rather than sinking to the bottom.

Alkanes form a homologous series with the general formula CnH2n+2. How many hydrogen atoms are in a molecule of the alkane with 5 carbon atoms?

• A22
• B12✓
• C14
• D10

Answer: Substituting n equals 5 into the general formula CnH2n+2 gives 2 times 5 plus 2, which equals 12 hydrogen atoms. Using just 2n without adding the extra 2 gives 10, which misses the final step of the formula. Wrongly adding the 2 before multiplying, as 2 times 7, gives 14, and 22 comes from doubling the correct answer rather than applying the formula correctly.

Long-chain hydrocarbons are heated with a catalyst to break them into shorter, more useful molecules. What is this process called, and what does it typically produce?

• APolymerisation, which joins the short hydrocarbons into long-chain polymers
• BFermentation, which produces ethanol and carbon dioxide from the hydrocarbons
• CCracking, which produces a single long-chain alkane with no other products at all
• DCracking, which produces a mixture of shorter alkanes and reactive alkenes✓

Answer: This process is called cracking, and it breaks large hydrocarbon molecules down into a mixture of smaller, more useful alkanes and alkenes. Cracking produces a mixture of shorter molecules, not a single long-chain product, since the whole point is to break the chain apart. Fermentation is a biological process that makes ethanol from sugars, and polymerisation joins small molecules together rather than breaking large ones apart, so neither describes cracking.

Iron is extracted from its ore by reduction with carbon, but aluminium is extracted using electrolysis instead. Why is this?

• AAluminium ore does not contain enough oxygen for a reduction reaction to occur
• BIron is more reactive than carbon, so it must be extracted using electrolysis instead
• CAluminium is less reactive than carbon, so it cannot be displaced by carbon at all
• DAluminium is more reactive than carbon, so carbon cannot displace it from its compound, unlike iron✓

Answer: Aluminium is more reactive than carbon, so carbon is unable to displace it from its ore, meaning electrolysis has to be used instead. Iron, by contrast, is less reactive than carbon, so carbon can reduce iron ore directly and cheaply, which is why iron does not need electrolysis. The issue is reactivity rather than a lack of oxygen in the ore, since both iron and aluminium ores are oxides.

Earth's early atmosphere is thought to have formed mainly from volcanic activity. Which two gases most likely dominated this early atmosphere?

• AOxygen and nitrogen
• BCarbon dioxide and water vapour✓
• CCarbon dioxide and oxygen
• DMethane and hydrogen

Answer: Volcanic activity released large amounts of carbon dioxide and water vapour, which are thought to have made up most of Earth's early atmosphere. Oxygen and nitrogen dominate today's atmosphere, but oxygen only built up much later through photosynthesis, so this pairing describes the modern atmosphere, not the early one. Methane and hydrogen, and any mixture including free oxygen, are also not considered accurate descriptions of the earliest volcanic atmosphere.

Over billions of years, the proportion of oxygen in Earth's atmosphere increased substantially. What is thought to have caused this change?

• AAlgae and plants releasing oxygen as a product of photosynthesis✓
• BEarly animals releasing oxygen as a waste product of respiration
• CVolcanic eruptions releasing large quantities of oxygen gas directly
• DOceans releasing dissolved oxygen as they gradually cooled down

Answer: The rise in atmospheric oxygen is attributed to algae and, later, plants carrying out photosynthesis, which releases oxygen as a by-product over billions of years. Volcanic eruptions release gases such as carbon dioxide and water vapour, not oxygen, so they cannot explain this change. Animal respiration removes oxygen rather than releasing it, and gradual ocean cooling is not the process credited with building up atmospheric oxygen.

Greenhouse gases such as carbon dioxide and methane help to keep the Earth's surface warm. How do they achieve this?

• AThey absorb infrared radiation emitted from the Earth's surface and re-radiate some of it back down✓
• BThey convert infrared radiation from the sun directly into oxygen gas
• CThey reflect incoming sunlight away from the Earth's surface before it can be absorbed
• DThey block all radiation from reaching the Earth's surface, trapping heat in space instead

Answer: Greenhouse gases absorb infrared radiation given off by the warmed Earth's surface and radiate some of that energy back towards the ground, keeping the surface warmer than it would otherwise be. They do not work by reflecting incoming sunlight, and they do not block radiation from reaching the surface at all, since sunlight still needs to reach the ground to warm it in the first place. Greenhouse gases also do not convert radiation into oxygen gas.

Scientists point to a correlation between rising atmospheric carbon dioxide levels and increasing fossil fuel consumption since industrialisation. What does this correlation provide evidence for?

• AHuman activity is a significant contributor to the increase in atmospheric carbon dioxide✓
• BRising carbon dioxide levels have caused humans to burn more fossil fuels
• CFossil fuel consumption has no measurable effect on atmospheric carbon dioxide levels
• DAtmospheric carbon dioxide levels have remained constant throughout industrialisation

Answer: The close match between rising fossil fuel use and rising atmospheric carbon dioxide since industrialisation is used as evidence that human activity is a major contributor to the increase. Saying fossil fuels have no measurable effect directly contradicts this correlation. The correlation also reflects fossil fuel burning driving carbon dioxide levels up, not the reverse, and the data shows a clear rise rather than levels staying constant.

Carbon monoxide is a toxic gas released when carbon-based fuels undergo incomplete combustion. Why is it dangerous to human health?

• AIt displaces nitrogen in the atmosphere, lowering the oxygen concentration of the air
• BIt binds to haemoglobin in red blood cells, reducing the blood's ability to carry oxygen✓
• CIt binds irreversibly to bone tissue, weakening the skeleton over time
• DIt reacts with water vapour in the lungs to form dilute sulfuric acid

Answer: Carbon monoxide binds to haemoglobin in red blood cells more readily than oxygen does, which reduces the blood's ability to transport oxygen around the body and can starve tissues of it. It does not react with water vapour to form sulfuric acid, which is instead associated with sulfur dioxide pollution. Carbon monoxide also does not displace atmospheric nitrogen or bind to bone tissue, so its danger comes specifically from its effect on blood chemistry.

Burning fossil fuels that contain sulfur impurities releases sulfur dioxide into the atmosphere. What environmental problem does this cause?

• AIt reacts with oxygen in the air to form ozone, which actually improves air quality at ground level
• BIt dissolves in atmospheric moisture to form acid rain, which damages plants and buildings✓
• CIt combines with atmospheric nitrogen to form a harmless, unreactive gas that lingers in the air
• DIt increases the alkalinity of rainwater generally, raising the overall pH of lakes and rivers nearby

Answer: Sulfur dioxide dissolves in moisture in the atmosphere to form acidic solutions that fall as acid rain, damaging plants, aquatic life and stone buildings. It does not form ozone, and ground-level ozone is itself a harmful pollutant rather than something that improves air quality. Sulfur dioxide does not combine with nitrogen to form a harmless gas, and it lowers, rather than raises, the pH of rainwater by making it more acidic.

A student mixes citric acid solution with sodium hydrogencarbonate. The temperature of the mixture falls from 20.0 °C to 14.5 °C.

What type of reaction is this?

• AExothermic, as energy is given out to the surroundings
• BEndothermic, as energy is given out to the surroundings
• CEndothermic, as energy is taken in from the surroundings✓
• DExothermic, as energy is taken in from the surroundings

Answer: The temperature of the mixture falls because the reaction takes in energy from its surroundings, including the solution itself and the thermometer: an endothermic reaction. An exothermic reaction gives out energy, so the temperature would rise. Pairing either name with the opposite energy transfer mixes up the definitions.

Methane burns in oxygen: CH₄ + 2O₂ → CO₂ + 2H₂O Bond energies (kJ/mol): C–H 413, O=O 498, C=O 805, O–H 464

What is the overall energy change for the reaction?

• A+818 kJ/mol
• B−1316 kJ/mol
• C+110 kJ/mol
• D−818 kJ/mol✓

Answer: Energy to break bonds = 4 × 413 + 2 × 498 = 2648 kJ. Energy released making bonds = 2 × 805 + 4 × 464 = 3466 kJ. Overall change = 2648 − 3466 = −818 kJ/mol: more energy is released than taken in, so the reaction is exothermic. The tempting +818 subtracts the wrong way round, −1316 breaks only one O=O bond, and +110 forms only one water molecule.

Equal lengths of magnesium ribbon were added to excess hydrochloric acid of different concentrations, and the time for the magnesium to disappear was recorded. Concentration / mol/dm³: 0.5, 1.0, 1.5, 2.0 Time / s: 160, 80, 54, 40

Which statement describes and explains the pattern?

• ADoubling the concentration halves the time, as each collision has more energy than it did before
• BDoubling the concentration doubles the time, as the extra acid particles get in each other's way
• CDoubling the concentration halves the time, as the magnesium now has a larger surface area
• DDoubling the concentration halves the time, as acid particles collide with the magnesium more often✓

Answer: From 0.5 to 1.0 mol/dm³ the time falls from 160 s to 80 s, and from 1.0 to 2.0 it falls from 80 s to 40 s: doubling the concentration halves the time, so the rate doubles. A more concentrated acid has more particles in the same volume, so collisions with the magnesium are more frequent. The energy of each collision depends on temperature, not concentration, and the ribbon's surface area was the same each time.

A 0.24 g piece of magnesium reacts completely with acid in 40 s. (Relative atomic mass: Mg = 24)

What is the mean rate of reaction in mol/s?

• A2.5 × 10⁻⁴ mol/s✓
• B6.0 × 10⁻³ mol/s
• C4.0 × 10³ mol/s
• D1.0 × 10⁻² mol/s

Answer: Moles of magnesium = 0.24 ÷ 24 = 0.010 mol. Mean rate = amount used ÷ time = 0.010 ÷ 40 = 2.5 × 10⁻⁴ mol/s. The tempting 6.0 × 10⁻³ is the rate in g/s (0.24 ÷ 40), not mol/s; 4.0 × 10³ divides the time by the moles, and 1.0 × 10⁻² is the number of moles without dividing by the time.

For a reaction, the activation energy is 180 kJ/mol without a catalyst and 110 kJ/mol with a catalyst. The overall energy change is −60 kJ/mol without the catalyst.

Which statement about the catalysed reaction is correct?

• AThe catalyst lowers the activation energy by 70 kJ/mol and the energy change becomes −130 kJ/mol
• BThe catalyst lowers the activation energy by 110 kJ/mol and the energy change stays at −60 kJ/mol
• CThe catalyst lowers the activation energy by 70 kJ/mol and the energy change stays at −60 kJ/mol✓
• DThe catalyst lowers the activation energy by 70 kJ/mol and the energy change becomes +10 kJ/mol

Answer: A catalyst provides a different reaction pathway with a lower activation energy: 180 − 110 = 70 kJ/mol lower. The reactants and products are the same, so the overall energy change is unchanged at −60 kJ/mol. The catalyst is not used up and does not alter the energy of the reactants or products. The value 110 kJ/mol is the new activation energy, not the drop.

Brown nitrogen dioxide gas forms an equilibrium with colourless dinitrogen tetroxide in a sealed tube: 2NO₂(g) ⇌ N₂O₄(g) The forward reaction is exothermic.

What happens to the colour of the mixture when the tube is heated?

• AIt turns paler, as the equilibrium shifts in the endothermic direction
• BIt turns darker brown, as the equilibrium shifts in the endothermic direction✓
• CIt turns paler, as the equilibrium shifts in the exothermic direction
• DIt turns darker brown, as the equilibrium shifts in the exothermic direction

Answer: Raising the temperature shifts the equilibrium in the endothermic direction, which takes in energy and opposes the change. The forward reaction is exothermic, so the reverse reaction (N₂O₄ → 2NO₂) is endothermic: more brown NO₂ forms and the mixture darkens. Paler would mean more N₂O₄, which is the exothermic direction and would be favoured by cooling.

In paper chromatography, an unknown dye moved 3.0 cm from the start line while the solvent front moved 8.0 cm. Known dyes have these Rf values: Dye P: 0.25 Dye Q: 0.38 Dye R: 0.63 Dye S: 0.75

Which dye is the unknown most likely to be?

• ADye P
• BDye R
• CDye Q✓
• DDye S

Answer: Rf = distance moved by the substance ÷ distance moved by the solvent = 3.0 ÷ 8.0 = 0.375, which rounds to 0.38: dye Q. The tempting dye R has Rf 0.63, which comes from measuring the 5.0 cm between the spot and the solvent front instead of from the start line. Rf values only match like this when the same solvent and paper are used.

Pure aspirin melts at 136 °C. A student's sample of aspirin melts over the range 128–133 °C.

What does this show about the sample?

• AIt is pure, as it melts at a temperature close to that of aspirin
• BIt is impure, as impurities raise the melting point of a substance
• CIt is impure, as a pure substance melts at a specific temperature✓
• DIt is pure, as a melting range shows that all of it has melted

Answer: In chemistry, a pure substance is a single element or compound, and it melts at a specific temperature. Impurities lower the melting point and make the substance melt over a range, which is what the sample does: 128–133 °C is below and wider than the 136 °C of pure aspirin. Impurities lower the melting point rather than raising it, and melting over a range is the sign of an impure sample.