Five ways to speed it up — answer key

1. UQ8 · Practice

Options as numbered on the pupil sheet: 1 — Slower; 2 — Faster.

Model answer. More concentrated solutions, higher pressures of gases, smaller pieces (more surface), higher temperatures and catalysts all make a reaction faster. A lower temperature and larger pieces make it slower.

A full answer includes

• I matched 1, more concentrated acid, to faster.
• I matched 2, cooling, to slower.
• I matched 3, larger pieces of the solid, to slower.
• I matched 4, adding a catalyst, to faster.
• I matched 5, a higher pressure of reacting gases, to faster.
• I did not match larger pieces to faster because there is more solid.

Also accept

• Concentrated, catalyst, higher pressure: faster; colder, larger pieces: slower

Watch for

• “Larger pieces react faster because there is more solid.” — Only the surface of a lump meets the acid. Larger pieces have less surface for the same mass.
• “Cooling makes a reaction faster, because the particles have more time to meet.” — Cold particles move more slowly, so they meet less often: the reaction is slower.
• “Pressure makes no difference to a reaction of gases.” — For gases, a higher pressure crowds the particles into a smaller space, like a more concentrated solution: the reaction is faster.

2. UQ9 · Practice

Model answer. A catalyst speeds up a reaction and is unchanged at the end: the same substance and the same mass. It does not change how much product is made.

A full answer includes

• I chose: it increases the rate of a reaction and is unchanged at the end.
• I did not choose that a catalyst is used up.

Also accept

• Increases the rate; unchanged at the end

Watch for

• “A catalyst is used up in the reaction.” — A catalyst is unchanged at the end: the same mass of the same substance, ready to speed up the reaction again.
• “A catalyst increases the amount of product made.” — A catalyst makes the same product faster. How much is made depends on the limiting reactant.
• “A catalyst slows down a reaction that is too fast.” — A catalyst increases the rate. It speeds a reaction up and is still there at the end.

3. UQ10 · Practice

Model answer. Temperature is the one thing you change. The time to turn cloudy is the result. Everything else that affects the rate must stay the same: the volume and concentration of each solution. Then any change in rate is caused by the temperature.

A full answer includes

• I chose the volume of the sodium thiosulfate solution.
• I chose the concentration of the sodium thiosulfate solution.
• I chose the volume of the hydrochloric acid.
• I chose the concentration of the hydrochloric acid.
• I did not choose the temperature: it is the one thing I change.
• I did not choose the time to turn cloudy: it is the result I measure.

Also accept

• Volume and concentration of both solutions

Watch for

• “The temperature must be kept the same.” — Temperature is the one thing you change on purpose in this test. Everything else that affects the rate stays the same.
• “The time must be kept the same.” — The time to turn cloudy is the result you measure. It changes because the temperature changes.
• “It is fair as long as you measure carefully.” — Careful measuring does not help if two things change at once: you cannot tell which one changed the rate.

4. UQ11 · Practice

Model answer. Cooking is a set of chemical reactions in the food. In the pressure cooker the water, and so the food, reaches about 120 °C instead of 100 °C. At a higher temperature these reactions are faster. The pressure matters only because it lets the water get hotter before it boils.

A full answer includes

• I chose: the food is hotter, so the cooking reactions are faster.
• I named temperature as the factor that changes.
• I did not choose the pressure squeezing the food.

Also accept

• The food is hotter, so the reactions are faster

Watch for

• “The high pressure squeezes the food, so its reactions are faster.” — Pressure speeds up reactions of gases. Food is solid and liquid, which pressure hardly squeezes. Here the pressure matters only because it lets the water get hotter.
• “The steam acts as a catalyst.” — There is steam over an open pan too. What is different is the temperature: 120 °C instead of 100 °C.
• “The boiling breaks the food up, so it has a larger surface.” — The food is in the same pieces in both pans. What is different is the temperature.

5. UQ12 · Practice

Model answer. (a) Catalase is a catalyst: it speeds up the breakdown of hydrogen peroxide into water and oxygen. (b) The same cubes speed up the reaction again, so the catalase was not used up: a catalyst is unchanged at the end. (c) Little or no froth. Enzymes are biological catalysts that stop working if they get too hot, and boiling is far too hot, so the hydrogen peroxide breaks down only very slowly, as it does on its own.

A full answer includes

• (a) I wrote that the catalase is a catalyst.
• (a) I wrote that it speeds up the breakdown of hydrogen peroxide.
• (b) I wrote that the catalase was not used up.
• (b) I wrote that it still works, as a catalyst unchanged at the end should.
• (c) I predicted little or no froth.
• (c) I wrote that enzymes stop working if they get too hot.
• (c) I did not write that boiling makes the enzyme work faster.

Also accept

• A catalyst; not used up; little or no froth, because enzymes stop working when too hot

Watch for

• “A catalyst is used up in the reaction.” — A catalyst is unchanged at the end: the same mass of the same substance, ready to speed up the reaction again.
• “Boiling makes an enzyme work faster, because heat speeds up reactions.” — Only up to a point. Enzymes stop working if they get too hot, and boiling is far too hot.

6. UQ13 · Practice

Model answer. Pressure affects reactions of gases: a higher pressure crowds the gas particles together. Nitrogen and hydrogen are both gases, so that reaction speeds up. The others have a solid or a solution reacting, which pressure hardly affects.

A full answer includes

• I chose nitrogen gas with hydrogen gas.
• I used the fact that both reactants are gases.
• I did not choose a reaction that only gives off a gas.

Also accept

• Nitrogen with hydrogen

Watch for

• “A reaction that gives off a gas is sped up by pressure.” — Pressure matters when the reactants are gases. In copper(II) carbonate with acid the gas is a product; the reactants are a solid and a solution.
• “Pressure squeezes a solid, so magnesium reacts faster.” — Solids and liquids are hardly squeezed at all. Only gas particles are pushed closer together.
• “Pressure crowds the particles in a solution.” — A solution is a liquid, which hardly changes volume under pressure, so its particles are not crowded closer.

7. UT2 · Apply

Model answer. (a) Enzymes are catalysts made by living things. (b) Unlike other catalysts, enzymes stop working when they get too hot. (c) An enzyme, like any catalyst, gives the reaction a route with a lower activation energy. The particles collide just as often, but a larger share of the collisions have at least Ea, so more succeed each second. (d) More slowly: removing the catalyst slows the reaction back down.

A full answer includes

• (a) I wrote that an enzyme is a catalyst.
• (a) I wrote that it is made by living things: a biological catalyst.
• (b) I wrote that enzymes stop working if they get too hot.
• (b) I did not write that a hotter wash is always faster.
• (c) I wrote that the enzymes give the breakdown a route with a lower activation energy.
• (c) I wrote that a larger share of the collisions have at least Ea, so more of them succeed.
• (c) I did not write that the enzymes give the particles more energy.
• (d) I predicted that it removes the stain more slowly.
• (d) I wrote that without the catalyst the reaction is slower.

Also accept

• Biological catalyst; too hot stops enzymes; lower Ea, more collisions succeed; slower

Watch for

• “The hotter the water, the faster an enzyme works.” — Up to a point. Too hot, and the enzyme stops working.
• “Enzymes work by giving the particles extra energy.” — An enzyme lowers the activation energy. The particles have the same energy, but more of their collisions have enough.