Rates of reaction — final check key

1. UK1 · Final check

Model answer. (a) The volume stops at 57 cm³. (b) 24 cm³ ÷ 10 s = 2.4 cm³/s. (c) From 50 s the volume stays at 57 cm³, so no more gas is made: the reaction stopped at about 50 s.

Checked automatically

• (a) Total volume: 57 cm³ (exact value)
• (b) Rate over the first 10 s: 2.4 cm³/s (within ±0.05 cm³/s)
  • Common wrong answer 5.7 : That is the total ÷ 10. Use the gas made in the first 10 s: 24 ÷ 10.
• (c) Time the reaction stopped: 50 s (exact value)
  • Common wrong answer 70 : That is the last time in the table. Find when the volume first stops changing.

A full answer includes

• (a) I read the volume where it stops changing: 57 cm³.
• (b) I worked out 24 cm³ ÷ 10 s.
• (b) I got 2.4 cm³/s.
• (c) I wrote 50 s.
• (c) I wrote that the volume stays at 57 cm³ from 50 s.
• (c) I did not give 70 s, the last time in the table.

Also accept

• 57 cm³; 2.4 cm³/s; about 50 s

Watch for

• “The reaction stops at the last time in the table.” — The table goes on after the reaction has stopped. Look for when the volume stops changing.

2. UK2 · Final check

Model answer. (a, b) The flask of acid and powder stands on a balance and the reading is taken at set times from a stopclock started as the powder goes in. The mass falls as carbon dioxide escapes. (c) Cotton wool lets the gas out but keeps acid droplets in, so only the gas counts as mass lost. (d) The balance shows the change from the start, it does not fill up like a 100 cm³ syringe, and nothing sticks.

A full answer includes

• (a) I named a conical flask standing on a balance.
• (a) I named cotton wool for the neck of the flask.
• (a) I named a stopclock.
• (b) I wrote that she reads the mass on the balance.
• (b) I wrote that she reads it at set times, for example every 30 s.
• (b) I wrote that she starts the clock as the powder goes into the acid.
• (c) I wrote that the cotton wool lets the carbon dioxide out.
• (c) I wrote that it stops acid spray escaping, which would add to the mass lost.
• (c) I did not write that it stops the gas escaping.
• (d) I gave one: no gas is lost before the measuring starts; there is no volume limit like a full syringe; there is no plunger to stick.

Also accept

• Balance and stopclock; mass at set times; cotton wool keeps acid spray in; no gas lost, no syringe limit

Watch for

• “The cotton wool stops the gas escaping.” — The gas must escape: its loss is what the balance measures. The cotton wool keeps acid spray in.

3. UK3 · Final check

Model answer. (a) A powder has a much larger surface for the acid to reach: faster. (b) A lower temperature: slower. (c) Water lowers the concentration of the acid: slower. (d) The same gas in half the volume is at a higher pressure: faster.

A full answer includes

• (a) I wrote faster.
• (a) I named surface area.
• (a) I did not write that the powder has more particles.
• (b) I wrote slower.
• (b) I named temperature.
• (c) I wrote slower.
• (c) I named concentration: the acid is less concentrated.
• (c) I did not write that more liquid makes it faster.
• (d) I wrote faster.
• (d) I named the pressure of the gases.

Also accept

• Faster, slower, slower, faster, with the factor

Watch for

• “A powder has more particles than lumps of the same mass.” — The same mass has the same number of particles. In a powder far more of them are on a surface, where the acid can reach them.
• “Adding water makes more liquid to react, so it is faster.” — The water does not react. It spreads out the same acid, so the acid is less concentrated.
• “Pushing the plunger in has no effect, because no gas is added.” — The same gas in half the space is at a higher pressure: more particles in each cm³, so the reaction is faster.

4. UK4 · Final check

Model answer. A catalyst increases the rate (50 cm³ of oxygen in 40 s instead of 300 s) and is unchanged at the end (the same 1.00 g of the same black powder). A blue solution would show that the powder had reacted, and a catalyst does not change the amount of product.

A full answer includes

• I chose the shorter time for 50 cm³ of oxygen: 40 s instead of 300 s.
• I chose the same mass at the end, 1.00 g.
• I chose the powder still being black at the end.
• I did not choose more oxygen in total.
• I did not choose the solution turning blue.

Also accept

• Faster; the same mass; still black

Watch for

• “The solution turning blue shows that the powder is a catalyst.” — A blue solution would mean that a copper compound had dissolved: the powder reacted and was used up. A catalyst is unchanged.
• “More oxygen in total shows that the powder is a catalyst.” — A catalyst does not change the amount of product, only how fast it forms.

5. UK5 · Final check

Model answer. (a) More concentrated acid has more acid particles in each cm³, so they collide with the magnesium surface more often: more successful collisions per second. (b) As the reaction goes on, acid particles are used up; with fewer left in the same volume, collisions happen less often and the rate falls.

A full answer includes

• (a) I wrote that there are more acid particles per unit volume.
• (a) I wrote that collisions with the magnesium are more frequent.
• (a) I did not write that the particles collide harder.
• (b) I wrote that the acid particles are used up as the reaction goes on.
• (b) I wrote that fewer particles per unit volume means less frequent collisions.
• (b) I did not write that the particles get tired or slow down.

Also accept

• More particles per unit volume, more frequent collisions; fewer left as it goes on

Watch for

• “The reaction slows down because the particles get tired.” — Particles do not slow down as the reaction goes on. There are just fewer acid particles left to collide.
• “More concentrated acid makes each collision more energetic.” — Concentration changes how often the particles collide, not how hard. Temperature changes the energy.

6. UK6 · Final check

Model answer. A catalyst lowers the activation energy, so more of the collisions that happen have enough energy to succeed. A higher concentration, a powder and a higher pressure all work by making collisions more frequent.

A full answer includes

• I chose adding a catalyst.
• I did not choose a change that makes collisions more frequent.

Also accept

• Adding a catalyst

Watch for

• “More concentrated acid makes a larger share of collisions succeed.” — More concentrated acid puts more particles in each cm³, so they collide more often. The share that succeed is the same.
• “A powder makes a larger share of collisions succeed.” — A powder exposes more particles, so collisions are more frequent. The share that succeed is the same.
• “A higher pressure makes a larger share of collisions succeed.” — A higher pressure crowds the gas particles, so they collide more often. The share that succeed is the same.