A rate question that says “explain” needs two things: the trend and the particle reason. “The reaction is faster at higher concentration” is only the trend. The marks usually sit in the sentence about collisions that follows.
This page gives a short routine and a worked example with numbers. It supports rates of reaction.
What is actually going wrong?
Most lost marks fall into one of these patterns:
- Trend only. “Higher temperature makes the reaction faster.” True, but it repeats the question.
- Collisions without the word successful. “More collisions” does not say why the extra collisions lead to reaction.
- Wrong cause. “The particles expand” or “the acid gets stronger” when only concentration or temperature changed.
- No reference to the same volume. Concentration means particles per unit volume, not just more particles in total.
All four are fixed by writing the answer in the same shape every time.
A three-part answer
- State the trend with the variable. “As concentration increases, the rate increases.”
- Name the particle change. “There are more particles in the same volume.” (Temperature: “the particles move faster and have more energy.” Surface area: “more particles are exposed at the surface.”)
- Link to successful collisions. “So collisions happen more often, and more successful collisions occur each second.”
Reading the three parts aloud is a good check. If part two is missing, you have described, not explained.
Worked example
A fictional experiment reacts magnesium ribbon with dilute acid and collects the hydrogen gas.
With acid of concentration 1.0 mol/dm³, 40 cm³ of gas is collected in 40 s. With 2.0 mol/dm³, the same 40 cm³ is collected in 20 s.
The temperature and the ribbon are the same. Calculate both mean rates and explain the difference.
Step 1, mean rate = volume of gas ÷ time.
- 1.0 mol/dm³: 40 ÷ 40 = 1.0 cm³/s
- 2.0 mol/dm³: 40 ÷ 20 = 2.0 cm³/s
Step 2, describe the trend. Doubling the concentration doubled the mean rate in this data.
Step 3, explain with particles. At 2.0 mol/dm³ there are more acid particles in the same volume of solution. Acid particles therefore collide with the magnesium surface more often, so more successful collisions occur each second and the reaction is faster.
Full answer: “The mean rate rose from 1.0 cm³/s to 2.0 cm³/s as concentration increased. There are more acid particles in the same volume, so collisions with the magnesium happen more often and more successful collisions happen each second.”
A quick check on the numbers: the same gas volume in half the time must be double the rate, and 2.0 ÷ 1.0 = 2. That agrees.
The mistake to watch for
Mistaken answer: “The reaction is faster because the concentration is higher, so there are more collisions.”
The trend is stated and the collision idea is half there, but it never says more particles in the same volume, and “more collisions” is missing the word successful.
The correction adds the particle change and the frequency. For example: “There are more particles in the same volume, so successful collisions happen more often”. It is one extra clause, and it is usually what the question is asking for.
Check yourself
1. Explain why the reaction is faster at 40 °C than at 20 °C.
Show answer
At the higher temperature the particles move faster and have more energy. They collide more often, and a greater proportion of the collisions have enough energy to react, so the frequency of successful collisions increases and the rate rises.2. The same mass of calcium carbonate reacts faster with acid as a powder than as large lumps. Explain.
Show answer
The powder has a larger surface area, so more carbonate particles are exposed to the acid. Acid particles collide with the solid more often, so the frequency of successful collisions is higher and the rate is greater. The mass is the same, so the amount that can react is unchanged.3. A reaction collects 48 cm³ of gas in the first 30 s and a further 18 cm³ in the next 30 s. Calculate the mean rate for each interval and explain why the second is lower.
Show answer
First interval: 48 ÷ 30 = 1.6 cm³/s. Second interval: 18 ÷ 30 = 0.6 cm³/s. The reactants are being used up, so their concentration falls. There are fewer particles in the same volume, collisions are less frequent, and the rate slows.What can you do next?
Use the mole and equation-ratio tutor to check the amounts behind a rate question and the equation balance reasoning trainer to confirm the equation you are explaining is balanced. To practise more of the skill, try the rates of reaction lessons.
If you can calculate rates but explanations still come out as trends, online one-to-one Chemistry tuition lets a teacher question each sentence as you write it. The Chemistry learning guide shows how rates connect to the other topics.