Each question gives you data and asks you to argue from it. Try them on paper before opening the answers. They extend the thermochemistry chapter into reasoning about evidence.
Questions
Question 1. A reaction profile shows reactants at 50 kJ, a peak at 130 kJ and products at 20 kJ. With a catalyst the peak is at 100 kJ. State what changed and what stayed the same.
Answer
Ea without the catalyst = 130 − 50 = 80 kJ mol⁻¹. With the catalyst it is 100 − 50 = 50 kJ mol⁻¹, so Ea fell.
ΔH = 20 − 50 = −30 kJ mol⁻¹ in both cases, because the reactant and product levels did not move.
Question 2. A student says: “The catalyst makes the reaction more exothermic because it goes faster.” Correct this.
Answer
A catalyst changes the rate, not the amount of heat released. The energy given out depends on the reactant and product levels, and those do not change, so ΔH stays at −30 kJ mol⁻¹.
Going faster means the same heat is released in a shorter time.
Question 3. Two experiments use the same mass of marble chips and an excess of acid.
Experiment A (20 °C) collects 60 cm³ of gas in 100 s. Experiment B (40 °C) collects 60 cm³ in 45 s, and both stop at 60 cm³. Explain the result.
Answer
B is faster: 60 cm³ in 45 s is a higher rate than 60 cm³ in 100 s.
Both give the same total, 60 cm³, because the mass of marble, which limits the gas, is the same. Temperature changed how fast the reaction went, not how much gas the marble can make.
Question 4. A cup is needed for a heat experiment.
Material X conducts heat well and reacts with acid. Material Y conducts heat poorly and does not react with acid. Choose one and use the supplied properties.
Answer
Choose Y.
Its poor heat conduction reduces heat loss to the surroundings, so the temperature change is measured more accurately. Its lack of reaction with acid means the cup will not interfere with the reaction.
Question 5. A reaction takes 60 s at 30 °C, 35 s at 40 °C and 20 s at 50 °C. Calculate 1 ÷ time for each and state what the data shows and what it does not.
Answer
1 ÷ time: 1 ÷ 60 = 0.017 s⁻¹, 1 ÷ 35 = 0.029 s⁻¹ and 1 ÷ 20 = 0.050 s⁻¹.
The rate increases as the temperature rises, and each 10 °C step gives a larger increase.
The data does not show the cost of heating, safety, or what a factory would choose, because the experiment measured only time at three temperatures.
Question 6. A student claims from the data in Question 5: “A factory should always use 50 °C.” Is this supported?
Answer
No. The data shows only that the rate is higher at 50 °C in this experiment. It does not include cost, safety or the result at other temperatures.
A supported claim would be: “In this experiment the reaction is fastest at 50 °C.”
If you got these wrong
- Questions 1 and 2 use reading energy-level diagrams.
- Questions 3, 5 and 6 need the habit of quoting the data and naming its limits.
- Question 4 is about choosing a material from supplied properties.
Log your slips in the mistake log, and build a timed set with the timed original practice session builder. For a teacher to question your reasoning, see online one-to-one Chemistry tuition.