Topic 9

Question 1

What is instantaneous rate?

Answer 1

The rate at a particular instant of the reaction is called the instantaneous rate.

Question 2

Describe the characteristics of a rate curve. [3 M]

Answer 2

• At the beginning, the slope is steepest because the rate of reaction is faster since the concentration of reactants is higher.
• In the middle, the slope becomes less steep because some of the reactant is used up and the reaction rate decreases.
• In the end, the slope equals zero since the reaction rate equals zero as all the reactants have been used up.

Question 3

If a mixture of gases is allowed to react at the same temperature but in a closed container of larger volume instead, will the time required for the reaction to complete remain the same? Explain. [1M]

Answer 3

The time required will become longer.
In a larger container, the concentrations of reactants become less and hence the collision frequency decreases.

Question 4

Explain why the collisions between two molecules will not necessary lead to a reaction. [2M]

Answer 4

Colliding molecules will undergo reaction only if they
- possess an energy greater than the activation energy and
- collide in the right orientation.

Question 5

2MnO₄^- (aq) + 5C₂O₄^2- (aq) + 16H⁺ (aq) → 2Mn²⁺ (aq) + 10CO₂ (g) + 8H₂O (l)
What evidence from the concentration-time graph of MnO₄^- supports that one of the products may have catalyzed the reaction? Explain. [2M]

Answer 5

• The rate of consumption of MnO4 (aq) ions is slow at the beginning (from 0 to 180 s) and then increases rapidly. [1 M]
• It is likely to be due to the building up of the concentration of the product which catalyses the reaction. [1 M]

Question 6

2MnO₄^- (aq) + 5C₂O₄^2- (aq) + 16H⁺ (aq) → 2Mn²⁺ (aq) + 10CO₂ (g) + 8H₂O (l)

Suggest how to show whether or not Mn²⁺ (aq) is a catalyst for this reaction. [2 M]

Answer 6

• Repeat the experiment with a few drops of Mn²⁺ (aq) firstly added to the reaction mixture. [1 M]
• Consumption of MnO₄^- (aq) ions will be faster at the beginning if Mn²⁺ (aq) is a catalyst. [1 M]

Question 7

In an experiment, 50 cm³ of 2.0 M HCl (aq) was added to a conical flask containing 2.0 g of Zn (s) powder.

Explain qualitatively the effect on the initial rate of the reaction of replacing the 2.0 M HCl (aq) with 2.0 M H₂SO₄ (aq). [1M]

Answer 7

HCl is a monobasic acid, while H₂SO₄ is a dibasic acid. Initial rate increases if H₂SO₄ is used.

Question 8

You are provided with common laboratory apparatus, calcium carbonate and 1 M hydrochloric acid. Outline how you would perform a fair comparison in studying the effect of different concentrations of acid on the rate of production of carbon dioxide from the following reaction:

CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + H₂O (l) + CO₂ (g)

(5 marks)
Note: 1 mark is for communication.

[2014 DSE]

Answer 8

Measurement:
- Proper way to follow the progress of the reaction (e.g. measure the volume of CO₂ (g) evolved / measure the loss in mass of the reaction mixture over a certain time interval / measure the pressure of the CO₂ (g) formed in a sealed reaction container.) [1 M] (Accept graphical representation; no communication mark if no description about time.)

Experiment process:
- Dilute 1 M HCl to different concentrations by adding water [1 M]
- Repeat the experiment with the diluted HCl. [1 M]

Controlled variable:
State one condition for performing fair comparison (e.g. CaCO₃ (s) used should be of the same amount / under same experimental conditions such as same temperature or pressure.) [1 M]

Communication mark [1 M]
Chemical knowledge = 0 to 2, communication = 0;
Chemical knowledge = 3 to 4, communication = 0 or 1
Incomplete answer / difficult to understand, communication mark = 0