Exercises: Factors affecting reaction rate

Name four factors that can change the rate of a chemical reaction. For each factor, state whether increasing it (or adding it) normally makes a reaction faster or slower.

A student in a school in Galway adds a whole marble chip (calcium carbonate) to dilute hydrochloric acid in one beaker. In a second beaker she adds the same mass of marble powder to the same volume of the same acid at the same temperature.

1. a) In which beaker will the reaction be faster?

1. b) Explain your answer using the idea of surface area.

Explain in your own words what a catalyst is. Does a catalyst get used up in the reaction? Give one example of a catalyst used in everyday life or in industry.

A student in Cork measures the time taken for a piece of magnesium ribbon to disappear completely in $50\ \text{cm}^3$ of hydrochloric acid at different concentrations. The temperature is kept constant.

1. a) State the relationship between concentration and the time taken.

1. b) Predict the time for a concentration of $4.0\ \text{mol/dm}^3$.

1. c) Explain, using particles and collisions, why a higher concentration gives a faster reaction.

Sodium thiosulfate solution reacts with acid to form a cloudy yellow precipitate of sulfur. A student places the flask over a cross marked on paper and times how long it takes for the cross to disappear. The results are:

1. a) What is the effect of increasing the temperature by $10\ °\text{C}$ on the time taken?

1. b) Calculate the rate as $\dfrac{1}{\text{time}}$ in $\text{s}^{-1}$ at $25\ °\text{C}$ and at $45\ °\text{C}$.

1. c) Explain why particles react faster at a higher temperature.

Hydrogen peroxide, $\text{H}_2\text{O}_2$, decomposes slowly into water and oxygen. When a small amount of manganese(IV) oxide, $\text{MnO}_2$, is added, bubbles of gas form rapidly. After the reaction, the $\text{MnO}_2$ is filtered off, dried and found to have the same mass as at the start.

1. a) Write a word equation for the decomposition.

1. b) Name the role of $\text{MnO}_2$ and explain how the mass result supports your answer.

1. c) Describe a test to identify the gas produced.

A student investigates the reaction between calcium carbonate and excess hydrochloric acid by measuring the volume of carbon dioxide collected in a gas syringe.

1. a) Calculate the average rate of reaction (in $\text{cm}^3/\text{s}$) during the first $30\ \text{s}$ and between $90\ \text{s}$ and $120\ \text{s}$.

1. b) Explain why the rate decreases as the reaction proceeds.

1. c) Explain why the volume stops changing after $150\ \text{s}$.

1. d) The experiment is repeated with the same mass of calcium carbonate but using powder instead of chips. Sketch in words how the curve would differ and state what would be the final volume. Justify your answer.

A factory in Cork Harbour uses a solid catalyst in the form of a fine mesh rather than in large lumps, and operates the reaction at a temperature of $450\ °\text{C}$ rather than at room temperature.

1. a) Explain why a fine mesh of catalyst is more effective than large lumps.

1. b) Explain, using the idea of activation energy, how a catalyst increases the rate without being used up.

1. c) Suggest one reason why a company might choose not to raise the temperature much further, even though this would increase the rate.

Two experiments are carried out with $1.0\ \text{g}$ of zinc and excess dilute sulfuric acid. In experiment A the acid concentration is $1.0\ \text{mol/dm}^3$ at $20\ °\text{C}$. In experiment B the acid concentration is $1.0\ \text{mol/dm}^3$ at $40\ °\text{C}$.

1. a) Name the gas produced and write a balanced chemical equation.

1. b) Which experiment finishes first? Explain using collision theory, mentioning both the frequency and the energy of collisions.

1. c) Both experiments produce the same total volume of gas. Explain why.

1. d) Experiment C uses $1.0\ \text{g}$ of zinc with $0.5\ \text{mol/dm}^3$ acid at $20\ °\text{C}$ and the same volume of acid, still in excess. Predict how the initial rate compares with A and justify.