The Rate and Extent of Chemical Change (review)

1. Hydrogen peroxide (H2O2 ) can decompose in UV light to produce water and a gas. The gas will relight a glowing splint.

a) Identify the gas produced.

Answer: Oxygen can relight a glowing splint.

b) Manganese dioxide is a catalyst for this reaction. Explain the purpose of a catalyst.

Answer: A catalyst is a chemical that speeds up a reaction but does not get used up itself.

c) Suggest one observation for this reaction.

Answer: In this reaction you would observe bubbles (or effervesces, fizzing) as a gas is being made.

d) The figure below is an incomplete energy level diagram for this reaction. Label A, B, C and D.

Answer: A = reactants ; B = activation energy without catalyst ; C = activation energy with catalyst ; D = products

2. A student investigated the rate of reaction between calcium carbonate cubes and hydrochloric acid to produce a salt, water and carbon dioxide.

a) Write a word equation for this reaction.

Answer: calcium carbonate + hydrochloric acid  →  calcium chloride + water + carbon dioxide

b) Describe a chemical test to show that the gas produced was carbon dioxide.

Answer: To test for carbon dioxide, pass the gas through limewater (calcium hydroxide solution). If the solution turns from colourless to cloudy, it is carbon dioxide.

c) The student carried out the investigation twice. First, she used a single cube of calcium carbonate. Then she used four smaller cubes of calcium carbonate. The dimensions of the cubes are shown in the figure below. Calculate the surface area of cube (R) and the total volume of the cubes that make up (P).

Answer: 2 × 2 × 6 = 24 cm2  ;  2 × 2 × 2 = 8 cm3

d) Use your own knowledge of collision theory and the figure below to predict whether the cube (R) or the four smaller cubes (P) would have the fastest rate of reaction.

Answer: Both cubes have the same volume and therefore the same amount of calcium carbonate and will eventually make the same amount of products. However, (P) has a bigger surface area compared to cube (R). This means that there are more particles available for collision at any one time in (P). So, there will be more successful collisions in a given time and this will give a faster rate of reaction for (P).

e) Sketch a graph to show how the amount of gas collected would change throughout the reaction for the cube (R) and the four smaller cubes (P) that the student investigated.


3. Methane (CH4 ) can undergo a reversible reaction with steam to make hydrogen and carbon monoxide (CO). The reaction is endothermic. In a closed system, a dynamic equilibrium is established.

a) Write a balanced symbol equation for this reaction.

Answer: CH4 + H2O  ⇋  3H2 + CO

b) Describe the effect of increasing the temperature on this reaction.

Answer: The system will oppose the change. It will follow Le Chatelier’s principle and try to reduce the temperature. This will favour the forwards endothermic reaction and the position of equilibrium will move to the right and so increase the yield of the products.