HIGHER CHEMISTRY REVISION. Unit 1:- Reaction Rates 1. A student was asked to write a plan of the procedure for an investigation. The entry made in her laboratory notebook is shown below. (a) This method does not keep the overall volume of the reaction mixture constant. Do not use 1 measuring cylinder to measure out all the solutions. Use separate ones with the size related to the volumes of solutions used. Start timing as soon as the hydrogen peroxide is added to the other reactants. Why is instruction 4 not the best way of altering the concentration of the potassium iodide solution? State two other ways of improving the student’s plan of this investigation procedure.

The balanced equation for the decomposition of hydrogen peroxide into water and oxygen is: 2H2O2 (l)  2H2O(l) + O2 (g) The following graph is obtained for the volume of oxygen released over time. Calculate the average rate of reaction between 10 s and 20 s. Rate = 36-24/10 = 1.2 cm3 s-1.

3. The decomposition of hydrogen peroxide solution into water and oxygen can be catalysed by an enzyme. 2H2O2 (l)  2H22O(l) + O2 (g) The rate of reaction can be followed by recording the mass loss over a period of time. (a) The following graph was obtained from experiments to find the effect of pH on the efficiency of the enzyme. (a) (i) rate = (60.01)-(59.68)/20 = 0.165g min-1. (ii) The enzyme is most efficient between a pH of 7 and 10. (b) The rate could be measured by collecting the oxygen gas in a gas syringe and plotting the volume of gas collected against time. (i) Calculate the average rate of the reaction over the first 20 minutes, in g min-1, for the experiment at pH 10. (ii) From the results shown on the graph, what can be concluded about the efficiency of the enzyme over the pH range used in the experiment? (b) Give one other way of following the rate of this reaction

Identify the graph which shows how the rate of reaction varies with 4. The grid shows the possible effect of temperature change on reaction rate. Identify the graph which shows how the rate of reaction varies with temperature in the decomposition of hydrogen peroxide solution using catalase, an enzyme, found in potatoes. A B C D E F Graph in box D.

5. The effect of temperature changes on reaction rate can be studied using the reaction between an organic acid solution and acidified potassium permanganate solution. 5(COOH)2(aq) + 6H+(aq) + 2MnO4-(aq)  2Mn2+(aq) + 10CO2(g) + 8H2O(l) The apparatus required is shown in the diagram. (a) Name the organic acid. (b) Describe how the reaction time can be measured. (c) The headings for a set of results are shown below. Temperature of reaction /°C Reaction time /s Reaction rate / Complete the headings by entering the correct unit in the third column. Oxalic acid. Start timing when solutions are mixed and stop timing when purple colour appears. s-1.

6. Methanoic acid, HCOOH, can break down to carbon monoxide and water by two different reactions, A and B. Reaction A (catalysed) HCOOH(aq) + H+(aq)  CO(g) + H2O(l) + H+ (aq) Reaction B (uncatalysed) HCOOH(aq)  CO(g) + H2O(l) (a) (i) What is the evidence in the equation for Reaction A that the H +(aq) ion acts as a catalyst? (ii) Explain whether reaction A is an example of heterogeneous or homogeneous catalysis. ( b) The energy diagram for the catalysed reaction is:- Copy the diagram and draw a line to show the reaction pathway for the uncatalysed reaction. (a) (i) Hydrogen ions are not used up in the reaction. (ii) Homogeneous – the catalyst and reactant are in the same physical state. (b) The uncatalysed reaction has a higher activation energy.