Unit 4 Rates, equilibria and further organic chemistry

Recall factors that affect the rate of reaction By the end of the lesson you should be able to: Recall factors that affect the rate of reaction Describe different techniques to measure the rate of reaction Suggest and justify suitable techniques for measuring rate of reaction Compare two techniques for measuring gas volumes, taking into consideration accuracy and gas properties

In preparation for this lesson You were asked to: Review the factors that affect the rate of reactions Make notes on the experimental techniques for measuring the rate of reactions Check point: You have been given a number of reactions on the method you would use to measure the rate. confident? Justify your choice

Using a gas syringe Colorimetry Titrimetric analysis The reaction of granulated zinc with dilute hydrochloric acid The reaction of copper(II) sulphate solution with granulated zinc metal Using a gas syringe Could you draw it? Colorimetry The reaction of bromoethane with sodium hydroxide solution to produce ethanol and sodium bromide solution Gas phase decomposition of N2O5 forming NO2 gas and oxygen Titrimetric analysis Can you suggest a method? Using a closed flask and monitoring the volume change

RATES OF REACTION Rate of change of concentration of a named reactant or product with respect to time Rate of reaction = Time / s Concn / mol dm-3 Rate at this point in time X Y = Gradient of tangent = Y / X Rate is measured in mol dm-3 s-1 Rate decreases as time proceeds

“Initial Rate” = the rate at the start of a reaction = the gradient of a concentration-time graph at t=0 “Initial rates” are preferred because, at the start of a reaction, ALL variables (concentrations, temperature, catalyst etc) are known by design. Time / s Concn / mol dm-3 X Y Gradient of tangent (Y/X) at t = 0 = "INITIAL RATE"

C B A 1 Which has fastest initial rate, A, B or C? A B C 2 For reactions A & B, how does rate vary with time? Inc Dec Constant 3 For reaction C, how does rate vary with time? 4 What is the initial rate for reaction A? 0.100 8.0 0.0125

Monitoring can be: (1) Continuous (2) For initial rate only A single reaction mixture is used and the concentration of the chosen reactant or product is measured at regular time intervals throughout the course of the reaction. Rate at time t = gradient of concentration-time graph at time = t. Several different reaction mixtures are used. Each with different value of variable being studied, e.g. temp. For each, the time taken (trx) for a fixed, small change in concentration (ΔC) of the chosen reactant or product is measured.

e.g. CONTINUOUS MONITORING for Mg + 2HCl  MgCl2 + H2 Fixed volume 0.1M HCl(aq) reacted with fixed amount of Mg at fixed temperature. Volume H2 collected in gas syringe measured at 5s intervals. 5 1.5 5 2.5 Vol H2 formed / cm3 5.0 10.0 15.0 20.0 Rate of H2 formation / cm3 s-1 5 3 4.0/5 = 0.80 5 4 3.0/5 = 0.60 2.5/5 = 0.50 1.5/5 = 0.30

Magnesium metal was reacted with dilute hydrochloric acid in an experiment to follow the rate of the reaction. 1.0g of magnesium ribbon was added to 50cm3 of 0.12mol dm-3 acid and the hydrogen gas collected in a gas syringe. The volume of gas collected at set time intervals was recorded as follows (1 mole of gas at rtp = 24 dm3) Time / s Vol of H2 / cm3 20 25 40 43 60 55 80 64 100 69 120 71 140 72 160 Draw a diagram of suitable apparatus for this reaction Plot a graph of these results How many moles of Mg ribbon were used? How many moles of hydrochloric acid were present at the start of this reaction? What volume of gas was produced by the end? Show, by calculation that this is the expected volume of gas at reaction completion Calculate the initial rate for this reaction

Recall factors that affect the rate of reaction By the end of the lesson you should be able to: Recall factors that affect the rate of reaction Describe different techniques to measure the rate of reaction Suggest and justify suitable techniques for measuring rate of reaction Compare two techniques for measuring gas volumes, taking into consideration accuracy and gas properties Which outcomes have you met today? Further questions can be found on page 13 of your textbook

Progress Checker

Answers: a. There is an increase in number of ions (1 mark) so follow the reaction by measuring electrical conductivity (1 mark) OR there is an increase in H+ concentration (1 mark) so follow with a pH meter (1 mark). b. To find the rate of reaction plot a graph of propanone concentration against time (1 mark). The gradient of the tangent to the line is equal to the rate of reaction (1 mark) so draw a tangent at a particular time and calculate the gradient (1 mark). 2. a. Graph with concentration on y-axis and time on x-axis (1 mark) with points plotted accurately (1 mark); smooth fit curve (1 mark) with tangent drawn at 15s (1 mark); 1 mark for rate within range 0.004 ± 0.001, 1 mark for correct units of mol dm-3 s-1. b. The volume of nitrogen (1 mark), using a gas syringe (1 mark) OR the pH (1 mark) using a pH meter (1 mark).