NATIONAL 4/5 CHEMISTRY CHEMICAL CHANGES AND STRUCTURE LESSON 2 MEASURING AND CALCULATING THE RATE OF REACTION

Previous Learning from S3: A working knowledge of the factors affecting rates of reaction is required for this Course. To compare rates of chemical reactions, changes in mass, volume and other quantities can be measured. Graphs can then be drawn to help this comparison. Reactions can be followed by measuring changes in concentration, mass and volume of reactants and products. The average rate of a reaction, or stage in a reaction, can be calculated from initial and final quantities and the time interval. The rate of a reaction, or stage in a reaction, is proportional to the reciprocal of the time taken, ie if the rate is high the time taken will be small, and vice versa. National 4 Chemistry: Rates of reaction Reactions monitored and graphs interpreted and rate calculated from a plotted graph Previous Learning from S3: Through experimentation, I can identify indicators of chemical reactions having occurred. I can describe ways of controlling the rate of reactions and can relate my findings to the world around me. SCN 3-19a

True or false?

Calculating the Rate of Reaction The rate of reaction can be calculated by measuring the : 1. Time taken for the reaction and 2. One other variable (something that changes) e.g. Volume (cm3) Mass (g) Concentration (moles per litre = moll-1) Rate = Change (in variable) Time taken

Example 1 Use the graph below to calculate the rate of reaction for the first 20 seconds. Average rate = (change) / time 20 cm3/ 20 s 1 cm3 S-1

Example 2 Use the graph below to calculate the rate of reaction between 5.2 and 10.8 seconds. Average rate = (change) / time 0.4 - 0.2/ (10.8 - 5.2) 0.2 mol l-1/ 5.6 s 0.036 mol l-1 s-1

Rates and Graphs These show the increasing amount of product or the decreasing amount of reactant. Amount of product Time Amount of reactant Time Steep gradient Fast reaction Shallow gradient Slow reaction Steep gradient Fast reaction Shallow gradient Slow reaction

Rate graphs and reactant concentrations Amount of product Time All product Mix of reactant And product Reactant Concentration falls Rate of Reaction falls reactants product Gradient of graph decreases All reactant

Calcium carbonate + hydrochloric acid  calcium chloride + carbon dioxide + water CaCO3 + HCL  CaCl2 + CO2 + H2O Time (s) Decrease in mass (g) Concentration of CO2 - 4.00 30 1.25 2.86 60 2.92 2.25 90 2.40 1.82 120 3.76 1.49 150 3.06 1.22 180 3.32 0.98 210 3.52 0.80 240 3.68 0.65 270 3.81 0.54 300 3.92 0.44 360 4.11 0.27 420 4.23 0.15 480 4.30 0.09 540 4.35 0.04 600 Time (s) Volume of CO2 (cm3) 30 60 90 130 120 142 150 154 180 162 210 171 240 173 270 174 300 177 360 420 195 480 200 540 600 Activity 1 plot either of these graphs on graph paper and work out the average rate for the first 120 seconds and then between 300 – 600 seconds

Rate and Time For some reactions, a colour change will indicate the end of the reaction. The only measurement carried out during this type of experiment is time. Therefore the rate equation is slightly changed from: No other variable measured Rate = Change (in variable) Time taken To: Rate = 1 Time taken Units = per second (s-1)

Concentration of Potassium Iodide (KI) Mol/l Concentration of sodium thiosulphate and the time taken for the cross to disappear Concentration of Potassium Iodide (KI) Mol/l Time (S) Rate (1/t) S-1 1.0 23 0.8 29   0.6 39 0.4 60 0.2 111 Activity 2 - Plot a Time vs Rate Graph for this reaction and identify the relationship between concentration and rate of reaction 0.0435 0.0345 0.027 0.02 0.009

Rate = (1/t) S-1 Concentration of sodium thiosulphate (mol/l) Q) How long did it take for the cross to be obscured when the concentration of sodium thiosulphate was 0.7 mol / l? A) Rate = 1/t… so we must rearrange the equation to Time = 1/rate and obtain the rate from the graph 0.7 mol l = 0.032 S-1 ; Time = 1/0.032  31.25 seconds