WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.221 Unit C C 14: Rates of reaction. 00:00:09.900 --> 00:00:14.396 Reaction rate describes how quickly reactants are used up or products are formed. 00:00:14.546 --> 00:00:20.635 Average rate equals amount of change divided by time taken, with units appropriate to the measurement. 00:00:23.300 --> 00:00:33.241 Choose a method that fits the reaction: measure gas volume with a gas syringe, mass loss on a balance if gas escapes, or a change in colour or cloudiness (turbidity). 00:00:35.900 --> 00:00:44.036 For 40 cubic centimetres gas in 20 seconds, average rate is 40 divided by 20 equals 2 cubic centimetres per second. 00:00:44.186 --> 00:00:50.159 A mass loss of 0.6 grams in 30 seconds gives 0.02 grams per second. 00:00:52.833 --> 00:00:56.655 On a product-volume versus time graph, gradient gives rate. 00:00:56.805 --> 00:01:03.907 A steeper curve means faster production; the plateau means gas production has stopped, usually because a reactant is used up. 00:01:04.057 --> 00:01:09.006 Same reactant amounts and completion; speed differs but final yield need not. 00:01:13.667 --> 00:01:20.289 The rate at one particular moment (instantaneous rate) is the gradient of a tangent to the curve at that time. 00:01:20.439 --> 00:01:30.421 Average rate over an interval is change in measured quantity divided by change in time: the gradient of a straight line joining the two ends of that interval (a secant). 00:01:30.571 --> 00:01:35.446 Choose a sufficiently large triangle; the tangent estimates rate at one instant. 00:01:35.596 --> 00:01:39.813 Tangent geometry is calculated at a point on this schematic curve. 00:01:44.467 --> 00:01:52.829 On a reactant-mass graph the gradient can be negative because mass falls; rate of consumption is usually reported as a positive magnitude. 00:01:52.979 --> 00:01:55.347 Check what the vertical axis measures. 00:01:58.000 --> 00:02:04.869 Reacting particles must collide with sufficient energy, at least the activation energy, for a successful reaction. 00:02:05.019 --> 00:02:07.525 Not every collision produces reaction. 00:02:10.200 --> 00:02:15.543 Higher concentration means more reactant particles per volume and more frequent collisions. 00:02:15.693 --> 00:02:19.147 Keep other variables controlled when explaining the effect. 00:02:21.800 --> 00:02:28.135 Higher pressure for reacting gases puts more particles into a given volume, increasing collision frequency. 00:02:28.285 --> 00:02:32.175 This does not apply directly to merely pressing a solid reactant. 00:02:34.833 --> 00:02:39.387 Smaller solid pieces have a larger surface area for the same total mass. 00:02:39.537 --> 00:02:45.572 More exposed particles are available for collisions, so powder usually reacts faster than large lumps. 00:02:45.722 --> 00:02:51.598 Schematic faces represent cubes; cutting increases exposed area at fixed total volume. 00:02:56.267 --> 00:03:01.025 Increasing temperature makes particles move faster and collide more frequently. 00:03:01.175 --> 00:03:06.977 More importantly, a greater fraction of collisions have enough energy to overcome activation energy. 00:03:07.127 --> 00:03:10.971 Temperature affects both frequency and the energy distribution. 00:03:15.633 --> 00:03:21.798 As reactants are used up, concentration or available solid decreases, so the rate often slows. 00:03:21.948 --> 00:03:25.114 A reaction graph commonly becomes less steep with time. 00:03:27.767 --> 00:03:32.142 Smaller solid pieces or a catalyst can make a reaction finish sooner. 00:03:32.292 --> 00:03:39.043 They do not change the final amount of product if the same amounts of reactants react completely in the same reaction. 00:03:41.700 --> 00:03:51.053 Calcium carbonate marble reacts with hydrochloric acid: C A C O 3 plus 2 H C L produces C A C L 2 plus H 2 O plus C O 2. 00:03:51.203 --> 00:03:54.635 Carbon dioxide volume can be recorded against time. 00:03:57.300 --> 00:04:02.190 Use a conical flask connected through a stopper and delivery tube to a gas syringe. 00:04:02.340 --> 00:04:09.463 Check connections for leaks and ensure the plunger moves freely; do not create a sealed pressure vessel with no expansion route. 00:04:09.613 --> 00:04:13.182 Check for leaks; gas must be able to expand into the syringe. 00:04:17.833 --> 00:04:24.638 Measure acid volume and concentration, add measured marble chips, connect promptly and start the timer consistently. 00:04:24.788 --> 00:04:27.486 Record gas volume at regular intervals. 00:04:30.167 --> 00:04:38.411 To compare surface area, use the same mass of marble as different-sized chips, with the same acid volume and concentration and temperature. 00:04:38.561 --> 00:04:43.455 If acid is sufficient for complete reaction, final gas volume should be the same. 00:04:46.133 --> 00:04:51.871 To compare acid concentration, control acid volume, chip mass and size and temperature. 00:04:52.021 --> 00:05:00.319 Make sure the chosen limiting reactant is understood: changing concentration can also change final yield if acid limits reaction. 00:05:03.000 --> 00:05:08.929 An alternative is recording mass loss as C O 2 escapes from an unsealed flask on a balance. 00:05:09.079 --> 00:05:14.987 A loose cotton-wool plug can reduce spray without trapping the gas; cotton wool is not a gas-tight stopper. 00:05:17.667 --> 00:05:21.914 Use eye protection, appropriate dilute acid and teacher supervision. 00:05:22.064 --> 00:05:29.338 Repeat trials, compare results and investigate anomalies rather than discarding inconvenient readings without reason. 00:05:32.000 --> 00:05:37.940 Sodium thiosulfate reacts with hydrochloric acid to form a cloudy sulfur precipitate. 00:05:38.090 --> 00:05:42.878 Place a flask over a drawn cross and time until the cross can no longer be seen. 00:05:43.028 --> 00:05:48.169 Use the same depth, cross and viewing conditions; endpoint judgement has uncertainty. 00:05:52.833 --> 00:06:01.035 The method uses the same visual endpoint to compare relative rate: for the same endpoint, rate is proportional to one divided by time. 00:06:01.185 --> 00:06:05.654 Reciprocal time has units per second, not a measured gas-volume rate. 00:06:08.333 --> 00:06:15.563 To vary thiosulfate concentration, dilute measured portions with water while keeping total mixture volume the same. 00:06:15.713 --> 00:06:21.238 Keep acid volume and concentration, temperature, flask and viewing conditions controlled. 00:06:23.900 --> 00:06:28.220 Record the time from consistent mixing to disappearance of the cross. 00:06:28.370 --> 00:06:35.782 A shorter time indicates faster reaction; repeat and use a suitable mean, noting uncertainty in judging the endpoint. 00:06:38.433 --> 00:06:45.931 The reaction releases sulfur dioxide, so use suitable ventilation, small volumes and the specified safe school procedure. 00:06:46.081 --> 00:06:49.824 Stop and follow the teacher's guidance rather than breathing fumes. 00:06:52.500 --> 00:06:57.503 The cross disappearing is a visual endpoint, not necessarily the end of the reaction. 00:06:57.653 --> 00:07:03.689 How cloudy the liquid is, the lighting and the observer’s judgement all affect when the cross seems to disappear. 00:07:06.367 --> 00:07:10.156 A catalyst speeds up a reaction and is not used up overall. 00:07:10.306 --> 00:07:13.916 It is chemically unchanged and has the same mass at the end. 00:07:14.066 --> 00:07:18.952 It may react in individual steps, but is made again (regenerated) by the end. 00:07:21.633 --> 00:07:29.311 A catalyst provides an alternative reaction pathway with lower activation energy, so more collisions succeed at the same temperature. 00:07:29.461 --> 00:07:32.074 It does not make every collision successful. 00:07:32.224 --> 00:07:38.271 The catalyst is regenerated overall; final products and equilibrium position are unchanged. 00:07:42.933 --> 00:07:50.270 For a complete reaction with the same limiting-reactant amount, a catalyst does not increase theoretical final product amount. 00:07:50.420 --> 00:07:54.226 It also does not change equilibrium yield under fixed conditions. 00:07:56.900 --> 00:08:02.371 Catalysts can allow useful industrial rates at lower temperatures, reducing energy costs. 00:08:02.521 --> 00:08:07.309 A suitable catalyst depends on the reaction; there is no universal catalyst. 00:08:09.967 --> 00:08:17.505 Vehicle catalytic converters convert pollutants such as carbon monoxide and nitrogen oxides to less harmful gases. 00:08:17.655 --> 00:08:24.055 Carbon dioxide produced still contributes to greenhouse effects, so “all products harmless” is too broad. 00:08:26.733 --> 00:08:29.293 Enzymes are biological catalysts. 00:08:29.443 --> 00:08:37.613 Yeast enzymes catalyse fermentation of sugars in alcoholic-drink production; temperature and conditions affect enzyme activity. 00:08:40.267 --> 00:08:43.211 For a rate experiment with and without a catalyst, 00:08:43.361 --> 00:08:46.433 keep reactant amounts and other conditions the same, 00:08:46.583 --> 00:08:51.425 then compare rates rather than just noting a different total time without controls. 00:08:54.100 --> 00:08:56.286 That completes Rates of reaction. 00:08:56.436 --> 00:08:59.781 Revisit the notes and test yourself on the revision website.