WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.169 Unit S C 18: Rates of reaction. 00:00:09.833 --> 00:00:14.329 Reaction rate describes how quickly reactants are used up or products are formed. 00:00:14.479 --> 00:00:20.569 Average rate equals amount of change divided by time taken, with units appropriate to the measurement. 00:00:23.233 --> 00:00:33.174 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). 00:00:35.833 --> 00:00:43.969 For 40 cubic centimetres gas in 20 seconds, average rate is 40 divided by 20 equals 2 cubic centimetres per second. 00:00:44.119 --> 00:00:50.093 A mass loss of 0.6 grams in 30 seconds gives 0.02 grams per second. 00:00:52.767 --> 00:00:56.589 On a product-volume versus time graph, gradient gives rate. 00:00:56.739 --> 00:01:03.841 A steeper curve means faster production; the plateau means gas production has stopped, usually because a reactant is used up. 00:01:03.991 --> 00:01:08.939 Same reactant amounts and completion; speed differs but final yield need not. 00:01:13.600 --> 00:01:20.223 The rate at one particular moment (instantaneous rate) is the gradient of a tangent to the curve at that time. 00:01:20.373 --> 00:01:30.355 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). 00:01:30.505 --> 00:01:35.379 Choose a sufficiently large triangle; the tangent estimates rate at one instant. 00:01:35.529 --> 00:01:39.747 Tangent geometry is calculated at a point on this schematic curve. 00:01:44.400 --> 00:01:52.762 On a reactant-mass graph the gradient can be negative because mass falls; rate of consumption is usually reported as a positive magnitude. 00:01:52.912 --> 00:01:55.280 Check what the vertical axis measures. 00:01:57.933 --> 00:02:04.802 Reacting particles must collide with sufficient energy, at least the activation energy, for a successful reaction. 00:02:04.952 --> 00:02:07.458 Not every collision produces reaction. 00:02:10.133 --> 00:02:15.476 Higher concentration means more reactant particles per volume and more frequent collisions. 00:02:15.626 --> 00:02:19.081 Keep other variables controlled when explaining the effect. 00:02:21.733 --> 00:02:28.069 Higher pressure for reacting gases puts more particles into a given volume, increasing collision frequency. 00:02:28.219 --> 00:02:32.109 This does not apply directly to merely pressing a solid reactant. 00:02:34.767 --> 00:02:39.320 Smaller solid pieces have a larger surface area for the same total mass. 00:02:39.470 --> 00:02:45.505 More exposed particles are available for collisions, so powder usually reacts faster than large lumps. 00:02:45.655 --> 00:02:51.531 Schematic faces represent cubes; cutting increases exposed area at fixed total volume. 00:02:56.200 --> 00:03:00.958 Increasing temperature makes particles move faster and collide more frequently. 00:03:01.108 --> 00:03:06.910 More importantly, a greater fraction of collisions have enough energy to overcome activation energy. 00:03:07.060 --> 00:03:10.905 Temperature affects both frequency and the energy distribution. 00:03:15.567 --> 00:03:21.731 As reactants are used up, concentration or available solid decreases, so the rate often slows. 00:03:21.881 --> 00:03:25.047 A reaction graph commonly becomes less steep with time. 00:03:27.700 --> 00:03:32.075 Smaller solid pieces or a catalyst can make a reaction finish sooner. 00:03:32.225 --> 00:03:38.976 They do not change the final amount of product if the same amounts of reactants react completely in the same reaction. 00:03:41.633 --> 00:03:50.987 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. 00:03:51.137 --> 00:03:54.569 Carbon dioxide volume can be recorded against time. 00:03:57.233 --> 00:04:02.123 Use a conical flask connected through a stopper and delivery tube to a gas syringe. 00:04:02.273 --> 00:04:09.397 Check connections for leaks and ensure the plunger moves freely; do not create a sealed pressure vessel with no expansion route. 00:04:09.547 --> 00:04:13.115 Check for leaks; gas must be able to expand into the syringe. 00:04:17.767 --> 00:04:24.571 Measure acid volume and concentration, add measured marble chips, connect promptly and start the timer consistently. 00:04:24.721 --> 00:04:27.419 Record gas volume at regular intervals. 00:04:30.100 --> 00:04:38.344 To compare surface area, use the same mass of marble as different-sized chips, with the same acid volume and concentration and temperature. 00:04:38.494 --> 00:04:43.389 If acid is sufficient for complete reaction, final gas volume should be the same. 00:04:46.067 --> 00:04:51.805 To compare acid concentration, control acid volume, chip mass and size and temperature. 00:04:51.955 --> 00:05:00.253 Make sure the chosen limiting reactant is understood: changing concentration can also change final yield if acid limits reaction. 00:05:02.933 --> 00:05:08.862 An alternative is recording mass loss as C O 2 escapes from an unsealed flask on a balance. 00:05:09.012 --> 00:05:14.921 A loose cotton-wool plug can reduce spray without trapping the gas; cotton wool is not a gas-tight stopper. 00:05:17.600 --> 00:05:21.847 Use eye protection, appropriate dilute acid and teacher supervision. 00:05:21.997 --> 00:05:29.271 Repeat trials, compare results and investigate anomalies rather than discarding inconvenient readings without reason. 00:05:31.933 --> 00:05:37.873 Sodium thiosulfate reacts with hydrochloric acid to form a cloudy sulfur precipitate. 00:05:38.023 --> 00:05:42.811 Place a flask over a drawn cross and time until the cross can no longer be seen. 00:05:42.961 --> 00:05:48.102 Use the same depth, cross and viewing conditions; endpoint judgement has uncertainty. 00:05:52.767 --> 00:06:00.968 The method uses the same visual endpoint to compare relative rate: for the same endpoint, rate is proportional to one divided by time. 00:06:01.118 --> 00:06:05.587 Reciprocal time has units per second, not a measured gas-volume rate. 00:06:08.267 --> 00:06:15.497 To vary thiosulfate concentration, dilute measured portions with water while keeping total mixture volume the same. 00:06:15.647 --> 00:06:21.171 Keep acid volume and concentration, temperature, flask and viewing conditions controlled. 00:06:23.833 --> 00:06:28.153 Record the time from consistent mixing to disappearance of the cross. 00:06:28.303 --> 00:06:35.715 A shorter time indicates faster reaction; repeat and use a suitable mean, noting uncertainty in judging the endpoint. 00:06:38.367 --> 00:06:45.864 The reaction releases sulfur dioxide, so use suitable ventilation, small volumes and the specified safe school procedure. 00:06:46.014 --> 00:06:49.757 Stop and follow the teacher's guidance rather than breathing fumes. 00:06:52.433 --> 00:06:57.436 The cross disappearing is a visual endpoint, not necessarily the end of the reaction. 00:06:57.586 --> 00:07:03.623 How cloudy the liquid is, the lighting and the observer’s judgement all affect when the cross seems to disappear. 00:07:06.300 --> 00:07:10.089 A catalyst speeds up a reaction and is not used up overall. 00:07:10.239 --> 00:07:13.849 It is chemically unchanged and has the same mass at the end. 00:07:13.999 --> 00:07:18.885 It may react in individual steps, but is made again (regenerated) by the end. 00:07:21.567 --> 00:07:29.245 A catalyst provides an alternative reaction pathway with lower activation energy, so more collisions succeed at the same temperature. 00:07:29.395 --> 00:07:32.007 It does not make every collision successful. 00:07:32.157 --> 00:07:38.204 The catalyst is regenerated overall; final products and equilibrium position are unchanged. 00:07:42.867 --> 00:07:50.203 For a complete reaction with the same limiting-reactant amount, a catalyst does not increase theoretical final product amount. 00:07:50.353 --> 00:07:54.159 It also does not change equilibrium yield under fixed conditions. 00:07:56.833 --> 00:08:02.304 Catalysts can allow useful industrial rates at lower temperatures, reducing energy costs. 00:08:02.454 --> 00:08:07.242 A suitable catalyst depends on the reaction; there is no universal catalyst. 00:08:09.900 --> 00:08:17.439 Vehicle catalytic converters convert pollutants such as carbon monoxide and nitrogen oxides to less harmful gases. 00:08:17.589 --> 00:08:23.988 Carbon dioxide produced still contributes to greenhouse effects, so “all products harmless” is too broad. 00:08:26.667 --> 00:08:29.227 Enzymes are biological catalysts. 00:08:29.377 --> 00:08:37.547 Yeast enzymes catalyse fermentation of sugars in alcoholic-drink production; temperature and conditions affect enzyme activity. 00:08:40.200 --> 00:08:43.144 For a rate experiment with and without a catalyst, 00:08:43.294 --> 00:08:46.366 keep reactant amounts and other conditions the same, 00:08:46.516 --> 00:08:51.358 then compare rates rather than just noting a different total time without controls. 00:08:54.033 --> 00:08:56.219 That completes Rates of reaction. 00:08:56.369 --> 00:08:59.715 Revisit the notes and test yourself on the revision website.