WEBVTT 00:00:00.500 --> 00:00:03.976 Welcome to GCSE Edexcel Science revision. 00:00:04.126 --> 00:00:07.181 Unit C C 12: Dynamic equilibria. 00:00:09.833 --> 00:00:15.591 A reversible reaction can proceed in both directions: products can react to reform reactants. 00:00:15.741 --> 00:00:19.619 The reversible arrow symbol shows forward and reverse processes. 00:00:22.300 --> 00:00:25.254 Changing conditions can favour one direction. 00:00:25.404 --> 00:00:31.387 Reversibility is chemical reaction in both directions, not just two unrelated processes. 00:00:34.067 --> 00:00:45.350 When ammonium chloride is heated, it can decompose to ammonia and hydrogen chloride gases: N H 4 C L solid reversibly produces N H 3 gas plus H C L gas. 00:00:48.000 --> 00:00:53.471 On cooling, ammonia and hydrogen chloride recombine to form solid ammonium chloride. 00:00:53.621 --> 00:00:57.114 The second gas is hydrogen chloride, not hydrogen. 00:00:57.264 --> 00:01:01.776 The gaseous products are ammonia and hydrogen chloride, not hydrogen. 00:01:06.433 --> 00:01:13.504 The forward and reverse reactions have opposite energy changes: if one is exothermic, the reverse is endothermic. 00:01:13.654 --> 00:01:16.513 Heating can favour the endothermic direction. 00:01:19.167 --> 00:01:28.947 This decomposition is a supervised demonstration using suitable ventilation and safety controls; the gases are not suitable for an unsupervised home experiment. 00:01:31.600 --> 00:01:38.490 A reversible reaction can reach dynamic equilibrium in a closed system, where reactants and products cannot escape. 00:01:41.167 --> 00:01:49.485 At equilibrium the forward and reverse reactions continue at equal rates. “Dynamic” means reactions are still happening at particle level. 00:01:52.167 --> 00:01:58.661 The amounts or concentrations of reactants and products remain constant while conditions remain unchanged. 00:01:58.811 --> 00:02:01.137 They do not have to be equal to each other. 00:02:03.800 --> 00:02:09.538 Starting with reactants only, the forward rate can initially be high while the reverse rate is zero. 00:02:09.688 --> 00:02:13.469 As product forms, the reverse rate rises until both match. 00:02:13.619 --> 00:02:18.546 At equilibrium reactions continue; their rates match rather than becoming zero. 00:02:23.200 --> 00:02:29.076 If products escape from an open system, the reverse reaction may not establish the same equilibrium. 00:02:29.226 --> 00:02:31.530 Closed-system conditions matter. 00:02:34.200 --> 00:02:39.362 The equilibrium position describes how much reactant and product the mixture contains. 00:02:39.512 --> 00:02:46.977 An equilibrium further to the right has a greater proportion of products; further to the left means a greater proportion of reactants. 00:02:49.633 --> 00:02:58.731 A change in conditions disturbs an equilibrium; the system responds in a direction that partly opposes the change until a new equilibrium is reached. 00:03:01.400 --> 00:03:06.925 Increasing a reactant's concentration favours the direction that uses it, often forward. 00:03:07.075 --> 00:03:13.175 Removing a product can favour more product formation; adding product tends to favour the reverse reaction. 00:03:15.833 --> 00:03:19.443 Increasing temperature favours the endothermic direction. 00:03:19.593 --> 00:03:23.111 Decreasing temperature favours the exothermic direction. 00:03:23.261 --> 00:03:26.543 Use the energy change of the stated forward reaction. 00:03:29.200 --> 00:03:36.090 Increasing pressure favours the side with fewer gaseous particles; decreasing pressure favours more gaseous particles. 00:03:36.240 --> 00:03:39.613 Count gaseous coefficients, not atoms or solids. 00:03:39.763 --> 00:03:45.179 Concentration changes favour consuming what was added or replacing what was removed. 00:03:49.833 --> 00:03:56.840 If both sides have equal numbers of gaseous particles, pressure change has no equilibrium-position effect in this model. 00:03:56.990 --> 00:04:00.753 Solid and liquid terms do not count towards the gaseous total. 00:04:03.433 --> 00:04:08.723 A catalyst speeds both forward and reverse reactions and reaches equilibrium sooner. 00:04:08.873 --> 00:04:14.323 It does not change equilibrium position or the equilibrium yield at the same conditions. 00:04:17.000 --> 00:04:25.553 Rate and equilibrium yield are different: conditions giving more product at equilibrium may make the reaction too slow for an economical process. 00:04:28.233 --> 00:04:36.978 The Haber process makes ammonia by the reversible reaction N 2 gas plus 3 H 2 gas reversibly produces 2 N H 3 gas. 00:04:37.128 --> 00:04:41.627 Nitrogen is obtained from air and hydrogen commonly from natural gas. 00:04:44.300 --> 00:04:46.796 The forward reaction is exothermic. 00:04:46.946 --> 00:04:53.163 Four moles of reactant gas correspond to two moles of product gas, so pressure favours the product side. 00:04:53.313 --> 00:04:57.655 Count coefficients of gases, not the total atoms in their molecules. 00:05:02.333 --> 00:05:10.374 The specified industrial conditions are about 450 degrees Celsius, about 200 atmospheres and an iron catalyst. 00:05:10.524 --> 00:05:16.037 These are a practical compromise, not conditions giving the greatest possible equilibrium yield. 00:05:18.700 --> 00:05:23.637 A lower temperature gives a higher equilibrium ammonia yield but a slower rate. 00:05:23.787 --> 00:05:29.589 A moderate high temperature allows useful production speed with a lower equilibrium yield. 00:05:32.267 --> 00:05:39.285 Higher pressure increases equilibrium yield and can increase rate, but compression and strong equipment cost energy and money. 00:05:39.435 --> 00:05:43.429 Very high pressure is not automatically the best economic choice. 00:05:46.100 --> 00:05:50.687 The iron catalyst gives a faster rate without increasing equilibrium yield. 00:05:50.837 --> 00:05:54.791 It lets the process operate efficiently under the chosen conditions. 00:05:57.467 --> 00:06:00.886 Ammonia is cooled so it condenses and can be removed. 00:06:01.036 --> 00:06:05.229 Unreacted nitrogen and hydrogen are recycled through the reactor. 00:06:05.379 --> 00:06:10.851 This uses the starting materials more efficiently, even though only some react each time through. 00:06:11.001 --> 00:06:16.759 Recycling improves overall conversion without requiring complete reaction in one pass. 00:06:21.433 --> 00:06:33.475 For exothermic N 2 plus 3 H 2 reversibly produces 2 N H 3, raising temperature shifts left, raising pressure shifts right, and removing ammonia favours further ammonia formation. 00:06:36.133 --> 00:06:40.092 For an endothermic forward reaction, heating instead shifts right. 00:06:40.242 --> 00:06:45.842 Always identify which written direction is endothermic before making a temperature prediction. 00:06:48.500 --> 00:07:00.369 For H 2 gas plus I 2 gas reversibly produces 2 H I gas, gaseous totals are two on both sides, so a pressure change does not change equilibrium position in this GCSE model. 00:07:03.033 --> 00:07:09.033 On a rate, time graph, equilibrium begins where forward and reverse rates are equal and stay equal. 00:07:09.183 --> 00:07:13.842 On a concentration, time graph, constant values need not have the same height. 00:07:16.500 --> 00:07:26.674 Explain industrial conditions with both chemistry and costs. “High temperature makes more ammonia” is wrong for equilibrium yield even though heating increases rate. 00:07:29.333 --> 00:07:31.765 That completes Dynamic equilibria. 00:07:31.915 --> 00:07:35.261 Revisit the notes and test yourself on the revision website.