WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:08.075 Unit S C 12: Reversible reactions and equilibria. 00:00:10.733 --> 00:00:16.491 A reversible reaction can proceed in both directions: products can react to reform reactants. 00:00:16.641 --> 00:00:20.519 The reversible arrow symbol shows forward and reverse processes. 00:00:23.200 --> 00:00:26.154 Changing conditions can favour one direction. 00:00:26.304 --> 00:00:32.287 Reversibility is chemical reaction in both directions, not just two unrelated processes. 00:00:34.967 --> 00:00:46.250 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. 00:00:48.900 --> 00:00:54.371 On cooling, ammonia and hydrogen chloride recombine to form solid ammonium chloride. 00:00:54.521 --> 00:00:58.014 The second gas is hydrogen chloride, not hydrogen. 00:00:58.164 --> 00:01:02.676 The gaseous products are ammonia and hydrogen chloride, not hydrogen. 00:01:07.333 --> 00:01:14.404 The forward and reverse reactions have opposite energy changes: if one is exothermic, the reverse is endothermic. 00:01:14.554 --> 00:01:17.413 Heating can favour the endothermic direction. 00:01:20.067 --> 00:01:29.847 This decomposition is a supervised demonstration using suitable ventilation and safety controls; the gases are not suitable for an unsupervised home experiment. 00:01:32.500 --> 00:01:39.390 A reversible reaction can reach dynamic equilibrium in a closed system, where reactants and products cannot escape. 00:01:42.067 --> 00:01:50.385 At equilibrium the forward and reverse reactions continue at equal rates. “Dynamic” means reactions are still happening at particle level. 00:01:53.067 --> 00:01:59.561 The amounts or concentrations of reactants and products remain constant while conditions remain unchanged. 00:01:59.711 --> 00:02:02.037 They do not have to be equal to each other. 00:02:04.700 --> 00:02:10.438 Starting with reactants only, the forward rate can initially be high while the reverse rate is zero. 00:02:10.588 --> 00:02:14.369 As product forms, the reverse rate rises until both match. 00:02:14.519 --> 00:02:19.446 At equilibrium reactions continue; their rates match rather than becoming zero. 00:02:24.100 --> 00:02:29.976 If products escape from an open system, the reverse reaction may not establish the same equilibrium. 00:02:30.126 --> 00:02:32.430 Closed-system conditions matter. 00:02:35.100 --> 00:02:40.262 The equilibrium position describes how much reactant and product the mixture contains. 00:02:40.412 --> 00:02:47.877 An equilibrium further to the right has a greater proportion of products; further to the left means a greater proportion of reactants. 00:02:50.533 --> 00:02:59.631 A change in conditions disturbs an equilibrium; the system responds in a direction that partly opposes the change until a new equilibrium is reached. 00:03:02.300 --> 00:03:07.825 Increasing a reactant's concentration favours the direction that uses it, often forward. 00:03:07.975 --> 00:03:14.075 Removing a product can favour more product formation; adding product tends to favour the reverse reaction. 00:03:16.733 --> 00:03:20.343 Increasing temperature favours the endothermic direction. 00:03:20.493 --> 00:03:24.011 Decreasing temperature favours the exothermic direction. 00:03:24.161 --> 00:03:27.443 Use the energy change of the stated forward reaction. 00:03:30.100 --> 00:03:36.990 Increasing pressure favours the side with fewer gaseous particles; decreasing pressure favours more gaseous particles. 00:03:37.140 --> 00:03:40.513 Count gaseous coefficients, not atoms or solids. 00:03:40.663 --> 00:03:46.079 Concentration changes favour consuming what was added or replacing what was removed. 00:03:50.733 --> 00:03:57.740 If both sides have equal numbers of gaseous particles, pressure change has no equilibrium-position effect in this model. 00:03:57.890 --> 00:04:01.653 Solid and liquid terms do not count towards the gaseous total. 00:04:04.333 --> 00:04:09.623 A catalyst speeds both forward and reverse reactions and reaches equilibrium sooner. 00:04:09.773 --> 00:04:15.223 It does not change equilibrium position or the equilibrium yield at the same conditions. 00:04:17.900 --> 00:04:26.453 Rate and equilibrium yield are different: conditions giving more product at equilibrium may make the reaction too slow for an economical process. 00:04:29.133 --> 00:04:37.878 The Haber process makes ammonia by the reversible reaction N 2 gas plus 3 H 2 gas reversibly produces 2 N H 3 gas. 00:04:38.028 --> 00:04:42.527 Nitrogen is obtained from air and hydrogen commonly from natural gas. 00:04:45.200 --> 00:04:47.696 The forward reaction is exothermic. 00:04:47.846 --> 00:04:54.063 Four moles of reactant gas correspond to two moles of product gas, so pressure favours the product side. 00:04:54.213 --> 00:04:58.555 Count coefficients of gases, not the total atoms in their molecules. 00:05:03.233 --> 00:05:11.274 The specified industrial conditions are about 450 degrees Celsius, about 200 atmospheres and an iron catalyst. 00:05:11.424 --> 00:05:16.937 These are a practical compromise, not conditions giving the greatest possible equilibrium yield. 00:05:19.600 --> 00:05:24.537 A lower temperature gives a higher equilibrium ammonia yield but a slower rate. 00:05:24.687 --> 00:05:30.489 A moderate high temperature allows useful production speed with a lower equilibrium yield. 00:05:33.167 --> 00:05:40.185 Higher pressure increases equilibrium yield and can increase rate, but compression and strong equipment cost energy and money. 00:05:40.335 --> 00:05:44.329 Very high pressure is not automatically the best economic choice. 00:05:47.000 --> 00:05:51.587 The iron catalyst gives a faster rate without increasing equilibrium yield. 00:05:51.737 --> 00:05:55.691 It lets the process operate efficiently under the chosen conditions. 00:05:58.367 --> 00:06:01.786 Ammonia is cooled so it condenses and can be removed. 00:06:01.936 --> 00:06:06.129 Unreacted nitrogen and hydrogen are recycled through the reactor. 00:06:06.279 --> 00:06:11.751 This uses the starting materials more efficiently, even though only some react each time through. 00:06:11.901 --> 00:06:17.659 Recycling improves overall conversion without requiring complete reaction in one pass. 00:06:22.333 --> 00:06:34.375 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. 00:06:37.033 --> 00:06:40.992 For an endothermic forward reaction, heating instead shifts right. 00:06:41.142 --> 00:06:46.742 Always identify which written direction is endothermic before making a temperature prediction. 00:06:49.400 --> 00:07:01.269 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. 00:07:03.933 --> 00:07:09.933 On a rate, time graph, equilibrium begins where forward and reverse rates are equal and stay equal. 00:07:10.083 --> 00:07:14.742 On a concentration, time graph, constant values need not have the same height. 00:07:17.400 --> 00:07:27.574 Explain industrial conditions with both chemistry and costs. “High temperature makes more ammonia” is wrong for equilibrium yield even though heating increases rate. 00:07:30.233 --> 00:07:33.368 That completes Reversible reactions and equilibria. 00:07:33.518 --> 00:07:36.863 Revisit the notes and test yourself on the revision website.