WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:08.671 Unit S C 19: Heat energy changes in chemical reactions. 00:00:11.333 --> 00:00:18.777 An exothermic change transfers heat energy from the reacting system to the surroundings, so the surrounding temperature can rise. 00:00:18.927 --> 00:00:21.359 The system loses energy overall. 00:00:24.033 --> 00:00:28.299 An endothermic change takes in heat from surroundings, which can cool. 00:00:28.449 --> 00:00:30.839 The system gains energy overall. 00:00:30.989 --> 00:00:34.940 Temperature observations depend on heat exchange and insulation. 00:00:39.600 --> 00:00:47.043 Common exothermic examples include combustion and many acid, alkali neutralisations and displacement reactions. 00:00:47.193 --> 00:00:52.644 Do not infer every reaction's energy change from its category alone without evidence. 00:00:55.300 --> 00:01:00.132 Dissolving salts in water can release or absorb heat depending on the salt. 00:01:00.282 --> 00:01:04.565 Precipitation reactions can also show measurable temperature changes. 00:01:07.233 --> 00:01:14.399 The same energy transfer can be described from two perspectives: an exothermic system loses heat while surroundings gain it. 00:01:14.549 --> 00:01:19.967 A temperature change in a solution reflects exchange with that solution and apparatus. 00:01:22.633 --> 00:01:26.815 For the reverse of a reaction, the energy change has the opposite sign. 00:01:26.965 --> 00:01:31.382 An exothermic forward reaction has an endothermic reverse reaction. 00:01:34.033 --> 00:01:44.207 For a solution reaction, measure suitable reagent volumes and starting temperatures, mix in an insulated container and record the highest or lowest temperature reached. 00:01:46.867 --> 00:01:51.997 Use a polystyrene cup with a lid where appropriate to reduce heat exchange with the room. 00:01:52.147 --> 00:01:57.373 Support the cup securely in a beaker and use an appropriate thermometer or temperature probe. 00:01:57.523 --> 00:02:03.219 Insulation and a lid reduce exchange with the room; stir and record the extreme temperature. 00:02:07.900 --> 00:02:13.755 Temperature change delta T equals final relevant temperature minus initial temperature. 00:02:13.905 --> 00:02:19.695 A rise gives positive delta T for the measured surroundings; a fall gives negative delta T. 00:02:22.367 --> 00:02:28.913 For a comparison, control reagent amounts and concentrations, initial temperature, container and mixing. 00:02:29.063 --> 00:02:32.133 Repeat runs and use comparable recording intervals. 00:02:34.800 --> 00:02:40.079 Heat loss during an exothermic reaction can make the observed maximum rise too small. 00:02:40.229 --> 00:02:44.520 Heat entering from the room can make an endothermic cooling appear smaller. 00:02:47.200 --> 00:02:51.352 Wear eye protection and follow the specified dilute-reagent method. 00:02:51.502 --> 00:02:58.028 A rise alone does not identify every product, and a small change may be limited by instrument resolution. 00:03:00.700 --> 00:03:05.664 A reaction profile plots energy vertically against reaction progress horizontally. 00:03:05.814 --> 00:03:08.553 Reaction progress is not a time axis. 00:03:11.233 --> 00:03:16.459 Reactants begin at one energy level, the curve reaches a peak, and products end at another. 00:03:16.609 --> 00:03:21.932 The activation energy is the difference from the reactant level to the peak for the forward reaction. 00:03:24.600 --> 00:03:29.708 Activation energy is the minimum energy barrier required for successful reaction. 00:03:29.858 --> 00:03:36.673 Exothermic reactions still need an initial energy input; a fuel does not necessarily ignite spontaneously. 00:03:39.333 --> 00:03:43.316 For an exothermic reaction the product level is below reactants. 00:03:43.466 --> 00:03:49.961 Overall energy change delta H equals energy of products minus energy of reactants is negative. 00:03:50.111 --> 00:03:54.463 Products lower than reactants; activation energy remains positive. 00:03:59.133 --> 00:04:04.914 For an endothermic reaction the product level is above reactants and delta H is positive. 00:04:05.064 --> 00:04:08.896 The activation-energy arrow must still start at the reactant level. 00:04:09.046 --> 00:04:14.477 Products higher than reactants; distinguish overall delta H from the activation barrier. 00:04:19.133 --> 00:04:24.382 A catalyst gives an alternative pathway with a lower peak, reducing activation energy. 00:04:24.532 --> 00:04:29.906 Reactant and product energy levels and overall energy change remain unchanged. 00:04:30.056 --> 00:04:34.662 A catalyst changes the pathway and activation energy, not delta H. 00:04:39.333 --> 00:04:45.241 Forward and reverse activation energies are measured from different starting levels to the relevant peak. 00:04:45.391 --> 00:04:47.503 They are not necessarily equal. 00:04:50.167 --> 00:04:54.202 Breaking chemical bonds requires energy and is endothermic. 00:04:54.352 --> 00:04:58.508 It is incorrect to say breaking a fuel's bonds itself releases energy. 00:05:01.167 --> 00:05:05.198 Forming chemical bonds releases energy and is exothermic. 00:05:05.348 --> 00:05:10.127 Overall reaction energy depends on both bond breaking and bond making. 00:05:12.800 --> 00:05:19.636 An exothermic reaction releases more energy in forming product bonds than it takes in to break reactant bonds. 00:05:19.786 --> 00:05:23.237 An endothermic reaction takes in more than it releases. 00:05:23.387 --> 00:05:28.230 An exothermic result releases more in making bonds than it needs for breaking them. 00:05:32.900 --> 00:05:37.183 Average bond energies are usually given in kilojoules per mole of bonds. 00:05:37.333 --> 00:05:45.565 They are average values, so calculations using them may differ from the actual measured energy change of a reaction (its enthalpy change). 00:05:48.233 --> 00:05:53.523 Count all bonds in every molecule, multiplying by the balanced-equation coefficients. 00:05:53.673 --> 00:05:59.187 A double bond has its given double-bond energy, not necessarily twice a single-bond value. 00:06:01.867 --> 00:06:11.081 Estimated reaction energy change equals total energy needed to break reactant bonds minus total energy released forming product bonds. 00:06:11.231 --> 00:06:14.781 Use the sign to identify endothermic or exothermic. 00:06:17.433 --> 00:06:20.975 For H 2 plus C L 2 produces 2 H C L, 00:06:21.125 --> 00:06:24.090 using H to H 436, 00:06:24.240 --> 00:06:36.269 C L to C L 243 and H to C L 431 kilojoules per mole: break 436 plus 243 equals 679; 00:06:36.419 --> 00:06:40.759 form 2 times 431 equals 862; 00:06:40.909 --> 00:06:51.041 delta H equals 679 minus 862 equals minus 183 kilojoules per mole for the equation as written. 00:06:51.191 --> 00:06:57.737 Equation: H 2 plus C L 2 produces 2 H C L; negative delta H is exothermic. 00:07:02.400 --> 00:07:05.856 For 2 H 2 plus O 2 produces 2 H 2 O, 00:07:06.006 --> 00:07:08.971 using H to H 436, 00:07:09.121 --> 00:07:21.311 O double bonded to O 498 and O to H 463: break 2 times 436 plus 498 equals 1370; 00:07:21.461 --> 00:07:26.739 form 4 times 463 equals 1852; 00:07:26.889 --> 00:07:33.138 delta H equals minus 482 kilojoules per mole for two moles of water as written. 00:07:35.800 --> 00:07:48.865 For C H 4 plus 2 O 2 produces C O 2 plus 2 H 2 O, count four C to H bonds and two O double bonded to O bonds broken; two C double bonded to O bonds and four O to H bonds formed. 00:07:49.015 --> 00:07:50.914 Coefficients are essential. 00:07:53.567 --> 00:07:59.241 If a question asks per mole of one product rather than per balanced reaction, adjust accordingly. 00:07:59.391 --> 00:08:06.727 For the water equation above, per mole of water is minus 241 kilojoules per mole using those average values. 00:08:09.400 --> 00:08:12.781 Show the bond counts and both totals before subtracting. 00:08:12.931 --> 00:08:21.121 A negative answer indicates heat released overall; do not drop a minus sign just because the released energy has a positive magnitude. 00:08:23.800 --> 00:08:27.329 That completes Heat energy changes in chemical reactions. 00:08:27.479 --> 00:08:30.824 Revisit the notes and test yourself on the revision website.