WEBVTT 00:00:00.500 --> 00:00:03.976 Welcome to GCSE Edexcel Science revision. 00:00:04.126 --> 00:00:08.723 Unit C C 15: Heat energy changes in chemical reactions. 00:00:11.400 --> 00:00:18.843 An exothermic change transfers heat energy from the reacting system to the surroundings, so the surrounding temperature can rise. 00:00:18.993 --> 00:00:21.425 The system loses energy overall. 00:00:24.100 --> 00:00:28.366 An endothermic change takes in heat from surroundings, which can cool. 00:00:28.516 --> 00:00:30.905 The system gains energy overall. 00:00:31.055 --> 00:00:35.007 Temperature observations depend on heat exchange and insulation. 00:00:39.667 --> 00:00:47.110 Common exothermic examples include combustion and many acid, alkali neutralisations and displacement reactions. 00:00:47.260 --> 00:00:52.711 Do not infer every reaction's energy change from its category alone without evidence. 00:00:55.367 --> 00:01:00.199 Dissolving salts in water can release or absorb heat depending on the salt. 00:01:00.349 --> 00:01:04.631 Precipitation reactions can also show measurable temperature changes. 00:01:07.300 --> 00:01:14.466 The same energy transfer can be described from two perspectives: an exothermic system loses heat while surroundings gain it. 00:01:14.616 --> 00:01:20.034 A temperature change in a solution reflects exchange with that solution and apparatus. 00:01:22.700 --> 00:01:26.882 For the reverse of a reaction, the energy change has the opposite sign. 00:01:27.032 --> 00:01:31.449 An exothermic forward reaction has an endothermic reverse reaction. 00:01:34.100 --> 00:01:44.274 For a solution reaction, measure suitable reagent volumes and starting temperatures, mix in an insulated container and record the highest or lowest temperature reached. 00:01:46.933 --> 00:01:52.064 Use a polystyrene cup with a lid where appropriate to reduce heat exchange with the room. 00:01:52.214 --> 00:01:57.440 Support the cup securely in a beaker and use an appropriate thermometer or temperature probe. 00:01:57.590 --> 00:02:03.285 Insulation and a lid reduce exchange with the room; stir and record the extreme temperature. 00:02:07.967 --> 00:02:13.821 Temperature change delta T equals final relevant temperature minus initial temperature. 00:02:13.971 --> 00:02:19.762 A rise gives positive delta T for the measured surroundings; a fall gives negative delta T. 00:02:22.433 --> 00:02:28.979 For a comparison, control reagent amounts and concentrations, initial temperature, container and mixing. 00:02:29.129 --> 00:02:32.200 Repeat runs and use comparable recording intervals. 00:02:34.867 --> 00:02:40.145 Heat loss during an exothermic reaction can make the observed maximum rise too small. 00:02:40.295 --> 00:02:44.587 Heat entering from the room can make an endothermic cooling appear smaller. 00:02:47.267 --> 00:02:51.419 Wear eye protection and follow the specified dilute-reagent method. 00:02:51.569 --> 00:02:58.095 A rise alone does not identify every product, and a small change may be limited by instrument resolution. 00:03:00.767 --> 00:03:05.731 A reaction profile plots energy vertically against reaction progress horizontally. 00:03:05.881 --> 00:03:08.620 Reaction progress is not a time axis. 00:03:11.300 --> 00:03:16.526 Reactants begin at one energy level, the curve reaches a peak, and products end at another. 00:03:16.676 --> 00:03:21.999 The activation energy is the difference from the reactant level to the peak for the forward reaction. 00:03:24.667 --> 00:03:29.775 Activation energy is the minimum energy barrier required for successful reaction. 00:03:29.925 --> 00:03:36.739 Exothermic reactions still need an initial energy input; a fuel does not necessarily ignite spontaneously. 00:03:39.400 --> 00:03:43.383 For an exothermic reaction the product level is below reactants. 00:03:43.533 --> 00:03:50.027 Overall energy change delta H equals energy of products minus energy of reactants is negative. 00:03:50.177 --> 00:03:54.530 Products lower than reactants; activation energy remains positive. 00:03:59.200 --> 00:04:04.981 For an endothermic reaction the product level is above reactants and delta H is positive. 00:04:05.131 --> 00:04:08.963 The activation-energy arrow must still start at the reactant level. 00:04:09.113 --> 00:04:14.543 Products higher than reactants; distinguish overall delta H from the activation barrier. 00:04:19.200 --> 00:04:24.449 A catalyst gives an alternative pathway with a lower peak, reducing activation energy. 00:04:24.599 --> 00:04:29.973 Reactant and product energy levels and overall energy change remain unchanged. 00:04:30.123 --> 00:04:34.729 A catalyst changes the pathway and activation energy, not delta H. 00:04:39.400 --> 00:04:45.307 Forward and reverse activation energies are measured from different starting levels to the relevant peak. 00:04:45.457 --> 00:04:47.569 They are not necessarily equal. 00:04:50.233 --> 00:04:54.269 Breaking chemical bonds requires energy and is endothermic. 00:04:54.419 --> 00:04:58.575 It is incorrect to say breaking a fuel's bonds itself releases energy. 00:05:01.233 --> 00:05:05.265 Forming chemical bonds releases energy and is exothermic. 00:05:05.415 --> 00:05:10.194 Overall reaction energy depends on both bond breaking and bond making. 00:05:12.867 --> 00:05:19.703 An exothermic reaction releases more energy in forming product bonds than it takes in to break reactant bonds. 00:05:19.853 --> 00:05:23.303 An endothermic reaction takes in more than it releases. 00:05:23.453 --> 00:05:28.297 An exothermic result releases more in making bonds than it needs for breaking them. 00:05:32.967 --> 00:05:37.249 Average bond energies are usually given in kilojoules per mole of bonds. 00:05:37.399 --> 00:05:45.632 They are average values, so calculations using them may differ from the actual measured energy change of a reaction (its enthalpy change). 00:05:48.300 --> 00:05:53.590 Count all bonds in every molecule, multiplying by the balanced-equation coefficients. 00:05:53.740 --> 00:05:59.253 A double bond has its given double-bond energy, not necessarily twice a single-bond value. 00:06:01.933 --> 00:06:11.147 Estimated reaction energy change equals total energy needed to break reactant bonds minus total energy released forming product bonds. 00:06:11.297 --> 00:06:14.847 Use the sign to identify endothermic or exothermic. 00:06:17.500 --> 00:06:21.041 For H 2 plus C L 2 produces 2 H C L, 00:06:21.191 --> 00:06:24.157 using H to H 436, 00:06:24.307 --> 00:06:36.336 C L to C L 243 and H to C L 431 kilojoules per mole: break 436 plus 243 equals 679; 00:06:36.486 --> 00:06:40.825 form 2 times 431 equals 862; 00:06:40.975 --> 00:06:51.108 delta H equals 679 minus 862 equals minus 183 kilojoules per mole for the equation as written. 00:06:51.258 --> 00:06:57.804 Equation: H 2 plus C L 2 produces 2 H C L; negative delta H is exothermic. 00:07:02.467 --> 00:07:05.923 For 2 H 2 plus O 2 produces 2 H 2 O, 00:07:06.073 --> 00:07:09.038 using H to H 436, 00:07:09.188 --> 00:07:21.378 O double bonded to O 498 and O to H 463: break 2 times 436 plus 498 equals 1370; 00:07:21.528 --> 00:07:26.806 form 4 times 463 equals 1852; 00:07:26.956 --> 00:07:33.205 delta H equals minus 482 kilojoules per mole for two moles of water as written. 00:07:35.867 --> 00:07:48.932 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. 00:07:49.082 --> 00:07:50.981 Coefficients are essential. 00:07:53.633 --> 00:07:59.307 If a question asks per mole of one product rather than per balanced reaction, adjust accordingly. 00:07:59.457 --> 00:08:06.794 For the water equation above, per mole of water is minus 241 kilojoules per mole using those average values. 00:08:09.467 --> 00:08:12.847 Show the bond counts and both totals before subtracting. 00:08:12.997 --> 00:08:21.187 A negative answer indicates heat released overall; do not drop a minus sign just because the released energy has a positive magnitude. 00:08:23.867 --> 00:08:27.395 That completes Heat energy changes in chemical reactions. 00:08:27.545 --> 00:08:30.891 Revisit the notes and test yourself on the revision website.