Calculating And Explaining Energy Change
Visão Geral
Set a small burner of ethanol under a metal can holding water, light it, and watch a thermometer climb. Nothing about that arrangement looks like measurement, and yet it is: the rise in temperature is a proxy for the energy the flame gave away, and with two weighings and one multiplication it becomes a figure in kilojoules that can be set beside the figure for any other fuel on Earth. This is the lesson where chemical energy stops being an adjective and starts being a number.
You will learn to run that number in both directions. Forwards, from a thermometer and a balance to the energy a fuel or a reaction gives out, quoted per gram or per mole depending on the question being asked. Backwards, from a table of bond energies to a prediction of the energy change before anybody lights anything at all. In between sits the energy level diagram, the one picture in this course that shows you the whole journey: the hill a reaction has to climb before it can start, and the drop it makes on the far side.
Objetivos
- The relative amounts of energy released when substances burn can be measured by simple calorimetry, eg by heating water in a glass or metal container. This method can be used to compare the amount of energy produced by fuels. Students should be able to calculate and compare the amount of energy released by different fuels given the equation: Q = mc ΔT
- Energy is normally measured in joules (J) or kilojoules (kJ) for a given mass or amount of substance eg kilojoules per gram or kilojoules per mole.
- The amount of energy produced by a chemical reaction in solution can be calculated from the measured temperature change of the solution when the reagents are mixed in an insulated container. This method can be used for reactions of solids with water or for neutralisation reactions.
- Simple energy level diagrams can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction. Students will be expected to understand simple energy level diagrams showing the relative energies of reactants and products, the activation energy and the overall energy change, with a curved arrow to show the energy as the reaction proceeds. Students should be able to relate these to exothermic and endothermic reactions.
- During a chemical reaction: energy must be supplied to break bonds; energy is released when bonds are formed. Students should be able to calculate the energy transferred in reactions and interpret simple energy level diagrams in terms of bond breaking and bond formation (including the idea of activation energy and the effect on this of catalysts).
- In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds. Students should be able to calculate the energy transferred in reactions using bond dissociation energies supplied.
- In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds.
- Catalysts provide a different pathway for a chemical reaction that has a lower activation energy. Students should be able to represent the effect of a catalyst on an energy level diagram.
Mapa mental
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Nota de Aula
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- A student burns a fuel to heat water in a metal can and uses the equation Q = mcT. What does m represent? A. the mass of fuel burned B. the mass of water heated C. the mass of the metal can D. the total mass of the fuel and the water Answer: B
- 50 g of water is heated by 12 degrees Celsius. The specific heat capacity of water is 4.2 J/g per degree Celsius. How much energy was transferred to the water? A. 210 J B. 600 J C. 2520 J D. 25200 J Answer: C
- Which statement about bonds during a chemical reaction is correct? A. Breaking bonds releases energy and making bonds takes in energy. B. Breaking bonds takes in energy and making bonds releases energy. C. Breaking bonds and making bonds both release energy. D. Breaking bonds and making bonds both take in energy. Answer: B
- In a reaction, breaking the bonds in the reactants requires 1350 kJ and forming the bonds in the products releases 1500 kJ. What is the energy change for the reaction? A. +150 kJ B. -150 kJ C. +2850 kJ D. -2850 kJ Answer: B
- A catalyst is added to a reaction. Which quantity changes? A. the energy of the reactants only B. the activation energy only C. the overall energy change only D. both the activation energy and the overall energy change Answer: B
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