Calculating And Explaining Energy Changes
Visão Geral
A can of petrol and the air above it will sit in a garage for a decade without anything happening. Touch a spark to the vapour and the same two substances release, in a fraction of a second, thousands of times more energy than the spark ever put in. Nothing was added except a moment of heat, and nothing was taken away. Both of those facts, the waiting and the violence, come out of one picture and one subtraction, and this is the lesson where you learn to draw the picture and do the sum.
The picture is the energy level diagram: a hill a reaction has to climb before it can start, and the drop it makes on the far side. The subtraction is the bond ledger, where every bond that comes apart is an outgoing and every bond that forms is an incoming, and what is left over is the energy change you would have measured with a thermometer. By the end you will be able to predict whether a reaction warms its surroundings or cools them before anybody mixes anything at all, and to say exactly what a catalyst changes on that picture and what it leaves untouched.
Objetivos
- 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.
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- 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
- On an energy level diagram for an endothermic reaction, where is the activation energy measured? A. from the bottom of the graph to the top of the peak B. from the reactant level to the top of the peak C. from the product level to the top of the peak D. from the reactant level to the product level Answer: B
- Three molecules of carbon dioxide are formed in a reaction. Each molecule contains two C=O bonds and the bond energy of C=O is 803 kJ per mole. How much energy is released when these bonds form? A. 803 kJ B. 1606 kJ C. 2409 kJ D. 4818 kJ Answer: D
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