Exothermic And Endothermic Reactions
Visão Geral
Twist the base of a self-heating can of coffee and, without a flame, a plug, or a single moving part, the drink inside climbs to something like drinking temperature in about three minutes. Squeeze a sports injury pack on the touchline and the opposite happens: the pack turns cold enough to be uncomfortable to hold, with nothing having been put in a freezer. Both devices are sealed plastic containers of ordinary chemicals. The only thing either of them does is start a reaction.
This lesson is about the direction energy travels when a reaction happens, and about the small piece of notation chemists use to record it. You will learn what separates a reaction that warms its container from one that chills it, why a thermometer sitting in the beaker reports the opposite of what the chemicals are doing, and how the same two ideas explain a hand warmer, a cold pack and a furnace. By the end you will be able to look at a reaction, a temperature reading or a value with a minus sign in front of it and say confidently which way the energy went.
Objetivos
- When chemical reactions occur, energy is transferred to or from the surroundings. Knowledge of delta H (ΔH) conventions and enthalpy changes, including the use of positive values for endothermic reactions and negative values for exothermic reactions, is required.
- An exothermic reaction is one that transfers energy to the surroundings. Examples of exothermic reactions include combustion, many oxidation reactions and neutralisation Students should be able to give examples of exothermic reactions including combustion, oxidation and neutralisation. Everyday uses of exothermic reactions include self-heating cans (eg for coffee) and hand warmers.
- An endothermic reaction is one that takes in energy from the surroundings. Endothermic reactions include thermal decompositions. Some sports injury packs are based upon endothermic reactions.
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Nota de Aula
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- Which of these changes is exothermic? A. cracking a long chain alkane in a refinery B. photosynthesis in a green leaf C. the neutralisation of hydrochloric acid by sodium hydroxide D. dissolving ammonium nitrate in water Answer: C
- What is the sign of the enthalpy change for an exothermic reaction? A. always positive B. always negative C. always zero D. positive or negative depending on the temperature Answer: B
- During a reaction in a beaker the temperature of the solution falls from 21 degrees Celsius to 16 degrees Celsius. Which statement is correct? A. The reaction is exothermic and energy is transferred to the surroundings. B. The reaction is exothermic and energy is transferred from the surroundings. C. The reaction is endothermic and energy is transferred to the surroundings. D. The reaction is endothermic and energy is transferred from the surroundings. Answer: D
- A reaction has an enthalpy change of plus 50 kJ/mol. Which statement about this reaction is correct? A. The products store less enthalpy than the reactants and the surroundings warm up. B. The products store more enthalpy than the reactants and the surroundings cool down. C. The products store more enthalpy than the reactants and the surroundings warm up. D. The reactants and the products store the same enthalpy. Answer: B
- A sports injury pack becomes cold when it is squeezed. Which statement explains this? A. The pack releases coldness into the surroundings. B. The pack contains a process that takes energy in from the surroundings. C. The pack contains a process that transfers energy to the surroundings. D. The pack contains no energy change at all. Answer: B
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