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, about the small piece of notation chemists use to record it, and about how you put a thermometer in a cup and turn that direction into a measurement. 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 a single reaction can be exothermic when you drive it one way and endothermic when you drive it back. 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.
- In some chemical reactions, the products of the reaction can react to produce the original reactants. Such reactions are called reversible reactions and are represented as follows: A + B <=> C + D For example: hydrated copper sulfate (blue) <=> anhydrous copper sulfate (white) + water, where the forward reaction is endothermic and the reverse reaction is exothermic.
- 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.
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- Which of these reactions is exothermic? A. the thermal decomposition of calcium carbonate B. photosynthesis in a green leaf C. the neutralisation of hydrochloric acid by sodium hydroxide D. the breakdown of a metal carbonate by heating 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
- The forward reaction of a reversible reaction has an enthalpy change of minus 92 kJ/mol. What is the enthalpy change of the reverse reaction? A. minus 92 kJ/mol B. minus 46 kJ/mol C. plus 46 kJ/mol D. plus 92 kJ/mol Answer: D
- A student measures the temperature change of a reaction in solution. Why is a polystyrene cup used instead of a glass beaker? A. Polystyrene is cheaper than glass. B. Polystyrene is a better insulator, so less energy is lost to the surroundings. C. Polystyrene reacts with the solution and releases energy. D. Polystyrene conducts energy quickly so the reading settles sooner. Answer: B
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