Heat Energy Changes in Chemical Reactions Flashcards
All 16 cards in this deck
What is an exothermic change or reaction?
One in which heat energy is given out (released to the surroundings), so the temperature rises.
e.g. a hand warmerWhat is an endothermic change or reaction?
One in which heat energy is taken in (absorbed) from the surroundings, so the temperature falls.
e.g. a cold packName four types of change in solution that are accompanied by heat energy changes.
Salts dissolving in water, neutralisation reactions, displacement reactions and precipitation reactions.
How is the temperature change worked out when a solid dissolves in water in a polystyrene cup?
Temperature change = final temperature − initial temperature (in °C). A rise means the change is exothermic; a fall means it is endothermic.
Why is a polystyrene cup used instead of a glass beaker when measuring a temperature change?
Polystyrene is a good heat insulator (poor conductor of heat), so it reduces heat energy transfer.
Is the breaking of bonds endothermic or exothermic?
Endothermic — heat energy must be taken in to break bonds.
Is the making of bonds endothermic or exothermic?
Exothermic — heat energy is given out when bonds form.
In terms of bonds, when is the overall heat energy change for a reaction exothermic?
When more heat energy is released in forming bonds in the products than is required to break bonds in the reactants.
True or false? If more heat energy is needed to break the bonds in the reactants than is released when bonds form in the products, the reaction is exothermic.
False. The reaction is endothermic, because less energy is released than is taken in.
What is meant by activation energy?
The minimum energy that the reacting particles must have for a reaction to take place.
On a reaction profile for an exothermic reaction, how do the heat energy levels of the reactants and products compare?
The products are at a lower heat energy level than the reactants.
On a reaction profile, where is the activation energy measured from and to?
From the heat energy level of the reactants up to the top (peak) of the curve.
True or false? An exothermic reaction always gives out light.
False. An exothermic reaction always gives out heat energy; it does not have to give out light, sound or a gas.
Are the products of an endothermic reaction drawn above or below the reactants on a reaction profile?
Above: heat energy has been taken in from the surroundings, so the products end with more energy than the reactants.
On a reaction profile, what does the difference in height between the reactants and the products show?
The overall heat energy change of the reaction: the heat given out (exothermic) or taken in (endothermic).
Is dissolving ammonium chloride in water endothermic or exothermic?
Endothermic: it takes in heat energy, so the temperature of the solution falls.