Heat Energy Changes in Chemical Reactions Flashcards
All 18 cards in this deck
What is an exothermic reaction or change?
One in which heat energy is given out (released) to the surroundings.
What is an endothermic reaction or change?
One in which heat energy is taken in (absorbed) from the surroundings.
Name four types of change that are accompanied by heat energy changes.
Salts dissolving in water, neutralisation reactions, displacement reactions and precipitation reactions.
How is the heat energy change of a reaction in solution measured?
e.g. neutralising dilute acid with alkali in a polystyrene cup
By the temperature change: record the starting temperature, mix and stir, then record the highest (or lowest) temperature reached.
True or false? Energy is released when a bond is broken.
False. Energy is absorbed (taken in) when a bond is broken — bond breaking is endothermic.
Is making (forming) a bond exothermic or endothermic?
Exothermic — energy is released when a bond is made.
In terms of bond breaking and bond making, when is a reaction overall exothermic?
When more heat energy is given out forming bonds in the products than is taken in breaking bonds in the reactants.
In terms of bond breaking and bond making, when is a reaction overall endothermic?
When less heat energy is released forming bonds in the products than is required to break bonds in the reactants.
What is the equation for the overall energy change of a reaction from bond energies?
Energy change = (sum of bond energies broken) − (sum of bond energies made).
In a bond energy calculation, what does a negative energy change tell you about the reaction?
The reaction is exothermic (energy is given out overall).
What unit is used for bond energies and for an energy change calculated from them?
kJ mol (kilojoules per mole).
Explain the term activation energy.
The minimum amount of energy that colliding particles must have for a reaction to occur.
How is the activation energy shown on a reaction profile?
An arrow from the reactants' energy level up to the peak of the curve, labelled activation energy.
On a reaction profile for an exothermic reaction, how do the energy levels of the products and reactants compare?
The products line is lower than the reactants line, so the energy change is negative.
True or false? On a reaction profile for an endothermic reaction, the products are at a lower energy level than the reactants.
False. The products have more energy than the reactants, so the energy change is positive.
When measuring a temperature change in a polystyrene cup, why is the cup packed in cotton wool inside a beaker and covered with a lid?
To reduce heat transfer between the mixture and the surroundings: the cotton wool insulates the cup and the lid cuts heat lost by evaporation, so the measured temperature change is more accurate.
Does dissolving a salt in water always make the temperature rise?
No. It depends on the salt: some dissolve exothermically (temperature rises) and others endothermically (temperature falls).
e.g. anhydrous magnesium sulfate warms the water; ammonium nitrate cools it.How can a thermometer show whether a reaction in solution is exothermic or endothermic?
If the temperature of the mixture goes up, heat has been given out (exothermic); if it goes down, heat has been taken in (endothermic).