7.2 Energy changes Flashcards
Heat energy and temperature changes
Chemical reactions occur to achieve a stable energy state by forming new bonds. This involves energy transfer between the system (the reaction) and the surroundings. Chemical bonds store energy, mainly released or absorbed as heat, but sometimes as light or sound. Heat changes can be measured using a thermometer or calorimeter.
Examples of heat changes in reaction
Salts dissolving in water:
These can either take energy in or give it out
Neutralisation reactions:
These always give energy out
Displacement reactions:
These can either take energy in or give it out
Precipitation reactions:
These always give energy out
Exothermic reactions
In exothermic reactions energy is given out to the surroundings so the temperature of the surroundings increases.
Combustion, oxidation, and neutralisation reactions are typical exothermic reactions.
Endothermic reactions
In endothermic reactions energy is taken in from the surroundings so the temperature of the surroundings decreases.
These types of reactions are much less common than the exothermic reactions.
Electrolysis, thermal decomposition reactions and the first stages of photosynthesis are typical endothermic reactions.
Breaking and forming bonds
Energy is needed to break bonds which is absorbed from the reaction surroundings, so bond breaking is an endothermic process.
The opposite occurs for forming bonds as it releases energy back to the surroundings in an exothermic process.
Endothermic bonds
If more energy is absorbed than is released, this reaction is endothermic.
More energy is required to break the bonds than that gained from making the new bonds.
The change in energy is positive since the products have more energy than the reactants.
Therefore an endothermic reaction has a positive change in energy.
Exothermic bonds
If more energy is released than is absorbed, then the reaction is exothermic.
More energy is released when new bonds are formed than energy required to break the bonds in the reactants.
The change in energy is negative since the reactants have more energy than the products.
Therefore an exothermic reaction has a negative change in energy.
Bond energy calculations
Each chemical bond has a specific bond energy associated with it.
This is the amount of energy required to break the bond or the amount of energy given out when the bond is formed.
Energy change = Energy taken in - Energy given out
Activation energy
The minimum energy the colliding particles need in order to react is known as the activation energy.
The activation energy can be lowered by the addition of a catalyst.
This means that a higher percentage of the particles will have the minimum energy required to react, hence there are a higher number of successful collision.
Therefore more product molecules are produced in a shorter time, thus increasing the rate of reaction.
Determining the rate of a reaction
To measure the rate of a reaction, we need to be able to measure either how quickly the reactants are used up or how quickly the products are formed.
The method used for measuring depends on the substances involved.
Changes in mass
When a gas is produced in a reaction it usually escapes from the reaction vessel, so the mass decreases.
This can be used to measure the rate of reaction.
Volume of gases
When a gas is produced in a reaction, it can be trapped and its volume measured over time.
This can be used to measure the rate of reaction.
Calculating rates of reaction
Reactions take place at different rates depending on the identities and conditions.
Some are extremely slow e.g. rusting and others are extremely fast e.g. explosives.
Reactant used or product formed = rate of reaction x time taken
