Top ↑

Chemistry

Thermochemistry:

CONTENTS


EXOTHERMIC AND ENDOTHERMIC REACTIONS

Exothermic Reactions:

DEFINITION: Exothermic Reaction

An exothermic reaction is one which gives out energy to the surroundings, usually in the form of heat and usually shown by a rise in temperature.

View Glossary

Examples of Exothermic Reactions:


Endothermic Reactions:

DEFINITION: Endothermic Reaction

An endothermic reaction is one which takes in energy from the surroundings, usually in the form of heat and usually shown by a fall in temperature.

View Glossary

Examples of Endothermic Reactions:


ENTHALPY CHANGE

ΔH (read as 'delta H') is the symbol for the enthalpy change of a reaction. Enthalpy change refers to the change in energy in a reaction. ΔH is negative for an exothermic reaction (i.e. the products have less energy than the reactants) and positive for an endothermic reaction (i.e. the product have more energy than the reactants).


Energy Level Diagrams:

Enthalpy change can be illustrated using an energy level diagram:

Energy Level Diagrams

Sometimes, a simplified version of the above diagrams are drawn, without the reaction pathway curve and activation energy drawn on.

DEFINITION: Activation Energy

Activation energy is the minimum energy required for reactants to undergo a chemical reaction. It can be lowered by using a catalyst.

View Glossary

CALORIMETRY CALCULATIONS

Formula for Energy Change:

FORMULA:

Energy Change Symbol Formula
Where:
q = energy change
m = mass of water
c = specific heat capacity
ΔT = temperature change

Specific Heat Capacity (c) is the quantity of energy, in Joules (J), needed to change the temperature of one gram (g) of a substance by one degree Celsius (oC).

e.g. The specific heat capacity of water is 4.18 J/g/oC


Formula for Enthalpy Change:

FORMULA:

Enthalpy Change Symbol Formula
Where:
ΔH = enthalpy change
q = energy change
n = number of moles


BOND ENERGY

We can work out the energy needed to break different chemical bonds. This is called the bond energy. These bond energies can be used to calculate ΔH for a reaction.

DEFINITION: Bond Energy

Bond energy is the amount of energy required to break one mole of a stated bond, measured in kJ/mol. It can be used to calculate ΔH for a reaction.

View Glossary

Bond energies are average values for the bonds in different molecules. Therefore, these calculations only give us a rough value for ΔH.

Making bonds is EXOTHERMIC
Breaking bonds is ENDOTHERMIC