Exothermic reactions are most common, however, an important example of an endothermic reaction isphotosynthesis in plants, where the energy supplied is from sunlight.
Law of conservation of energy: Energy cannot be destroyed or created but only transferred from one form to another. The total energy of a system of reacting chemicals and surroundings remains constant.
Enthalpy change is the term used to describe the energy exchange that takes place with the surroundings at a constant pressure and is given the symbol ΔH.
Enthalpy is the total energy content of the reacting materials. It is given the symbol, H.
ΔH = ΔH products - ΔH reactants
The standard conditions are:
- A pressure of 100kilopascals (102kPa)
- A temperature of 298K (25oC)
- Reactants and products in physical states, normal for the above conditions.
- A concentration of 1.0mol dm-3 for solutions.
Types of reactions
ΔHor The standard enthalpy change of reaction is the enthalpy change when the amounts of reactants shown in the equation for the reaction, react under standard conditions to give the products in their standard states.
ΔHof The standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its elements under standard conditions; both compound and elements are in standard states.
ΔHoc The standard enthalpy change of combustion is the enthalpy change when one mole of an element or compound reacts completely with oxygen under standard conditions.
Bond Making and Breaking
For a chemical reaction to occur bonds must break before new bonds can be made. When bonds break energy is absorbed (endothermic). When bonds form, energy is released (exothermic).
If the energy absorbed whilst making bonds is greater than the energy transferred to the surroundings as bonds are made, then an endothermic reaction will occur.
Whereas, if the energy released on bond formation is greater than that absorbed through breaking bonds then anexothermic reaction is observed.
Bond energy: This is the amount of energy required to break a covalent bond, it indicates the strength of a bond. Values are always quoted as bond energy per mole, E.
Calculating Enthalpy changes
Energy transfer = mcΔT (joules)
m= mass (g)
c= Specific heat capacity of water
ΔT= Change in temperature
Hess' Law States: the total enthalpy change for a chemical reaction is independent of the route by which the reaction takes place.