Enthalpy, H, is the heat content stored in a chemical system.
If heat is released, the amount of energy that leaves a chemical system is exactly the same as the amount that goes into the surroundings. This is the law of conservation.
Heat loss in a chemical system = heat gain to the surroundings
Heat gain in a chemical system = heat loss from surroundings
Enthalpy change, ΔH, is;
- the heat exchange with the surroundings during a chemical reaction, at constant pressure
- the difference between the enthalpy of the products and the enthalpy of the reactants
During an exothermic reaction, bonds are made. This releases energy. Therefore the enthalpy of the products is less than that of the reactants. The ΔH is therefore negative.
Heat is lost from the system.
Combustion of fuels and respiration are examples of exothermic reactions.
During an endothermic reaction, bonds are broken. This uses energy. Therefore the enthalpy of the products is greater that the enthalpy of the reactants.The ΔH is therefore positive.
The system gains heat.
Thermal decomposition and photosynthesis are examples of endothermic reactions.
Enthalpy profile diagrams and activation energy
Draw an enthalpy profile diagram for an exothermic reaction and for an endothermic reaction.
Activation energy is the minimum energy required to start a reaction by breaking bonds.
Standard conditions are;
- a pressure of 100 kPa
- a temperature of -298K
- a concentration of 1mol/dm-3
For a standard enthalpy change, reactant must be in their standard states under standard conditions;
- Mg = solid
- H = gas
- Water = liquid
Standard enthalpy change of reaction
Standard enthalpy change of reaction is the enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states
Standard enthalpy change of combustion definition
Standard enthalpy change of combustion is the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants being in their standard states.
Standard enthalpy change of formation
Standard enthalpy change of formation is the enthalpy change that takes place when one mole of an ionic compound is formed from its constituent elements in their standard states, under standard conditions.
Determination of enthalpy changes
- m - the mass of the surroundings involved in heat exchange
- c - the specific heat capacity of the surroundings
- ΔT - the temperature change of the surroundings
ΔH = Q/moles
If the reaction is exothermic, ΔH will be negative
If the reaction is endothermic, ΔH will be positive
Bond enthalpy is the enthalpy change that takes place when breaking by homolytic fission 1 mol of a given bond in the molecules of gaseous species.
H-H --> 2H ΔH=+436 kJ/mol
H-Cl --> H + Cl ΔH=+432 kJ/mol
Average bond enthalpy is the average enthalpy change that takes place when breaking by homolytic fission 1 mole of a given type of bond in the molecules of a gaseous species.
Exo or endo?
- exothermic --> bond formed are stronger than bonds that are broken
- endothermic-> bonds formed are weaker than bonds that are broken
ΔH = ∑(bond enthalpies of bonds broken) - ∑(bond enthalpies of bonds made)