Topic 4
ENERGETICS
Measuring and Calculating Enthalpy Changes
Mean Bond Dissociation Enthalpies
Hess’ Law
ENTHALPY CHANGES
1. Exothermic and endothermic reactions
When a chemical reaction takes place, the products and reactants have different stabilities and thus there is a change in potential energy. However since total energy is always conserved, any change in potential energy must be balanced by an equal and opposite change in kinetic energy.
The lower the chemical potential energy of a given chemical species, the more stable it is. This means that stable species have a lower potential energy than unstable species.
In some reactions, the products are more stable than the reactants. The products therefore have less potential energy than the reactants, and the potential energy of the reacting species decreases.
Since the total energy is always conserved, it follows that the kinetic energy of the species must increase. The particles thus move faster and the temperature increases. Reactions in which the products are more stable than the reactants thus involve a transfer of energy from potential to kinetic and an increase in temperature.
PE ( KE
Such reactions give out heat and are thus said to be EXOTHERMIC.
In other reactions, the reactants are more stable than the products. The products therefore have more potential energy than the reactants, and the potential energy of the reacting species increases.
Since the total energy is always conserved, it follows that the kinetic energy of the species must decrease. The temperature of the system thus decreases. Reactions in which the reactants are more stable than the products thus involve a transfer of energy from kinetic to potential and a decrease in temperature.
KE ( PE
Such reactions absorb heat and are said to be ENDOTHERMIC.
2. Standard enthalpy changes
The change in chemical potential energy during a chemical reaction is known as the enthalpy change for that reaction. It is given the symbol H.
By convention, if the reaction is exothermic (ie heat is given out) the enthalpy change is said to be negative: H = -ve. If the reaction is endothermic (ie heat is absorbed) the enthalpy change is said to be positive: H = +ve.
The enthalpy change of a reaction depends on the reaction conditions. It is therefore necessary to specify standard conditions for the measurement of enthalpy changes. These are taken to be atmospheric pressure (1 atm) and room temperature (298K). Enthalpy changes measured under standard conditions are known as standard enthalpy changes and are given the symbol Ho. During these chemical changes, the pressure should be kept constant.
The enthalpy change for a reaction is the heat energy change measured under conditions of constant pressure.
The standard enthalpy change for a reaction is the heat energy change measured under standard conditions: 100 kPa and a stated temperature (usually 298K).
The enthalpy change also depends on the amount of substance used. It is theref...
