Enthalpy and heat
For most reactions the total energy of products is different to the total energy of the reactants.
Therefore as the reaction proceeds heat energy is either released to or consumed from the surroundings.
Thermal changes for chemical reactions are described using a quantity enthalpy (H) where the change in enthalpy (ΔH) equals qp, the heat transferred at constant pressure. The unit for ΔH is the same as the unit for heat q (J or kJ).
The enthalpy change is positive if the reaction system consumes heat energy from the surroundings.
These reactions are endothermic (from Greek for heat in).
N2(g) + O2(g)
2NO(g)
ΔH = +180 kJ
These reactions are exothermic (from Greek meaning heat out).
The enthalpy change depends on the amount of substance reacting.
The enthalpy of reaction (ΔrH) is equal to the heat transfer at constant pressure (qp) for a specific amount in moles of reaction (n) as defined by the balanced equation. The unit is kJ mol–1.
Consider the reaction:
2Na(s) + Cl2(g) 2NaCl(s)
ΔrH = -822 kJ mol–1
1 mol reaction in this case involves the reaction of 2 moles of Na with 1 mol of Cl2 to form 2 moles of NaCl.
The amount in moles of reaction for a given amount in moles of reactant or product, is that amount in moles divided by the coefficient in the balanced equation.
If 1 mol Cl2 reacts, there is 1 mol of reaction because the coefficient of Cl2 is 1, and 822 kJ of heat energy is released.
If 1 mol Na reacts, there is 0.5 mol of reaction because the coefficient of Na is 2, and 411 kJ of heat energy is released.
If 1 mol NaCl is formed, there is 0.5 mol of reaction because the coefficient of NaCl is 2 and 411 kJ of heat energy is released.
Therefore as the reaction proceeds heat energy is either released to or consumed from the surroundings.
Thermal changes for chemical reactions are described using a quantity enthalpy (H) where the change in enthalpy (ΔH) equals qp, the heat transferred at constant pressure. The unit for ΔH is the same as the unit for heat q (J or kJ).
Many reactions are carried out at constant pressure in containers open to the atmosphere. For these volume occupied and the external pressure are constant AND ΔH is also equal to the difference between the energy of the products and the reactants.
| ΔrH = | qp |
| n |
The enthalpy change is positive if the reaction system consumes heat energy from the surroundings.
These reactions are endothermic (from Greek for heat in).
N2(g) + O2(g)
2NO(g) ΔH = +180 kJ
As shown in the diagram at the right, consumption of heat as reaction proceeds means that the products have a higher energy than reactants.
The enthalpy change is negative if the reaction system releases heat energy to the surroundings.These reactions are exothermic (from Greek meaning heat out).
2H2(g) + O2(g) 2H2O(g)
ΔH = -484 kJ
2H2O(g)ΔH = -484 kJ
As shown in the diagram at the right, release of heat as the reaction proceeds means that the products have a lower energy than reactants.
The enthalpy change depends on the amount of substance reacting.
The enthalpy of reaction (ΔrH) is equal to the heat transfer at constant pressure (qp) for a specific amount in moles of reaction (n) as defined by the balanced equation. The unit is kJ mol–1.
Consider the reaction:
2Na(s) + Cl2(g)
2NaCl(s)ΔrH = -822 kJ mol–1
1 mol reaction in this case involves the reaction of 2 moles of Na with 1 mol of Cl2 to form 2 moles of NaCl.
The amount in moles of reaction for a given amount in moles of reactant or product, is that amount in moles divided by the coefficient in the balanced equation.
If 1 mol Cl2 reacts, there is 1 mol of reaction because the coefficient of Cl2 is 1, and 822 kJ of heat energy is released.
If 1 mol Na reacts, there is 0.5 mol of reaction because the coefficient of Na is 2, and 411 kJ of heat energy is released.
If 1 mol NaCl is formed, there is 0.5 mol of reaction because the coefficient of NaCl is 2 and 411 kJ of heat energy is released.