For most reactions the total energy of products is different to the total energy of reactants, and heat is either released to or absorbed from the surroundings by the reaction system. 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 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.
Calculate the amount in moles of reaction for a given amount in moles of reactant or product by dividing that amount in moles by the coefficient on the substance in the balanced equation.
Consider the reaction:
2Na(s) + Cl2(g)
2NaCl(s)
ΔrH = -822 kJ mol–1
1 mol reaction involves the reaction of 2 moles of Na with 1 mole of Cl2 to form 2 moles of NaCl.
If 1 mol Cl2 reacts, there is 1 mol of reaction and 822 kJ of heat energy is released.
If 1 mol Na reacts, there is 0.5 mol of reaction and 411 kJ of heat energy is released.
| ΔrH = | qp |
| n |
qp is positive if heat has been transferred to the reaction system from the surroundings
qp is negative if heat has been transferred from the reaction system to the surroundings
It is usually assumed that heat transfer is complete; thus qp(reaction) = –qp(surroundings).
qp is negative if heat has been transferred from the reaction system to the surroundings
It is usually assumed that heat transfer is complete; thus qp(reaction) = –qp(surroundings).
Calculate the amount in moles of reaction for a given amount in moles of reactant or product by dividing that amount in moles by the coefficient on the substance in the balanced equation.
Consider the reaction:
2Na(s) + Cl2(g)
2NaCl(s)ΔrH = -822 kJ mol–1
1 mol reaction involves the reaction of 2 moles of Na with 1 mole of Cl2 to form 2 moles of NaCl.
If 1 mol Cl2 reacts, there is 1 mol of reaction and 822 kJ of heat energy is released.
If 1 mol Na reacts, there is 0.5 mol of reaction and 411 kJ of heat energy is released.