The quantity of heat transferred during a reaction depends on the amount of material undergoing chemical change.
For purposes of comparing thermal changes in different chemical reactions, it is convenient to define a quantity that is characteristic of the chemical reaction.
Because most reactions are carried out at constant pressure in containers open to the atmosphere, the quantity enthalpy (H) was defined.
The change in enthalpy (ΔH) equals qp, the heat transferred at constant pressure. The unit for ΔH is the same as the unit for q (J or kJ).
The enthalpy of reaction (ΔrH) is characteristic of the reaction and can be calculated by measuring the heat transfer at constant pressure (qp) for a specific amount in moles of reaction (n). The unit is kJ mol–1.
Once the enthalpy change of reaction is known, it can be used to calculate the heat absorbed or released for any amount of reaction.
For purposes of comparing thermal changes in different chemical reactions, it is convenient to define a quantity that is characteristic of the chemical reaction.
Because most reactions are carried out at constant pressure in containers open to the atmosphere, the quantity enthalpy (H) was defined.
The change in enthalpy (ΔH) equals qp, the heat transferred at constant pressure. The unit for ΔH is the same as the unit for q (J or kJ).
| ΔrH = | qp |
| n |
The enthalpy of reaction (ΔrH) is characteristic of the reaction and can be calculated by measuring the heat transfer at constant pressure (qp) for a specific amount in moles of reaction (n). The unit is kJ mol–1.
Consider the reaction:
Na2CO3 + 2HCl
CO2 + H2O + 2NaCl
The amount in moles of reaction is equal to n(Na2CO3) reacting.
The amount in moles of reaction is also equal to ½ × n(HCl) reacting or ½ × n(NaCl) formed.
Na2CO3 + 2HCl
CO2 + H2O + 2NaClThe amount in moles of reaction is equal to n(Na2CO3) reacting.
The amount in moles of reaction is also equal to ½ × n(HCl) reacting or ½ × n(NaCl) formed.
Once the enthalpy change of reaction is known, it can be used to calculate the heat absorbed or released for any amount of reaction.