Defining enthalpy
The quantity of heat transferred during a reaction depends on
Most reactions are carried out at constant pressure in containers open to the atmosphere. The external pressure on these reaction systems is constant during reaction.
To discuss the thermal changes for these, it is useful to define 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 q (J or kJ).
The enthalpy of reaction (ΔrH) is calculated by measuring the heat transferred during a reaction at constant pressure (qp) for a specific amount in moles of reaction (n). The unit is kJ mol–1(kJ per mole of reaction).
How is the amount in moles of reaction occurring determined?
Consider the reaction:
Na2CO3 + 2HCl
Na2SO4 + 2NaCl
The maount in moles of reaction occurring for a specified amount of substance is equal to the amount of substance divided by its coefficient in the balanced equation.
n(Na2CO3) reacting or n(Na2SO4) formed is equal to the amount in moles of reaction occurring..
The amount in moles of reaction is also equal to ½ × n(HCl) reacting or ½ × n(NaCl) formed.
the amount of material undergoing chemical change.
whether the volume occupied by the reaction mixture
or the external pressure on the reaction mixture changes during reaction.
whether the volume occupied by the reaction mixture
or the external pressure on the reaction mixture changes during reaction.
Most reactions are carried out at constant pressure in containers open to the atmosphere. The external pressure on these reaction systems is constant during reaction.
To discuss the thermal changes for these, it is useful to define a quantity enthalpy (H) where the change in enthalpy (ΔH) equals qp, the heat transferred at constant pressure.
| ΔrH = | qp |
| n |
For reactions where both the volume occupied and the external pressure are constant, ΔH is also equal to the difference between the energy of the products and the reactants.
The unit for ΔH is the same as the unit for q (J or kJ).
The enthalpy of reaction (ΔrH) is calculated by measuring the heat transferred during a reaction at constant pressure (qp) for a specific amount in moles of reaction (n). The unit is kJ mol–1(kJ per mole of reaction).
How is the amount in moles of reaction occurring determined?
Consider the reaction:
Na2CO3 + 2HCl
Na2SO4 + 2NaClThe maount in moles of reaction occurring for a specified amount of substance is equal to the amount of substance divided by its coefficient in the balanced equation.
n(Na2CO3) reacting or n(Na2SO4) formed is equal to the amount in moles of reaction occurring..
The amount in moles of reaction is also equal to ½ × n(HCl) reacting or ½ × n(NaCl) formed.