The enthalpy (energy) change accompanying a reaction (ΔrH) depends on the amount of material undergoing chemical change.
The unit for reaction enthalpy change (ΔrH) is kJ mol–1. kJ (kilojoules) is a unit of energy. mol is a unit of amount.
In this case mol–1 means per mole of reaction (per mole of chemical change) by the balanced equation specified.
1 mole of reaction occurs when the amount in moles of each reactant consumed and product produced is equal to its coefficient in the balanced equation for the reaction.
(1) 2H2(g) + O2(g)
2H2O(l)
ΔrH = –572 kJ mol–1
One mole of reaction (1) produces 572 kJ of heat, consumes 2 mol H2 and 1 mol O2, and produces 2 mol H2O.
(2) 4H2(g) + 2O2(g) 4H2O(l)
ΔrH = –1144 kJ mol–1
One mole of reaction (2) produces 1144 kJ of heat.
consumes 4 mol H2 and 2 mol O2,and produces 4 mol of H2O.
ΔH is the observed enthalpy change for a particular reaction system and has the unit kJ.
If the reaction system involves 2 mol H2 and 1 mol O2 reacting to give 2 mol H2O,
572 kJ heat will be released, and ΔH is 572 kJ because 1 mol of reaction has occurred.
For reactions using different amounts in moles of reactants, the magntiude ΔH will be different BUT the sign is the same.
The amounts in moles of reactants required can be calculated from the amount in moles of reaction required.
To calculate the amount of reaction required to produce enthalpy change ΔH, divide the enthalpy change required by the reaction enthalpy. The unit of n(reaction) is mol.
Examples of calculating n(reaction) required for a specified ΔH:
To produce 286 kJ of heat (ΔH = –286 kJ) requires n(reaction 1) of 0.50 mol and thus requires reaction of 1 mol H2 and ½ mol O2
To produce 286 kJ of heat requires n(reaction 2) = 0.25 mol and thus requires reaction of 1 mol H2 and ½ mol O2
Thus the same amounts of the reactants are required to liberate a specific amount of heat BUT the amount of reaction that must occur depends on which of the balanced equations for the reaction is used.
The unit for reaction enthalpy change (ΔrH) is kJ mol–1. kJ (kilojoules) is a unit of energy. mol is a unit of amount.
In this case mol–1 means per mole of reaction (per mole of chemical change) by the balanced equation specified.
1 mole of reaction occurs when the amount in moles of each reactant consumed and product produced is equal to its coefficient in the balanced equation for the reaction.
(1) 2H2(g) + O2(g)
2H2O(l)ΔrH = –572 kJ mol–1
One mole of reaction (1) produces 572 kJ of heat, consumes 2 mol H2 and 1 mol O2, and produces 2 mol H2O.
(2) 4H2(g) + 2O2(g)
4H2O(l) ΔrH = –1144 kJ mol–1
One mole of reaction (2) produces 1144 kJ of heat.
consumes 4 mol H2 and 2 mol O2,and produces 4 mol of H2O.
ΔH is the observed enthalpy change for a particular reaction system and has the unit kJ.
If the reaction system involves 2 mol H2 and 1 mol O2 reacting to give 2 mol H2O,
572 kJ heat will be released, and ΔH is 572 kJ because 1 mol of reaction has occurred.
For reactions using different amounts in moles of reactants, the magntiude ΔH will be different BUT the sign is the same.
For example you may wish to use the burning of H2 in O2 to produce 3 kJ of heat to warm your cup of coffee.
Obviously smaller amounts of H2 and O2 are required than in the cases described above.
Obviously smaller amounts of H2 and O2 are required than in the cases described above.
The amounts in moles of reactants required can be calculated from the amount in moles of reaction required.
To calculate the amount of reaction required to produce enthalpy change ΔH, divide the enthalpy change required by the reaction enthalpy. The unit of n(reaction) is mol.
| n(reaction) = | ΔH |
| ΔrH |
Examples of calculating n(reaction) required for a specified ΔH:
To produce 286 kJ of heat (ΔH = –286 kJ) requires n(reaction 1) of 0.50 mol and thus requires reaction of 1 mol H2 and ½ mol O2
To produce 286 kJ of heat requires n(reaction 2) = 0.25 mol and thus requires reaction of 1 mol H2 and ½ mol O2
Thus the same amounts of the reactants are required to liberate a specific amount of heat BUT the amount of reaction that must occur depends on which of the balanced equations for the reaction is used.