To review the concept of amount of reaction and calculating amount of reaction:
The reaction enthalpy (energy) change (ΔrH) has the unit kJ mol–1 and is the energy change per mole of reaction by the particular balanced equation.
NEW IDEAS!! Using the amount in moles of reaction calculated above to calculate the amount in moles of reactants.
The amount in moles of any of the reactants required (or products produced) for a given amount in reaction can be calculated by multiplying the amount in moles of reaction by the coefficient of the substance in the balanced equation for the reaction.
EXAMPLE:
For reaction of H2 and O2 by reaction 1
For an experiment where ΔH = –286 kJ
n(reaction 1) = 0.5 mol
The amount of H2 and O2 required and H2O produced can be calculated from this:
n(H2) = 2 × 0.5 mol = 1 mol
n(O2) = 1 × 0.5 mol = 0.5 mol
n(H2O) = 2 × 0.5 mol = 1 mol
The reaction enthalpy (energy) change (ΔrH) has the unit kJ mol–1 and is the energy change per mole of reaction by the particular balanced equation.
(1) 2H2(g) + O2(g) 
2H2O(l)
ΔrH = –572 kJ mol–1
For (1) 572 kJ heat is released when the amount in moles of reaction is 1, that is 2 mol H2 react with 1 mol O2 to produce 2 mol H2O.
2H2O(l) ΔrH = –572 kJ mol–1
For (1) 572 kJ heat is released when the amount in moles of reaction is 1, that is 2 mol H2 react with 1 mol O2 to produce 2 mol H2O.
ΔH is the enthalpy change for systems where the amount in moles of reaction is not equal to 1. The unit of ΔH is kJ.
The amount of reaction required for a specified enthalpy change ΔH can be calculated by dividing the enthalpy change by the enthalpy change per mol of reaction. The unit of the result is mol.
| n(reaction) = | ΔH |
| ΔrH |
NEW IDEAS!! Using the amount in moles of reaction calculated above to calculate the amount in moles of reactants.
The amount in moles of any of the reactants required (or products produced) for a given amount in reaction can be calculated by multiplying the amount in moles of reaction by the coefficient of the substance in the balanced equation for the reaction.
For the reaction below:
aA + bB cC + dD
where A, B, C and D are substances and
a, b, c and d are coefficients
n(A) = a × n(reaction)
n(B) = b × n(reaction)
n(C) = c × n(reaction)
n(D) = d × n(reaction)
aA + bB
cC + dDwhere A, B, C and D are substances and
a, b, c and d are coefficients
n(A) = a × n(reaction)
n(B) = b × n(reaction)
n(C) = c × n(reaction)
n(D) = d × n(reaction)
EXAMPLE:
For reaction of H2 and O2 by reaction 1
For an experiment where ΔH = –286 kJ
n(reaction 1) = 0.5 mol
The amount of H2 and O2 required and H2O produced can be calculated from this:
n(H2) = 2 × 0.5 mol = 1 mol
n(O2) = 1 × 0.5 mol = 0.5 mol
n(H2O) = 2 × 0.5 mol = 1 mol