Enthalpy change and mass

The focus of previous pages has been calculating the mass of a reaction component A associated with a given enthalpy change (ΔH).

The pathway for doing this is summarised below with the appropriate mathematical relationships given over the arrows. 

Note the row that indicates how the unit of the answer changes.  It is good practice to keep track of units.  Through unit analysis you can avoid making silly mistakes like multiplying where you should have divided.
 

ΔH	n(reaction)= ΔH ΔrH	n(reaction)	n(A) = a × n(reaction)
kJ	divide by kJ mol–1	gives mol	times a number

n(A)	m(A) = n(A) × M(A)	m(A)
gives mol	times g mol–1	gives g

The reverse process, namely calculating the enthalpy change ΔH associated with reaction of a specified mass of reactant or product is also possible using rearranged forms of the same mathematical relationships.  The plan for doing this is shown below.  Note that the sign of ΔH is the same as the sign of Δ_rH.
 

m(A)	n(A)= m(A) M(A)	n(A)	n(reaction)= n(A) a
g	divide by g mol–1	gives mol	divide by a number

n(reaction)	ΔH = n(reaction) × ΔrH	ΔH
gives mol	times kJ mol–1	gives kJ

Note that In this case the amount in moles of reaction is calculated by dividing the amount in moles of substance A by its coefficient in the balanced equation.  Once the amount in moles of reaction is known, the enthalpy change in kilojoules is calculated by multiplying the amount in moles of reaction by the reaction enthalpy.

IMPORTANT:  Note that the steps before n(reaction) are divisions AND the steps after n(reaction) are multiplications!!