# Enthalpy change of reaction.

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#1
My year 1 book says that for an enthalpy change of combustion to calculate delta H, you do q=m*c* delta T, then delta H= q/n, where n is the moles of the fuel, and I understand this.

But in the enthalpy of a reaction, you change the q/n to (q/n) x number of moles reacting in the balanced chemical equation), where n is the moles of one of the reactants.

When I do questions, they always use the first equation even when it is not a combustion. How do I know which equation to use? and which reactant to use for the moles in an enthalpy of reaction, where nothing is being combusted?

Thanks
Last edited by JJJJJAAAAMES; 2 years ago
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2 years ago
#2
Enthalpy of combustion, by definition, involves the combustion of 1 mol of reactant. Therefore "(q/n) x number of moles reacting in the balanced chemical equation" is simply x 1, so can be ignored.
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#3
(Original post by Pigster)
Enthalpy of combustion, by definition, involves the combustion of 1 mol of reactant. Therefore "(q/n) x number of moles reacting in the balanced chemical equation" is simply x 1, so can be ignored.
Im not sure what you mean by x 1 in a combustion reaction, e.g. in the combustion of methane, there's 3 moles reacting ( 1CH4 + 2O2).

My book gives the example of saying NH4Cl being dissolved in water making NH4+ and Cl- (NH4Cl --> NH4+ + Cl-), theres only one mole of reactant, so they x1 in the q/n step. I'm asking about reactions that involve more than 1 reactant, and which reactant to use to find the moles of.
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2 years ago
#4
Enthalpy of combustion is the enthalpy change when 1 mole of a substance is completely combusted.
So you only use the moles of the substance being combusted (the fuel) and therefore not oxygen.
For CH4 + 2O2 --> CO2 + 2H2O there is one mole of CH4 being burned.
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2 years ago
#5
(Original post by JJJJJAAAAMES)
Im not sure what you mean by x 1 in a combustion reaction, e.g. in the combustion of methane, there's 3 moles reacting ( 1CH4 + 2O2).

My book gives the example of saying NH4Cl being dissolved in water making NH4+ and Cl- (NH4Cl --> NH4+ + Cl-), theres only one mole of reactant, so they x1 in the q/n step. I'm asking about reactions that involve more than 1 reactant, and which reactant to use to find the moles of.
Enthalpy change of reaction is "per mole of reaction as stated in the quantities in the balanced equation". One mol of methane combustion reaction does involve 3 mol in total.
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