# Heat Of Reaction

chikis

## Homework Statement

Given that for the reaction KOH(aq) + HCL(aq) ----> KCL(aq) + H2O(l) H = - 54kjmol^-1.
What is the quantity of heat involved in the reaction? 2NaOH(aq) + H2SO4(aq) ----> Na2SO4(aq) + 2H2O(l)?

## Homework Equations

Heat of reaction Hrn =heat content of product Hp - heat content of reactant Hr. It can be written as Hrn= Hp-Hr

## The Attempt at a Solution

Since Hrn=Hp-Hr. I don't know what to do again in other to get the value of Hp and Hr so that I can take the difference and Hrn will appear from no where.

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Mentor
You don't need "heat content" of reactants/products.

First things first - write net ionic reaction.

Gold Member
This is a neutralization reaction between a strong acid and a strong base. Any neutralization reaction between a strong base and a strong acid yields the same heat of neutralization, -54 KJ/mol.

To better understand this, as Borek said, try to write the net ionic equation for both the reaction. You will see the magic then.

chikis
You don't need "heat content" of reactants/products.

First things first - write net ionic reaction.
For the reaction KOH(aq) + HCL(aq) ----> KCL(aq) + H2O(l)
Net ionic equation
H++OH---->H2O
For the reaction:
2NaOH(aq) + H2SO4(aq) ----> Na2SO4(aq) + 2H2O(l)
Net ionic equation
2OH-+2H+
---->2H2O
So what is the heat of reaction?

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Mentor
If burning 1 candle produces x kJ, how many kJ will be produced when you burn 2 candles?

chikis
If burning 1 candle produces x kJ, how many kJ will be produced when you burn 2 candles?

If 1 mole of water produces -54kJ, then 2 moles of water will produce -108kJ. Could that be what you are tying to ask?

Mentor
That's not what I am trying to ask, that's what you are trying to understand :tongue2:

It is not "moles of water produce 108kJ", it is "108kJ evolved while producing 1 mole of water in the neutralization reaction". But otherwise you are right. Note the sign!

chikis
That's not what I am trying to ask, that's what you are trying to understand :tongue2:

It is not "moles of water produce 108kJ", it is "108kJ evolved while producing 1 mole of water in the neutralization reaction". But otherwise you are right. Note the sign!

What does the sign have to the with the computation?

Mentor
Earlier you wrote:

1 mole of water produces -54kJ

That's incorrect. If anything, when 1 mole of water is made in neutralization reaction, 54 kJ are produced. That means standard enthalpy of reaction is -54 kJ - because by convention enthalpy change for exothermic reaction is negative (lost by the system, not gained by the surroundings).

When you write "-54 kJ is produced" it suggests system gained 54 kJ, and the reaction was endothermic.

chikis
Why do I have to write the net ionic equation before solving?

Gold Member
You have to understand first what is actually going on before trying to intercept the situation. You don't eat food without seeing what is served to you, do you?

chikis
You have to understand first what is actually going on before trying to intercept the situation.
You don't eat food without seeing what is served to you, do you?

No I don't, unless am blind.

Gold Member
Same here. Try to actually see what reaction is going on. This case, just simple neutralization, formation of water by strong acid and strong base.

Always try to write down the reaction when solving the problem. This makes the problem visually easy and enables you to solve it rather easily, now that you are able to see the reaction.

chikis
That's not what I am trying to ask, that's what you are trying to understand :tongue2:

It is not "moles of water produce 108kJ", it is "108kJ evolved while producing 1 mole of water in the neutralization reaction". But otherwise you are right.
Note the sign!

I Now understand what you meant properly now. Because the sign is negative, the reaction is exothermic, therefore heat in kJ is evolved when 1 mole of water is produced. Isn't it?

chikis
Here are the possible answers which the authors of the question set for the problem A.-108kj B. -54kj C. -27kj D. +27kj E. +54kj.
They went and choose C. = -27kj instead of A. = -108kj. I don't blame them as such, maybe that mistake is due to typo error. Don't you think the same?

Mentor
27 kJ (regardless of the sign) doesn't make any sense.

chikis
27 kJ (regardless of the sign) doesn't make any sense.

I don't understand what you mean.

Mentor
Neither 27 kJ nor -27 kJ is a correct answer.

chikis
Neither 27 kJ nor -27 kJ is a correct answer.

Thanks for the asistance. I do appreciate!