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1. Can you please explain how to do this question?

Q1. Find the delta H reaction for the following reaction:

4C (s) + 5H2 (g) = C4H10 (g)

Use Hess's Law and the following information in order to work this out.

2 C4H10 (g) + 13 O2 (g) = 8 CO2 (g) + 10 H2O (g) delta H reaction = -5413 kJ

C (s) + O2 (g) = CO2 (g) delta H reaction = -393.5 kJ

2H2 (g) + O2 (g) = 2H2O (g) delta H reaction = -483.6 kJ
2. (Original post by m_nusrat)
Can you please explain how to do this question?

Q1. Find the delta H reaction for the following reaction:

4C (s) + 5H2 (g) = C4H10 (g)

Use Hess's Law and the following information in order to work this out.

2 C4H10 (g) + 13 O2 (g) = 8 CO2 (g) + 10 H2O (g) delta H reaction = -5413 kJ

C (s) + O2 (g) = CO2 (g) delta H reaction = -393.5 kJ

2H2 (g) + O2 (g) = 2H2O (g) delta H reaction = -483.6 kJ
Hess's law allows you to go via any other route to obtain your products.

Start off with the reactants and then go via their combustion products to the final products
3. This is how I was taught in school:

The equation in black "4C (s) + 5H2 (g) = C4H10 (g)" is your target equation i.e. thats what your final equation needs to look like.

Now using the 3 the equations in red, either multiple/reverse & cancel out common species to get to the target equation.

I'm not going to work through it all, but I'll do the first one:
In the T.E. it says you need 4C(s)..

Using this equation (the 2nd one down): C(s) + O2 (g) = CO2 (g) delta H reaction = -393.5 kJ
multiply EVERYTHING(including the delta H value) by 4 to get the 4C(s)..
Since the 4C(s) are on the left side of the arrow in both cases you don't need to reverse it or anything.

Do that for the other species in the T.E. (you should have 3 equations now) and cancel out common species until you get your target equation.

Then all you need to do, is add the delta H values together and that's you got the delta H for the whole reaction

And one more thing, if there is an equation which you need to reverse remember to reverse the sign of the delta H aswell.. I always used to forget that!

Hope that helps
4. Delta H= sum Products-sum Reactants
5. (Original post by m_nusrat)
Can you please explain how to do this question?

Q1. Find the delta H reaction for the following reaction:

4C (s) + 5H2 (g) = C4H10 (g)

Use Hess's Law and the following information in order to work this out.

2 C4H10 (g) + 13 O2 (g) = 8 CO2 (g) + 10 H2O (g) delta H reaction = -5413 kJ

C (s) + O2 (g) = CO2 (g) delta H reaction = -393.5 kJ

2H2 (g) + O2 (g) = 2H2O (g) delta H reaction = -483.6 kJ
Ok, most important thing is elements can't have any enthalpy change.

in the question you are given 3 bits of information. (I have underlined)

Using equation 2, we can tell CO2 is =-393.5 deltaH (because all the others are elements)

Using equation 3 we can tell 2H20 is -483.6 (because all the others are elements). This means H20 is -241.8 )

Sub these values in to equation 1 and you should get delta H for C4H10 (remember to divide final answer by 2 because you have 2 molar)

Now you have Delta H for H20, CO2 and C4H10.

If it is a combustion reaction, you change the delta H of the products and add all the values.
If it is a formation reaction, you change the delta H of the reactants and add all the values.
In this case you are working out delta H of reaction so I think you just add all the delta H values and dont change the sign for anything. I may be wrong here but all else is correct

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Updated: March 24, 2011
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