4.4 Hess's law

Question 1

What does Hess’s law state

Answer 1

The enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products

∆H1 = ∆H2 + ∆H3

Question 2

What is the law of conservation of energy, and how does it link to Hess’s law

Answer 2

It states that energy can never be created or destroyed , so as long as the starting and finish points of a reaction are the same, the energy change must be the same

Question 3

Eg C2H2 is converted to C2H6 by two different routes

The direct one

C2H2(g) + 2H2(g) —> C2H6(g)

Or there is a second route, what is this

Answer 3

C2H2 + H2 –> C2H4
It reacts with one mole of hydrogen to make ethene
And the energy change (∆H1) is -176 KJmol-1

Then:
C2H4(g) + H2(g) —> C2H6(g)
And energy change (∆H2) is -137 KJMol

Here, ethene reacts with a second mole of hydrogen to give ethane

Hess’s law means that ∆H1 = ∆H2 + ∆H3

So add together the energy to get a value of -313KJmol-1

Question 4

What is the enthalpy of formation ∆fH

Answer 4

The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions, all reactants and products being in their standard states

Question 5

Another way to make ethyne to ethane is via the elements carbon and hydrogen, what occurs here

And what type of enthalpy change is it

Answer 5

Ethyne is converted to its constituent elements carbon and hydrogen

It is the reverse of formation so the enthalpy change is the negative of the enthalpy of formation
So has a negative ∆H value

Then the carbon and hydrogen react to form ethane

Question 6

What are the reactions for when ethyne is broken up into constituent reactants before reacting

Answer 6

C2H2 + 2H2 —> 2C + 3H2

As you can see, there are 2 moles of hydrogen spare, as only one mol of hydrogen needs to be involved

These 2 moles of hydrogen remain in their standard states so no enthalpy change is involved