DF2: Enthalpy cycles + Hess' Law

Question 1

What is Hess’s Law?

Answer 1

If the starting and ending points are the same, so is the total enthalpy change of a reaction, no matter what route is taken.

Question 2

What is the formula for finding the enthalpy change of reaction from enthalpy changes of combustion?

Answer 2

Change in H1 = Change in H2 - Change in H3

Question 3

What is the formula for finding the enthalpy change of reaction from enthalpy changes of formation?

Answer 3

Enthalpy change of reaction = sum of enthalpy changes of formation of products - sum of enthalpy changes of formation of reactants

Question 4

Which ways do the arrows point in an enthalpy cycle for combustion?

Answer 4

Down

Question 5

What is placed at the bottom in an enthalpy cycle for combustion?

Answer 5

Combustion products - CO2 + H20 usually

Question 6

Which ways do the arrows point in an enthalpy cycle for formation?

Answer 6

Up

Question 7

What is placed at the bottom in an enthalpy cycle for formation?

Answer 7

The elements that formed the compound reactants

Question 8

Why is the amount of oxygen used in combustion not critical?

Answer 8

It is normally used in excess

Question 9

Why are the enthalpy changes for elements always 0?

Answer 9

They have already been formed and do not change their amount so enthalpy change is 0