Hess's Law Practice

We can determine the change in enthalpy for a reaction by using _____ values for ΔH.

• positive

• published

• negative

• experimental

If a reaction is carried out in a series of steps, what will we do with the constituent ΔH values?

• multiply them

• subtract them

• add them

• divide them

The reactions must all add up to the _____ reaction for Hess's Law.

A constituent reaction has a ΔH = -758 kJ. If you need to reverse the reaction to obtain the final one, what will you need to do to the ΔH value?

• reverse the sign to +758 kJ

• divide -758 kJ by 2

• multiply -758 kJ by 2

• nothing, it will remain the same

You need to multiply the coefficients in a constituent reaction by 3 to obtain the correct number of moles of a reactant in the overall equation. What will you need to do to the ΔH for this reaction?

• divide by 3

• reverse the sign and multiply by 3

• multiply by 3

• reverse the sign

Given the following equations, calculate the ΔH for NO(g) + O(g) --> NO2(g)

NO(g) + O3(g) --> NO2(g) + O2(g) ΔH = -198.9 kJ/mol

O3(g) --> 3/2O2(g) ΔH = -142.3 kJ/mol

O2(g) --> 2O(g) ΔH = +495 kJ/mol

• -588.7 kJ/mol

• -304 kJ/mol

• -247.5 kJ/mol

• 304 kJ/mol

Given the following equations calculate ΔH for P₄O₆(s) + 2O₂(g) --> P₄O₁₀(s)

P₄(s) + 3O₂(g) --> P₄O₆(s) ΔH = -1640.1 kJ/mol

P₄(s) + 5O₂(g) --> P₄O₁₀(s) ΔH = -2940.1 kJ/mol

• -1640 kJ/mol

• -4580.2 kJ/mol

• +1300 kJ/mol

• -1300 kJ/mol

We are able to use constituent reactions in our Hess's Law calculations because ΔH is a _____ function.

You have concluded that the ΔH values you need to determine the overall ΔH are as follows: -576 KJ, +1169 kJ, and -639 kJ. What is the overall ΔH?

• -46 kJ

• -2384 kJ

• 1106 kJ

• 4.3 x 10⁸ kJ