Finding enthalpy change using Hess's Law
Practice problems on Hess's Law of heat summation
Chemists can determine the enthalpy change of any reaction using an important law, known as Hess’s law of heat summation. This law states that the enthalpy change of a physical or chemical process depends only on the beginning conditions (reactants) and the end conditions (products). The enthalpy change is independent of the pathway of the process and the number of intermediate steps in the process. It is the sum of the enthalpy changes of all the individual steps that make up the process.
Ethene, C2H4, reacts with water to form ethanol, CH3CH2OH(l) .
C2H4(g) + H2O(l) → CH3CH2OH(l)
Determine the enthalpy change of this reaction, given the following thermochemical equations.
(1) CH3CH2OH(l) + 3O2(g) → 3H2O(l) + 2CO2(g) ΔH° = −1367 kJ
(2) C2H4(g) + 3O2(g) → 2H2O(l) + 2CO2(g) ΔH° = −1411 kJ
Enthalpy change for the target equation can be determined by adding equation 1 and equation 2.
You need to flip the first equation so that CH3CH2OH goes to the product side. When ever you flip an equation the energy sign is revered.You dont need to flip the 2nd equation.
So, the 1st equation looks as follows after flipping.
3H2O(l) + 2CO2(g)→CH3CH2OH(l) + 3O2(g) ΔH° = +1367 kJ
C2H4(g) + 3O2(g) → 2H2O(l) + 2CO2(g) ΔH° = −1411 kJ
C2H4(g) + H2O(l) → CH3CH2OH(l) ΔH° =-44 kJ
Practice Problem 2:
A typical automobile engine uses a lead-acid battery. During discharge, the following chemical reaction takes place.
Pb(s) + PbO2(s) + 2H2SO4(l) → 2PbSO4(aq) + 2H2O(l)
Determine the enthalpy change of this reaction, given the following
(1) Pb(s) + PbO2(s) + 2SO3(g) → 2PbSO4(s) ΔH° = −775 kJ
(2) SO3(g) + H2O(l) → H2SO4(l) ΔH° = −133 kJ
1st equation don't need to be flipped . 2nd equation needs to be flipped and times by 2 to get 2 mol of H2SO4
Pb(s) + PbO2(s) + 2SO3(g) → 2PbSO4(s) ΔH° = −775 kJ
2 H2SO4(l) →2SO3(g) + 2H2O(l) ΔH° = +266 kJ
Pb(s) + PbO2(s) + 2H2SO4(l) → 2PbSO4(aq) + 2H2O(l) ΔH° = -509 kJ
Practice Problem 3:
Mixing household cleansers can result in the production of hydrogen chloride gas, HCl(g). Not only is this gas dangerous in its own right, but it also reacts with oxygen to form chlorine gas and water vapour.
4HCl(g) + O2(g) → 2Cl2(g) + 2H2O(g)
Determine the enthalpy change of this reaction, given the following equations.
(1) H2(g) + Cl2(g) → 2HCl(g) ΔH° = −185 kJ
(2) H2(g) + 1/2 O2(g) → H2O(l) ΔH° = −285.8 kJ
(3) H2O(g) → H2O(l) ΔH° = −40.7 kJ
Note: A substance can be cancelled on both sides of the equation only when they both are in same phase(slid/liquid/gas)
Equation 1: Flip and times by 2.
Equation 2: Multiply by 2
Equation 3: Flip and multiply by 2
2HCl(g)→2H2(g) +2 Cl2(g) ΔH° = +370 kJ
2H2(g) + O2(g) →2 H2O(l) ΔH° = −571.6kJ
2H2O(g) → 2H2O(l) ΔH° = +81.4kJ
4HCl(g) + O2(g) → 2Cl2(g) + 2H2O(g) ΔH° = -120.2 kJ
Practice problem 4:
Calculate the enthalpy change of the following reaction between nitrogen gas and oxygen gas, given thermochemical equations (1), (2), and (3).
2N2(g) + 5O2(g) → 2N2O5(g)
(1) 2H2(g) + O2(g) → 2H2O(l) ΔH° = −572 kJ
(2) N2O5(g) + H2O(l) → 2HNO3(l) ΔH° = −77 kJ
(3) 1/2 N2(g) + 3/2 O2(g) + 1/2 H2(g) → HNO3(l) ΔH° = −174 kJ
Equation 1: Leave it alone
Equation 2: flip and multiply by 2
Equation 3: Multiply by 4 ( you get 2 N2, 6 O2 on left side)
Equation 1: need to be flipped to cancel the oxygen.
2H2O(l) → 2H2(g) + O2(g) ΔH° = +572 kJ
4HNO3(l) →2N2O5+2H2O(l) ΔH° = +154 kJ
2 N2(g) + 6O2(g) + 2 H2(g) → 4HNO3(l) ΔH° = −696 kJ
2N2(g) + 5O2(g) → 2N2O5(g) ΔH° = +30kJ