Hess's Law

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Hess's law

HESS"S LAW: says that when there is more that one reaction, the total enthalphy change is the sum of the enthalpy changes of each reaction.

CS₂(l) + 3 O₂(g) → CO₂(g) + 2 SO₂(g)

Using the equations below calculate the enthalpy of the reaction.

C(s) + O₂(g) → CO₂(g) ΔH= -393.5 kJ/mol
S(s) + O₂(g) → SO₂(g) ΔH = -296.8 kJ/mol
C(s) + 2 S(s) → CS₂(l) ΔH = 87.9 kJ/mol

The solution

Leave the first reaction alone	C(s) + O₂(g) → CO₂(g) ΔH= -393.5 kJ/mol
Double the second reaction	2S(s) + 2O₂(g) → 2SO₂(g) ΔH = -296.8 kJ/mol
Reverse the third reaction	CS₂(l)→C(s) + 2 S(s) ΔH = -87.9 kJ/mol
Combine the reactions and cancel species that appear on both sides. Add up the ΔH.	CS₂(l) + 3 O₂(g) → CO₂(g) + 2 SO₂(g) ΔH = -1075 kJ/mol

Example 2

HCl(g) + NaNO₂(s) → HNO₂(l) + NaCl(s)

Using the equations below calculate the enthalpy of the reaction.

2NaCl(s) + H₂O(l) →  2HCl(g) + Na₂O(s)                     ΔH = 507 kJ
NO(g) + NO₂(g) + Na₂O(s) → 2NaNO₂(s)                    ΔH = -427 kJ
NO(g) + NO₂(g) → N₂O(g) + O₂(g)                               ΔH = -43 kJ
2HNO₂(l) → N₂O(g) + O₂(g) + H₂O(l)                          ΔH = 34 kJ

Reverse and 1/2 the first reaction	HCl(g) + 1/2 Na₂O(s)→NaCl(s) + 1/2H₂O(l) ΔH = -1/2(507 kJ)
Reverse and 1/2 the second reaction	NaNO₂(s)→1/2 NO(g) +1/2 NO₂(g) +1/2 Na₂O(s) ΔH = -1/2(-427 kJ)
1/2 the third reaction	1/2NO(g) + 1/2NO₂(g) → 1/2N₂O(g) + 1/2O₂(g) ΔH = 1/2(-43 kJ)
Reverse and 1/2 the second reaction	1/2N₂O(g) + 1/2O₂(g) + 1/2H₂O(l)→HNO₂(l) ΔH = -1/2(34 kJ)
Combine the reactions and cancel species that appear on both sides. Add up the ΔH.	HCl(g) + NaNO₂(s) → HNO₂(l) + NaCl(s) ΔH =-78.5kJ

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