18. Calculate ƒ´H for the following reaction using Hess law
H2 (g) + Br2 (g) ¡÷ 2 HBr (g)
Experimental information
2 H+ ¡÷ H2 ƒ´H -436.4 kJ
2 Br- ¡÷ Br2 ƒ´H -192.5 kJ
2 H+ + 2 Br- ¡÷2 HBr ƒ´H -144.8 kJ
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Reverse equation 1 and add to the reverse of equation 2 and add equation 3 as is. Remember to change the sign of dH when reversing an equation.
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To calculate ΔH for the given reaction using Hess's law, we need to find a combination of reactions whose ΔH values add up to the ΔH of the desired reaction. Here's how we can do it step by step:
1. Start with the given reaction:
H2(g) + Br2(g) → 2 HBr(g)
2. Look for reactions involving the same species on the reactant and product sides. In this case, we have H2 and Br2 on the left side, and HBr on the right side.
3. We are given the following experimental information:
a) 2 H+ → H2, ΔH = -436.4 kJ
b) 2 Br- → Br2, ΔH = -192.5 kJ
c) 2 H+ + 2 Br- → 2 HBr, ΔH = -144.8 kJ
4. Rearrange the given reactions to match the desired reaction. In this case, we need to double reaction c to get two moles of HBr:
2 H+ + 2 Br- → 2 HBr, ΔH = -144.8 kJ
5. Multiply the ΔH value of reaction c by 2 to match the stoichiometric coefficient of HBr in the desired reaction:
2 × (-144.8 kJ) = -289.6 kJ
6. Now, we need to add the ΔH values of reactions a and b to get the overall ΔH for the desired reaction:
-436.4 kJ + (-192.5 kJ) + (-289.6 kJ)
7. Calculate the sum of the ΔH values:
-436.4 kJ - 192.5 kJ - 289.6 kJ = -918.5 kJ
Therefore, the ΔH for the given reaction using Hess's law is -918.5 kJ.