alleryStart learning →Given Fe + Cl2 giving FeCl2 at -341.8 kJ, and FeCl2 + half Cl2 giving FeCl3 at -57.7 kJ, what is the formation enthalpy of FeCl3?
A-284.1 kJ/mol
B-399.5 kJ/mol
C+399.5 kJ/mol
D-170.9 kJ/mol
Answer & Solution
Correct answer: B. -399.5 kJ/mol
1. The target reaction is Fe plus three halves Cl2 giving 1 mol of FeCl3.
2. Neither given step needs reversing, since Fe appears as a reactant and FeCl3 as a product.
3. Neither step needs multiplying, since each already carries the coefficients required.
4. The FeCl2 made in the first step is consumed in the second, so it cancels.
5. The chlorine amounts add to 1 plus one half, that is three halves Cl2, as wanted.
6. Add the values: minus 341.8 plus minus 57.7.
7. The sum is minus 399.5 kJ, so the formation enthalpy of FeCl3 is minus 399.5 kJ/mol.
8. Since no step was reversed, no sign was flipped anywhere in this calculation.
_Source: OpenStax Chemistry (CC BY 4.0), Ch 5 "Thermochemistry", section 5.3 Enthalpy_
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