In a generic chemical reaction involving reactants A and B and products C and D, aA + bB → cC + dD, the standard enthalpy ΔH∘rxn of the reaction is given by
ΔH∘rxn = cΔH∘f(C) + dΔH∘f(D) − aΔH∘f(A) − bΔH∘f(B)
Notice that the stoichiometric coefficients, a, b, c, d, are an important part of this equation. This formula is often generalized as follows, where the first sum on the right-hand side of the equation is a sum over the products and the second sum is over the reactants:
ΔH∘rxn = ∑products nΔH∘f − ∑reactants mΔH∘f
where m and n represent the appropriate stoichiometric coefficients for each substance.
A. What is ΔH∘rxn for the following chemical reaction?
H2O(l) + CCl4(l) → COCl2(g) + 2HCl(g)
You can use the following table of standard heats of formation (ΔH∘f) to calculate the enthalpy of the given reaction.
Element/ Standard Heat of Element/ Standard Heat of
Compound Formation (kJ/mol) Compound Formation (kJ/mol)
H(g) 218 N(g) 473
H2(g) 0 O2(g) 0
CCl4(l) −139.5 O(g) 249
H2O(l) −285.8 HCl(g) −92.30kJ
C(g) 71 COCl2(g) −218.8kJ
C(s) 0 HNO3(aq) −206.6
B. Chemical energy is released or absorbed from reactions in various forms. The most easily measurable form of energy comes in the form of heat, or enthalpy. The enthalpy of a reaction can be calculated from the heats of formation of the substances involved in the reaction: ΔH∘rxn = Σ npΔH∘f(products) − Σ nrΔH∘f(reactants)where n represents the stoichiometric coefficients.
Calculate the standard enthalpy change for the reaction 2A + B ⇌ 2C + 2DUse the following data:
Substance ΔH∘f(kJ/mol)A -225B -377C 181D -523
C. Ethanol can be made from the fermentation of crops and has been used as a fuel additive to gasoline. Write a balanced equation for the combustion of ethanol.