The combustion of ammonia by the following reaction yields nitric oxide and water

4 NH3 (g) + 5 O2 (g) → 4 NO (g) + 6 H2O (g)

Determine the heat of reaction (ΔHrxn) for this reaction by using the following thermochemical data:

N2 (g) + O2 (g) → 2 NO (g) ΔH = 180.6 kJ

N2 (g) + 3 H2 (g) → 2 NH3 (g) ΔH = - 91.8 kJ

2 H2 (g) + O2 (g) → 2 H2O (g) ΔH = - 483.7 kJ

Hess' Law Problems

2. Determine the heat of reaction (ΔHrxn) for the process by which hydrazine (N2H4) is formed from its elements:

N2 (g) + 2 H2 (g) → N2H4 (g)

by using the following thermochemical data:

N2H4 (g) + O2 (g) → N2 (g) + 2 H2O (g) ΔH = - 622.2 kJ

H2 (g) + 1/2 O2 (g) → H2O (g) ΔH = - 285.8 kJ

Hess' Law Problems

3. Determine the heat of reaction (ΔHrxn) for the process by which lead (II) chloride (PbCl2) is formed from its elements:

Pb (s) + Cl2 (g) → PbCl2 (s)

by using the following thermochemical data:

Pb (s) + 2 Cl2 (g) → PbCl4 (l) ΔH = - 329.3 kJ

PbCl2 (s) + Cl2 (g) → PbCl4 (l) ΔH = + 30.1 kJ

Hess' Law Problems

4. Determine the heat of reaction (ΔHrxn) for the process by which acetic acid (CH3COOH) is formed from its elements:

C (graphite) + 2 H2 (g) + O2 (g) → CH3COOH (l)

by using the following thermochemical data:

CH3COOH (l) + O2 → 2 CO2 (g) + 2 H2O(l) ΔH = - 871 kJ

C (graphite) + O2 (g) → 2 CO2 (g) ΔH = - 394 kJ

H2 (s) + 1/2 O2 (g) → H2O(l) ΔH = - 286 kJ

Heat of Formation Problems

5. Determine the heat of reaction (ΔHrxn) for the combustion of ethanol (C2H5OH) by using heat of formation data:

C2H5OH (l) + 3 O2 (g) → 2 CO2 + 3 H2O (g)

Heat of Formation Problems

6. Determine the heat of reaction (ΔHrxn) for the chlorination of methane (CH4) by using heat of formation data:

CH4 (g) + 4 Cl2 (g) → CCl4 (g) + 4 HCl (g)

Heat of Formation Problems

7. Determine the heat of reaction (ΔHrxn) for the reduction of acetaldehyde (CH3CHO) to ethanol (C2H5OH) by using heat of formation data:

CH3CHO (g) + H2 (g) → C2H5OH (l)

Heat of Formation Problems

8. Determine the heat of reaction (ΔHrxn) for the reaction of calcium carbonate (CaCO3) with HCl to produce CO2 by using heat of formation data:

CaCO3 (s) + 2 HCl (g) → CaCl2 (s) + CO2 (g) + H2O (g)

Expert Answer

It is necessary to consider that the Hess Law states "that the change of enthalpy in a chemical reaction (i.e. the heat of reaction at constant pressure) is independent of the pathway between the initial and final states", in such a way, one modifies each sub-reaction to attain the main one, of course, the enthalpy of reaction must be inverted as the inverse reaction will account for the overall process. In some exercises, it was necessary to multiply the chemical reaction by a whole number to attain the main reaction, for instance, in the first exercise, the second subreaction was inverted and the first and the second multiplied by two and the third one by 3, therefore, both the stoichiometric coefficients and the given enthalpies of reaction are multiplied as well in order to compute the enthalpy of that reaction.

On the other hand, the enthalpy of reaction for the exercises 5 to 8 were computed as widely worked:

[tex]\Delta _rH=\Sigma \nu_i \Delta _fH_{products}-\Sigma \nu_i \Delta _fH_i ,_{reactants}[/tex]

Whereas [tex]\nu_i[/tex] accounts for the stoichiometric coefficient of the ith compound based on the standard enthalpies of formation.

Best regards.