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6.E: Thermochemistry


Draw an energy diagram for an exothermic reaction where \(\mathrm{\Delta{H} = - 20\; kJ}\).


Classify each of the following processes as endothermic or exothermic. In each case, indicate which has greater heat content (enthalpy), reactants or products.

  1. combustion of natural gas
  2. condensation of water vapor
  3. splitting of carbon dioxide into carbon and oxygen
  4. solidification of melted wax
  5. formation of sodium chloride (\(\ce{NaCl}\)) from it elements
  6. evaporation of alcohol


Propane gas, \(\mathrm{C_3H_8}\), is a common fuel for camp stoves.

  1. What is \(\mathrm{\Delta H_f^\circ}\) for propane? (Use\(\mathrm{\Delta H_f^\circ}\) Table.)
  2. Write a balanced thermochemical equation for the formation of propane from its elements.
  3. Is this reaction endothermic or exothermic?
  4. How many moles of \(\ce{H2}\) are needed to produce 1090 kJ according to the equation in b)?
  5. How much heat is produced during the formation of 30.1 g of propane?
  6. Write a balanced chemical equation for the complete combustion of propane.
  7. Find \(\mathrm{\Delta H}\) for the reaction in f) using \(\mathrm{\Delta H_f^\circ}\) Table and Hess’ Law.


For each of the reactions below, calculate \(\mathrm{\Delta H}\) and indicate whether the reaction is endothermic or exothermic. (Use \(\mathrm{\Delta H_f^\circ}\) values in Table T1 as needed.)

  1. \(\mathrm{2 NO(g) + O_2(g) \rightarrow 2 NO_2(g)}\)
  2. \(\mathrm{6 PbO(s) + O_2(g) \rightarrow 2 Pb_3O_4(s)}\)


A coffee cup calorimeter is used to calculate the heat change when \(\mathrm{NH_4Cl}\) is dissolved in water. Use the data table below to calculate the heat change when 1.00 mole of \(\mathrm{NH_4Cl}\) is dissolved in water. (Assume that the heat change for the solution is the same as that of water alone and that you can ignore the mass of solid in the water, so use only the mass of water and the specific heat of water, 4.18 J/g °C, in calculating the heat change.)

                       \(\mathrm{Mass\: of\: NH_4Cl = 5.03\: g}\)

                       \(\mathrm{Mass\: of\: water\: in\: the\: coffee\: cup = 60.0\: g}\)

                       \(\mathrm{Initial\: temperature\: of\: the\: water\: = 24.78\: ^\circ C}\)

                       \(\mathrm{\textrm{Final temperature of the water} = 19.23\: ^\circ C}\)

Is this process endothermic or exothermic?


To change the temperature of a particular calorimeter and the water it contains by one degree Celsius requires 6485 Joules. The complete combustion of 1.40 g of ethylene gas, \(\mathrm{C_2H_4}\), in the calorimeter causes a temperature rise of 10.7 degrees. Find the heat of combustion per mole of ethylene.


Calculate \(\mathrm{\Delta H}\) for the reaction:  \(\mathrm{N_2H_{4\;(l)} + O_{2\; (g)} \rightarrow N_{2\;(g)} + 2 H_2O_{(l)}}\) given the following data:

\[ \mathrm{2 NH_{3\; (g)} + 3 N_2O_{\:(g)} \rightarrow  4 N_{2\:(g)} + 3 H_2O_{\:(l)}} \tag{Δ H = -1010 kJ}\]

\[\mathrm{N_2O_{\:(g)} + 3 H_{2\;(g)} \rightarrow  N_2H_{4\:(l)} + H_2O_{\:(l)}} \tag{Δ H = -317 kJ}\]

\[\mathrm{2 NH_{3\;(g)} + \frac{1}{2} O_{2\;(g)}  \rightarrow  N_2H_{4\:(l)} + H_2O_{\:(l)}} \tag{Δ H = -143 kJ}\]

\[\mathrm{H_{2\:(g)} + \frac{1}{2} O_{2\,(g)}  \rightarrow  H_2O_{\:(l)}} \tag{Δ H = -286 kJ}\]

Extra Questions


Use Hess’ Law to calculate ΔH° for each of the following reactions and indicate whether the reaction is endothermic or exothermic. (Use the ΔHf° Table as needed below.)

  1. 2 HCl(g) + Br2(l)  →  2 HBr(g) + Cl2(g)
  2. 4 NH3(g) + 3 O2(g)  →  2 N2(g) + 6 H2O(l)
  3. N2H4(g) + 3 O2(g)  →  2 NO2(g) + 2 H2O(l); ΔHf°( N2H4) = +50.6 kJ


Given the following reaction: 2 P(s) + 3 Cl2(g) 2 PCl3(g) ΔH = -574 kJ

  1. How many moles of phosphorus are needed to produce 488 kJ?
  2. How much heat is released when 122 g of PCl3 are produced?
  3. How many grams of Cl2 are needed to produce 27.0 kJ?


When 1.00 g of KClO3 is dissolved in 50.0 g of water in a coffee-cup calorimeter, the temperature drops from 25.00 to 23.36°C. Calculate ΔH for the process

\[KClO_3(s)  →  KClO_3(aq)\]


Write a balanced chemical equation for the complete combustion of liquid benzene, C6H6. Then use Hess’ Law to calculate ΔH for this reaction. ΔHf°( C6H6) = +48.5 kJ


Given the reaction:

4 PH3(g) + 8 O2(g  P4O10(s) + 6 H2O(g) ΔH = -4500 kJ.

The heat of formation of phosphine, PH3, is +9.2 kJ/mole. Calculate the heat of formation of P4O10.