Worksheet: Hess Cycles Using Enthalpies of Formation

In this worksheet, we will practice calculating reaction enthalpies by comparing the standard enthalpies of formation of the reactants and the products.


In the early days of automobiles, illumination at night was provided by burning acetylene, CH22. Though no longer used in headlamps, acetylene is still used as a source of light by some cave explorers. Acetylene can be prepared by the reaction of water with calcium carbide, CaC2: CaC()+2HO()Ca(OH)()+CH()22222slsg. What is the standard enthalpy of the reaction?

Δ𝐻f (kJ/mol) 241.82285.8359.8985.2227.4
  • A155 kJ
  • B222 kJ
  • C104 kJ
  • D126 kJ
  • E214 kJ


The white pigment TiO2 is prepared by the reaction of titanium tetrachloride, TiCl4, with water vapor in the gas phase. TiCl()+2HO()TiO()+4HCl()422ggsg How much heat is evolved in the production of exactly 1 mole of TiO()2s under standard state conditions?



Among the substances that react with oxygen and that have been considered as potential rocket fuels are diborane BH()26g, produces BO()23s and HO()2g, methane CH()4g, produces CO()2g and HO()2g, and hydrazine NH()24l, produces N()2g and HO()2g. On the basis of the heat released by 1.00 g of each substance in its reaction with oxygen, which of these compounds offers the best possibility as a rocket fuel?

Δ𝐻f (kJ/mol) 36.41,273.574.6393.5150.63241.82
  • AThe amount of heat they release is too small for them to be used as fuels.
  • BDiborane
  • CHydrazine
  • DMethane


Ethylene, CH22, a byproduct from the fractional distillation of petroleum, is fourth among the 50 chemical compounds produced commercially in the largest quantities. About 80% of synthetic ethanol is manufactured from ethylene by its reaction with water in the presence of a suitable catalyst. CH()+HO()CHOH()24225ggl Using the data shown, what is Δ𝐻 for the reaction?



Using the data below, calculate to 3 significant figures the standard molar enthalpy of combustion of diborane (BH)26.



How does the standard enthalpy of formation of HO()2g differ from Δ𝐻m for the given reaction expressed per mole of oxygen reacted? 2H()+O()2HO()222ggg

  • AThe standard enthalpy of formation is exactly double the molar enthalpy of the given reaction.
  • BThe standard enthalpy of formation is exactly half the molar enthalpy for the given reaction.
  • CThere is no difference: they are identical.


Using the data in the table, calculate, to the nearest kilojoules per mole, the standard enthalpy of combustion of hydrazine (NH)24. One of the products of the reaction is nitrogen.

Δ𝐻f (kJ/mol)50.63285.83


Using the data in the table, calculate, to the nearest kilojoules per mole, the standard enthalpy of combustion of methane.

Δ𝐻f (kJ/mol)74.6393.51285.83


Consider the following dimerization reaction. 2NO()NO()224gg This reaction is exothermic, since the major chemical change is the formation of a new bond (the NN bond). The value of Δ𝐻r for this reaction carried out at a temperature of 298 K is 57.2 kJ. The standard molar heat capacity of NO()24g is 𝐶=77.28/JKmol, and the standard molar heat capacity of NO()2g is 𝐶=37.2/JKmol. Assuming the reaction enthalpy and standard molar heat capacities do not vary with temperature, what is the value of Δ𝐻r for the dimerization reaction carried out at a temperature of 1,298 K?

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