# 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.

Q1:

In the early days of automobiles, illumination at night was provided by burning acetylene, . 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, : What is the standard enthalpy of the reaction?

 Substance Δ𝐻⦵f (kJ/mol) HO()2g HO()2l CaC()2s Ca(OH)()2s CH()22g −241.82 −285.83 −59.8 −985.2 227.4
• A kJ
• B kJ
• C kJ
• D kJ
• E kJ

Q2:

The white pigment is prepared by the reaction of titanium tetrachloride, , with water vapor in the gas phase. How much heat is evolved in the production of exactly 1 mole of under standard state conditions?

 Substance Δ𝐻⦵f(kJ/mol) TiCl()4g HO()2l HO()2g TiO()2s HCl()g −763.2 −285.83 −241.82 −944.0 −92.307

Q3:

Among the substances that react with oxygen and that have been considered as potential rocket fuels are diborane , produces and , methane , produces and , and hydrazine , produces and . 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?

 Substance Δ𝐻⦵f (kJ/mol) BH()26g BO()23s CH()4g CO()2g NH()24l HO()2g 36.4 −1,273.5 −74.6 −393.51 50.63 −241.82
• AThe amount of heat they release is too small for them to be used as fuels.
• BDiborane
• CHydrazine
• DMethane

Q4:

Ethylene, , a byproduct from the fractional distillation of petroleum, is fourth among the 50 chemical compounds produced commercially in the largest quantities. About of synthetic ethanol is manufactured from ethylene by its reaction with water in the presence of a suitable catalyst. Using the data shown, what is for the reaction?

 Substance Δ𝐻⦵f(kJ/mol) CH()24g HO()2l HO()2g CHOH()25l 227.4 −285.83 −241.82 −277.6

Q5:

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

 Substance Δ𝐻⦵f(kJ/mol) BH()26g BO()23s HO()2l 36.4 −1,273.5 −285.83

Q6:

How does the standard enthalpy of formation of differ from for the given reaction expressed per mole of oxygen reacted?

• 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.

Q7:

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

Substance Δ𝐻⦵f (kJ/mol) NH()24l HO()2l 50.63 −285.83

Q8:

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

 Substance Δ𝐻⦵f (kJ/mol) CH()4g CO()2g HO()2l −74.6 −393.51 −285.83

Q9:

Consider the following dimerization reaction. This reaction is exothermic, since the major chemical change is the formation of a new bond (the bond). The value of for this reaction carried out at a temperature of 298 K is kJ. The standard molar heat capacity of is , and the standard molar heat capacity of is . Assuming the reaction enthalpy and standard molar heat capacities do not vary with temperature, what is the value of for the dimerization reaction carried out at a temperature of 1,298 K?