Lesson: Hess Cycles Using Enthalpies of Formation

In this lesson, we will learn how to calculate reaction enthalpies by comparing the standard enthalpies of formation of the reactants and the products.

Sample Question Videos

  • 05:48

Worksheet: 9 Questions • 1 Video

Q1:

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?

Substance H O ( ) 2 g H O ( ) 2 l C a C ( ) 2 s C a ( O H ) ( ) 2 s C H ( ) 2 2 g
Δ 𝐻 ⦵ f (kJ/mol) − 2 4 1 . 8 2 − 2 8 5 . 8 3 − 5 9 . 8 − 9 8 5 . 2 227.4

Q2:

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?

Substance T i C l ( ) 4 g H O ( ) 2 l H O ( ) 2 g T i O ( ) 2 s H C l ( ) g
Δ 𝐻 ⦵ f ( k J / m o l ) − 7 6 3 . 2 − 2 8 5 . 8 3 − 2 4 1 . 8 2 − 9 4 4 . 0 − 9 2 . 3 0 7

Q3:

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?

Substance B H ( ) 2 6 g B O ( ) 2 3 s C H ( ) 4 g C O ( ) 2 g N H ( ) 2 4 l H O ( ) 2 g
Δ 𝐻 ⦵ f (kJ/mol) 36.4 − 1 , 2 7 3 . 5 − 7 4 . 6 − 3 9 3 . 5 1 50.63 − 2 4 1 . 8 2
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