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Lesson: Hess Cycles Using Enthalpies of Formation

Worksheet • 8 Questions

Q1:

How does the standard enthalpy of formation of H O ( ) 2 g differ from Ξ” 𝐻 ⦡ m for the reaction 2 H ( ) + O ( ) 2 H O ( ) 2 2 2 g g g expressed per mole of oxygen reacted?

  • A The 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.

Q2:

Using the data in the table, calculate to the nearest kJ/mol the standard enthalpy of combustion of hydrazine ( N H ) 2 4 . One of the products of the reaction is nitrogen.

Substance N H ( ) 2 4 l H O ( ) 2 l
Ξ” 𝐻 ⦡ f (kJ/mol) 50.63 βˆ’ 2 8 5 . 8 3

Q3:

In the early days of automobiles, illumination at night was provided by burning acetylene, C H 2 2 . 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, C a C 2 : 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 )
Ξ” 𝐻 ⦡ f ( k J / m o l ) βˆ’ 2 4 1 . 8 2 βˆ’ 2 8 5 . 8 3 βˆ’ 5 9 . 8 βˆ’ 9 8 5 . 2 227.4

Q4:

Using the data in the table, calculate to the nearest kJ/mol the standard enthalpy of combustion of methane.

Substance C H ( ) 4 g C O ( ) 2 g H O ( ) 2 l
Ξ” 𝐻 ⦡ f (kJ/mol) βˆ’ 7 4 . 6 βˆ’ 3 9 3 . 5 1 βˆ’ 2 8 5 . 8 3

Q5:

Using the data below, calculate to 3 significant figures the standard molar enthalpy of combustion of diborane ( B H ) 2 6 .

Substance B H ( ) 2 6 g B O ( ) 2 3 s H O ( ) 2 l
Ξ” 𝐻 ⦡ f ( k J / m o l ) 36.4 βˆ’ 1 2 7 3 . 5 βˆ’ 2 8 5 . 8 3

Q6:

The white pigment T i O 2 is prepared by the reaction of titanium tetrachloride, T i C l 4 , with water vapor in the gas phase: How much heat is evolved in the production of exactly 1 mole of T i O 2 ( 𝑠 ) 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 )
Ξ” 𝐻 ⦡ f ( k J / m o l ) βˆ’ 7 6 3 . 2 βˆ’ 2 8 5 . 8 3 βˆ’ 2 4 1 . 8 2 βˆ’ 9 4 4 . 0 βˆ’ 9 2 . 3 0 7

Q7:

Among the substances that react with oxygen and that have been considered as potential rocket fuels are diborane ( B H ( ) 2 6 g , produces B O ( ) 2 3 s and H O ( ) 2 g ), methane ( C H ( ) 4 g , produces C O ( ) 2 g and H O ( ) 2 g ), and hydrazine ( N H ( ) 2 4 l , produces N ( ) 2 g and H O ( ) 2 g ). 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
Ξ” 𝐻 ⦡ f ( k J / m o l ) 36.4 βˆ’ 1 , 2 7 3 . 5 βˆ’ 7 4 . 6 βˆ’ 3 9 3 . 5 1 50.63 βˆ’ 2 4 1 . 8 2
  • ADiborane
  • BMethane
  • CThe amount of heat they release is too small for them to be used as fuels.
  • DHydrazine

Q8:

Ethylene, C H 2 2 , a byproduct from the fractional distillation of petroleum, is fourth among the 50 chemical compounds produced commercially in the largest quantities. About 8 0 % of synthetic ethanol is manufactured from ethylene by its reaction with water in the presence of a suitable catalyst: Using the data below, what is Ξ” 𝐻 ⦡ for the reaction?

Substance C H 2 4 ( 𝑔 ) H O 2 ( 𝑙 ) H O 2 ( 𝑔 ) C H O H 2 5 ( 𝑙 )
Ξ” 𝐻 ⦡ f ( k J / m o l ) 227.4 βˆ’ 2 8 5 . 8 3 βˆ’ 2 4 1 . 8 2 βˆ’ 2 7 7 . 6
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