# Worksheet: Hess Cycles Using Enthalpies of Formation

Q1:

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

• AThere is no difference: they are identical.
• B The standard enthalpy of formation is exactly double the molar enthalpy of the given reaction.
• CThe standard enthalpy of formation is exactly half the molar enthalpy for the given reaction.

Q2:

Using the data in the table, calculate to the nearest kJ/mol the standard enthalpy of combustion of hydrazine . One of the products of the reaction is nitrogen.

Substance Ξ π» β¦΅ f (kJ/mol) N H ( ) 2 4 l H O ( ) 2 l 50.63 β 2 8 5 . 8 3

Q3:

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 Ξ π» β¦΅ f ( k J / m o l ) H O 2 ( π ) H O 2 ( π ) C a C 2 ( π  ) C a ( O H ) 2 ( π  ) C H 2 2 ( π ) β 2 4 1 . 8 2 β 2 8 5 . 8 3 β 5 9 . 8 β 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 Ξ π» β¦΅ f (kJ/mol) C H ( ) 4 g C O ( ) 2 g H O ( ) 2 l β 7 4 . 6 β 3 9 3 . 5 1 β 2 8 5 . 8 3

Q5:

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

 Substance Ξ π» β¦΅ f ( k J / m o l ) B H ( ) 2 6 g B O ( ) 2 3 s H O ( ) 2 l 36.4 β 1 2 7 3 . 5 β 2 8 5 . 8 3

Q6:

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 Ξ π» β¦΅ f ( k J / m o l ) T i C l 4 ( π ) H O 2 ( π ) H O 2 ( π ) T i O 2 ( π  ) H C l ( π ) β 7 6 3 . 2 β 2 8 5 . 8 3 β 2 4 1 . 8 2 β 9 4 4 . 0 β 9 2 . 3 0 7

Q7:

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 Ξ π» β¦΅ f ( k J / m o l ) 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 36.4 β 1 2 7 3 . 5 β 7 4 . 6 β 3 9 3 . 5 1 50.63 β 2 4 1 . 8 2
• AHydrazine
• BMethane
• CThe amount of heat they release is too small for them to be used as fuels.
• DDiborane

Q8:

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 below, what is for the reaction?

 Substance Ξ π» β¦΅ f ( k J / m o l ) C H 2 4 ( π ) H O 2 ( π ) H O 2 ( π ) C H O H 2 5 ( π ) 227.4 β 2 8 5 . 8 3 β 2 4 1 . 8 2 β 2 7 7 . 6