TYPES OF HEAT REACTIONS ASSIGNMENT

Answer:

ΔH = −153 kJ

2. Another Example of the Measurement of an Enthalpy Change
A gummy bear contains 2.67 g sucrose, C₁₂H₂₂O₁₁. When it reacts with 7.19 g potassium chlorate, KClO₃, 43.7 kJ of heat are produced. Determine the enthalpy change for the reaction

Solution
We have  available, and  available. Since  is needed, C₁₂H₂₂O₁₁ is the excess reactant and KClO₃ is the limiting reactant.

The reaction uses 8 mol KClO₃, and the conversion factor is , so we have . The enthalpy change for this reaction is −5960 kJ, and the thermochemical equation is:

Check Your Learning
When 1.42 g of iron reacts with 1.80 g of chlorine, 3.22 g of FeCl₂(s) and 8.60 kJ of heat is produced. What is the enthalpy change for the reaction when 1 mole of FeCl₂(s) is produced?

Answer:

ΔH = −338 kJ

3.

Using Enthalpy of Combustion
As Figure 3 suggests, the combustion of gasoline is a highly exothermic process. Let us determine the approximate amount of heat produced by burning 1.00 L of gasoline, assuming the enthalpy of combustion of gasoline is the same as that of isooctane, a common component of gasoline. The density of isooctane is 0.692 g/mL.

Solution
Starting with a known amount (1.00 L of isooctane), we can perform conversions between units until we arrive at the desired amount of heat or energy. The enthalpy of combustion of isooctane provides one of the necessary conversions. Table 2 gives this value as −5460 kJ per 1 mole of isooctane (C₈H₁₈).

Using these data,

The combustion of 1.00 L of isooctane produces 33,100 kJ of heat. (This amount of energy is enough to melt 99.2 kg, or about 218 lbs, of ice.)

Note: If you do this calculation one step at a time, you would find:

Check Your Learning
How much heat is produced by the combustion of 125 g of acetylene?

Answer:

6.25 × 10³ kJ

Evaluating an Enthalpy of Formation
Ozone, O₃(g), forms from oxygen, O₂(g), by an endothermic process. Ultraviolet radiation is the source of the energy that drives this reaction in the upper atmosphere. Assuming that both the reactants and products of the reaction are in their standard states, determine the standard enthalpy of formation,  of ozone from the following information:

Solution
 is the enthalpy change for the formation of one mole of a substance in its standard state from the elements in their standard states. Thus,  for O₃(g) is the enthalpy change for the reaction:

For the formation of 2 mol of O₃(g), . This ratio, , can be used as a conversion factor to find the heat produced when 1 mole of O₃(g) is formed, which is the enthalpy of formation for O₃(g):

Therefore, .

Check Your Learning
Hydrogen gas, H₂, reacts explosively with gaseous chlorine, Cl₂, to form hydrogen chloride, HCl(g). What is the enthalpy change for the reaction of 1 mole of H₂(g) with 1 mole of Cl₂(g) if both the reactants and products are at standard state conditions? The standard enthalpy of formation of HCl(g) is −92.3 kJ/mol.

For the reaction 

Writing Reaction Equations for 
Write the heat of formation reaction equations for:

(a) C₂H₅OH(l)

(b) Ca₃(PO₄)₂(s)

Solution
Remembering that  reaction equations are for forming 1 mole of the compound from its constituent elements under standard conditions, we have:

(a) 

(b) 

Note: The standard state of carbon is graphite, and phosphorus exists as P₄.

Check Your Learning
Write the heat of formation reaction equations for:

(a) C₂H₅OC₂H₅(l)

(b) Na₂CO₃(s)