Chapter 25 Energy from chemical reactions

Q1.

Sodium nitrate dissolves readily in water according to the equation

NaNO3(s) NaNO3(aq); H = +21.0 kJ mol–1

a Determine the energy change when 1.0 g of solid NaNO3 is dissolved in water.

b Would you expect the temperature to rise or fall when sodium nitrate dissolves in water? Explain your answer.

A1.

a n(NaNO3) = =0.0117 mole

1 mole NaNO3 absorbs 21 kJ energy

0.0117 mole absorbs 0.0117 × 21 kJ = 0.25 kJ

b The temperature will fall. Since the reaction is endothermic the enthaply of reactants is higher than the enthalpy of products, i.e. the reaction absorbs energy.

Q2.

Calculate the energy released when the following quantities of ethane gas burn according to the equation:

2C2H6(g) + 7O2(g) 4CO2(g) + 6H2O(l); H = –3120 kJ mol–1

a 3.00 mol

b 100 g

c 10.0 L at SLC

A2.

a From the equation, use stoichiometry to find energy released.

2 mol C2H6 releases 3120 kJ of energy

3.00 mol C2H6 releases ´ 3.00 kJ

So energy = 4680 kJ

= 4.68 ´ 103 kJ

b Step 1 Calculate the amount of ethane, using n = .

n(C2H6) =

= 3.326 mol

Step 2 Use stoichiometry to find energy released.

2 mol C2H6 releases 3120 kJ of energy

3.326 mol C2H6 releases 3120 ´ kJ

So energy released = 5189 kJ

= 5.19 ´ 103 kJ

c Step 1 Calculate the amount of ethane, using n = at SLC.

n(C2H6) =

= 0.4082 mol

Step 2 Use stoichiometry to find energy released.

2 mol C2H6 releases 3120 kJ of energy

0.4082 mol C2H6 releases 3120 ´ kJ

So energy released = 636.7 kJ

= 637 kJ

Q3.

What volume of ethane, measured at STP, must be burnt according to the equation in Question 2 in order to yield 100 kJ of heat energy?

A3.

Step 1 Use stoichiometry to find the amount of C2H6.

3120 kJ is released by 2 mol

100 kJ is released by 2 ´ mol

So n(C2H6) = 2 ´

= 0.641 mol

Step 2 Calculate the volume of C2H6 at STP, using n = .

V(C2H6) = n ´ Vm

= 0.641 mol ´ 22.4 L...