Question

6-9 Three of the reactions that occur when the paraffin of a candle (typical formula C₂₁H₄₄(s) burns are:

1. Complete combustion forms CO₂ and water vapor
2. Incomplete combustion forms CO and water vapor
3. Some Wax is oxidized to elemental carbon (soot) and water vapor

1. Find the ΔH_rxn for each reaction (ΔH_f of C₂₁H₄₄(s) = -476 kJ/mole) Use graphite for elemental carbon. Use the table in problem 6.9 for other enthalpies of formation.

6-10 Using the results from problem 6-9, Find q in kJ when a 250 gram candle burns completely.

Find q in kJ when 8.00% by mass of the candle burns incompletely and 5.00 % undergoes soot formation.

Answer 1

6-9) C₂₁H₄₄(s)

i) Complete combustion of C₂₁H₄₄(s) forms CO₂(g) and water vapor as below:

   C₂₁H₄₄(s) + 32 O₂ (g) ------------------> 21 CO₂(g) + 22 H₂O (g)

∆Hrxn = ∆H^o_f of products - ∆H^o_f of reactants

= 21 [∆H^o_f CO₂ (g) ] + 22 [∆H^o_f H₂O (g) ] - {[ ∆H^o_f  [C₂₁H₄₄(s)] + 32 ∆H^o_f [O₂(g)] ]}

= 21 [-393.5 kJ ] + 22 [-241.83 kJ ] - { -476 kJ + 32 x 0 kJ}

= -13107 kJ

Therefore,

∆Hrxn = -13107 kJ

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ii) Complete combustion of C₂₁H₄₄(s) forms CO(g) and water vapor as below:

2C₂₁H₄₄(s) + 43 O₂ (g) ------------------> 42 CO(g) + 44 H₂O (g)

∆Hrxn = ∆H^o_f of products - ∆H^o_f of reactants

= 42 [∆H^o_f CO (g) ] + 44 [∆H^o_f H₂O (g) ] - {2 [ ∆H^o_f  [C₂₁H₄₄(s)] + 43 ∆H^o_f [O₂(g)] ]}

= 42 [-110.5 kJ ] + 44 [-241.83 kJ ] - { 2 (-476 kJ) + 43 x 0 kJ }

= -14329 kJ

Therefore,

∆Hrxn = -14329 kJ

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iii) Given that Some Wax is oxidized to elemental carbon (soot) and water vapor.

C₂₁H₄₄(s) + 11 O₂ (g) ------------------> 21 C (graphite) + 22 H₂O (g)

∆Hrxn = ∆H^o_f of products - ∆H^o_f of reactants

= 21 [∆H^o_f C (s) ] + 22 [∆H^o_f H₂O (g) ] - { [ ∆H^o_f  [C₂₁H₄₄(s)] + 11 ∆H^o_f [O₂(g)] ]}

= 21 [0 kJ ] + 22 [-241.83 kJ ] - { (-476 kJ) + 11 x 0 kJ }

= -4844 kJ

Therefore,

∆Hrxn = -4844 kJ

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