Adrian Dingle\'s chemistry Pages and ChemEducator LLc 2013 -All rights reserved

Question

Adrian Dingle's chemistry Pages and ChemEducator LLc 2013 -All rights reserved 2. Butane, CH,o has a standard enthalpy of combustion 2881 kJ mol (a) What is the significance of the negative sign associated with the enthalpy change? (1) (b) Calculate the enthalpy change when 4.000 g of butane is completely burned. (2) (c) Assuming the specific heat capacity of water to be 4 200 J g 'K', and that in such a reaction only 63.15% of the energy generated is actually transferred to the water, calculate the change in temperature of 2.000 kg of water that is heated by burning the butane in part (b) above. (4)

Explanation / Answer

a)

negative sign is essentially an exothermic reaction, meaing that there is a RELEASE of energy from the system TO the surrundings

b)

m = 4 g of butane

mol = mass/MW = 4/58.12 = 0.068823 mol

so

Q = n*HRxn = 0.068823*-2881 = -198.279 kJ

c)

Qwater = 63.15/100*Qbutane

Qwater = 0.6315*198.279 *1000 = 125213.18 J

so..

Q water = m*C*(Tf-Ti)

dT = 125213.18 / (2000*4.184) = 14.96 °C (increase)

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