5.44 Consider the following reaction: 5.5 2 CH OH (g)- 2CH,(g) + O2(g) AH= +252.

Question

5.44 Consider the following reaction: 5.5 2 CH OH (g)- 2CH,(g) + O2(g) AH= +252.8 kJ (a) Is this reaction exothermic or endothermic? (b) Calculate the amount of heat transferred when 24.0 g of CH,OH(g) is decomposed by this reaction at constant pressure. (c) For a given sample of CH OH, the enthalpy change during the reaction is 82.1 kJ. How many grams of methane gas are produced? (d) How many kilojoules of heat are released when 38.5 g of CH4(g) reacts completely with O2(g) to form CH,OH(g) at constant pressure? 5.5

Explanation / Answer

The reaction is

2 CH3OH ---------------> 2CH4 +O2 delta H = + 252.8kJ

a) The reaction is endothermic as the delta H is with positive sign.

b)According to the reaction

2 moles 0f CH3OH (32g/mol) th eheat transferred = 252.8kJ

when 24/32 mol CH3OH reacts the heat transferred = (24/32)x 252.8/2

= 94.8kJ

c)For a gaiven sample the heat transfered = 82.1kJ

When 2 moles 2x(16g/mol)of methane is formed the heat change = 252.8

mass of methane formed when 82.1kJ is changed = 82.1kJx2x16g /252.8kJ

= 10.39 g of methane

d) when 2 moles of CH4 (2x16g ) reacts with O2 the heat liberated = -252.8 kJ

hence when 38.5g of CH4 reacts the heat produced = 38.5gx252.8 kJ/ 2x16g

= -304.15 kJ

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