Define standard heat of formation. Explain why it is important in thermoche?

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Feb 8, 2018


Heat of formation is the change in heat when a compound is formed from its elements. Standard heat of formation is the heat of formation when the synthesis is performed at 298 K and 1 atm


Many times we use Hess's Law to determine the change in heat for a reaction that was not measured by adding together reaction equations to produce the desired reaction equation. Standard heats of formation are known for many compounds and can be used this way. For example:

#2CO(g) + O_2(g) rarr 2CO_2(g)# #DeltaH = -566.0 kJ#

#C(s) + O_2(g)rarrCO_2(g)# #DeltaH = -393.5kJ#

If I reverse its direction of the first reaction, the sign of delta H changes, then multiply second reaction by 2 and add it :
#2CO_2(g)rarr 2CO + O_2(g)# #DeltaH = +566.0 kJ#
#2C(s) + 2O_2(g)rarr2CO_2(g)# #DeltaH = 2(-393.5kJ)= -787kJ#
Adding the two reactions yields
#2C(s) + O_2(g) rarr2CO(g)# which must have a #Delta H = (566 kJ + (-787 kJ) or -221kJ#

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