The standard heat of formation, Delta H_f degree, is defined as the enthalpy cha
ID: 497883 • Letter: T
Question
The standard heat of formation, Delta H_f degree, is defined as the enthalpy change for the formation of one mole of substance from its constituent elements in their standard states. Thus, elements in their standard states have Delta H_f degree = 0. Heat of formation values can be used to calculate the enthalpy change of any reaction. Consider, for example, the reaction 2NO(g) + O_2(g) 2NO_2(g) with heat of formation values given by the following table. Then the standard heat of reaction for the overall reaction is Delta H_rxn degree = Delta H_f degree (products) - Delat H_f degree (reactants) = 2(33.2) - [2(90.2) + 0] = -114 kJ For which of the following reactions is Delta H_rxn degree equal to Delta H_f degree of the product(s)? You do not need to look up any values to answer this question. Check all that apply. Li(s) + 1/2 F_2 (I) rightarrow LiF(s) C(s, graphite) + O_2(g) rightarrow CO_2(g) 2Li(s) + F_2 (s) rightarrow 2LiF(s) CO(g) + 1/2 O_2(g) rightarrow CO_2 (g) Li(s) + 1/2 F_2 (g) rightarrow LiF(s) BaCO_3(s) rightarrow BaO(s) + CO_2(g) The combustion of propane C_3H_8, occurs via the reaction C_3H_8(g) + 5O_2(g) rightarrow 3CO_3(g) + 4H_3O(g) with heat at formation values given by the following table: Calculate the enthalpy for the combustion of 1 mole of propane. Express your answer to four significant figures and include the appropriate units.Explanation / Answer
Part A
The enthalpy of reaction is the heat of reaction either absorbed or liberated when the reaction occurs according to the stoichiometric equation.
The enthalpy / heat of formation is the heat liberated or absorbed when one mole of the substance is formed form the elements in their standard states.
From the given set of reactions,
the reactions in which one mole of substance is formed from its elements in teir standard states will have the delta H formation = delta H reaction.
They are
Li(s) + 1/2F2(g) -----> LiF (s)
C(s,graphite) + O2(g) ------> CO2(g) only.
The standard state of a substance is defined as the most stable state of that element/substance at 25C.
The standard state are
Li -solid
F2 -gas
C-solid graphite
O2 - gas
CO2 - gas
LiF -solid
Part B
Enthalpy of combustion is defined as the heat liberated when one moles of substance is completely burnt in oxygen to its oxidised state.
The reaction given is the reaction for the combustion of propane. Thus the heat of reaction = heat of combustion.
= enthalpy of formation of products - enthalpy of formation of reactants
= [3 (-393.5) + 4(-241.8) ] - [(-104.7) + 5 (0)]
= - 2043 kJ /mol
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