For each reaction, identify another quantity that is equal to Δ H ∘ rxn . CH 4 ( g ) + 2 O 2 ( g ) ⟶ CO 2 ( g ) + 2 H 2 O ( l ) enthalpy of formation of CO 2 ( g ) − 4 × bond energy of C − H 4 × bond energy of C − H enthalpy of combustion of CH 4 CH 4 ( g ) ⟶ C ( g ) + 4 H ( g ) 4 × bond energy of C − H − 4 × bond energy of C − H enthalpy of formation of C ( g ) enthalpy of combustion of CH 4 C ( graphite ) + 2 H 2 ( g ) ⟶ CH 4 ( g ) enthalpy of combustion of CH 4 4 × bond energy of C − H − 4 × bond energy of C − H enthalpy of formation of CH 4 ( g ) C ( g ) + 4 H ( g ) ⟶ CH 4 ( g ) − 4 × bond energy of C − H enthalpy of formation of CH 4 ( g ) enthalpy of combustion of C 4 × bond energy of C − H

Explanation:

CH 4 ( g ) + 2 O 2 ( g ) ⟶ CO 2 ( g ) + 2 H 2 O ( l )

enthalpy of formation of CO 2 ( g )

− 4 × bond energy of C H

4 × bond energy of C − H

enthalpy of combustion of CH 4

enthalpy of combustion of CH 4: This is because we usually calculate the enthalpy change of combustion from enthalpies of formation. This is given as;

ΔHoc = ∑ΔH∘f(products) − ∑ΔH∘f(reactants)

CH 4 ( g ) ⟶ C ( g ) + 4 H ( g )

1. enthalpy of combustion of CH4

2. enthalpy of formation of C(g)

3. 4× bond energy of C–H

4. –4× bond energy of C–H

4× bond energy of C–H: Bond breaking is an endothermic process. So it cannot be option 4, enthalpy of formation of a pure element is 0 KJ/mol. This is not a combustion reaction, so options 1 and 2 are aslo wrong.

c) C(graphite) + 2H2(g) CH4(g)

1. enthalpy of combustion of CH4

2. enthalpy of formation of CH4(g)

3. 4× bond energy of C–H

4. –4× bond energy of C–H

The standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy during the formation of 1 mole of the substance from its constituent elements. The correct option is; enthalpy of formation of CH4(g)

d) C(g) + 4H(g) CH4(g)

1. enthalpy of combustion of C

2. enthalpy of formation of CH4(g)

3. 4× bond energy of C–H

4. –4× bond energy of C–H

–4× bond energy of C–H; bond formation is always exothermic, it cannot be option 3. This is not a combustion reaction so it cannot be option 1.  enthalpy of formation of a pure element is 0 KJ/mol, so it cannot be option 2.

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