Indicate whether each of these reactions is endothermic or exothermic.

1. [tex]NaOH (s) \rightarrow Na^{+}(aq) + OH^{-}(aq) \quad \Delta H_{rxn} = -44.5 \, \text{kJ}[/tex]
[ ] Endothermic
[ ] Exothermic

2. [tex]CH_4(g) + 2 \, O_2(g) \rightarrow CO_2(g) + 2 \, H_2O(g) \quad \Delta H_{rxn} \ \textless \ 0[/tex]
[ ] Endothermic
[ ] Exothermic

3. [tex]CH_3COOH(aq) + NaHCO_3(s) \rightarrow CO_2(g) + H_2O(l) + Na^{+}(aq) + CH_3COO^{-}(aq) \quad \Delta H_{rxn} \ \textgreater \ 0[/tex]
[ ] Endothermic
[ ] Exothermic



Answer :

To determine whether each of the reactions is endothermic or exothermic, we need to examine the sign of the enthalpy change ([tex]\(\Delta H_{\text{rxn}}\)[/tex]) for each reaction:

1. Reaction 1:
[tex]\[ NaOH (s) \rightarrow Na^{+}(aq) + OH^{-}(aq) \quad \Delta H_{\text{rxn}} = -44.5 \, \text{kJ} \][/tex]
The enthalpy change [tex]\(\Delta H_{\text{rxn}}\)[/tex] is given as [tex]\(-44.5\)[/tex] kJ. Because [tex]\(\Delta H_{\text{rxn}}\)[/tex] is negative, this indicates that the reaction releases heat to the surroundings. Therefore, this reaction is exothermic.

2. Reaction 2:
[tex]\[ CH_4(g) + 2 O_2(g) \rightarrow CO_2(g) + 2 H_2O(g) \quad \Delta H_{\text{rxn}} < 0 \][/tex]
The enthalpy change [tex]\(\Delta H_{\text{rxn}}\)[/tex] is indicated to be less than zero ([tex]\(\Delta H_{\text{rxn}} < 0\)[/tex]). When [tex]\(\Delta H_{\text{rxn}}\)[/tex] is less than zero, it means the reaction releases heat to the surroundings. As a result, this reaction is also exothermic.

3. Reaction 3:
[tex]\[ CH_3COOH(aq) + NaHCO_3(s) \rightarrow CO_2(g) + H_2O(l) + Na^{+}(aq) + CH_3COO^{-}(aq) \quad \Delta H_{\text{rxn}} > 0 \][/tex]
The enthalpy change [tex]\(\Delta H_{\text{rxn}}\)[/tex] is given as greater than zero ([tex]\(\Delta H_{\text{rxn}} > 0\)[/tex]). When [tex]\(\Delta H_{\text{rxn}}\)[/tex] is positive, it means the reaction absorbs heat from the surroundings. Thus, this reaction is endothermic.

Summarizing the results:

1. [tex]\[ NaOH (s) \rightarrow Na^{+}(aq) + OH^{-}(aq) \quad \Delta H_{\text{rxn}} = -44.5 \, \text{kJ} \][/tex]
Exothermic

2. [tex]\[ CH_4(g) + 2 O_2(g) \rightarrow CO_2(g) + 2 H_2O(g) \quad \Delta H_{\text{rxn}} < 0 \][/tex]
Exothermic

3. [tex]\[ CH_3COOH(aq) + NaHCO_3(s) \rightarrow CO_2(g) + H_2O(l) + Na^{+}(aq) + CH_3COO^{-}(aq) \quad \Delta H_{\text{rxn}} > 0 \][/tex]
Endothermic