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Consider the following intermediate chemical equations:

[tex]\[
\begin{array}{ll}
C ( s ) + O_2( g ) \rightarrow CO_2( g ) & \Delta H_1 = -393.5 \, \text{kJ} \\
2 CO ( g ) + O_2( g ) \rightarrow 2 CO_2( g ) & \Delta H_2 = -566.0 \, \text{kJ} \\
2 H_2O ( g ) \rightarrow 2 H_2( g ) + O_2( g ) & \Delta H_3 = 483.6 \, \text{kJ}
\end{array}
\][/tex]

The overall chemical equation is [tex]\(C ( s ) + H_2O ( g ) \rightarrow CO ( g ) + H_2( g )\)[/tex].

To calculate the final enthalpy of the overall chemical equation, which step must occur?

A. Reverse the first equation, and change the sign of the enthalpy. Then, add.
B. Reverse the second equation, and change the sign of the enthalpy. Then, add.
C. Divide the third equation by two, and double the enthalpy. Then, add.

Sagot :

To calculate the final enthalpy change ([tex]\(\Delta H_{\text{total}}\)[/tex]) for the overall chemical equation [tex]\(C(s) + H_2O(g) \rightarrow CO(g) + H_2(g)\)[/tex], we need to manipulate the given intermediate equations correctly. Here are the detailed steps:

1. Reverse the first equation:
The given equation is [tex]\(C(s) + O_2(g) \rightarrow CO_2(g)\)[/tex] with [tex]\(\Delta H_1 = -393.5 \, \text{kJ}\)[/tex].

Reversing this equation gives:
[tex]\(CO_2(g) \rightarrow C(s) + O_2(g)\)[/tex], and the enthalpy change also reverses its sign:
[tex]\(\Delta H_1' = 393.5 \, \text{kJ}\)[/tex].

2. Reverse the second equation:
The given equation is [tex]\(2 CO(g) + O_2(g) \rightarrow 2 CO_2(g)\)[/tex] with [tex]\(\Delta H_2 = -566.0 \, \text{kJ}\)[/tex].

Reversing this equation gives:
[tex]\(2 CO_2(g) \rightarrow 2 CO(g) + O_2(g)\)[/tex], and the enthalpy change also reverses its sign:
[tex]\(\Delta H_2' = 566.0 \, \text{kJ}\)[/tex].

3. Divide the third equation by two:
The given equation is [tex]\(2 H_2O(g) \rightarrow 2 H_2(g) + O_2(g)\)[/tex] with [tex]\(\Delta H_3 = 483.6 \, \text{kJ}\)[/tex].

Dividing this equation by two gives:
[tex]\(H_2O(g) \rightarrow H_2(g) + \frac{1}{2} O_2(g)\)[/tex], and the enthalpy change is also halved:
[tex]\(\Delta H_3' = \frac{1}{2} \times 483.6 \, \text{kJ} = 241.8 \, \text{kJ}\)[/tex].

4. Final enthalpy change for the overall reaction:
Now we need to sum up the enthalpy changes of the reversed and modified equations to find the total enthalpy change:
[tex]\[ \Delta H_{\text{total}} = \Delta H_1' + \Delta H_2' + \Delta H_3' \][/tex]
Substituting the values we get:
[tex]\[ \Delta H_{\text{total}} = 393.5 \, \text{kJ} + 566.0 \, \text{kJ} + 241.8 \, \text{kJ} = 1201.3 \, \text{kJ} \][/tex]

Therefore, the final enthalpy change for the overall chemical equation [tex]\(C(s) + H_2O(g) \rightarrow CO(g) + H_2(g)\)[/tex] is [tex]\(1201.3 \, \text{kJ}\)[/tex].
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