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Consider the following intermediate chemical equations.

[tex]\[
\begin{array}{ll}
NO(g) + O_3(g) \rightarrow NO_2(g) + O_2(g) & \Delta H_1 = -198.9 \text{ kJ} \\
\frac{3}{2} O_2(g) \rightarrow O_3(g) & \Delta H_2 = 142.3 \text{ kJ} \\
O(g) \rightarrow \frac{1}{2} O_2(g) & \Delta H_3 = -247.5 \text{ kJ}
\end{array}
\][/tex]

What is the enthalpy of the overall chemical equation [tex]\(NO(g) + O(g) \rightarrow NO_2(g)\)[/tex]?

A. -305 kJ
B. -304.1 kJ
C. -93.7 kJ
D. 588.7 kJ


Sagot :

To solve this problem, we use Hess's Law, which states that the total enthalpy change for a reaction is the sum of the enthalpy changes for the steps that lead to the final reaction. We are given three intermediate equations with their respective enthalpy changes:

[tex]\[ \begin{array}{ll} 1) & NO ( g ) + O_3 ( g ) \rightarrow NO_2 ( g ) + O_2 ( g ) \quad \Delta H_1 = -198.9 \, \text{kJ} \\ 2) & \frac{3}{2} O_2 ( g ) \rightarrow O_3 ( g ) \quad \Delta H_2 = 142.3 \, \text{kJ} \\ 3) & O ( g ) \rightarrow \frac{1}{2} O_2 ( g ) \quad \Delta H_3 = -247.5 \, \text{kJ} \end{array} \][/tex]

We seek the enthalpy change for the overall reaction:
[tex]\[ NO ( g ) + O ( g ) \rightarrow NO_2 ( g ) \][/tex]

To find this, we need to combine the given reactions such that the resultant equation matches the desired overall reaction [tex]\( NO ( g ) + O ( g ) \rightarrow NO_2 ( g ) \)[/tex].

First, note the target reaction:
[tex]\[ NO ( g ) + O ( g ) \rightarrow NO_2 ( g ) \][/tex]

Let's examine how to combine the given equations:

1. Start with the first equation:
[tex]\[ NO ( g ) + O_3 ( g ) \rightarrow NO_2 ( g ) + O_2 ( g ) \][/tex]

2. Then, consider the reverse of the second equation (because we need [tex]\( O_3 \)[/tex] to cancel out and match oxygen atoms):
[tex]\[ O_3 ( g ) \rightarrow \frac{3}{2} O_2 ( g ) \quad \Delta H = -142.3 \, \text{kJ} \][/tex]

3. Lastly, take the third equation:
[tex]\[ O ( g ) \rightarrow \frac{1}{2} O_2 ( g ) \][/tex]

Now, add up these reactions:

[tex]\[ \begin{align*} NO ( g ) + O_3 ( g ) & \rightarrow NO_2 ( g ) + O_2 ( g ) & (\Delta H_1 = -198.9 \, \text{kJ}) \\ O_3 ( g ) & \rightarrow \frac{3}{2} O_2 ( g ) & (\Delta H = -142.3 \, \text{kJ}) \\ O ( g ) & \rightarrow \frac{1}{2} O_2 ( g ) & (\Delta H_3 = -247.5 \, \text{kJ}) \end{align*} \][/tex]

Summing these reactions:

[tex]\[ \begin{array}{ll} NO ( g ) + O_3 ( g ) & \rightarrow NO_2 ( g ) + O_2 ( g ) \\ O_3 ( g ) & \rightarrow \frac{3}{2} O_2 ( g ) \\ O ( g ) & \rightarrow \frac{1}{2} O_2 ( g ) \end{array} \][/tex]

Simplifying by canceling the common terms on both sides, we get:

[tex]\[ NO ( g ) + O ( g ) \rightarrow NO_2 ( g ) \][/tex]

The sum of the enthalpy changes is:

[tex]\[ \Delta H_{\text{overall}} = \Delta H_1 + \Delta H_2 + \Delta H_3 \][/tex]

Substituting the given values:

[tex]\[ \Delta H_{\text{overall}} = -198.9 \, \text{kJ} + 142.3 \, \text{kJ} + (-247.5 \, \text{kJ}) \][/tex]

[tex]\[ \Delta H_{\text{overall}} = -198.9 + 142.3 - 247.5 = -304.1 \, \text{kJ} \][/tex]

Therefore, the enthalpy change for the overall reaction [tex]\( NO ( g ) + O ( g ) \rightarrow NO_2 ( g ) \)[/tex] is:
[tex]\[ \boxed{-304.1 \, \text{kJ}} \][/tex]
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