Consider the following intermediate chemical equations:
[tex]$\begin{array}{l}
2 P(s)+3 Cl_2(g) \rightarrow 2 PCl_3(l) \\
PCl_3(l)+Cl_2(g) \rightarrow PCl_5(s)
\end{array}$[/tex]
When you combine the intermediate chemical equations, which substance do you cancel out?
A. P
B. [tex]$Cl _2$[/tex]
C. [tex]$PCl _3$[/tex]
D. [tex]$PCl _5$[/tex]
Answer (1)
Combine the two given chemical equations.
Identify PC l 3 as the substance present on both sides of the equation.
Cancel out PC l 3 by subtracting it from both sides.
Conclude that the substance cancelled out is PC l 3 .
Explanation
Understanding the Problem We are given two chemical equations:
2 P ( s ) + 3 C l 2 ( g ) → 2 PC l 3 ( l )
PC l 3 ( l ) + C l 2 ( g ) → PC l 5 ( s )
Our goal is to determine which substance cancels out when we combine these two equations.
Combining the Equations To combine the equations, we add them together:
2 P ( s ) + 3 C l 2 ( g ) + PC l 3 ( l ) + C l 2 ( g ) → 2 PC l 3 ( l ) + PC l 5 ( s )
Now, we simplify by combining like terms:
2 P ( s ) + 4 C l 2 ( g ) + PC l 3 ( l ) → 2 PC l 3 ( l ) + PC l 5 ( s )
Cancelling Common Substances Next, we look for substances that appear on both sides of the equation. In this case, PC l 3 appears on both the reactant and product sides. To cancel it out, we subtract the smaller amount from both sides. We have 1 PC l 3 on the left and 2 PC l 3 on the right, so we subtract 1 PC l 3 from both sides:
2 P ( s ) + 4 C l 2 ( g ) + PC l 3 ( l ) − PC l 3 ( l ) → 2 PC l 3 ( l ) − PC l 3 ( l ) + PC l 5 ( s )
This simplifies to:
2 P ( s ) + 4 C l 2 ( g ) → PC l 3 ( l ) + PC l 5 ( s )
Final Answer The substance that was cancelled out in the process of combining the equations is PC l 3 .
Examples
In chemical manufacturing, intermediate reactions often produce byproducts that are recycled within the process. Identifying and canceling out these intermediate substances, like PC l 3 in this example, is crucial for optimizing the overall reaction and minimizing waste. This ensures a more efficient and environmentally friendly production process, where resources are used effectively and unwanted substances are reduced.
