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 (2)
When combining the two given chemical equations, the substance that gets canceled out is PCl_3. This substance appears as a product in the first equation and as a reactant in the second equation. Therefore, the correct answer is PCl_3.
;
Balance the chemical equations to have the same amount of PC l 3 on both sides.
Add the balanced equations together.
Cancel out the PC l 3 from both sides of the combined equation.
The substance cancelled out is PC l 3 .
The final answer is PC l 3 .
Explanation
Analyzing the Equations We are given two chemical equations:
Equation 1: 2 P ( s ) + 3 C l 2 ( g ) → 2 PC l 3 ( l )
Equation 2: PC l 3 ( l ) + C l 2 ( g ) → PC l 5 ( s )
We want to find the substance that is cancelled out when we combine these two equations. This means we need to identify a substance that appears on both the reactant and product sides.
Balancing the Equations First, we need to balance the equations so that the substance we want to cancel out has the same coefficient on both sides. In this case, PC l 3 appears on the product side of Equation 1 and the reactant side of Equation 2. To cancel it out, we need to multiply Equation 2 by 2:
2 { PC l 3 ( l ) + C l 2 ( g ) → PC l 5 ( s )} becomes 2 PC l 3 ( l ) + 2 C l 2 ( g ) → 2 PC l 5 ( s )
Adding the Equations Now we have the following equations:
Equation 1: 2 P ( s ) + 3 C l 2 ( g ) → 2 PC l 3 ( l )
Equation 2 (modified): 2 PC l 3 ( l ) + 2 C l 2 ( g ) → 2 PC l 5 ( s )
Adding the two equations together, we get:
2 P ( s ) + 3 C l 2 ( g ) + 2 PC l 3 ( l ) + 2 C l 2 ( g ) → 2 PC l 3 ( l ) + 2 PC l 5 ( s )
Cancelling and Simplifying Now we can cancel out the 2 PC l 3 ( l ) from both sides:
2 P ( s ) + 3 C l 2 ( g ) + 2 C l 2 ( g ) → 2 PC l 5 ( s )
Simplifying, we get:
2 P ( s ) + 5 C l 2 ( g ) → 2 PC l 5 ( s )
Identifying the Cancelled Substance The substance that was cancelled out is PC l 3 .
Examples
In chemical reactions, intermediate products are often formed and consumed. Balancing and combining chemical equations helps us understand the overall reaction and identify the substances that are not part of the final product. This is useful in industrial processes to optimize the production of desired chemicals and minimize waste.
