Composite Equations

[Savannah Mance 4G]

For the homework problem 5H.3, it asks for the K value at 300 kelvin for the reaction 2BrCl(g) + H2(g) > Br2(g) + 2HCl(g) using the Table 5G.2. I wanted to combine H2+Cl2>2HCl which had a K of 4.0x10^31 at 300K and then Br2>2Br. But for the Br equation the kelvins listed were either 1000 or 1200. Is it okay to use these if it asks for the K value at 300? Do I have to divide the the K value by 300 or something like that?

[LBacker_2E]

When I did this problem, I used:
H2 + Cl2 <--> 2HCl
2BrCl <--> Br2 + Cl2
The Cl2's cancel out to get the complete reaction, and both are listed at 300K in the book.

[Keya Jonnalagadda 1A]

For composite equations, the product of one reaction must correspond to the reactant of the other reaction that you are trying to combine. As the other reply stated, you would want to use the two equations in the chart that cancel out Cl2 to give you the desired equation. As it happens, these are both listed at 300K so you don’t have to divide anything.

[Dina Marchenko 2J]

Can someone explain the process of combining two equations and determining how to combine their K's? How do you know which reactants will cancel each other out when you combine two equations? Also how do stoichiometric coefficients tie into this?