Answer:
-65 kJ
Explanation:
To use Hess's Law, you need to combine the equations in a way that results in the desired equation. To add reactants/products, just add the equations and their subsequent heat of reactions. Subtracting reactants/products involves flipping the signs of enthalpies, duplicating reactants/products involves multiplying the coefficients and enthalpies by a desired number.
A + 2B --> C + D
+ B --> 2C + D
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A + 3B --> 3C + 3D
In this case, you only need to add the equations together to get the desired equation. Because you just add the equations, combine each equations' heat of reaction.
(-20 kJ) + (-45 kJ) = -65 kJ
The overall heat of reaction of the desired reaction is -65 kJ.
