Why Does Thermochemistry Equate Q of Solution to Negative Q of Reaction?

[Ritzycat]

Hello! I have a few conceptual questions regarding thermochemistry.

In thermochemistry, why is Q of solution equal to negative Q of reaction?
Why is a volume decrease equal a POSITIVE value for w?

 

[Simon Bridge]

You should be able to answer those questions yourself from a consideration of which quantities the letters in question represent and where those quantities come from - the properties that the relations are illustrating.

i.e. if a solution heats up, where did the heat energy come from?