Textbook Problem 4B.7

[Naibe Reynoso 2C]

Why is q negative for this problem? How should we know when q is positive or negative?

[isabella1012]

q = negative when the reaction is exothermic (feels hot)
q = positive when the reaction is endothermic (feels cold)

[Erin Haisan 1B]

Hello! Q is negative in this problem since the system is losing heat and thus is losing 947KJ (-947KJ) of energy. The heat is then transferred to its surroundings. If q were positive, that would indicate that the system is gaining heat. I hope this helps!

[justinamartelli]

It says "the cooling system that surrounds the chamber absorbs 947 kJ as heat", and I was also confused by this initially because I thought it required a positive Q since it seemed as if a cold system gained heat. However we must think of it this way: If surrounding heat increases, heat of the system decreases because heat is not created nor destroyed. This ultimately takes the heat away from the system, making it exothermic and negative. The value of Internal Energy will be the negative value of the heat absorbed by the surroundings.

[Jayla Riley]

in this problem, q is negative because this reaction is a combustion reaction. combustion reactions will give off energy as heat which is denoted as a negative delta U. A positive delta U means that the reaction requires energy to form the bond and is therefore endothermic.