## Nali 3 years ago A volume of 50.0 mL of 0.400 M HBr at 24.35°C is added to 50.0 mL of 0.400 M NaOH, also at 24.35°C. The final temperature is 27.06°C. Calculate the enthalpy change, H, in kJ for the following reaction: HBr(aq) + NaOH(aq) NaBr(aq) + H2O(l) (Heat capacity of the system is 0.418 kg/°C.) −28.4 kJ −113 kJ −324 kJ −56.7 kJ

1. Nali

How would i know if it is exothermic or endothermic? in this case it is exothermic, but i do not get it

2. Preetha

Nali, the final temp is greater than the initial temp. That tells you that it is exothermic.

3. Nali

but that means that it had gained heat meaning it is endothermic

4. Nali

@Preetha

5. Omniscience

well first do you know what endothermic means?

6. Omniscience

endothermic = absorb heat exothermic = release heat

7. Preetha

The reaction releases heat. The water absorbs heat. The reaction is exothermic. The water just absorbs heat. So we call the reaction exothermic.

8. Nali

wait i do not get it the temperature change made the reaction absorb heat so it is endo. not exo. @Preetha

9. Preetha

Nali: There are two things going on. 1. Reaction occurs. Due to the reaction heat is given off by the reactants. So it is exothermic. 2. The surroundings - in this case water - absorb heat. There is no reaction. It just absorbs heat. ( IF you heated a metal rod and plunged it into a bowl of water, the temp of the water will go up. There is NO Exo thermic reaction, because here - with the metal rod, there is no reaction). Reactions are called exo or endo thermic. Not the surroundings. So the REACTION is EXOthermic.

10. Nali

so why does it say the final reaction is 27.06 and the reaction of the reactants is 24.35 does not that mean that the reactants had 24.35 degrees and after the reaction happened it became 27.06 which means it gained heat meaning it is endo.

11. Preetha

First you take the first solution, that is at 24.35. Then you add the second solution, which is at 24.35. Then you mix it. Reaction occurs. You get the products. Then the temp of the mixture (water and the products) is 27.06. How did that happen? How did the temp rise from 24.35 to 27.06?

12. Nali

meant final temperature*

13. Nali

because the reaction absorbed energy

14. Nali

in order to occur

15. Preetha

Hmmm. Let me think of how to explain this differently. Think of it as two different steps. First dry HBr and dry NaOh are combined. They react. They form NaBr and H2O. This reaction gives off heat. Heat is released to the surroundings. THEN the water which was at 24.35 absorbs this heat. And its temp becomes 27.06. Reaction gives off heat (exo) and the water absorbs this heat.

16. Nali

u r repeating the same point and i am doing the same thing plz let me understand how the reaction gives off heat when it actually absorbed heat and that the reason the temperature increased

17. Preetha

I give up.

18. Nali

it si ok u could of just reminded me of the rule q(solution)= -q (reaction)

19. Nali

thanku for trying to make me get this patiently :)