If you have a solution of HCl in a copper cup which is suspended in a cup of water and both solutions are at 20 degrees Celsius, and you add Magnesium to the HCl, what happens to the temperatures of the individual liquids and why?

Question

anonymous · Answer

I believe the temperature of the liquids should increase. The addition of magnesium to HCl is quite exothermic (and quite reactive). The heat produced by this reaction must be transferred somewhere, namely the liquids.

anonymous · Answer

If the Magnesium/HCl reaction is exothermic, wouldn't it transfer its heat to the water? Or am I way off?

anonymous · Answer

well it is going to transfer some heat to the water. Remember, the reaction is not actually taking place in the water, but in a container that is surrounded by the water. So the liquid in that container will definitely increase in temperature. Heat will be transferred from that liquid, through the copper container wall, and then into the water.

anonymous · Answer

So the temperature of both goes down? Or up? I'm really confused, my teacher hasn't gone over this well yet...

anonymous · Answer

I would say they both go up.

anonymous · Answer

I would say the solution is warmer than the water however, as the heat will not have transferred completely from the container to the water.

anonymous · Answer

Okay... And then I have to choose an option for why that is, so out of these multiple choice answers, which sounds the most like the reason why: 
     In an exothermic reaction, the reactants give off heat to the surroundings and form cooler products.
     Thermal energy cannot be created or destroyed - it can only be transferred from one place to another.
     The temperature difference causes heat to leave the inner vessel and enter the outer one.
     ΔH < 0 means a decrease in energy and therefore a decrease in temperature.
     The chemical change in the system absorbs heat.

anonymous · Answer

Sorry... looking at those choices, the first one is definitely correct. I actually may have contradicted myself earlier (brain freeze haha).

anonymous · Answer

:-P oh gosh, now that confuses me. So the first reason is right, and the temperature of the acid goes down, whereas the temperature of the water goes up?

anonymous · Answer

OK, sorry I will try to explain why that is the correct answer. So that reaction between magnesium and HCl... that is exothermic. That means that reaction is giving of heat to its surroundings. Because heat energy cannot be created nor destroyed, that heat that is given off to the surroundings must come from somewhere, namely from the chemical bonds found in HCl and magnesium. Because heat energy is being taken from the HCl and magnesium, in order to be transferred to the surroundings (a.k.a. the surrounding water), the container where the HCl was will become colder.

anonymous · Answer

I'm sorry for the earlier confusion :S did that explanation help?

anonymous · Answer

Yes it did! Thank you soo much, it makes so much more sense now!

anonymous · Answer

OK I'm glad! :)