Chem May 8 2014 revision of c and Q, delta H calculations - 3

Chem May 8 2014 revision of c and Q, delta H calculations 3

1

With a partner or small group, talk about the different specific heat capacities of each substance in the above list.

a) What is the link between the value of specific heat capacity and energy?b) Is energy absorbed during an increase or decrease of temperature?c) Which one would release /absorb the biggest amount of energy when its temperature is decreased/increased? Why?d) Which one would release/absorb the least amount of energy when its temperature is decreased/increased? Why?


2

Quickly sketch or describe a phase diagram for a substance with a big c value and another with a small c value.


3

Examine the following incorrect statement:

make the necessary corrections in your own words

The following are the possible corrections for the statement above:


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When the water absorbs heat, it means that the reaction lost it (exothermic reaction), but when the water loses heat, it means that the reaction absorbed it (endothermic reaction).

In an endothermic reaction, the reaction absorbs heat from the water (because water is the surroundings). So, the temperature of the water will drop.

In an exothermic reaction, the reaction releases heat into the water (because the water is the surroundings). So, the temperature of the water will increase.


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In the next part, we'll be looking at heat calculations. We are really interested in knowing the heat absorbed or released by each reaction, but often times we can't. One of the reasons is that we can't stick a thermometre in a reaction that only involves solid reactants, so we dissolve them in water (which is only the solvent, not a reactant.)

That way, when the heat is absorbed by the reaction, we can find out because the water will be losing heat (its temperature is dropping), but when the reaction is releasing heat, the water will absorb it (so its temperature is rising.)

We just have to remember that we are assuming no heat loss at all; therefore, the water is absorbing all the heat released by the reaction, and the reaction is absorbing heat from the water. The quantity of heat of both the water and the reaction is the same.

Heat has the symbol Q. Q will have a positive sign when heat is absorbed and a negative sign when heat is released.


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Since the Q of the reaction is positive, we know that the reaction is endothermic.


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H = (Qrxn)(Molar mass of reactant) (mass of reactant)

Chem May 8 2014 revision of c and Q, delta H calculations - 3

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