Enthalpy Help

[dcowboys107]

I have just recieved my grade on my enthalpy test (78) and have a few questions in which my teacher was unable to answer to my understanding.

 

1. In exothermic reactions, the products contain more heat than do the reactans. Sometimes True, Never True, or Always True? I put AT because wouldn't the products gain heat from the reactant?

 

2. Here is a multiple choice question in which I also missed: A limiting Reactant: a. may be heat but only in an exothermic reaction.

b. may be heat but only in an endothermic reaction

c. does not involve the heat change at all

d. both a and b are correct

e. none of the above.

 

The answer for 2 is "B" but why?

[dttom]

1) well, heat means the transfer of thermal energy, so I don't think the product could contain heat, nor the reactant; if you talk about enthalpy contained in product, compared with that in reactant, in an exothermic reaction, which net energy is given out by the reaction, I think the answer should be sometimes true.

for an exothermic reaction that A--> B

as energy is given out in this reaction, enthalpy of B is probably lower than that in A, hence in this case, enthalpy of product is lower than the reactant's.

while for an exothermic reaction that C + D--> E

let C has an enthalpy of x J mol^-1; D y J mol^-1; 1 mole of C and D is used in the reaction;

even it is an exothermic reaction, I think E could somehow possess x+y-z J mol^-1 of enthalpy, where z is positve real number, and z J of energy is given out in the reaction. so in this case, the product contains an enthalpy higher than either reactant.

Hence I think the conclusion would be 'sometimes true'.

[dttom]

for question two, though I am not sure I could get the explanation correct, I would like to have a try.

for an exothermic reaction, says, glucose --heat--> carbon dioxide + water

this reaction could not take place at room temperature, while not enough heat given to it seems not be a limiting agent. the above reaction could not take place just because the reaction does not get enough activation energy, so I think activation energy should not be regarded as a sort of limiting reactant/agent.

while for an endothermic reaction, the enthalpy of the product should be greater than that of reactant; so energy absorbed during the reaction is used to enhence the enthalpy of chemical, hence this energy (heat) could be considered as a reactant, hence if the energy given is not enough, the percentage of completion of the reaction, assuming the reaction is irreversible, would not attend one hundred percent. in this way looking into the matter, in an endothermic reaction, heat could be a limiting agent.