Question

A neutralisa tionreaction occurs when 120 millilitres of 0.5 mol/l lithium hydroxide(aq) and 160 millilitres of 0.375 mol/l and HNO3(aq) are mixed in a insulated Cup. initially the solutions are at the same temperature. If the highest temperature reached during the mixing with 24.5 degrees celsius what was the initial temperature of the solution?
liOH(aq) + HNO3(aq) -> LiNO3(aq) + H2O plus 53.1 kilojoules
assume that both of these Solutions has a density of 1 gram per millilitre and the specific heat capacity of 4.19 joules per gram degrees Celsius

After working out their moles they both have the same one of 0.0 mol but I wanted to know if the Q of the reaction would be -53.1 or + 53.1 because Q=mcT so
53.1= 0.28g×4.19×(Tf-24.5)
So what does Tf equal to

Answer 1

1. The 53.1 you quote is kJ/mol and you need to change that to J/mol.
2. You have calculated the mols but you don't have as much as a whole mole. Therefore, 53,100 J/mol x # mols in reaction = J and that is Q.

3. This is an exothermic reaction and q goes in this equation as +. Actually, you can put it in both ways and see what you get. If you use +Q, Tf comes out higher (which you know is right since the reaction is exothermic). And if you insert Q as -, Tf comes out LOWER than you started and you know that is not right.