# thermochemistry - energy and enthalpy of different substances in water

I conducted an experiment in which I added Sodium Nitrate, Potassium Nitrate, Potassium Chloride, and Sodium Hydroxide - all separately into water - then measured the temperature change of each. The results were endothermic reactions for the first three reactions, and it was exothermic for Sodium Hydroxide. (sulfuric acid was used as a catalyst for the exothermic reaction).

Information that I have: Mass/volume for each substance, Initial Temperature, and Final Temperature.

Now I want to find the q (energy) for each solution, the enthalpy change for each solution, and the enthalpy change for the reaction.

I'm thinking I can use the heat capacity of water (4.18 J/gK) to calculate, however, I'm not sure.

• What relationships (mathematical equations) are you familiar with that relate heat (q) and the heat capacity of water or a solute in water? – bobthechemist Nov 6 '13 at 20:25
• I am familiar with q= mct , -q reactants = q surroundings , Hess's Law ... Grade 12 chemistry level. – Ds.109 Nov 7 '13 at 3:14
• Yes. The water is absorbing the heat from the reaction, so 4.18 J/g is the appropriate value for c to use. I presume you also know the mass of water used so you can use $q=mc \Delta T$ – bobthechemist Nov 7 '13 at 3:39

Assuming that the heat capacity of the system is constant, you could use the temperature differences you measured and calculate the enthalpy change: $\Delta H = C_P\Delta T$. At constant pressure (which you presumably have), the enthalpy change is equal to the heat entering or leaving the system, i.e. $q=\Delta H$.