Thermodynamics Enthalpy of Reaction Hesss Law PreLab Discussion

Thermodynamics – Enthalpy of Reaction & Hess’s Law Pre-Lab Discussion

What you need to know: • Hess’s Law and how to solve for DHrxn • q=m. Cp. DT • how to relate q & DHrxn using concentrations of reactants

Purpose • Verify Hess’s Law • Use 3 reactions: – Reaction 3 = reaction 1 – reaction 2 – DHrxn 3 = DHrxn 1 – DHrxn 2 • Monitor temperature changes during the reaction • Calculate enthalpy and heat energy changes

Things to note: • Some of the temperature changes will be really small, so we will use digital thermometers • DHrxn for # 2 is really small, so we will pool our data and come up with an average DHrxn

Pre-Lab Questions • Define DHrxn • Define specific heat • Questions 3 -5 are calculation questions on the next few slides

Pre-Lab Question 3 The specific heat of a solution is 4. 18 J/(g C) and its density is 1. 02 g/m. L. The solution is formed by combining 25. 0 m. L of a solution A with 25. 0 m. L of solution B with each solution initially at 21. 4 C. The final temperature of the combined solutions is 25. 3 C. Calculate the heat of reaction, q, assuming no heat loss to the colorimeter. Use correct sig figs.

Pre-Lab Question 4 In Problem 3, the calorimeter has a heat capacity of 8. 20 J/C. If a correction is included to account for the heat absorbed by the calorimeter, what is the heat of reaction, q?

Pre-Lab Question 5 If the reaction in question 3 is A + B AB And the molarity of A in solution is 0. 60 M and the molarity of B in solution B is 0. 60 M, calculate the enthalpy of reaction, D Hrxn, for the formation of 1 mole of AB in solution.

During Lab • Even though we are using vernier, you still need your own data table and graphs • Part 1 is determining the heat capacity of your calorimeter • Part 2 is determining the heats of reaction for each of the 3 reactions

After Lab • The calculations seem difficult, but all the equations are given to you. You just have to keep track of and plug in the data. • Really don’t put this post-lab off to the last minute, because it is going to take you a while!