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=mCpDT how to relate q & DHrxn using concentrations of reactants

Purpose Verify Hess’s Law Use 3 reactions: Reaction 3 = reaction 1 – reaction 2 DHrxn3 = DHrxn1 – DHrxn2 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/mL. The solution is formed by combining 25.0 mL of a solution A with 25.0 mL 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!