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!