Hess's Law

Topics: Thermodynamics, Temperature, Enthalpy Pages: 7 (1439 words) Published: November 26, 2012
Enthalpy and Hess’s Law Lab
Every chemical reaction is accompanied by a change in heat. Thermochemical reactions include the heat of reaction as part of the equation. The heat released or absorbed in a reaction at constant pressure is the Enthalpy change (∆Hrxn) for the reaction. The enthalpy change for each reaction is unique to that reaction. Many values for ∆Hrxn were experimentally determined, and many were calculated used Hess’s law. This lab will illustrate the principle of Hess’s law: if a reaction can be carried out in a series of steps, the sum of the enthalpies for each step equals the enthalpy change for the overall reaction. ∆Hrxn = ∆Hstep1 + ∆Hstep2 . The three reactions that we will be using are as follows:

Reaction 1:
Reaction 2:
Reaction 3:

NaOH(s) + HCl(aq)  NaCl(aq) + H2O(l)
NaOH(aq) + HCl(aq)  NaCl(aq) + H2O(l)
NaOH(s)  NaOH(aq)

Since reaction 1 can be obtained by adding reactions 2 and 3, the ∆Hrxn1 should equal ∆Hrxn2 + ∆Hrxn3. At constant pressure ∆Hrxn = qp. We cannot directly measure ∆Hrxn or qp, but we can measure the change in temperature for a solution, and using the specific heat of the solution, and the grams of solution, we can find qp using the following equation: Equation 1:

q = (grams of solution) x (specific heat of solution) x ∆T

The heat that is released by the reaction will be absorbed by both the surroundings; in this case, the water in the solution and the calorimeter itself:
Equation 2:

qrxn = -(qsolution + qcalorimeter)

Since every group’s calorimeter is slightly different, the heat capacity for the calorimeter will need to experimentally determine before it is used.

Pre-Lab Questions

Define ∆Hrxn.
Define specific heat, and heat capacity. How are these two terms different? The specific heat of a solution is 4.18 J/gK and its density is 1.02g/mL. The solution is formed by combining 25.0mL of solution A with 25.0mL of solution B, with each solution initially at 21.4C. The final temperature of the combined solutions is 25.3C. Calculate the heat of reaction, qrxn, assuming no heat loss to the calorimeter. If the calorimeter in the reaction above has a Heat Capacity of 8.20J/C, recalculate the qrxn, taking the heat loss to the calorimeter into account.

If the reaction above between solutions A and B goes as follows: A(aq) + B(aq)  AB(aq), and the molarity of A in solution A is 0.60M, and the molarity of B in solution B is 0.60M, what is the enthalpy of reaction (∆Hrxn), for the formation of 1 mole of AB in solution. Express ∆Hrxn in kJ/mol

HCl(aq, 1.0M)
250mL Beaker

Digital Thermometer
Graduated Cylinder

Distilled Water
NaOH(aq, 1.0M)

Calorimeter w/ Lid
Magnetic Stirrer and Stir Bar

Part 1: Determining the heat capacity of the Calorimeter


Set up a calorimeter of two nested cups with a cover inside a beaker. Measure 50.0mL of room temp distilled water into the calorimeter. Place the calorimeter on a magnetic stirrer and add a stir bar, set the stir bar to stir slowly. (Alternatively, gently stir the solution with the thermometer.)

Record the Temperature of the water in the calorimeter.
Heat or obtain roughly 75mL of 70C water.
Measure out 50.0mL of this water using a graduated cylinder. Record the temperature of the hot water, and pour the hot water into the room temp water in the calorimeter. Cover the calorimeter and insert the thermometer.

9. Stir and record the temperature every 20 seconds for three minutes. 10. Empty and dry the inside of the calorimeter, thermometer, and stir bar.

Part 2: Determining the heats of Reaction
Reaction 1: NaOH(s)  NaOH(aq)
1. Weigh out about 2.0g of NaOH(s). Record the actual weight. 2. Add 100.0mL of room temperature water to the calorimeter. 3. Start the stir bar and record the temperature of the water. 4. Add the NaOH(s), and record the temperature once every 20 seconds,...
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