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Partners: Jeff Somvorachith, Connor Engels

10. Additivity of Heats of Reaction: Hess's Law

Purpose:

To practice applying Hess's Law using a coffee cup calorimeter and confirming that the heat of one reaction should be equal to the sum of the heats for the other reactions.

Pre-Lab:

(1) Na+(aq) + OH-(aq) --> NaOH(s)   -ΔH1

(2) NaOH(s) + H+(aq) + Cl-(aq) --> H2O(l) + Na+(aq) +Cl-(aq)   ΔH2

(3) Na+(aq) + OH-(aq) + H+(aq) + Cl-(aq) --> H2O(l) + Na+(aq) +Cl-(aq)   ΔH3

Procedure:

    1. Wear protective equipment.
    2. Open Vernier Graphical Analysis. Connect the temperature probe. 
    3. Use a utility clamp and a slit stopper to suspend the temperature probe from a ring stand.
    4. Place a Styrofoam cup into a 250mL beaker. Measure out 100.0mL of distilled water into the cup. Lower the temperature probe into the solution.
    5. Weigh out around 2 grams of solid sodium hydroxide and record the mass to the nearest 0.01g.
    6. Begin collecting data and obtain the initial temperature, t1. After obtaining the initial temperature, add the NaOH to the cup. Stir continuously until the temperature has maximize and has begun to drop. Record the maximum temperature, t2. After 3.3-4 minutes, data collection is completed.
    7. Graph temperature vs. time(done automatically). Examine the data points along the curve to verify the values of t1 and t2.
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