Hess Law

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Heats of Reaction – Hess’s Law
Overview
Hess’s Law states that the energy change for a reaction depends on the enthalpy of the reactants and products and is independent of the pathway of the reaction. In this experiment you will use calorimetry to measure the heats of reaction for three reactions: Reaction 1 Reaction Equation NaOH(s) → Na+(aq) + OH-(aq) Solid NaOH is dissolved in water 2 Na+(aq) + OH-(aq) + H+(aq) + Cl-(aq) → Na+(aq) + Cl-(aq) + H2O(l) Solutions of NaOH and HCl are mixed 3 NaOH(s) + H+(aq) + Cl-(aq) → Na+(aq) + Cl-(aq) + H2O(l) Solid NaOH is dissolved in a HCl solution The third reaction is actually a combination of the first two reactions. Notice that the equation for Reaction 3 can be obtained by adding together reactions 1 & 2. By calculating the heats of reaction for all three reactions you will be attempt to verify Hess’s Law: ΔH3 = ΔH1 + ΔH2 ΔH3 ΔH2 You will determine: ΔH1

Purpose •
To measure experimentally the amount of heat absorbed or released during the dissolving of ammonium nitrate and of sodium acetate in water.

2.0 g of solid NaOH is dissolved in 100 mL of water. The initial and final temperatures are measured and recorded. The heat of solution is calculated (ΔH1) Reaction 2. Na+(aq) + OH-(aq) + H+(aq) + Cl-(aq) → Na+(aq) + Cl-(aq) + H2O(l) 50 mL of 1.0 M hydrochloric acid solution is combined with 50 mL of 1.0 M sodium hydroxide solution. The initial and final temperatures are recorded, and the heat of the reaction is calculated (ΔH2) Reaction 3. NaOH(s) + H+(aq) + Cl-(aq) → Na+(aq) + Cl-(aq) + H2O(l) 2.0 g of solid NaOH is dissolved in 200 mL of 0.50 M HCl. The heat of reaction is calculated (ΔH3)

Equipment, Materials, and Procedure
Your teacher will provide you with a list of the materials and equipment required for this lab, and the procedures to follow. SAFETY PRECAUTIONS MUST BE STRICTLY FOLLOWED! If you are unable to actually perform the experiment, use the following set of experimental data to plot the graphs and answer the questions at the end of the lab.

Summary of Procedures
Reaction 1. NaOH(s) → Na+(aq) + OH-(aq)

Heats of Reaction – Hess’s Law

2

Data Analysis and Calculations Record your calculations in Table 2. Step 1. Calculate the mass of reaction mixture for each reaction. We will assume that the density of the solutions (the HCl and NaOH solutions) have the same density as pure water – 1.0 g/mL. Thus, 100.0 mL will have a mass of 100.0 g. Reaction 1: Add the mass of water used + mass NaOH Reaction 2: The volume of HCl used will be numerically equivalent to it’s mass, expressed in grams. Add mass NaOH + mass HCl Reaction 3: Add the masses of the two solutions: NaOH + mass HCl Step 2. Calculate the change in temperature for each of reactions Steps 3 – 6. Calculate the amount of heat released, in kJ, during each of the reactions, using Q = mcΔT Step 7 – 9. These steps convert mass or volume of materials used into moles. To convert mass of a solid into moles: moles = mass Molar _ Mass

To convert the volume of a solution into moles: The unit for concentration of solutions is M, which represents mol/L. moles = Volume of solution (in L) × concentration (in mol/L) For example: If 55.0 mL of a 1.0 M NaOH solution is used, then mol = (0.055 L) × (1.0 mol/L) = 0.055 mol

Step 10. Calculate the heat of reaction per mole of NaOH by dividing the heat released (your answer to Step 6) by moles NaOH used (answer to step 9).

Questions and Conclusions

1. Show that adding together the equations for Reaction 1 and Reaction 2 produce the equation for Reaction 3. 2. Calculate the sum of ΔH1 and ΔH2. How does this compare with the experimentally determined value for ΔH3? 3. Calculate the percent difference between ΔH3 and (ΔH1 + ΔH2). Percent Difference =

ΔH 3 − ( ΔH 1 + ΔH 2 ) × 100 ΔH 3

Heats of Reaction – Hess’s Law

3

Table 1A. Data recording sheet for Reactions 1 and 3

Reaction 1
NaOH (s) → Na + OH
+ -

Reaction 3
NaOH (s) + H + Cl → Na +...
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