Chemistry 30

• This experiment allows you to collect experimental evidence of Hess's Law: "the energy change for a reaction depends on the enthalpy of the reactants and products and is independent of the pathway of the reaction".
• You will calculate the heats of reaction for three reactions using a simple calorimeter.
• Sample data is provided for those unable to actually perform the experiment. This will allow you to do the calculations and answer the lab questions.
• This lab requires teacher supervision. Safety precautions must be carefully followed.

• The temperature change is recorded for each of the three reactions. Knowing the mass of the substances used, the amount of heat released during these exothermic reactions is calculated using:

Q = mcΔT

• Q is then used to find ΔH_reaction
  
  for each of the three reactions

Reaction 1. Solid NaOH is dissolved in water.    ΔH₁ is determined

NaOH _(s) → Na⁺_(aq) + OH^-_(aq)

Reaction 2. Aqueous solutions of NaOH and HCl are mixed.    ΔH₂ is determined

Na⁺_(aq) + OH^-_(aq) + H⁺_(aq) + Cl^-_(aq) → Na⁺_(aq) + Cl^-_(aq) + H₂O_(l)

Reaction 3. Solid NaOH is dissolved in an HCl solution.    ΔH₃ is determined

NaOH_(s)  + H⁺_(aq) + Cl^-_(aq) → Na⁺_(aq) + Cl^-_(aq) + H₂O_(l)

Hess's Law tells us that adding together ΔH₁ and  ΔH₂ should produce  ΔH₃ . The purpose of this lab is to test this experimentally.