CSS Hess Law Heat Capacity Mo


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. The enthalpy of the third reaction will be found directly through experimentation and by applying Hess’s Law. You will compare these two values to evaluate Hess’s Law. In this experiment you will determine and compare the quantity of heat energy released in three exothermic chemical reactions. Sodium hydroxide will be the limiting reactant in all cases.

Reaction 1: Solid sodium hydroxide dissolves in water to form an aqueous solution of ions.

NaOH(s) → Na+(aq) + OH-(aq)

Reaction 2: Solid sodium hydroxide reacts with an aqueous solution of hydrogen chloride to form water and an aqueous solution of sodium chloride.

NaOH(s) + H+(aq) + Cl-(aq) → H2O(l) + Na+(aq) + Cl-(aq)

Reaction 3: An aqueous solution of sodium hydroxide reacts with an aqueous solution of hydrogen chloride to form water and an aqueous solution of sodium chloride.

Na+(aq) + OH-(aq) + H+(aq) + Cl-(aq) → H2O(l) + Na+(aq) + Cl-(aq)

In order to accurately measure the heat released in each reaction, we will be using a calorimeter. (For this experiment a styrofoam cup will act as the calorimeter). The change in temperature that occurs for each reaction will be used to calculate the energy released in kilojoules per mole of sodium hydroxide used (molar enthalpy). For the calculations in this experiment, we will assume that any heat transferred to the styrofoam and surrounding air will be negligible. We can also assume that the specific heat of water or aqueous solutions is 4.18 J/g°C. We will also assume that water or aqueous solutions used have a density of 1g/mL.


Reaction #


Initial Temperature (°C)

Final Temperature (°C)

Total Volume (mL)












25.0mL of 1.00 mol/L NaOH





1. For each of the three reactions, calculate the heat released/absorbed (q), and the molar heat of reaction (DHreaction). Show all work.

2. With Hess’s Law, show how the first two chemical reactions can be used to form the third reaction. Use this, and the molar enthalpies from the first two experiments, to find the molar enthalpy for the third reaction.

3. Do the results from Q#2 confirm Hess’s Law?

4. Suppose you had used 4.00 grams of NaOH(s) in Reaction 1. How would the amount of heat released be affected? What effect would it have had on the molar enthalpy for Reaction 1?

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