Determination of Enthalpy

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Determining the Enthalpy
of a Chemical Reaction

All chemical reactions involve an exchange of heat energy; therefore, it is tempting to plan to follow a reaction by measuring the enthalpy change (∆H). However, it is often not possible to directly measure the heat energy change of the reactants and products (the system). We can measure the heat change that occurs in the surroundings by monitoring temperature changes. If we conduct a reaction between two substances in aqueous solution, then the enthalpy of the reaction can be indirectly calculated with the following equation.

q = Cp ( m ( ∆T

The term q represents the heat energy that is gained or lost. Cp is the specific heat of water, m is the mass of water, and ∆T is the temperature change of the reaction mixture. The specific heat and mass of water are used because water will either gain or lose heat energy in a reaction that occurs in aqueous solution. Furthermore, according to a principle known as Hess’s law, the enthalpy changes of a series of reactions can be combined to calculate the enthalpy change of a reaction that is the sum of the components of the series.

In this experiment, you will measure the temperature change of two reactions, and use Hess’s law to determine the enthalpy change, ΔH of a third reaction. You will use a Styrofoam cup nested in a beaker as a calorimeter, as shown in Figure 1. For purposes of this experiment, you may assume that the heat loss to the calorimeter and the surrounding air is negligible.


In this experiment, you will

• Use Hess’s law to determine the enthalpy change of the reaction between aqueous ammonia and aqueous hydrochloric acid. • Compare your calculated enthalpy change with the experimental results.


Figure 1


|LabQuest |2.0 M hydrochloric acid, HCl, solution | |LabQuest App |2.0 M sodium hydroxide, NaOH, solution | |Temperature Probe |2.0 M ammonium chloride, NH4Cl, solution | |glass stirring rod |2.0 M ammonium hydroxide, NH4OH, solution | |Styrofoam cup |ring stand | |250 mL or 400 mL beaker |utility clamp | |50 mL or 100 mL graduated cylinders |fume hood |


You will conduct the following three reactions in this experiment. In the space provided below, write the balanced net ionic reaction equations from the descriptions. Use the table of thermodynamic data in your text (or another approved resource) to calculate the molar enthalpy of the reactions.

Reaction 1: An aqueous solution of sodium hydroxide reacts with an aqueous solution of hydrochloric acid, yielding water.

Reaction 2: An aqueous solution of sodium hydroxide reacts with an aqueous solution of ammonium chloride, yielding aqueous ammonia, NH3, and water.

Reaction 3: An aqueous solution of hydrochloric acid reacts with aqueous ammonia, NH3, yielding aqueous ammonium chloride.

|Reaction |Balanced reaction equation |ΔH (kJ/mol) | |1 | | | |2 | | | |3 |...
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