Calorimetry

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Chem 17 ▪ General Chemistry Laboratory II

Experiment 1

Calorimetry

INTRODUCTION Chemical reactions are usually accompanied by the evolution (exothermic reaction) or absorption (endothermic reaction) of heat energy. When measured at constant pressure, the heat evolved (qp < 0) or absorbed (qp > 0) is equal to the enthalpy change, symbolized by ΔH. ΔH is positive for an endothermic process and negative for an exothermic one. If H f is the enthalpy of the final state and Hi of the initial state, the enthalpy change for a chemical reaction is given by Equation (1). ∆Hrxn = Hf - Hi (1)

The process of measuring ΔH is called calorimetry. This involves “trapping” the heat evolved (or absorbed) producing a measurable change in temperature in a device called a calorimeter. The calorimeter setup is shown in Figure 1.

Figure 1. Calorimeter Set-up The system involved here is adiabatic which means that no heat exchange occurs between the calorimeter (and its contents) and the immediate surroundings. The adiabatic system consists of the calorimeter and the reaction components, hence, heat exchange is only limited between these two components. That is, the heat change of the adiabatic system (qadiabatic system) is zero.

Institute of Chemistry, University of the Philippines, Diliman, Quezon City 1101, Philippines

1

Chem 17 ▪ General Chemistry Laboratory II

qadiabatic system = qrxn + qcal = 0

(2)

where qrxn and qcal are the heat involved in the reaction and the calorimeter, respectively. In the experiment, the changes in enthalpy for various reactions are determined by running them in an adiabatic calorimeter with a heat capacity (Ccal) obtained experimentally. From the recorded change in temperature, ΔT (T final – Tinitial) and Ccal, the heat change of the calorimeter, and thus the heat involved in the reaction are determined. The calorimeter consists of everything inside the Styrofoam box in which the reaction is carried out, the water in which the reactants are dissolved, the spectator ions, the excess reactants, the air and the thermometer. From qrxn and number of moles of reaction that took place, the enthalpy change per mole of reaction can be calculated. qcal = Ccal (∆T) ∆Hrxn = qrxn nrxn

(3) (4)

The calorimeter is calibrated to determine Ccal by running in it a reaction of known ΔH°. H+ (aq) + OH (aq) → H2 O(l) -

∆H° = -55.8 kJ rxn

(5)

For every mole of H2O(l) produced in Equation (5), 55.8 kJ of heat is evolved. The standard enthalpy of formation of a substance, Hf, is the enthalpy change involved in the formation of one mole of a substance from its elements under standard conditions. Tabulated Hf values are used to calculate for the change in enthalpy of any reaction under standard conditions. For any reaction under standard conditions, the reaction enthalpy is the sum of the standard enthalpies of formation of the products, multiplied by their stoichiometric coefficients in the reaction equation, minus the sum of the standard enthalpies of formation of the reactants, multiplied by their stoichiometric coefficients in the reaction equation.

∆H° = rxn

np ∆H° p

nr ∆H° r

(6)

The stoichiometric coefficients, np and nr are for each product and for each reactant, respectively.

Institute of Chemistry, University of the Philippines, Diliman, Quezon City 1101, Philippines

2

Chem 17 ▪ General Chemistry Laboratory II

EXPERIMENT Reagents and Materials 12.1 M HCl 15.6 M HNO3 17.4 M CH3COOH 14.5 M NH3 1.0 M CaCl2 0.1 M CuSO4 0.1 M AgNO3 0.5 M Na2CO3 NaOH pellets Procedure A. Solution Preparation Prepare the following solutions*: Solution Preparation 1.0 M NaOH Prepare a 500 mL solution from NaOH pellets 1.0 M HCl Prepare a 250 mL solution from concentrated HCl (12.1 M) 1.0 M NH3 Prepare a 100 mL solution from concentrated NH3 (14.5 M) 1.0 M CH3COOH Prepare a 100 mL solution from concentrated CH3COOH (17.4 M) 1.0 M HNO3 Prepare a 50 mL solution from concentrated HNO3...
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