Experiment no : 1
Experiment title : Determination of the enthalpy (heat) of reaction of a monobasic acid with sodium hydroxide
1) To understand the enthalpy chemistry.
2) To determine the calorimeter constant.
3) To determine the enthalpy reaction of acid-base reactions. 4) To study the exothermic reaction.
Apparatus and Materials :
* Dewar flask, stopwatch, thermometer (5 to 50 ˚C, graduated in 1/10 ˚C), * 50 cm3 graduated pipette fitted with a suction bulb, 10 cm3 graduated cylinder, * Conc. sulphuric acid (specific gravity 1.84, 98.5% H2SO4 ), conc. nitric acid, * 1 M sodium hydroxide, 0.1 M hydrochloric acid, methyl orange indicator.
Enthalpy is a measure of total energy of a thermodynamic system. It includes the internal energy of the system and the product of its volume multiplied by the pressure exerted on it by its surrounding.
H = U + p*V
H = EnthalpyV = Volume
p = Pressure
U = Internal Energy
The enthalpy is normally measure with S.I. unit of Joule, J , although other units still in use such as Calorie, C and calorie, c. However, we often measure the change in enthalpy, ∆H instead of measuring the enthalpy, H because the total enthalpy cannot be measured directly.
Enthalpy change is defined by the following equation :
∆H = HFinal – HInitial
∆H = Enthalpy change
HFinal = Enthalpy of the products
HInitial = Enthalpy of reactants
A positive value for ∆H indicates it is an endothermic reaction which involve energy released as heat into its surrounding, causing the temperature for the surrounding to rise. A negative value for ∆H indicates it is an exothermic reaction which involve energy acquired from its surrounding as heats, causing the surrounding temperature to drop. Change in enthalpy that occurs as a result of chemical reaction is numerically equal to the heat of reaction under constant (atmospheric) pressure conditions (∆H = q). The heat of reaction is conveniently measured adiabatically in a Dewar calorimeter by the rise or fall in temperature of the products produced by the reaction in solution. Dewar flask is used because it is designed to preserve heat and minimize heat loss to the surrounding. In addition, isolated system is also required in this experiment to obtain an accurate data. Since every Dewar flask has different calorimeter constant because of different substances used, so the calorimeter constant (Ccal) must first be determined, that is the quantity of heat required to increase the temperature of the calorimeter and its content by 1 ˚C.
Ccal = ∆H / ∆T
The constant is measured by supplying the calorimeter and contents with a definite known quantity of heat. This can be done by electrically or by adding a known amount of sulphuric acid, H2SO4.
Dewar flask is a suitable calorimeter.
A. Calorimeter constant
1) 100 cm3 of water was pipetted into the Dewar flask.
2) The water was stirred slowly and regularly with a 1/10 ˚C thermometer. 3) The temperature of the water was recorded at intervals of 1 minute over a period of 5 minutes. 4) 2 cm3 of conc. sulphuric acid was poured into the calorimeter at the end of the period. 5) The temperature was recorded at 1 minute intervals over a period of 5 minutes while stirring the mixture until the temperature fall became constant. 6) The mixture in the calorimeter was allowed to cool.
7) 25 cm3 of the mixture was titrated against 1 M sodium hydroxide, NaOH to determine the molarity of the mixture.
B. Enthalpy of reaction
1) 50 cm3 of 1 M sodium hydroxide, NaOH was mixed with 50 cm3 of water in the calorimeter. 2) Temperature observations were made as already described. 5 cm3 of 10 M nitric acid was added into the...