6.8
Constant Pressure Calorimetry
Calorimetry is a technique used to measure the amount of heat involved in a chemical or physical process or to measure the heat transferred to or from…
For a chemical reaction carried out under constant pressure, such as atmospheric pressure, the heat exchanged during the process is measured as the change in enthalpy ΔH. The enthalpy change for the reaction is manifested as a change in temperature, which is measured using a technique called calorimetry.
In calorimetry, the reaction is executed in a sealed and calibrated container called a calorimeter. The calorimeter is well insulated and prevents any heat flow between itself and its environment.
Thus, heat exchanged between the reactants and products (which constitute the system) and the calorimeter (which is the surroundings), can be accurately calculated by monitoring the subsequent changes in the temperature in the calorimeter.
If the reaction is exothermic, heat flows from the system into the surroundings, and the temperature rises. Conversely, if the reaction is endothermic, heat flows from the surroundings into the system, causing the temperature of the calorimeter to fall.
A simple coffee cup calorimeter measures change in enthalpy of a reaction occurring in a solution, under constant pressure conditions.
The calorimeter consists of two Styrofoam coffee cups nested together and equipped with a thermometer and a stirrer. The calorimeter is closed with a loose-fitting cork lid to maintain conditions of constant pressure that is open to the atmosphere.
Suppose 50.0 milliliters each of 1.0 molar aqueous hydrochloric acid and 1.0 molar aqueous potassium hydroxide, react within the calorimeter, to raise the temperature of the solution by 6.9 °C.
The heat absorbed by the solution, qsolution, is equal to its specific heat, Cs, (4.18 J/g·°C) — just below that of water — multiplied by its total mass, m (100.0 g), and the change in temperature.
The heat of the solution is 2.9 × 103 joules or 2.9 kilojoules. The heat of the reaction, qreaction, has the same value but with an opposite sign. Because the pressure is constant, the enthalpy change is the same as the heat of the reaction.
To find the enthalpy change of the reaction per mole, the enthalpy is divided by the number of moles of hydrochloric acid. The moles of hydrochloric acid are found by multiplying the volume in liters (0.05 L) with the molarity (1 molar). Therefore, negative 2.9 kilojoules divided by 0.050 moles gives negative 58 kilojoules per mole.
Q1: What is the purpose of insulation in a constant pressure calorimeter?
Insulation in a calorimeter prevents heat exchange between the device and its external environment, ensuring that all heat from the reaction is contained within the system. This allows accurate measurement of temperature changes, which directly reflect the enthalpy change of the reaction occurring inside.
Q2: How does temperature change indicate whether a reaction is exothermic or endothermic?
In an exothermic reaction, heat flows from the system into the calorimeter surroundings, causing the temperature to rise. Conversely, in an endothermic reaction, heat flows from the surroundings into the system, causing the temperature to fall. These temperature changes directly indicate the direction and magnitude of enthalpy exothermic and endothermic reactions.
Q3: What are the key components of a coffee cup calorimeter?
A coffee cup calorimeter consists of two nested Styrofoam cups, a thermometer to measure temperature changes, and a stirrer to ensure uniform mixing. A loose-fitting cork lid maintains constant atmospheric pressure while allowing the reaction to proceed in solution with minimal heat loss to the environment.
Q4: How is the heat of reaction calculated from calorimeter measurements?
The heat absorbed by the solution is calculated using the equation q = m × Cs × ΔT, where m is mass, Cs is specific heat capacity, and ΔT is temperature change. The heat of reaction has the same magnitude but opposite sign. At constant pressure, this heat equals the enthalpy change of the reaction.
Q5: Why is the enthalpy change measured at constant pressure in a coffee cup calorimeter?
The loose-fitting lid of a coffee cup calorimeter remains open to the atmosphere, maintaining constant atmospheric pressure throughout the reaction. Under constant pressure conditions, the heat exchanged during the reaction equals the enthalpy change, making this setup ideal for measuring reaction enthalpies in solution-based processes.
Q6: How do you determine the enthalpy change per mole of reactant?
First, calculate the total heat of reaction using q = m × Cs × ΔT. Then divide this heat value by the number of moles of the limiting reactant. For example, if 2.9 kilojoules is released by 0.050 moles of reactant, the enthalpy change per mole is -58 kilojoules per mole.
Q7: What is the relationship between heat absorbed by the solution and heat released by the reaction?
The heat absorbed by the solution and the heat released by the reaction are equal in magnitude but opposite in sign. This relationship, expressed as qrxn = -qsoln, reflects energy conservation: the first law thermodynamics conservation of energy ensures that energy lost by the reaction equals energy gained by the solution.