6.8
Calorimétrie à pression constante
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.