Calorimetry
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
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