Calorimetry - Review Sheet | Principles of Chemistry II | CH 302, Study notes of Chemistry

Download Calorimetry - Review Sheet | Principles of Chemistry II | CH 302 and more Study notes Chemistry in PDF only on Docsity! Calorimetry Concepts The whole point of calorimetry is to “trap” the heat that is entering/exiting the system and get a quantitative measure of it (how many joules?). Water is an excellent absorber of heat. It has one of the highest specific heats (Cs) there is at 4.184 J/g˚C. Unfortunately water must be in a container and we must therefore account for the heat entering/exiting the container (the hardware) as well as the water. You can treat the water and the container as one whole unit – the calorimeter. As a unit the whole calorimeter (water + hardware) will have a definite heat capacity, Ccal which will have units of J/˚C or kJ/K. Note that there is only energy per degree temperature for units for a plain heat capacity. All heat capacities have that for units and all heat capacities are extensive properties – meaning the bigger the calorimeter, the bigger the heat capacity will be. Heat capacities of calorimeters must be measured. You calibrate your calorimeter before you use it. How? Put a know amount of heat (qcal) into it via a standard reaction of some sort or a precise electric heater (see Calibration later in this review). Then measure the temperature response, ∆T. The heat capacity of the entire calorimeter will simply be Ccal = qcal/∆T Now you can turn that equation around and use Ccal and ∆T to get qcal. The heat from the chemical reaction you are studying (qrxn) is equal to but opposite in sign from the value of qcal. That is qrxn = -qcal Bomb Calorimetry and ∆U system adiabatic walls water ∆T qcal bomb calorimeter Bomb calorimetry is conducted under constant volume (isochoric) conditions. This is shown explicitly by writing qv for the heat term. The subscript “v” means conducted at constant volume. Because the volume is held constant and ∆V is forced to be zero, no expansion work can be done throughout the process (w = 0). This means that all heat flow (qv) is now equal to the internal energy change, ∆U. The calculations are as follows for the heat flow in/out of the calorimeter. (surroundings) qcal = Ccal∆T = - qv (system) Where Ccal is the heat capacity of the entire calorimeter (both water and hardware). A bomb calorimeter has lots of hardware (stirring blade, stainless steel reaction chamber, walls of container, etc…) and the heat capacity of just the hardware can be significant. It is a common practice to split up the overall calorimeter heat capacity into two components, the water part and the hardware part. This leads to the following equation. qcal = ( Cwater + Chardware)∆T The water part of this can be further split into the specific heat of water and its mass. Cwater = Cs,water · mwater which leads to qcal = mwater Cs,water ∆T + Chardware ∆T The specific heat for water (an intensive property) times the mass of the water (extensive property) equals the heat capacity of the water (an extensive property). Always check and see what you are given in a problem. Some give you the overall heat capacity for the whole calorimeter (water and hardware). Other problems will give you the mass of the water and the heat capacity of just the hardware of the calorimeter. Coffee-Cup Calorimetry and ∆H styrofoam cup ∆T system is dissolved in the water coffe-cup calorimeter Coffee cup calorimetry is conducted under constant pressure (isobaric) conditions. This means that work is allowed to be done if necessary and the heat flow (qp) is equal to the enthalpy change, ∆H. As with all calorimetry, qcal = Ccal∆T where Ccal is the heat capacity of the entire calorimeter (both water and hardware). However, since the “hardware” of a coffee cup calorimeter is really just a Styrofoam cup, you can assume the hardware heat Dr. McCord Calorimetry © 2007 McCord