WebLecture: Heat Capacity
Kotz and Triechel, Chemistry and Chemical Reactivity Chapter 5: Sections 1-3.
5.1 Energy is equivalent in physics to work. Work happens when something changes its state of motion as a result of applied force or when something moves. Kinetic energy is this "energy of motion". Potential energy is the capacity to do work. We use movement to determine the amount of work being done and the amount of energy involved. We can measure movement in different ways: temperature tells us something about the average speed of gas molecules, for example. Temperature changes can also tell us which way energy is moving (into or out of a system). If the system is a chemical reaction, temperature changes tell us how much energy was consumed or released in the reaction. We need a way to go from the temperature change to the energy change, which we do by equating heat energy (measured in calories) to energy units like joules.
5.2 Not everything is bound together with the same kinds of bonds. Two different materials may need to absorb different amounts of heat to achieve the same temperature change. The amount of heat necessary to change the temperature of one gram of a particular compound by one degree centigrade is its heat capacity.
5.3 We are going to use "changes of state" to refer to changes in matter structure from solid to liquid to gas. Do not confuse this term with a "state change", which we get to later in this chapter. Here, the main idea is that the energy that would normally change the temperature of a substance goes into changing the organization of the substance. In considering the energy involved in changing a 100g block of ice at -4C to water at +10C, we not only have to consider the heat to raise the temperature 14 degrees Celsius, we also have to figure in the amount of heat required to melt 100g of ice.
A state function is any change (chemical, mechanical, thermal, abstract mathematical) where the way the change occurs is irrelevant, and only the results count. In a physical system where no friction is involved, it doesn't matter whether we slide a box up an inclined plane, run around in circles with it as we change elevation, or use a pulley to pull it straight up: the amount of energy we expend to raise it equals the difference in potential gravitational energy between the low start state and the end high state. [Once we introduce friction, that all changes...and friction is the way we are used to dealing with energy changes of the gravitational type]. In a chemical system, friction-type energy losses are negligible. We don't have to worry about how the chemical reaction occured, only what the start and end points are.
ENERGY MUST BE CONSERVED! In all these situations, the total amount of energy in the system we are discussing must remain constant. Heat can change form to work, or move from one material to another, but the total amount must remain constant
1 calorie = 4.184 Joules
q = CmΔT
where q = the amount of heat gained or lost by
temperature change ΔT = Tfinal - Tinitial
in a mass m of some material
C is the heat capacity of the material, a measure of its ability to absorb or lose heat
Review the Videos at Thinkwell Video Lessons under THERMOCHEMISTRY.
Working with your teacher and classmates, design a calorimeter and propose a procedure to measure physical properties (melting point, boiling point, specific heat) of your calorimetery components and materials. You will use the calorimeter to determine the heat of enthalpy of an exothermic acid-base reaction. You will need to identify the materials required for your calorimeter and describe its assembly and use, and also determine its native heat capacity.
Determine the heat of solution and heat of fusion of a substance.
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