Measuring and Using Energy Changes Calorimetry. To measure the heat flow in a process, you need an isolated system You also need a known amount of a substance,

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Presentation transcript:

Measuring and Using Energy Changes Calorimetry

To measure the heat flow in a process, you need an isolated system You also need a known amount of a substance, usually water.  water absorbs the heat that is released by the process, or the water releases heat if the process is endothermic.

Calorimetry Water, a thermometer, and an isolated system are the basic components of a calorimeter. A calorimeter is a device that is used to measure changes in kinetic energy. The technological process of measuring changes in kinetic energy is called calorimetry.

Using a Calorimeter In a coffee-cup calorimeter, a known mass of water is inside the coffee cup. The water surrounds, and is in direct contact with, the process that produces the energy change. The initial temperature of the water is measured. Then the process takes place and the final temperature of the water is measured. This type of calorimeter can measure heat changes during processes such as dissolving, neutralization, heating, and cooling.

Assumptions The system is isolated. (No heat is exchanged with the surroundings outside the calorimeter.) The amount of heat that is exchanged with the calorimeter itself is small enough to be ignored. If something dissolves or reacts in the calorimeter water, the solution still retains the properties of water. (For example, density and specific heat capacity remain the same.)

q system = −q surroundings The system is the chemical or physical process you are studying. The surroundings consist of the water or solution in the calorimeter.

After a reaction has taken place the system and the surroundings will have the same final temperature. A system is said to be at thermal equilibrium when all its components have the same temperature.

Practice Page #1-4

Using a Calorimeter to Determine the Enthalpy of a Reaction A coffee-cup calorimeter is well-suited to determining the enthalpy changes of reactions in dilute aqueous solutions. The water in the calorimeter absorbs (or provides) the energy that is released (or absorbed) by a chemical reaction. When carrying out an experiment in a dilute solution, the solution itself absorbs or releases the energy. You can calculate the amount of energy that is absorbed or released by the solution using q = m c ΔT.

Practice P 667 #5,6,7

Bomb Calorimetry To measure precisely and accurately the enthalpy changes of combustion reactions, chemists use a calorimeter called a bomb calorimeter.

Bomb Calorimetry The bomb calorimeter works on the same general principle as the polystyrene calorimeter. The reaction, however, takes place inside an inner metal chamber, containing pure oxygen, called a “bomb.” The reactants are ignited using an electric coil. A known quantity of water surrounds the bomb and absorbs the energy that is released by the reaction.

Bomb Calorimeter The heat capacity of a calorimeter takes into account the heat that all parts of the calorimeter can lose or gain. C total = C water + C thermometer + C stirrer + C container

Uses of bomb calorimeters Energy of fuels such as coal, oil, gasoline and kerosene are tested by bomb calorimetry. The measure of energy give-off by the fuel is determined by the fuel's heat of combustion. Bomb calorimetry can be used to determine the calorie content of a product. This process is used in food and metabolic studies to examine the effects of energy content in food on humans and animals.