When matter undergoes a physical or chemical change there is also a transfer of energy. Energy is either transferred from the system to the surroundings or from the surroundings to the system. The system is whatever is being studied or observed and everything else is the surroundings.

HEAT (q)The transfer of energy is described as heat (q) and is measured in calories or joules (J). A calorie is the amount of heat needed to raise 1 gram of water 1 degree Celsius. This is not the same calorie used to describe the energy content in food. Food calories are actually kilocalories equal to 1000 heat calories. A joule is a derived SI unit and 4.184 joules equals 1 calorie. Chemists use both the calorie and the joule to measure heat.

EXOTHERMIC AND ENDOTHERMIC

When energy is transferred from the system to the surroundings the process is called exothermic. If energy is transferred from the surroundings to the system the process is called endothermic. All chemical changes are either endothermic reactions or exothermic reactions. The burning of a match is an exothermic reaction and photosynthesis is an endothermic reaction. However, the transfer of energy in a chemical change does not just happen. Both exothermic and endothermic reactions require a minimum amount of energy to occur. This minimum amount of energy needed is referred to as the activation energy and without it chemical reactions will not happen. It’s important to note that in an endothermic reaction the products contain more energy than the reactants, but in the exothermic reaction the products contain less energy than the reactants.

Physical changes also absorb or release energy. Any phase change in a substance results in an energy transfer. When ice melts it is because the ice absorbed energy from the surroundings and homemade ice cream freezes because it lost energy to the surroundings. But neither case resulted in new substances.

CALCULATING SPECIFIC

HEATA device used to measure changes in heat transfer is called a calorimeter. A calorimeter is an insulated container that contains water and a thermometer to measure the heat released or absorbed during an energy transfer. The amount of heat required to raise one gram of a substance one degree Celsius is called a substance’s specific heat (Cp). Since energy is conserved, a substance’s specific heat can be calculated if the system is insulated.Specific Heat Equation q = m×ΔT×Cp ΔT = Tf – TiA calorimeter and the above equation can be used quantitatively to determine the amount of heat released or absorbed. This same process can also be used to determine the specific heat of an unknown metal. By placing the heated metal in water and measuring the change in the water’s temperature and determining the water’s mass the amount of heat transferred from the metal can be calculated. Then using this data we can calculate the metal’s specific heat. (note: water has a specific heat of 4.184 J/g °C)Example: 40.0 g of water heated from 10.0°C to 30.0°C.Tell whether the above system is exothermic or endothermic. Then calculate the amount of heat released or absorbed.

The system is endothermic. The only way the water could reach 30.0°C is by absorbing energy. Use the equations above to calculate the change in heat.

 
Copyright © 2017 IDC. All Right Reserved.