Chapter 3 – Energy Transfer Thermodynamics Chapter 3 – Energy Transfer
Difference Between Heat and Temperature The motion of particles generates different forms of energy; the form of energy depends on whether there is order or disorder (random motion). When particle motion is orderly, kinetic energy is transferred between two systems in the form of work; work involves motion in a system under the effect of a force (W=F*d) When particle motion is random, kinetic energy is transferred between two systems in the form of heat; the particles move in all directions.
Difference Between Heat and Temperature Differences between work and heat A piston – the gas enclosed in a cylinder is compressed by the movement of the piston, causing all of the particles to move in the same direction. The piston generates work when force is applied. A lamp will warm the area surrounding it; if a chocolate bar is place underneath it, it will begin to melt.
Difference Between Heat and Temperature Heat is a transfer of thermal energy. In order for kinetic energy, in the form of heat, to transfer from one system to another, there needs to be a temperature difference between the two systems. Temperature is a measurement of the agitation of the particles in a system. The thermal agitation of particles is weak at low temperatures and strong at high temperatures. The Celsius scale is most commonly used to measure temperature. Absolute zero, 0K or -273C, is the temperature at which all molecules stop moving.
Difference Between Heat and Temperature When two bodies at different temperatures come into contact with one another, the agitation of the particles is transmitted from one to the other; the transfer occurs from the hotter body to the cooler body. If you have a cup of hot water and a cup of cold water and add them together, the heat transfer continues until the liquids both reach a temperature at which their molecules have the same thermal agitation. This temperature is called the equilibrium temperature. The equilibrium temperature depends on the nature and the quantity of the two liquids. An isolated system is one that does not permit heat exchange between the system and the surroundings. A closed system does not necessarily mean that it is isolated.
The Law of Conservation of Energy Lavoisier – “Nothing is lost, nothing is created, everything is transformed.” As applied to energy – energy can neither be created nor destroyed, but only changed from one form to another. Examples of energy transformation: Electric kettle – electrical energy thermal energy Light bulb – electrical energy light and thermal energy Photosynthesis – light energy chemical energy Washing machine – electrical energy mechanical and thermal energy
The Law of Conservation of Energy The principle of conservation of energy depends on the system, the location being observed, separated from its surrounding by an imaginary wall through which matter and energy are exchanged. Open system – the system exchanges matter and energy with the surroundings. Closed system – the system may exchange energy with the surroundings, but matter will not be exchanged between the system and the surroundings. Isolated system – no exchange of matter or energy will occur between the system and the surroundings.
The Law of Conservation of Energy A calorimeter is used to experimentally determine the heats of reaction. A calorimeter is used to measure the initial (before a chemical reaction occurs) and final (after a chemical reaction occurs) temperature of the water. If a chemical reaction releases heat, the temperature of the water will rise. If a chemical reaction requires heat, the temperature of the water will fall. Components of a calorimeter: isolation, thermometer, reaction vessel (barrel), stirring rod, water. A chemical reaction takes place in the barrel of the calorimeter, which is surrounded by water. This is a closed sub-system because matter is not exchanged, but there is a heat exchange between the chemical reaction and the water, all this is taking place within the isolated system of the calorimeter.
Thermal Energy Thermal energy depends on the agitation of the particles of a substance. When a substance experiences an increase in temperature it is absorbing thermal energy in the form of heat. When a substance experiences a decrease in temperature it is absorbing thermal energy in the form of heat. Thermal energy depends on: The mass of the substance The temperature change observed The nature of the substance
Thermal Energy The thermal energy of a substance is proportional to its mass. The amount of heat that is needed to increase the temperature of 1g of substance is double if there are 2g of the substance. The thermal energy of a substance is proportional to its temperature change. The amount of heat needed to change the temperature by 1C will be doubled if we wish to change the temperature by 2C (double).
Thermal Energy Would the same quantity of heat be required to raise the temperature of a mass of water and the same mass of oil by 10C?
Thermal Energy Answer: No, oil takes less time than water to reach the same temperature; this is because their specific heat capacities are not the same. The specific heat capacity of oil is 2.00 J/gC. The specific heat capacity of water is 4.18 J/gC.
Specific Heat Capacity The specific heat capacity of a substance is the quantity of energy required to increase the temperature of 1g of substance by 1C. This is a characteristic property and it is expressed in J/gC. Measures the difficulty with which the temperature of a substance can increase or decrease. The higher the specific heat capacity of a substance, the harder it is to change its temperature.
Specific Heat Capacity Depends on: The nature of the atoms and chemical bonds of the substance. The size and mass of the substance. The molecular structure and attractions between the molecules. The specific heat capacity of a substance varies as a function of temperature.
Specific Heat Capacity Lets take a look at water: Liquid: 4.18 J/gC Vapour: 1.41 J/gC Solid: 2.05 J/gC Water has its highest specific heat capacity in the liquid form. This shows that the specific heat capacity of a substance is in fact dependent on the state of the substance or the arrangement of the molecules in the substance. Metals generally have lower specific heat capacities, which explains why they are good conductors of heat; it does not take long for metals to heat up and they cool quickly too!
Thermal Energy As demonstrated, the thermal energy (Q) of a substance is proportional to: Mass, m Change in temperature, T Specific heat capacity, c We can therefore represent the relationship in the following equation: When Q is positive, the substance absorbed energy. When Q is negative, the substance released energy.
Thermal Energy Remember: the law of conservation of energy Energy is not created or destroyed, only transferred, therefore: The thermal energy that is released by a substance is absorbed by another substance.
Calculating Energy Transfer Thermal energy can be transferred from one system to another. An isolated system is one that does not allow matter or energy to be exchanged with the surroundings. Thermal energy may be calculated using Q=mcT. When two systems with different temperatures are put into contact with one another thermal energy will transfer from the hotter substance to the cooler substance until equilibrium is obtained.
Calculating Energy Transfer If there is an exchange of thermal energy inside an isolated system, the quantity of heat supplied by one system (-Q1) is equal to the quantity of heat absorbed by the second system (Q2): -Q1 = Q2 In other terms: -m1c1T1 = m2c2T2 Note: both substances will have the same final temperatures, which will be found between the initial temperatures of either substance, but their initial temperatures will differ.