Chapter 6

# Chapter 6 - Thermochemistry is the study of relationship...

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Kinetic Energy, Potential Energy and Thermal Energy: conservation Suppose we throw a stone into the air. The stone will go to a certain height turns around, comes down and hits the ground. As the stone goes up, its velocity (hence kinetic energy) decreases and comes to zero at turn-around point. Then kinetic energy is zero at the turn-around point. What happened to the kinetic energy of stone? As the stone comes down its kinetic energy increases until the point it hits the ground. How did kinetic energy disappear and then come back to the particle? ground turn-around point K. E = 0 maximum P.E P. E = 0 maximum K. E potential energy, PE = Mass (m) x acceleration due to gravity (g) x height (h)= mgh The answer is that the sum of kinetic and potential energy remains constant during the motion. When the stone hits the ground, PE=0, KE=0. All of the energy it had before (KE+PE) is now converted to thermal energy, which means the temperature of the stone raises, (even though the change in temperature may not be felt by your hand). The total energy is conserved. Thermochemistry : is the study of relationship between chemistry and energy.

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Units of Energy : SI unit of energy is Joule, J [1 J= kg.m 2 .s -2 ] To remember this, note that KE= (½) mv 2 . If m= 1kg, v= 1 m/s, then KE= (½)J 1 kJ= kiloJoules= 1000J=10 3 J 1 calorie= 4.184 J (1 calorie= the amount of energy required to raise the temperature of 1 g water by 1 o C. 1 food calorie= Calorie= 1000 cal = 1kcal 1 Watt = 1 J.s -1 ; kilowatt, kW=1000 J.s -1 ; =10 3 J.s -1 ; kilowatt-hour=kWh= 10 3 J.s -1 x3600s =3.6x10 6 J Kilogram standard : (C&E News, May 26, 2008; Page 43) It is currently equal to the mass of a golf-ball-sized cylinder of platinum- iridium alloy. This piece of metal, known as international prototype, is kept in a vault along with six copies at the International Bureau of Weights and Measures near Paris. Additional 90 calibrated copies exist throughout the world. The official US kilogram, K20, is secured in a vault at NIST (Gaithersburg, Maryland). NIST keeps working copies of K20 by which balances are calibrated.
Energy (E) Heat (q) Work (w) Heat : is a means of energy transfer from one system (or body) to another. The function of heat and work are inter-convertible. For example, one can raise the temperature of water either by heating it or by doing mechanical work on it (stirring with a paddle). Both heat and work have units of energy (Joule). Work : is another means of energy transfer from one system (or body) to another . [Temperature is a measure of energy, so it is not same as heat] State functions : If a gas is defined at a certain pressure, volume and temperature then the state of that gas is defined and certain energy is associated with that state. Energy is a state property, so one can say that a system has certain amount of energy. Therefore one can write the change in energy for a system in two different states as E=E 2 -E 1 . This change does not depend on the process used in taking the system from state 2 to state 1.

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Chapter 6 - Thermochemistry is the study of relationship...

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