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Brady Chapter07 - Chapter 7 Energy and Chemical Change...

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Chapter 7: Energy and Chemical Change Energy is the ability to do work and supply heat Work is motion against an opposing force Kinetic energy is the energy an object has because of its motion For an object of mass m with velocity v 2 2 1 KE Energy Kinetic mv = =
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The law of conservation of energy states that energy cannot be created or destroyed This can be applied to the collision of two particles with only kinetic energy Potential energy (PE) is the energy of position or internal arrangement Fast moving A transfers energy to B by colliding. The total energy is conserved.
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KE can be converted into PE and vice versa When the child is at points (a) and (c) they have only PE; at point (b) only KE. Total energy is conserved (PE + KE = constant). When fully compressed or extended only PE; at natural length only KE. Total energy is conserved.
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Work is required to pull the negatively charged electron away from the positively charged nucleus The gain and loss of PE can be summarized Pushing or pulling an object against an opposing force requires energy. The objects PE will rise. When the opposing force is not resisted, the object’s PE falls The SI unit of energy is the joule ( J )
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A 2 kg object moving at 1 meter per second has 1 J of kinetic energy You may also encounter the calorie ( cal ) The dietary Calorie (note capital), Cal , is actually 1 kilocalorie When a cold and hot object come into contact, they eventually reach thermal equilibrium (the same temperature) ( 29 (exactly) J 4.184 cal 1 s m kg 1 kg) (2 J 1 -2 2 2 s 1 m 1 2 1 = = =
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The energy that is transferred as heat comes from the object’s internal energy The energy associated with the motion of the object’s molecules is referred to as its molecular kinetic energy The internal energy is often given the symbol E or U We are interested in the change in E : reactants products initial final or - E E E - E E E = =
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The internal energy change is positive if the system absorbs energy from the surroundings and negative if it releases energy to its surroundings The temperature of an object is related to the average kinetic energy of its atoms and molecules The temperature for curve (1) is lower than for curve (2) because the average kinetic energy is lower.
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Heat is a transfer of energy due to a temperature difference a) Isolated warm (left) and cold (right) objects b) Thermal contact is made: thermal energy is transferred from left to right c) Thermal equilibrium: the same average KE for molecules in both objects
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The energy of an object depends only on its current condition The current condition is called the state Internal energy is a state function because it is a measure of energy An important property of state functions is that they are independent from the mechanism or method by which a change occurred
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The object we are interested in is called the system
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