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Energy

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Energy – the ability to do work or produce heat Energy exists in two different forms – kinetic energy & potential energy

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Potential Energy Potential energy – energy due to composition or position of an object Potential energy is stored energy that results from the attractions or repulsions of other objects

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Kinetic Energy Kinetic energy – the energy of motion Kinetic energy depends on as objects mass & its velocity Atoms has mass & they are in motion; therefore they will have kinetic energy

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Energy A roller coaster at the top of a hill has a great amount of potential energy. As the rollercoaster begins to speed down the hill, the potential energy is turned into kinetic energy

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Energy The SI unit for energy is the joule (J) 1 J = 1 Kgm 2 / s 2 Another unit of energy that you may be more familiar with is the calorie calorie – amount of energy required to raise 1 g of water 1°C 1 cal = 4.18 J

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Energy The calories that you eat are actually kilocalories or Calories (with a big C) 1000 calories = 1 Kilocalorie = 1 Calorie

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Energy Conversions Convert 15,500 joules into Calories J x 1 cal x 1 Cal = 4.18 J 1000 cal 3.71 Cal

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Formulas – Kinetic Energy Kinetic energy KE = ½ mv 2 KE = kinetic energy (joules) m = mass (must be in Kg) V = velocity (must be in m/s)

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Formulas – Potential Energy Potential Energy PE = mgh PE = Potential Energy (J) m = mass (Kg) g = gravitational constant = 9.8 m/s 2 h = height (m)

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Formulas - Work Work (w) – the energy used to move an object against a force Force (f) – a push or pull on an object W = mgd = fd = PE Work and potential energy can be looked at in the same light

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Work It is important to understand that if there is no movement, there is no work done If I push and push on the demonstration table with all of my might. I may get hot and sweaty and feel like I have done a TON of work, but in reality I have done NO work because the table has not moved

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Examples A bowler lifts a 5.4 kg bowling ball 1.6m and then drops it to the ground. How much work was required to raise the ball? W = mgd W = (5.4 kg)(9.8 m/s 2 )(1.6m) 85 Kgm 2 /s 2 = 85 J

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Examples How much potential energy does that ball have at this height? 85 J

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Examples If the bass is dropped and we assume that all of the potential energy is turned into kinetic energy, at what velocity will the bowling ball hit the ground? KE = PE = 85J m = 5.4 Kg V = ?

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Examples KE = ½ mv 2 85 J = ½ (5.4 Kg) v 2 v 2 = 31.5 v = 5.6 m/s

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More examples What is the kinetic energy of 1 atom of Ar moving at 650 m/s? KE = ½ mv 2 1atom Ar x 1mol Ar x g Ar x 1 Kg Ar = 6.02 x atoms 1 mol Ar 1 x 10 3 g Ar 6.64 x kg Ar

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More Examples KE = ½ mv 2 KE = ½ (6.64 x )(650 2 ) KE = 1.4 x J

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1 st Law of Thermodynamics 1 st Law of Thermodynamics – energy is conserved The law of conservation of energy states that in any chemical reaction or physical process, energy can be converted from one form to another, but it is neither created nor destroyed.

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1 st Law of Thermodynamics Since energy can neither be gained nor lost, the change in E can be calculated using: E = E f – E i In a chemical reaction i indicates reactants and f indicated products

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EE E has 3 parts: 1.A # indicating the magnitude 2.A sign (+/-) indicating the direction 3.A unit

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Thermochemistry Thermochemistry is the study of heat changes that accompany chemical reactions and phase changes. In thermochemistry, the system is the specific part of the universe that contains the reaction or process you wish to study.

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Thermochemistry Everything in the universe other than the system is considered the surroundings. Therefore, the universe is defined as the system plus the surroundings. universe = system + surroundings

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Relating E to heat & work The system can exchange energy with its surroundings in 2 ways: as heat or work E = q + w E = change in energy q = heat w = work

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q & w Don’t forget q & w must have signs In order to get the sign you must look at the system as a box and the surroundings as everything else System Surroundings

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q & w Anything going INTO the box will be + Anything going OUT of the box will be – + -

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q & w If heat is transferred from the surroundings to the system and work is done on the system what are the signs for q & w? q = + w = +

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q & w If heat is lost to the surroundings and work is done on the system what are the signs for q & w? q = - w = +

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Summary for q & w q + = heat into system q - = heat into surroundings w + = work done on the system w - = work done on the surroundings

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Examples A system loses 1150 J of heat to the surroundings and does 480 J of work on the surroundings. Calculate E. E = q + w E = (-1150J) + (-480J) E = J

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Examples A system absorbs 140 J of heat from the surroundings and does 85 J of work on the surroundings. Calculate E. E = q + w E = (+ 140J) + (-85J) E = + 55 J

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Endothermic & Exothermic Endothermic –system absorbs heat –Heat flows into the system –Temperature goes down Exothermic –Heat flows out of the system and into the surroundings –Temperature goes up Only look at heat (q) to determine if the system is endo or exo

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