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Energy Transfer

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One of the simplest ways energy is transferred is as heat. Though energy has many different forms, all energy is measured in units called joules (J). The amount of energy transferred from one sample MUST be equal to the amount of energy received by a second sample. Therefore, the total energy of the two samples remains exactly the same.

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When samples of different temperatures are in contact, energy is transferred from the sample that has the higher temperature to the sample that has the lower temperature. Temperature – a measure of how hot (or cold) something is; specifically, a measure of the average kinetic energy of the particles in an object.

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The temperature of a sample increases as the sample absorbs energy. The temperature of a sample depends on the average kinetic energy of the sample’s particles. The higher the temp of a sample is, the __________ the particles move.

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The temperature increase of a sample also depends on the mass of the sample. For example, a beaker with 100 mL of liquid in it will increase in temperature at about _______ the rate as a sample with only 50 mL of the same liquid.

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Temperature – Heat – The energy transferred between objects that are at different temperatures. Temp is an intensive property, which means that the temperature of a sample does not depend on the amount of the sample. Heat is an ___________ property, which means that the amount of energy transferred as heat in a sample DOES depend on the amount.

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Water in a glass vs. water in a pitcher: They can have the SAME temperature The water in the pitcher can transfer MORE energy as heat to another sample because the water in the pitcher has MORE particles that the water in the glass.

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All matter contains energy. Measuring the total amount of energy present in a sample of matter is impossible, but changes in energy content can be determined. These changes can be determined by measuring the energy that enters or leaves the sample of matter. Enthalpy, which is represented by the symbol H, is the total energy content of a sample.

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Enthalpy – the sum of the internal energy of a system plus the product of the system's volume multiplied by the pressure that the system exerts on its surroundings. If pressure remains constant, the enthalpy increase of a sample of matter equals the energy as heat that is received.

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Think about hot chocolate cooling: As kinetic energy decreases, enthalpy decreases This decrease is observed as a temperature decrease.

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When a substance receives energy in the form of heat, its enthalpy increases and the kinetic energy of the particles that make up the substance increases. The direction in which any particle moves is not related to the direction in which its neighboring particles move. The motions of these particles are RANDOM.

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The Specific Heat Capacity of a pure substance is the energy as heat needed to increase the temp of 1 gram of the substance by 1 K. Specific heat capacity has the symbol C p and the unit J/K*mol (J*k -1 mol -1 ) Specific heat capacity is accurately measured only if no other process, such as a chemical reaction, occurs.

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C p = q / m Δ T Or…. (algebraically manipulating it to solve for “q”) q = mC p Δ T Heat = (amt in grams)(molar heat cap)(change in temp)

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Determine the energy as heat needed to increase the temperature of 10.0 grams of mercury by 27.5 K. The value for C p for Hg is 27.8 J/g*K.

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A 14.5 gram sample of octane (C 8 H 18 ) absorbed 3.5x10 3 J of energy. Calculate the temperature increase of octane if the molar heat capacity of octane is 254.0 J/g*K.

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A 15.75-g piece of iron absorbs 1086.75 joules of heat energy, and its temperature changes from 25°C to 175°C. Calculate the specific heat capacity of iron.

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How many joules of heat are needed to raise the temperature of 10.0 g of aluminum from 22°C to 55°C, if the specific heat of aluminum is 0.90 J/g°C?

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Calculate the specific heat capacity of a piece of wood if 1500.0 g of the wood absorbs 67,500 joules of heat, and its temperature changes from 32°C to 57°C.

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Vocab Sheet and Section 1: Energy Transfer worksheet is due Friday!

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