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$$$ Review $$$ Thermochemistry
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Gives off heat (emits) exothermic
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Absorbs heat endothermic
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It flows from hot to cold objects and is known by the letter “q” heat
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The study of energy changes that occur during chemical reactions thermochemistry
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What is a calorie or joule? Unit of heat (q)
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Defined as the amount of heat needed to increase the temperature of an object by 1 o C. Heat capacity
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Defined as the amount of heat needed to increase 1 gram of an object by 1 o C. Specific heat or specific heat capacity
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The formula for q? q = C x m x T
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Solve the previous equation for the other three variables. C = q/m T m = q/C T T = q/Cm
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A balance bar has 200 Calories. How many kilojoules is this? How many joules is this? 836.8 kj 836,800 joules
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True or False. cal/ o C is an acceptable unit for specific heat. False, cal/g o C
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True or False. Metals generally have a higher specific heat capacity than water. False
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True or False. Metals generally have a higher specific heat capacity than molecular compounds. False
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You measure 1200 joules of heat during a 30 o C temperature change with a substance that weighs 100 g. What is the specific heat of the substance. 0.40 J/g o C
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During a phase change, the temperature of a substance ________. Remains constant
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A calorimeter can effectively measure the heat of another substance because of the ____________? Law of Conservation of Energy
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H of fusion involves which phase change? Melting
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H of solidification involves which more commonly known phase change? Freezing
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If the percent mass of a solution weighing 300 g is 6%, what is the mass of the solute?.06 = x/300g x = 18 g
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The heat content of a system at a constant pressure is known as the ________ of that system. enthalpy
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What is the enthalpy change in a chemical reaction known as? Heat of reaction, H
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If the H of a reaction is negative then the reaction is _______. exothermic
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The heat of the reaction for the complete burning of one mole of a substance. Heat of combustion
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The enthalpy change when a mole of solute is dissolved in a solvent. Heat of solution
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True or False. The quantity of heat absorbed when a solid melts is the same as the quantity released when the substance freezes. True. H fus = - H solid.
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What makes Hess’s law useful? It allows you to determine heats of reactions indirectly
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The change in enthalpy that involves the formation of one mole of a compound from its elements (at 25 o C) is known as? Standard heat of formation, ( H f o )
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True or False. The standard heat of a reaction can be calculated by the following equation … H o = H f o (products) - H f o (reactants) True
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How to solve a phase change problem You just multiply H of fusion/vaporization by mass Water is vaporizing H vap = 2260 J/g of H 2 O q = m x H vap Water is melting H fus = 334 J/g of H 2 O q = m x H fus (J)
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Heating/Cooling Curve: No Phase Changes What happens at A, C, and E?? (J) We use our old formula q = m x C x T q = m x C liq x T q = m x C solid x T q = m x C gas x T
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Heating/Cooling Curve: Putting it all Together (J) You should be able to calculate the total heat going all the way from heating a substance from its solid to its gas q = m x C liq x T q = m x C solid x T q = m x C gas x T q = m x H fus q = m x H vap
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Sample Problem You have a 4.30 grams of ice at -13.2 o C. You heat it until it completely vaporizes. How much heat was needed to complete this process? Here are some numbers you might need. (C ice = 2.10 J/g o C) (C water = 4.18 J/g o C) (C steam = 1.70 J/g o C) ( H fus = 334 J/g) ( H vap = 2260 J/g) q = m x C ice x T (4.30)(2.10)(0.00 - - 13.2)= 119.2 J q = m x H fus (4.30)(334)= 1436.2 J q = m x C wat x T (4.30)(4.18)(100. - 0.00)= 1797.4 J q = m x H vap = (4.30)(2260)= 9718 J To get answer you simply add these 4 numbers together: 13071 J or 13.1 kJ (3 sig. figs)
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