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$$$ Review $$$ Thermochemistry. Gives off heat (emits). exothermic. $1. Absorbs heat. endothermic. $1. It flows from hot to cold objects and is known by the letter “q”. heat. $1. The study of energy changes that occur during chemical reactions. thermochemistry. $1.
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$$$ Review $$$ Thermochemistry
Gives off heat (emits) exothermic $1
Absorbs heat endothermic $1
It flows from hot to cold objects and is known by the letter “q” heat $1
The study of energy changes that occur during chemical reactions thermochemistry $1
What is a calorie or joule? Unit of heat (q) $1
Defined as the amount of heat needed to increase the temperature of an object by 1oC. Heat capacity $2
Defined as the amount of heat needed to increase 1 gram of an object by 1oC. Specific heat or specific heat capacity $1
The formula for q? q = C x m x DT $2
Solve the previous equation for the other three variables. C = q/mDT m = q/CDT DT = q/Cm $3
A balance bar has 200 Calories. How many kilojoules is this? How many joules is this? 836.8 kj 836,800 joules $3
True or False. cal/oC is an acceptable unit for specific heat. False, cal/goC $1
True or False. Metals generally have a higher specific heat capacity than water. False $1
True or False. Metals generally have a higher specific heat capacity than molecular compounds. False $1
You measure 1200 joules of heat during a 30oC temperature change with a substance that weighs 100 g. What is the specific heat of the substance. 0.40 J/goC $3
During a phase change, the temperature of a substance ________. Remains constant $1
A calorimeter can effectively measure the heat of another substance because of the ____________? Law of Conservation of Energy $2
DH of fusion involves which phase change? Melting $1
DH of solidification involves which more commonly known phase change? Freezing $1
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 $2
The heat content of a system at a constant pressure is known as the ________ of that system. enthalpy $2
What is the enthalpy change in a chemical reaction known as? Heat of reaction, DH $2
If the DH of a reaction is negative then the reaction is _______ . exothermic $1
The heat of the reaction for the complete burning of one mole of a substance. Heat of combustion $1
The enthalpy change when a mole of solute is dissolved in a solvent. Heat of solution $1
True or False. The quantity of heat absorbed when a solid melts is the same as the quantity released when the substance freezes. True. DHfus = -DHsolid. $1
What makes Hess’s law useful? It allows you to determine heats of reactions indirectly $2
The change in enthalpy that involves the formation of one mole of a compound from its elements (at 25oC) is known as? Standard heat of formation, (DHfo) $2
True or False. The standard heat of a reaction can be calculated by the following equation … DHo = DHfo(products) - DHfo(reactants) True $1
How to solve a phase change problem You just multiply DH of fusion/vaporization by mass Water is vaporizing DHvap= 2260 J/g of H2O q = m xDHvap Water is melting DHfus = 334 J/g of H2O q = m xDHfus (J)
Heating/Cooling Curve: No Phase Changes What happens at A, C, and E?? We use our old formula q = m x C xDT q = m x Cgas x DT q = m x Cliq x DT q = m x Csolid x DT (J)
Heating/Cooling Curve: Putting it all Together 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 DHvap q = m x Cgas x DT q = m x Cliq x DT q = m x DHfus q = m x Csolid x DT (J)
Sample Problem You have a 4.30 grams of ice at -13.2oC. You heat it until it completely vaporizes. How much heat was needed to complete this process? Here are some numbers you might need. (Cice = 2.10 J/goC) (Cwater = 4.18 J/goC) (Csteam = 1.70 J/goC) (DHfus = 334 J/g) (DHvap = 2260 J/g) To get answer you simply add these 4 numbers together: 13071 J or 13.1 kJ (3 sig. figs) q = m x Cice x DT (4.30)(2.10)(0.00 - -13.2)= 119.2 J q = m x DHfus (4.30)(334)= 1436.2 J q = m x Cwat x DT (4.30)(4.18)(100. - 0.00)= 1797.4 J q = m x DHvap = (4.30)(2260)= 9718 J