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$$$ Review $$$ Thermochemistry. Gives off heat (emits) exothermic.

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Presentation on theme: "$$$ Review $$$ Thermochemistry. Gives off heat (emits) exothermic."— Presentation transcript:

1 $$$ Review $$$ Thermochemistry

2 Gives off heat (emits) exothermic

3 Absorbs heat endothermic

4 It flows from hot to cold objects and is known by the letter “q” heat

5 The study of energy changes that occur during chemical reactions thermochemistry

6 What is a calorie or joule? Unit of heat (q)

7 Defined as the amount of heat needed to increase the temperature of an object by 1 o C. Heat capacity

8 Defined as the amount of heat needed to increase 1 gram of an object by 1 o C. Specific heat or specific heat capacity

9 The formula for q? q = C x m x  T

10 Solve the previous equation for the other three variables. C = q/m  T m = q/C  T  T = q/Cm

11 A balance bar has 200 Calories. How many kilojoules is this? How many joules is this? 836.8 kj 836,800 joules

12 True or False. cal/ o C is an acceptable unit for specific heat. False, cal/g o C

13 True or False. Metals generally have a higher specific heat capacity than water. False

14 True or False. Metals generally have a higher specific heat capacity than molecular compounds. False

15 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

16 During a phase change, the temperature of a substance ________. Remains constant

17 A calorimeter can effectively measure the heat of another substance because of the ____________? Law of Conservation of Energy

18  H of fusion involves which phase change? Melting

19  H of solidification involves which more commonly known phase change? Freezing

20 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

21 The heat content of a system at a constant pressure is known as the ________ of that system. enthalpy

22 What is the enthalpy change in a chemical reaction known as? Heat of reaction,  H

23 If the  H of a reaction is negative then the reaction is _______. exothermic

24 The heat of the reaction for the complete burning of one mole of a substance. Heat of combustion

25 The enthalpy change when a mole of solute is dissolved in a solvent. Heat of solution

26 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.

27 What makes Hess’s law useful? It allows you to determine heats of reactions indirectly

28 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 )

29 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

30 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)

31 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

32 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

33 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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