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Unit 1 ENERGY 1.2 How do you calculate heat when there is a tempΔ? March 23, 2010 Objective 1: SWBAT calculate the amount of heat required to change the.

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Presentation on theme: "Unit 1 ENERGY 1.2 How do you calculate heat when there is a tempΔ? March 23, 2010 Objective 1: SWBAT calculate the amount of heat required to change the."— Presentation transcript:

1 Unit 1 ENERGY 1.2 How do you calculate heat when there is a tempΔ? March 23, 2010 Objective 1: SWBAT calculate the amount of heat required to change the temperature of a substance. DO NOW: Which line segment represents an increase in potential energy and no change in average kinetic energy? The graph below represents the uniform heating of a substance, starting with the substance as a solid below its melting point.

2 The amount of heat given off or absorbed in a reaction can be calculated using the following equation: q = mcΔT q = heat (joules) m = mass of the substance (grams) c = specific heat capacity of substance (J/g· ºC)  Table B for water ΔT = (temperature final – temperature initial )

3 When to use it? Whenever a substance undergoes a temperature change (TempΔ), the equation q = mcΔT can be used to calculate the heat involved.

4 Let’s try some 1. How many joules are absorbed when 50.0g of water are heated from 30.2ºC to 58.6ºC when the specific heat of water is 4.18 J/g· ºC? 2. pick out important information how much stuff  m 50g of water temperature change  ΔT30.2 o C to 58.6 o C Specific heat of water  c 4.18 J/g o C q = mcΔT 1. asking me to find out HOW MANY J  q

5 3. plug in q = mcΔT q = (50g) x (4.18J) x g o C (58.6 o C -30.2 o C) q = 209J x o C (28.4 o C) q = 5935.6J

6 FACT The equation q = mcΔT cannot be used to determine the amount of heat required to melt or boil a substance. Why? Temperature remains constant during a phase change, so there is no ΔT and the equation cannot be used

7 Let’s try some 2. How much heat is released when 33 grams of water is cooled from 85  C to 60  C? 2. pick out important information how much stuff  m 33g of water temperature change  ΔT85 o C to 60 o C What else do I need to know? TABLE B q = mcΔT 1. asking me to find out HOW MUCH HEAT  q

8 TABLE B

9 3. plug in q = mcΔT q = (33g) x (4.18J) x g o C (60 o C - 85 o C) q = 137.94J x o C (-25 o C) q = -3448.5J FOOD FOR THOUGHT: If it is cooling, is it exothermic or endothermic? EXOTHERMIC

10 INDEPENENT PRACTICE COMPLETE FOR HOMEWORK IF YOU ARE NOT DONE IN CLASS. ASSIGNMENT IS TO BE COMPLETED BY TOMORROW!!!

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