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 transcript:

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.

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 )

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

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

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

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

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

TABLE B

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

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