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Unit 5 ENERGY 5.4 What are endothermic and exothermic reactions? April 1, 2011 AIM: How do we identify a substance to be exothermic or endothermic? DO.

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Presentation on theme: "Unit 5 ENERGY 5.4 What are endothermic and exothermic reactions? April 1, 2011 AIM: How do we identify a substance to be exothermic or endothermic? DO."— Presentation transcript:

1 Unit 5 ENERGY 5.4 What are endothermic and exothermic reactions? April 1, 2011 AIM: How do we identify a substance to be exothermic or endothermic? DO NOW: q=(10g)(4.18J/g o C)(-10 o C) = -418J q = mcΔT 1. What much heat is released when 10 g of water are cooled from 20˚C to 10˚C? 2. What much heat is absorbed when 10 g of water are heated from 10˚C to 20˚C? 3. What is the difference between these two problems above? q=(10g)(4.18J/g o C)(10 o C) = 418J One has negative heat and the other has positive heat

2 REVIEW Heat of fusion and Heat of vaporization Explain the following in a coherent, well-written paragraph 1. Explain why we use the q=H f and q=H v 2. How we decide whether to use constant for the heat of fusion OR the heat of vaporization 3. Explain why we cant use Table B for any other substance besides water. 4. Explain why temperature does not change during a phase change but heat is required for the phase change.

3 ENDOTHERMIC -take in energy (heat) -heat is absorbed -heat is a positive number -ENDOTHERMIC REACTION -SAMPLE IS HEATED melting: vaporization: sublimation: solid gas liquid gas (boiling water) solid liquid (ice cream melting)

4 EXOTHERMIC -release energy (heat) -heat exits -heat is a negative number -EXOTHERMIC REACTION -SAMPLE IS COOLED freezing : condensation : deposition: liquid solid (making ice cubes) v gas liquid (rain) gas solid

5 Lets try some 1. What is the total number of joules of heat energy are absorbed when 100g of water are heated from 20˚C to 30˚C? 2. pick out important information how much stuff m 100g of water temperature change ΔT30 o C to 20 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

6 3. plug in q = mcΔT q = (100g) x (4.18J) x g o C (30 o C -20 o C) q = 418J x o C (10 o C) q = 4180J Positive so: ENDOTHERMIC

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


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