# Unit 5 ENERGY 5.4 What are endothermic and exothermic reactions?

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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? 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? DO NOW: q=(10g)(4.18J/g•oC)(-10oC) = -418J q=(10g)(4.18J/g• oC)(10oC) = 418J One has negative heat and the other has positive heat

Heat of fusion and Heat of vaporization
REVIEW Heat of fusion and Heat of vaporization Explain the following in a coherent, well-written paragraph 1. Explain why we use the q=Hf and q=Hv 2. How we decide whether to use constant for the heat of fusion OR the heat of vaporization 3. Explain why we can’t 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.

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

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

Let’s try some q = mcΔT 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? 1. asking me to find out HOW MANY J  q 2. pick out important information 100g of water how much stuff  m temperature change  ΔT 30oC to 20oC Specific heat of water  c 4.18 J/g • oC

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

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

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