Catalyst 2/ 24/ 12

1. Objectives • IWBAT identify the characteristics of exothermic and endothermic • IWBAT interpret a reaction energy diagram • Agenda • Catalyst • Intro to Heat • Endo/Exo Reaction • Practice • Homework: • none Catalyst 2/24/12 • If you wrap a fur coat around a thermometer, will its temperature rise? Explain why or why not. • If you hold one end of a metal nail against a piece of ice, your finger soon becomes cold. Does cold flow from the ice to your hand? Explain.

2. Thermochemistry

3. Heat?Houstonians Know All About It! Today we will: Learn what chemists mean by the term “heat” Learn what chemists mean by enthalpy change Understand what is meant by “systems” and “surroundings” Identify endothermic or exothermic reactions through temperature changes; Graph energy change vs. time Understand the meaning of Q positive or negative;

4. Essential Definitions Heat Change: (ΔQ) - The transfer of energy from one system to another due to a difference in temperature Here we’re talking about kinetic energy – how fast the particles are moving System: the part of the universewe are observing or looking (where the reaction is taking place. Surroundings: everything aroundthe system

5. Vocab: Does an XBOX give off heat? What do we call that? System: the thing we are studying (usually the reaction we are doing) Surrounding:whatever is touching the system So what is happening to the temperature of the air? What does that mean for the speed of the particles? Heat (Q) So if the XBOX is the system, what are its surroundings? The air

6. Essential Definitions Heat Change: (ΔQ) - The transfer of energy from one system to another due to a difference in temperature System: the part of the universewe are observing or looking (where the reaction is taking place. Surroundings: everything aroundthe system Temperature: a direct measure of kinetic energy (speed of particles)

7. In chemistry we talk about exothermic and endothermic reactions. What does the prefix exo sound like to you? What does the prefix endo sound like to you?

8. What do endo and exo mean? (top of your organizer) Endothermic reactions take heat in. Exothermic reactions releaseheat.

9. Demo Which reaction was exothermic? Which was endothermic? Ammonium nitrate (NH4NO3) + water (H2O) • What does it feel like? • Where is the heat going? (From where to where?) Sodium hydroxide (NaOH) + water (H2O) • What does it feel like? • Where is the heat going? (From where towhere?) What is the system in demo? What are the surroundings?

10. on your organizer try and work with a partner to write a definition for exothermic and endothermic reactions using the terms system and surroundings also draw yourself a picture to help remember exothermic and endothermic

11. Endothermic Exothermic System ___ΔH System ___ΔH Surroundings Surroundings Heat flows from surroundings to system Heat flows from system to surroundings

12. What is your system? The cup of coffee What are your surroundings? The air Is heat entering or exiting your system? Exiting What is that called? Exothermic

13. What is your system? The kettle What are your surroundings? The burner Is heat entering or exiting your system? Entering What is that called? Endothermic

14. Surroundings But what if YOU are the system? System Are you endothermic or exothermic? Exothermic (-H)

15. ….but what about a real reaction?

16. Essential Definitions Heat Change: (ΔQ) - The transfer of energy from one system to another due to a difference in temperature System: the part of the universewe are observing or looking (where the reaction is taking place. Surroundings: everything aroundthe system Temperature: a direct measure of kinetic energy (speed of particles) Enthalpy change (ΔH): used when we talk about chemical reactions: same as heat – except at constant pressure

17. Endothermic Exothermic System ___ΔH System ___ΔH Surroundings Surroundings Heat flows from surroundings to system Heat flows from system to surroundings

18. Characteristics Endothermic Exothermic An exothermic reaction feels hot because heat is released. An endothermic reaction feels cold because heat is absorbed. Think/Pair/Share: If in an endothermic reaction heat is absorbed, do you think heat will be a reactant or a product? NH4NO3(s) + heat/energy  NH4++ NO3- NaOH(s)  Na+ + OH- + heat/energy In an exothermic reaction as shown above, heat is a product. In an endothermic reaction as shown above, heat is a reactant.

19. Notation Endothermic Exothermic A + B + 50 kJ  C + D OR A+ B  C + D ΔH = +50 kJ/mol A + B +  C + D + 50kJ OR A+ B  C + D ΔH = -50 kJ/mol

20. Check for understanding! H2O(l)  →  H2O(g)                  ΔH = 44 kJ Is this reaction endothermic or exo? Should heat be on products or reactants side? H2O(l) + 44kJ   →  H2O(g)  2P(s)  +  3Cl 2(g)   →  2PCl3(g) +574 kJ Is thisreaction exo or endothermic? Will itsΔH = +574 kJ or -574 kJ? Endo Exo • -574 kJ?

21. Energy Diagrams • Like everything else in chemistry, we LOVE graphs! • Which one do you think represents an exothermic reaction and which one is endothermic? Think in terms of whether ΔH • is positive or negative.

22. Energy Diagrams Endothermic Exothermic If products have more heat than reactants, was heat released or absorbed? If products have less heat than reactants, was heat released or absorbed? Products Reactants Energy (kJ/mol) Energy (kJ/mol) • +ΔH • -ΔH Reactants Products Reaction Progress (time) Reaction Progress (time)

23. what’s that hump at the beginning? What do you need to start the engine?

24. Side note… what’s that hump about? Which graph represents the combustion reaction in the engine? Endothermic Exothermic Products • Activation Energy Reactants • Activation Energy Energy (kJ/mol) Energy (kJ/mol) • +ΔH • -ΔH Reactants Products Reaction Progress (time) Reaction Progress (time)