Post on 05-Jan-2016
description
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.
Objectives- IWBAT identify
the characteristics of exothermic and endothermic
- IWBAT interpret a reaction energy diagram
Agenda
1. Catalyst2. Intro to Heat3. Endo/Exo
Reaction4. Practice
Homework:none
Thermochemistry
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;
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 universe we are observing or looking (where the reaction is taking place.
Surroundings: everything around the system
Here we’re talking about kinetic energy – how fast the particles are moving
System: the thing we are studying (usually the reaction we are doing)
Surrounding: whatever is touching the system
Vocab:Does an XBOX give off heat? What do
we call that?
So if the XBOX is the system, what are its surroundings?
The air
Heat (Q)
So what is happening to the temperature of the air? What does that mean for the speed of the particles?
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 universe we are observing or looking (where the reaction is taking place.
Surroundings: everything around the system
Temperature: a direct measure of kinetic energy (speed of particles)
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?
What do endo and exo mean?(top of your organizer)
Exothermic reactions release heat.
Endothermic reactions take heat in.
DemoAmmonium 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 to where?)
What is the system in demo? What are the surroundings?
Which reaction was exothermic? Which was
endothermic?
on your organizer try and work with a partner to write a definition for exothermic and endothermic reactionsusing the terms system and surroundings
also draw yourself a picture to help remember exothermic and endothermic
Exothermic
System___ΔH
Surroundings
Endothermic
System___ΔH
Surroundings
Heat flows from surroundings to system
Heat flows from system to surroundings
What is your system?
What are your surroundings?
Is heat entering or exiting your system?
What is that called?
The cup of coffee
The air
Exiting
Exothermic
What is your system?
What are your surroundings?
Is heat entering or exiting your system?
What is that called?
The kettle
The burner
Entering
Endothermic
But what if YOU are the system?
System
Surroundings
Exothermic (-H)
Are you endothermic or exothermic?
….but what about a real reaction?
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 universe we are observing or looking (where the reaction is taking place.
Surroundings: everything around the 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
Exothermic
System___ΔH
Surroundings
Endothermic
System___ΔH
Surroundings
Heat flows from surroundings to system
Heat flows from system to surroundings
Characteristics
EndothermicAn endothermic reaction feels cold because heat is absorbed.
ExothermicAn exothermic reaction feels hot because heat is released.
NH4NO3(s) + heat/energy NH4+
+ NO3- NaOH(s) Na+ + OH- + heat/energy
In an endothermic reaction as shown above, heat is a reactant.
In an exothermic reaction as shown above, heat is a product.
Think/Pair/Share: If in an endothermic reaction heat is absorbed,
do you think heat will be a reactant or a product?
NotationEndothermic
A + B + 50 kJ C + D
OR
A+ B C + D ΔH = +50 kJ/mol
ExothermicA + B + C + D + 50kJ
OR
A+ B C + D ΔH = -50 kJ/mol
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 this reaction exo or endothermic?Will its ΔH = +574 kJ or -574 kJ?
Endo
Exo-574 kJ?
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.
Energy Diagrams
Endothermic Exothermic
Reactants
Products Reactants
Products
Reaction Progress (time) Reaction Progress (time)
Ener
gy (k
J/m
ol)
Ener
gy (k
J/m
ol)
-ΔH+ΔH
If products have less heat than reactants, was heat released or
absorbed?
If products have more heat than
reactants, was heat released or absorbed?
what’s that hump at the beginning?
What do you need to start the engine?
Side note… what’s that hump about?
Endothermic Exothermic
Reactants
Products Reactants
Products
Reaction Progress (time) Reaction Progress (time)
Ener
gy (k
J/m
ol)
Ener
gy (k
J/m
ol)
-ΔH+ΔH
Activation EnergyActivation Energy
Which graph represents the combustion reaction in the engine?