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?