CHAPTER 20 Chemical Reactions and Energy; Basic Thermodynamics - PowerPoint Presentation

Slide1

CHAPTER 20

Chemical Reactions and Energy; Basic Thermodynamics

Section 20.1

Slide2

Objectives ~

Compare and contrast exothermic and endothermic chemical reactions

Analyze the energy diagrams for typical chemical reactions

Illustrate the meaning of entropy and trace its role in various processes

Slide3

What is heat?

Energy that is transferred from an object at high temperature to an object at lower temperature.

Measured in Joules (J

) -

1 kilojoule (kJ) =

1000J

MOLE

= A unit of measure in a chemical reaction that contains the same number of particles

Slide4

Endothermic and Exothermic Reactions

Energy

changes that occur in a chemical reaction are a result of the bonds that are broken (reactants) and the bonds that are formed (products)

Slide5

What happens in an Exothermic reaction?

If more energy is released in forming new bonds than is required to break bonds, the reaction is

exothermic

If reactants go from higher energy to products at lower energy = exothermic

Slide6

What happens in an Exothermic reaction?

Slide7

What happens in an Endothermic reaction?

In an endothermic reaction, the energy required to break the bonds of the reactants is greater than the energy released by forming the bonds of the products.

If reactants go from lower energy to products at higher energy =

endothermic

Slide8

What happens in an Endothermic reaction?

Slide9

ENTHALPY ∆H

Symbolizes the difference in the energy between the products and

reactants

At constant pressure, the enthalpy of a reaction or system is the same as the heat that is gained or

lost

Exothermic = -∆

H

Endothermic = +∆

H

Δ

H

reaction

=

Δ

H

products

-

Δ

H

reactants

Slide10

Law of Conservation of Energy

Is…

Slide11

Slide12

Example

2H

2

(g) +

O

2

(g) →

2H

2

O

(l)

∆H = -572 kJ

In the synthesis of water, 2 moles of hydrogen gas react with 1 mole of oxygen gas to produce 2 moles of liquid water. The reaction is exothermic and produces 572 kJ of energy for every 2 moles water produced.

What would the enthalpy of the decomposition be?

What is the enthalpy value for 1 mole of water produced?

Slide13

What does decomposition look like?

2H

2

O

(l)



2H

2

(g) +

O

2

(g)

∆H = +572 kJ

If only 1 mole of water produced?

572kJ

= 2 x 286 kJ

286kJ

Slide14

http://education-portal.com/academy/lesson/endothermic-and-exothermic-reactions.html#lesson

Slide15

Activation Energy

the amount of energy that particles in a reaction must have when they collide in order for the reaction to occur. Figures 20.4 and

20.5

https

://

www.youtube.com/watch?v=OttRV5ykP7A

Slide16

Activation Energy

Slide17

Catalyst

Recall

that a catalyst will speed up the rate of a reaction without being

consumed

this occurs because the presence of a catalyst lowers the activation energy of a reaction.

Slide18

Entropy

∆S a measure of

disorder

The

natural tendency of entropy is to increase. MORE disorder is more favorable; Entropy is affected by temperature

+ ∆S means increasing disorder and is favorable

-∆S means increasing order and is not favorable

Slide19

Spontaneity

Once

activated, a reaction that uses some of its released energy to continue is considered spontaneous

When one considers ∆S and ∆H together, we know whether a reaction will be spontaneous or

not

Spontaneity depends on the balance between energy and entropy factors.

Slide20

Example

An exothermic reaction with increasing disorder is always spontaneous, BUT an endothermic reaction with increasing order is never spontaneous. (table 20.1 page 714)

Are reactions that occur spontaneously at room temperature generally

exo

or endothermic?

Exothermic - Remember the tendency is to change from a state of high energy to a state of low energy.

http://education-portal.com/academy/lesson/second-law-of-thermodynamics-entropy-and-systems.html#lesson

Slide21

Entropy – are these an increase or decrease of entropy?

Water in an ice-cube tray freezing?

You pick up scattered trash along a highway and pack it in a bag.

Your campfire burns, leaving gray ashes.

A cube of sugar dissolves in a cup of tea.

Slide22

Answers

Decrease

Decrease

Increase

Increase

Slide23

Which has greater entropy, a tablespoon of dry salt or a tablespoon of salt dissolved in water? Explain.

The salt dissolved in water has greater entropy because it has a less orderly structure.

Slide24

Assuming that temperature does not change, would there be an increase or decrease in entropy in the following reaction:

SO

2(g)

+ H

2

O

(l)

H

2

SO

4(

aq

)