Endothermic & Exothermic

Chemical Reactions and Their Thermal Properties

Endothermic Reactions

• The system gains energy in the form of heat.• Absorbs E from the

surroundings.• Requires an input of

energy in order to happen (react).SURROUNDINGS ENERGY

Reaction

energy

chloridewater

What Happens• Energy is drawn from the

surroundings (the environment).

• This fuels the start of the reaction.

ENERGY TO FLOWER

(Photosynthesis)

Physical Properties• Cold

energy drawn into the system is taken from its surroundings.

this is why it creates a “cold” feeling.

Chemical Properties

• Hard to identify

• Monitor △T (change in temp.)

• Endothermic reactions will ALWAYS drop in temperature.

(even if only a little)

Ways to Identify A Reaction

Chemical and Physical Properties

Is it cold? Was there a temperature change?

What kind of energy is available?

Using Calculations

The H for the reaction △will be positive

-This means that the system has a positive

energy change.

Examples

• Melting Ice: this is why it feels cold against your skin

1. • It draws heat from your body

2. • This allows the ice to continue melting.

YOU’RE PART OF THE REACTION!

Exothermic Reactions

• The system loses energy in the form of heat.• Loses E to the

surroundings. • Energy of the

surroundings will increase.

ENERGY FROM SUN SURROUNDINGS

What Happens

• Energy is given off to the environment.

• The reaction starts with more E than it ends with.

Physical Properties

Heat energy (warm/hot)

Light energy

Sound energy

All three

ENERG

Y

Chemical Properties

• Easier to identify

• Monitor △T (change in temp.)• System and its surroundings

• Endothermic reactions will ALWAYS drop in temperature.

(even if only a little)

Ways to Identify A Reaction

Chemical and Physical Properties

Is it HOT? Was there a temperature change?

What kind of energy is given off? (heat, light,

sound)

Using Calculations

The H for the reaction △will be negative

-This means that the system has a negative

energy change.

Other Examples

H2O(liquid) H2O(vapor) ∆H=32KJ

Positive ∆H

Energy

Input

Endothermic A positive H-value indicates the system gained energy during

the reaction.

Other Examples

H2O(liquid) H2O(solid) ∆H=-46KJ

Negative ∆H

Energy

output

Exothermic A negative H-value indicates the system

lost energy during the reaction.

Test Your Understanding

Would burning a fire have a positive or negative ∆H

value?• Is it exothermic or

endothermic?

Test Your Understanding

1. Think of a cold glass of lemonade on a hot summer day.

2. Think of the condensation on the outside of the glass…..

• Is this process endothermic or exothermic ?