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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 ?
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