Download Exothermic and Endothermic Reactions Intro

http://www.kentchemistry.com/links/Matter/EndoExo.htm
Energetics Cycle 1 - Part A
Exothermic and Endothermic Reactions Intro
Reference: Higher Level Chemistry, p. 158 - 163
1
Reactions Involve Energy Changes
Physical Changes
energy
absorbed
energy
released
melting
http://commons.wikimedia.org/wiki/File:Melting_ice_cream_on_a_bridge.JPG
condensation
http://wondermode.com/product_images/e/174/condensation-on-glass
Chemical Changes
energy
absorbed
energy
released
photosynthesis
http://ecolocalizer.com/2009/06/01/project-sprout-turns-vacant-lots-into-sunflower-gardens/
oxidation
http://www.skylighter.com/i/fireworks/shells/red-straw-hat.htm
2
Reactions Involve Energy Changes
What happens in a reaction?
reactants
2 H2O
products
2 H2 + O 2
bonds in reactants break
new bonds form in products
energy used
energy released
3
Reactions Involve Energy Changes
Energy changes in a chemical reaction are a balancing act.
reactants
energy used to
break bonds
products
energy released when
new bonds form
Energy change depends on the energy used vs energy released.
4
Reactions Involve Energy Changes
What happens in an ENDOTHERMIC reaction?
reactants
products
energy used
to break bonds
energy released when
new bonds form
energy used >
energy released
energy absorbed overall
5
Reactions Involve Energy Changes
What happens in an EXOTHERMIC reaction?
energy used
reactants
products
energy used
to break bonds
energy released when
new bonds form
< energy released
energy released overall
6
Reactions Involve Energy Changes
System vs Surroundings
Energy is transferred between a “system” and its “surroundings”.
system =
chemicals in the reaction
surroundings =
everything else
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch21/chemical.php
7
Reactions Involve Energy Changes
System vs Surroundings - Endothermic Reactions
energy is transferred to the system from the surrounding
energy is absorbed by the system
(energy is lost from the surroundings
and temperature decreases)
Example: a cold pack
system =
CaCl2(s) → CaCl2(aq)
surroundings =
air + your skin +
8
Reactions Involve Energy Changes
System vs Surroundings - Exothermic Reactions
energy is transferred from the system to the surroundings
energy is released by the system
(energy is gained by the surroundings
and temperature increases)
Example: a candle
system =
wax + O2 → CO2 + H2O
surroundings = air
http://www.easyvectors.com/browse/other/b23c580ca2dc99f8d1df709e0dde668f-candle-clip-art
9
Reactions Involve Energy Changes
Examples of Exothermic and Endothermic Reactions
state changes: solid → liquid → gas
melting and evaporation
endothermic
chemical changes:
various!
state changes: gas → liquid → solid
exothermic
condensation and solidification
chemical changes:
combustion reactions
neutralization reactions
(acid + base)
10