Unit 8 – Notes Page 7
• Endothermic and Exothermic Reactions
Energy:
• The ability to do work
• Chemical bonds store energy
• Breaking bonds takes energy
• Forming bonds releases energy
Activation Energy
• The minimum amount of energy required for a
reaction to take place
Heat of Reaction, ΔH
• A change in the internal energy of the reaction
• + ΔH: Reactants lower energy than products
• - ΔH: Reactants higher energy than products
Enthalpy
• Amount of energy a system or substance
contains
• Another way to say ΔH
Surroundings
• Where the reaction is taking place: the beaker,
the test tube, the classroom, the universe, etc.
System
• Substance(s) / chemicals involved in a reaction
or phase change.
Open System
Closed System
•
Isolated System
Exothermic Reactions
• Exo = out
• Thermic = heat
• Energy (heat, light) is given off from a system
when a reaction takes place
Endothermic Reactions
• Endo = into, inside
• Thermic = heat
• Energy (heat, light) is taken into a system
when a reaction takes place
Exothermic Reactions
• GENERAL EQUATION:
REACTANTS  PRODUCTS + ENERGY
Takes less energy
to break bonds in
reactants than is
released when
new bonds form in
products
Negative ΔH:
Exothermic
Example: Combustion
• Always exothermic
• Burns and combines with oxygen
• Give off heat as energy and light
Example: Hot Hands
How does an instant hot pack work?
Exothermic reaction of iron in oxygen:
4Fe(s) + 3O2(g)  2Fe2O3(s)
Endothermic Reactions
• GENERAL EQUATION:
REACTANTS + ENERGY  PRODUCTS
Needs a constant input
of energy to continue.
Takes more energy to
break bonds in
reactants than is
released when new
bonds form.
Positive ΔH:
Endothermic
Example: Photosynthesis
• 6 CO2 + 6 H2O → C6H12O6 + 6 O2
• Plants need to aborb energy from the sun (or
other source of light)
• Absorbs energy  endothermic
Example: Cold Pack
• How does an instant cold pack work?
(NO3)NH4 (s) + H2O(l)  NH4(OH) (aq) + HNO3 (aq)
• Squeezing the cold pack breaks an inner bag of
water, and the water mixes with a chemical inside
the pack. The chemical and water combine in an
endothermic reaction. The energy needed for the
reaction to take place comes from the water,
which gets colder as the reaction proceeds.
Phase Change
–Ice  Water
–Solid gold  molten gold
–Heat moves from the surroundings to
the system
• Solid  Liquid  Gas (Melt, evaporate)
• Requires heat (energy). It takes energy from
the environment. This is ENDOTHERMIC.
• Gas  Liquid  Solid (Condense, freeze)
• Requires absence of heat (energy). It releases
energy into its surroundings.
• Making ice is EXOTHERMIC!