Unit 11 Collision Theory and
Rate of Reaction
Major Quiz: Friday February 25
Reaction Rate
• Reaction rate – the change
in concentration of a
reactant or product per unit
of time
• Example:
• .50M at 2s & 0.20M at 5s
• So the rate is (.50M - .20M) /
(5s – 2s) = 0.10 mol/(Ls)
Factors Affecting Rate
1. Concentration
2. Temperature
3. Presence of a catalyst
4. Nature of reactants
Collision Theory
• Tries to explain why the 4 factors affect
the reaction rate
• Says that:
1. Molecules or ions must collide with each
other before a reaction can occur
2. Reacting substances must collide with
sufficient energy **
3. Collision frequency – number of collisions
that occur over a period of time; depends on
the 1) the temperature and 2) the
concentration
4. Collision effectiveness – indicates the %
of collisions that actually result in a chemical
reaction
Collision Theory & Energy
• **The minimum amount of energy that
reacting (colliding) particles need to react
is called activation energy (Ea).
• The lower the activation energy the faster
the reaction.
• (It is easier to climb a little hill than it is to
climb a mountain.)
Details about Factors that Affect
the Reaction Rate
1. Concentration – the higher the
concentration, the more particles there
are present, the higher the collision rate,
the higher the reaction rate
– Concentration and reaction rate are
directly related
Concentration Affected by:
• In solutions – molarity (moles solute per
liter of solution)
– Higher molarity, higher rate
• In solids – surface area
– Smaller the pieces, the higher the rate
• In gases – pressure
– The higher the pressure, the higher the rate
Which would react faster, 1M or 10 M
HCl? Why?
Surface Area
Which would rust faster. Powdered
Iron or a large iron nail?
2. Temperature
• The higher the temperature, the faster
the molecules are moving, the higher the
collision rate
• Temperature and reaction rate are
directly related
3. Catalyst
• Catalyst – a substance that increases
the rate of a reaction without getting
permanently involved in the reaction
• Presence of a catalyst increases the
reaction rate
• An inhibitor has the opposite effect of a
catalyst
Enthalpy (H)
• a measure of heat content of a system
• H = change in heat content that accompanies a
process
• Hrxn = Hfinal - Hinitial
• Hrxn = Hproducts - Hreactants
• ** ΔHrxn can also be written as ΔHf,
for heat of formation**
• **Chemical systems in the world tend to achieve
the lowest possible energy. Would this occur in
an exothermic or an endothermic reaction?
Exothermic reactions
• chemicals react and release
heat(feel hot); H is negative;
products are more stable
• 4Fe + 3O2 2Fe2O3 + 1625 kJ
Endothermic reactions
• chemicals need to absorb energy in order
for the reaction to take place (feel cool);
H is positive; reactants are more stable
• 27 kJ + NH4NO3 NH4+ + NO3-
Practice problems
• CO (g) + NO (g) CO2 (g) + N2 (g)
• CH4 + O2 CO2 + H2O
• N2 + O2 NO2