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There are 4 factors that affect the rate of a reaction:
1. Temperature:Increasing temperature = more energy= more collisions = more reactions
8b: Factors affecting reaction rate
2. Concentration:Increasing concentration = more particles = more collisions = more reactions
low concentration high concentration
study question 2
• What happens to the number of collisions when there is an increase in concentration?
3. Surface Area:Increasing surface area = more places to collide = more collisions = more reactions
Reaction occurs only at the surface
More surface area = More chance to react
study question 3
• 1. Which has more surface area? • A 1g sugar cube or 1g of granulated
sugar
• 2. Which would react faster?
4. Pressure (gases only):Increasing pressure = particles closer together = more collisions = more reactions
Increased pressure forces particles
together
low pressure higher pressure
study question 4
• 1. How would you increase the rate of a gas reaction?
• 2. Why does increasing pressure NOT help increase the rate of a liquid reaction?
Reaction mechanics
• A reaction can only occur if:1. Molecules collide2. Molecules collide with enough energy3. Molecules collide with the correct
orientation
Molecules must hit hard enough in just the right place
Reaction rate = speed of reaction• Example
•An explosion happens quickly•Rusting happens slowly
8a: Measuring reaction rate
• Reaction rate = change in concentration time
•∆[reactants] or ∆[products] time time
• ∆ = change• [ ] = concentration
Reversible reaction: X Y[X]
[Y]
time
X Y
Y X
timerate
concentration
•As concentration decreases - Rate decreases•As concentration increases - Rate increases•When concentration is constant - Rate is constant
en
erg
y
time
reactants
products
Activation energy
Reaction pathway• Reactions are not always
spontaneous• Activation energy = energy
needed for a reaction to happen
study question 8
• Paper burns in the presence of oxygen. Explain why this paper does not burst into flames.
Catalyst• A catalyst increases the rate of a
reaction by lowering ‘activation energy’ • A catalyst is not used up in the reaction• Example: a catalytic converter allows
CO to react more easily with O2 to make CO2
standard 8c: catalyst
Reaction pathwaycopy diagram on page 547
study question 9
1. In terms of ‘activation energy’, how does a catalyst increase the rate of reaction?
2. Does a catalyst affect the energy of reactants or products?
Standard 8d: the definition and role of activation energy in a chemical reaction.
• Reactants must form an ‘activated transition complex’
• This requires energy = activation energy
Honors.
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