What do you think the term “rate of reaction” means?
Reaction Rate is defined as the change in the amount
of either the reactants or the products over time. In
other words it is a measure of how fast the reactants
are used up, or how fast the products are produced.
Example
Calculate the rate of reaction
2.4 g of calcium reacts with water in 2.2 minutes.
Rate = 2.4 g reacted/2.2 minutes
= 1.1 g/min
Collision Theory
• There are two things that need to happen to the collisions that make a
reaction occur:
• Correct Orientation
• Enough Energy (speed)
• There are two methods to increase the rate of reaction:
• increase the number of collisions
• increase the energy of the collisions
Ways to Increase Reaction Rate
• Temperature
• Concentration
• Surface Area
• Catalysts
Temperature
Temperature is the measure of the average Kinetic
Energy of all the particles in an object.
• The more energy molecules have, the higher the
temperature.
• When molecules have more energy, they move
around more, collide into other molecules more,
and therefore react faster.
The rate of reaction changes with the temperature.
• Increasing temperature = increase in # of
collisions per second = increase in reaction rate
• Increasing temperature = Energy per collision
increases
Sometimes we want slower reactions (we use a
fridge to prevent food from rotting).
Sometimes we want faster reactions (we cook food
to speed up the production of new molecules)
Concentration
Concentration refers to how many molecules of a
substance exists in a certain amount.
• Concentration is how much solute is dissolved
in solution.
• Concentration is measured in mass per unit
volume (g/L).
Usually, the higher the concentration of reactants,
the faster the reaction occurs.
• Increased concentration = increase in # of
collisions per second = increase in reaction rate
• Increasing concentration DOES NOT change
energy per collision
A splint of wood glows brighter in highly
concentrated oxygen than in normal air with a
lower concentration of oxygen.
Surface Area
Increasing surface area increases the rate of reaction.
• Surface area can be increased by creating more pieces.
• A powdered substance has far more surface area than one large chunk.
• The increase in surface area must also be exposed for reaction
• A powder only reacts more quickly if it is spread into the air instead of lying on a pan (the outer particles block the
inner ones)
The more atoms and molecules there are to collide, the faster the reaction rate.
• Increasing SA = increase in # of collisions per second = increase in reaction rate
• Increasing concentration DOES NOT change energy per collision
Catalyst
Sometimes, increasing the temperature or concentration is not a desirable method to increase reaction rate.
A catalyst is a substance that allows a reaction to occur more quickly without actually participating in the reaction itself.
• The catalyst speeds up the reaction rate but does not get used up as a reactant.
• Catalysts often decrease the amount of Activation Energy necessary to break the bonds in the reactants for the reaction to occur.
• Catalysts line up particles in the correct orientation to collide for a reaction to occur (like a Matchmaker)
• Presence of Catalyst = DOES NOT increase the # of collisions per second / change energy per collision