Rates Flashcards
Rate of Reaction definition
The change in concentration per unit of time
Why do rate of reactions start of fast but then slow?
- Initially there is a high concentration of reactants so a large frequency of successful collisions.
- As the reactants start to get used up successful collisions become less frequent.
- Once one or more reactants are used up the can be zero successful collisions.
conc vs time graphs
A + 2B → C
What do different orders mean?
Zero order:
changing the concentration of a reactant has no effect on the rate of reaction
1st order:
The rate is directly proportional to the concentration of the reactant e.g. doubling the concentration of the reactant, doubles the rate.
2nd order:
The rate is proportional to the square of the change in concentration e.g. doubling the concentration causes the rate to increase by 4 (2^2)
The overall order is the sum of the orders
What does using a large excess of reactant mean?
Using a large excess of a reactant will mean the order of reaction is zero with respect to that reactant as the reactants concentration is effectively constant.
Units of k
Continuous Monitoring Method
1) By taking samples at regular intervals
2) By using a visible indicator/physical property such as gas volume
How to stop CMM for still reacting
To stop the reaction, quench the reaction by adding a large volume of cold distilled water.
This will both cool and dilute the reaction decreasing the rate.
CMM Monitoring using a physical property problem
This records the amount of product produced. The order of reaction is about the reactants. The volumes recorded would need converting to reactant concentration.
Concentration time tangent
gradient = rate
Straight line = constant = 0 order
Explain how the graph shows it is 0 order
as [H+] changes there is a constant gradient so rate stays the same
1st or 2nd order
- draw 1st tangent at t=0
- draw 2nd tangent at 1/2 initial conc
- compare increase in rate to conc change
Calculating rates orders
Initial Rates Method
the time it takes to get to a specific point in the reaction is recorded for each experiment.
For example the time is recorded when 20cm3 of gas are produced or when the mixture changes colour or a precipitate is formed.
The Thiosulphate – Acid Reaction
When the total volume is constant in each experiment the volume of the reactant is proportional to its concentration.
Rate = 1/t
The iodine clock reaction
S2O3 2- = sodium thiosulfate (slows down colour change of I2 - cannot add last)
- Measure known volumes of reactants. Keep them separate so the reaction doesn’t start.
- Measure a known volume of sodium thiosulphate.
- Start the stop clock when the last of the reactants is added to a conical flask. Do not add sodium thiosulphate last.
- Stop the timer when the mixture turns blue/black and record.
- Repeat the experiment using the same total volume altering the volume of KI (see table).
- Rate = 1/time
- Plot a graph of 1/t against vol of KI (draw all orders)
Initial rates orders
1st order conc. proportional to rate
Initial rates calculations
Arrhenius equation
ln k = -Ea/R x 1/T + ln a
y = m x + c
calculating Ea
Rate equations and mechanisms
THE SPECIES IN THE RATE DETERMINING STEP MUST MATCH THE SPECIES IN THE RATE EQUATION
