How Fast Flashcards
What are factors effecting rate?
Temp
Conc
SA
Catalyst
What is rate of reaction?
Change in conc of a reactant (or product) per unit time.
How quickly reactants are used up/ products formed
What is average rate?
Total change in conc of reactants (or products) divided by the time taken for the reaction to take place
What is instantaneous rate?
A reaction rate of change occurring at a specific time in the reaction
What is the rate of reaction equation?
Change in conc
———————— Units: moldm-3s-1
Time
What are the common reaction orders?
Zero
First
Second
What is zero order?
When the conc of a reactant has no effect on rate
What is first order?
When the rate depends on its concentrations raised to the power of one
What is second order?
When the rate depends on its conc raised to the power of two
What is the rate equation?
Rate = K[A] ^x [B]^y
Give examples of methods that continuously monitor conc
Change in mass Inverted measuring cylinder Gas syringe pH changes - using probe/ data logger Colorimetry - measure absorbance of reaction
What does the graph look like for zero order?
Conc-time graph
Produces straight line with negative gradient
Reaction rate does not change
What does the graph look like for first order?
Conc-time graph
Downward curve with decreasing gradient over time
As gradient decreases with time, reaction slows down
What does the graph look like for second order?
Conc-time graph
Downward curve but steeper at the start, tailing off more slowly
How can we tell if it is a first or second order conc-time graph?
Use half life
Fist order will have a constant half life (take same amount of time each at each half)
What is a method for measuring initial rate (conc)?
Clock method
What does the graph look like for zero order?
Rate-conc graph
Straight line - horizontal
What does the graph look like for first order?
Rate-conc graph
Straight line through the origin - diagonal
What does the graph look like for second order?
Rate-conc graph
Upwards curve with increasing gradient
What does the graph for zero order show?
Rate-conc graph
Rate doesn’t change
What does the graph for first order show?
Rate-conc graph
Rate directly proportional to conc
Why can’t you work out K from the second order rate-conc graph?
Because curve doesn’t acc confirm its 2nd. just its not 0 or 1
What must you do to work out K from 2nd order rate-conc graph?
Use results from graph to plot second graph
But plot rate against [A]^2
Then find K through line gradient
Should be straight line
What is a reaction mechanism?
Describes one or more steps involved in a reaction in a way which makes it clear how certain bonds are broken and made
What is the rate determining step?
The overall rate of reaction (the one you could measure if you did an experiment) is controlled by the rate of the slowest step. The slowest step is called the R.D.S
What does the rate expression tell us?
If the species is in the rate equation it is in the R.D.S
If it is not in the rate equation it is not in the R.D.S
The power of the substance is the number of the substance in the equation
What must the steps involved in the reaction add up to?
The overall chemical equation in the reaction
eg. some compounds/elements cancel out to make overall equation
Which step is the R.D.S?
Always the slowest
What is the Arrhenius equation?
K = Ae ^ -Ea/RT
What is e^ -Ea/RT?
Represents proportion of particles that exceed Ea and have sufficient energy to take place
What is A?
Arrhenius equation
Takes in account the frequency of collisions with correct orientation
Increases slightly with temp but overall constant
What happens to the rate constant in the Arrhenius equation when you increase temp?
The rate constant increases and so does rate
What happens to the rate constant in the Arrhenius equation when you use a catalyst?
Decreases Ea so increases rate constant and so increases rate
What is the more effect caused by when you increase temp in Arrhenius equation?
Caused by more particles exceeding Ea than increased collisions
When can’t you extrapolate the graph?
When the scale is broken
What is the inverse of Arrhenius equation?
Ln K = -Ea/R X 1/T + Ln A
How do you use the inverse of Arrhenius equation calculate
1. Work out Ln K and 1/T 1/ first column no. Ln(2nd column no.) 2. Plot graph - Ln K against 1/T 3. Workout gradient 4. Calculate A from y-intercept Extrapolate line
How do you find Ea from the gradient?
-Ea/R = X
-Ea = 8.31x
Ea = -8.31
How do you find A when you can’t extrapolate the graph?
Use Ln K = -Ea/R X 1/T + Ln A
Rearrange to get Ln A
Then A = E^ Ln A
