5.1.1 - How fast Flashcards
Order of a reaction
The power to which a conc. of a reactant is raised to in the rate equation
1st order graphs
Slightly downward slope (conc vs. time graph)
Drops w/ a constant half life
Rate=conc; diagonal line in rate vs. conc graph
2nd order graphs
Decline steeply then level off
Parabola in rate vs. conc graph
2nd order
Rate is proportional to the square of the con. (upward slope)
Zero order graphs
Constant decline in conc. (conc vs. time)
Rate is independent of []
Rate stays constant in rate vs. conc graph
Overall order
Obtained by adding all the orders of the reactants
Rate eqn.
Rate = k [A]^m [B]^n
Includes all reactants up to and including the rate determining step
May include catalysts
Rate constant
A number that allows the equation to work.
It is unchanged by changes to concentration but will increase at higher temperatures
Rate of reaction
Speed at which a reactant is destroyed
Speed at which a product is formed
conc/dm^3 s^1
Half life of a reaction
Time it takes for the concentration of a reactant to halve
Rate determining step
Reaction that occurs at a much slower rate and requires a much higher activation energy
Finding k using [conc] vs time graph
k = ln (2)/ half life
Units of k
s^-1
Increases w/ temp
Finding k using rate vs conc graphs
Gradient = m = k
Initial rates method
We always take the initial rate as that’s the maximum rate
Allows us to make a comparison of the effect of any changes of conditions as the rate changes during the reaction
Colorimetry
Measures absorbancy (and loss of colour) and you can see how this changes with conc
Units for zero order reaction
mol dm-3 s-1
Units for first-order reaction
s-1
Units for second order reaction
mol -1 dm3 s-1
Units for third order reaction
dm6 mol-2 s-1
Why is it better to measure a physical property than to do a chemical analysis of the reaction
Chemical requires reaction to stop
Physical does not interfere w/ progress
Why do we use a little ‘clock reactant’ (PAG 10.1)
So that the colour change happens as close to the inital rate as possible as the colour change begins when all he S2O8 2- ions have been used up. It is only then the I2 can react w/ the starch
Finding reaction mechanism
Mechanism adds up to overall
If species is both on reactants and products side, cancel out
Reactants in rate eqn are reactants of RDS
Linking k and T
As T increases as does k and therefore so does the rate
If Ea is reduced, k increases
For every 10 degree rise in T, k is doubled as is the rate
T and k have a +ve exponential relationship
Arrhenius eqn.
k = Ae^ -Ea/RT
A = pre exponential factor Ea = activation enrgy (always +ve) R = gas constant (8.314) T = temperature (K)
Finding Ea and A graphically
Plot 1/Temp against ln k
y - intercept = ln A
Gradient = -Ea/R
High Ea = steeper slope
Low Ea = shallower slope
How to perform colorimetry w/ a clock reactant
Carry out titration w/ A in the burette and constant vol of E in the conical flask w/ C
Zero colorimeter
Select a suitable filter
As A is being added measure the absorbance at set intervals
How to prove absorbance is proportional to the conc of E
Prepare standard sol of diff conc of E by serial dilutions
Measure the absorbance and plot abs vs [E]
How would you know how many steps the reaction mechanism takes
Stoichiometry of the rate eqn doesn’t match stoichiometry of the overall eqn
Must happen in more than one step
When to use colorimetry to determine rate
When you produce a coloured compound or using a clock reactant
Finding rate from clock reactions
C has a colour change and E uses this up
Rate = dE/dt
Rate is proportional to 1/t
Repeat w/ diff conc of A/B
Assumptions in clock reactions
E is used up before rate changes
Conc of A and B in such an excess that their conc doesn’t change significantly
Why does absorbance decrease during clock experiments
C is coloured and [C] decreases
