Physical Chemistry And Transition Elements Flashcards
Units for rate of reaction?
+ Calc for rate
Mol dm^-3 s^-1
Change in reaction/ time taken = rate
How do you measure rate of reaction?
How can you “stop” reaction chemically/ physically to measure for RofR
Measured by measuring some property of a reactant/ product at various times during reaction
E.g
CHEMICALLY
-titrating samples
- Quenching reactions by adding another chemical to remove one reactant
-Cooling sample
- diluting it massively
PHYSICALLY (measured by)
- change in volume/ pressure of gas
- changes in light absorbed/transmitted
- change in thermal conductivity
- change in mass
State the rate equation?
Rate= k[A]^m [B]^n
m = order of reaction with respect to A
n = order of reaction with respect to *
K= rate constant - separate for every reaction
*the oder is the power to which the concentration of a reactant is raised in the rate equation
If a 1st order reaction
1) write rate equation
2) units for K
1st order
Rate = K[A]
K is in S^-1
(See notes for more info on why)
If a 2nd order reaction
1) write rate equation
2) units for K
2nd order
Rate= K[A]^2
K is in Mol^-1 dm^3 s^-1
(See notes for more info on why)
To form concentration time graph what do you measure
(Continuous monitoring)?
If gas produced
- monitor by collecting gas produced in gas syringe
- monitor the mass loss
If colour change
- monitor using colorimeter
Sketch how a concentration (y) time (x) graph would look for
1) zero order reaction
2) first order reaction
3) second order reaction
See note for graphs
1) zero order
Produces a straight line with a negative gradient
2) first order
Downward curve with a decreasing gradient over time but with a constant half life
3) second order
Steep downward curve, steeper at the start but trailing off more slowly with not constant half life
Sketch how a rate (y) concentration (x) graph would look for
1) zero order reaction
2) first order reaction
3) second order reaction
See note for graphs
1) zero order
Rate remains constant- straight horizontal line
2) first order
Consistent linear increase
3) second order
Upwards curve
Def of half life?
Half-life (t1/2)
- the time taken for half of a reactant to be used up.
In a first order reaction what is the trend of half lives?
+ what is this called?
First order reactions have a constant half-life with the concentration halving every half life
+this pattern is called exponential decay.
How can you calculate the rate constant from half-life?
K = In 2
———
t 1/2
In 2 (natural log2 found on calculator on right of log
How do you calculate K from a rate (y) concentration (x) graph?
The rate constant is equal to the gradient of the graph
Things to consider when suggesting a mechanism?
1) the rate-determining step of reaction must fit with the rate equation
2) all of the equation in the mechanism must balance
3) the different steps of the reaction must add up to he overall equation
(Often several right answers for these questions)
State Arrhenius equation
(Given in exam)
K = Ae ^-Ea/RT
K= rate constant
Ea = activation energy (J mol-1)
T= temperature (K)
R= gas constant (8.314 J K-1 mol -1)
A= pre-exponential factor (same units as K)
*essentially a measure of the number of collisions)
Determination of Ea and A graphically from Arrhenius equation
Y = m x + c
LnK = -Ea. 1.
——. — +. LnA
R. T
If lnK is plotted against 1/t gradient is -Ea/R so we can calculate the activation energy
General equilibrium constant equation
Kc = Right
——
Left
aW (aq) + bX (aq) (reverse reaction arrow) cY(aq) = dZ(aq)
Kc = [Y]^c [Z] ^d
——————
[W]^a [X]^b
Position of equilibrium if more products than reactants + Kc size
When product concentration is high and reactant conc is low
Equilibrium will lie towards the left hand side
Kc will be small
Position of equilibrium if more reactants than products + Kc size
When reactant concentration is high and product conc is low
Equilibrium will lie towards the right hand side
Kc will be large
Why does equilibrium shift if change occurs
(Explain Le Chatelier’s using Kc)
Kc remains constant for a given temperature
This is why the reaction will oppose the change
How does Kc value change in different temps 1) for endothermic reaction
2) for exothermic reaction
For endothermic reaction
- Kc increases at higher temperatures
For exothermic reaction
-Kc decreases at higher temperatures
Change of equilibrium equation with heterogenous reaction
When different sates are involved then equilibrium = heterogenous
But products in liquid or solid states are omitted (as they concentrations are effectively unchanged)
This means only gas or aqueous reactants are included
Annotate an equilibrium graph
See notes
How do you work out equilibria of reaction
See notes
Describe the effect of condition changes on equilibrium
General
1) temp change
2) change pressure
1) temp change
- cooling temperature will favour the reaction that produces … to counteract the change because the …. Reaction is exothermic therefore the equilibrium shifted from left to right to reastablish the Kc value leading to more … being produce
(Opposite for higher temps but it depends on the reaction)
2) pressure change
- higher pressure will decrease production of … as to oppose the increase and re-establish the Kc value the reaction will shift equilibrium from the right to left favour reaction that produces less molecules.
