Kinetics Flashcards
pressure =
Conc of gas
⭐ can’t have ? on x axos of energy profile diagram?
time. must be ‘progres of reaction’
don’t put ? on EPD
Double headed arrows
kinetics is about?
ROR
Ea =
the min. energy a particle must have to be involved in a successful collision
Rate of reaction =?
the change in conc of products/ reactants with time
the units of rate are?
moldm-3
how can rate be measured from a graph?
- conc/time graph
- draw tangent to curve, calculate gradient
- draw tangent all the way to axis
- make the angle either side of the tangent roughly the same
slope of the curve =?
rate
⭐ IROR =
AT 0 SECONDS
what is the relationship between conc and rate?
conc ∝ rate
how are orders of reaction always deduced?
From experimental data
the order of reaction is to do w ?
how changing the conc of 1 reactant affects the ROR
First order reactions?
- x2 conc of reactant, rate does not change
- Rate = k[r] to the power of 0
- so rate is ∝ to [HCl]^0
anything to the power of 0 is 1
First order reactions?
- when conc of reactant x2, rate x2
- Rate = k[HCl]^1
- Rate ∝ [HCl]^1
Second order reactions?
- when the conc of reactant doubles, the rate increases by 2^2 = 4
- Rate = k[HCl] to the power of 2
Rate equation?
Rate = k[A][B] to the power of 0/1/2
if the reaction is first order?
don’t include that reactant in the rate equation
overall order?
just add indiv orders together
The rate constant, k, is
proportionality constant that relates rate to the conc
what do you have to work out for every single reaction?
units for k, just like we have to work out the units for the equilibrium constant
what is the impact of increasing temp on k?
Rate ⬆ and conc stays the same so ⬆ temp means that k has to ⬆.
when working out the units?
when bringing a unit to the top, change the signs
how do you work out the units for k ?
cancel units
k =
rate/ [reactants]
only include ? in the rate equation?
reactants not products
how do u know when a substance is a catalyst?
doesn’t appear in reaction but affects ROR
how else can we tell the order of a reaction?
- half life from a graph
- conc/time or conc/rate graph
half life definition?
the time for the conc of a reactant to drop to half of its original value
t subscript 1/2/
exam tip: half life
- show clearly on the graph 2 half lives
- calculation
- explain which order
Conc vs time graph: zero order
the [r] has no effect on time but conc decreases over time bc [r] is used up.
conc decreases at a constant rate.
STRAIGHT LINE
Conc vs time graph: first order
- as the conc x2, reaction time halves
- conc halves at equal time intervals - constant half life
- slight downwards curve
Conc vs time graph: second order
- as he conc x2, reaction time 1/4 so half life increases
- conc rapidly decreases
- downwards curve
rate vs conc graph: zero order
- reactant has no effect on rate
- horizontal line
rate vs conc graph: first order
- as the conc x2 the rate x2
- as conc x3, rate x3
- rate ∝ conc
- proportional straight line
rate vs conc graph: second order
- as the conc 2z, rate 4x
- rate ∝ conc^2
exponential graph when rate vs conc
but when rate vs conc^2, then straight line
initial rates method (using experimental data)
if 2 reactants. look at when one is constant to get the rate for the other
if 2 reactants, and both change ?
Work out the order for for, account for that change and deduce the order for the second (actually decent)
when working out the mechanism,
- TRY to make 1 of the products
- cancel intermediate to get overall equation
- reactions don’t have to make sense
- check by cancelling
- can only cancel something if it appears in the reactant and products
what is a reaction mechanism?
the series of steps of a reaction
the rate determining step?
the slow step.
how can the molecules in the rate determining step be worked out?
- from the rate equation
- if a reactant appears in the rate equation, it will appear in the RDS
- The order tells you how many particles to put into the RDS
[]^1 = 1 of that reactant, []^2 = 2 of that reactant
the 2nd step of the mechanism must?
use up the intermediate
Why might the initial rate of substances be different?
different molar ratios
diradical =?
2 unpaired e-s
what affects the value of the rate constant?
the temp
What is natural log the inverse of?
exponential
what can the Arrhenius equation do?
calculate the proportion of particles that can actually react
In means
natural log of
In k =
natural log of the rate constant
y axis
A =
the pre-exponential factor
tells us about the frequency and geometry of collisions
Ea=
Activate energy in J
R =
the gas constant(data sheet)
T =
temp in Kelvin
y = mx + c in relation to Arrhenius equation
Y = In K (y axis) M = -Ea/ R, gradient x = 1/T in kelvin (x axis) c = y intercept, natural log of A
graph for arrhenius?
- exponential
- when the temp x2, the fraction of molecules w E ≥ Ea more than x2
- so the relationship between rate constant and temp is NOT double
when MUST you substitute into the arrhenius equation to get A?
- when the x axis does not start at 0 (there’s a break)
- if it does, y intercept = inA
alternative method to find k?
k = in 2/ half life
BUT THIS ONLY WORKS FOR FIRST ORDER REACTIONS
Colorimetry:
darker solution…
more conc
Colorimetry: purpose
In any reaction involving a coloured substance (either reacting or being produced), the reaction can be followed using a colorimeter
Colorimetry: light
- colour of light can be changed by selecting a paticiualrly coloured filter
- complementary colour to the same used so the samle absorbs it
- e.g. copper sulphate is blue so a red light is used bc red is absorbed
Colorimetry: the more concentrated a solution is?
the more light it will absorb
Colorimetry: The substance used has to…
change colour as the reaction occurs
Colorimetry: what can u measure?
- the amount of light absorbed as it passes through a solution - recorded as the absorption of a solution
Colorimetry: how to get conc from absorbance?
- plot calibration curve by making up known concs of the coloured solution and measuring the absorbanc of each under the same conditions as the experiment that u will do
- plot absorbance against conc to get the CC
- read off CC
The rate constant always ?
STAYS THE SAME unless T changes
