Y13 Rates Flashcards
Rate of reaction definition
The change in concentration of a reactant or product per unit time
What are the units of rate?
mol dm-3 s-1
How can absorbance readings be used to plot a concentration-time graph (3 steps)?
- Measure the absorbance readings of standard solutions of known concentrations and plot a calibration curve
- Do the reaction and take absorbance readings at regular time intervals
- Use the calibration curve to convert absorbance into concentration
What does zero order mean?
Rate is NOT affected by [A]
e.g. if [A] = 4, rate = 1
What does first order mean?
Rate is DIRECTLY PROPORTIONAL to [A]
e.g. if [A] = 4, rate = 4
What does second order mean?
Rate is proportional to [A]^2
e.g. if [A] = 4, rate = 16
What is K in the rate equation (Rate = k [A][B]^2)?
Rate constant
What does it mean if a reactant is not in the rate equation?
It is zero order
What can orders NOT be determined by?
Molar ratios
How is the rate equation rearranged to get the value of K?
K = rate / [A][B]
What is used to calculate rate on a conc-time graph?
Gradient
What does zero order look like on a CONC-TIME graph?
[A] doesn’t affect rate so rate is CONSTANT so gradient is CONSTANT = straight diagonal line: \
What does first order look like on a CONC-TIME graph?
[A] DOES affect rate, as [A] decreases rate decreases so gradient becomes less steep = curved downwards line
What does second order look like on a CONC-TIME graph?
Second order has greater effect than 1st = steeper downwards curve
Half-life definition
Time taken for the CONCENTRATION of the reaction to reduce by half
How can half-life be used to prove a reaction is first order?
If half-life is CONSTANT, the reaction is FIRST order
What is the equation involving half-life used to find K?
K = ln2 / t 1/2
What does zero order look like on a RATE-CONC graph?
Straight line across —
What does first order look like on a RATE-CONC graph?
Straight line through the origin /
What does second order look like on a RATE-CONC graph?
Curved line going upwards
How can you prove a reaction is second order after plotting a rate-conc graph?
Plot a rate-conc^2 graph, if the line is now straight it is second order
Which graph do initial rates allow to be plotted?
Rate-conc (shape used to find order of reaction)
What is the initial rates method (7 steps)?
- Time how long it takes for a visual change to occur (colour change/precipitate)
- Run experiment with set concentrations of A and B
- Repeat but change [A] whilst keeping [B] constant
- Repeat again, but change [B] whilst keeping [A] constant (only change one at a time)
- Keep total volume and temp. the same (keeps all other reactants’ concs the same)
- Rate = 1 / t
- Plot RATE-CONC of each reactant and deduce order from shape
What is the rate-determining step?
Slowest step in the reaction mechanism of a multi-step reaction (usually the FIRST step)
What will the rate equation include which helps to determine the rate-determining step?
All reacting species in (or before) the rate-determining step
How do you deduce a mechanism from the rate equation (5 steps)?
- Left hand side of rate-determining step from rate equation (assume 1st step)
- Put remaining reactants into step 2
- Put one product into step 1, others into step 2
- Balance the rate-determining step with an intermediate (never single atom)
- Add up steps to check it makes the overall equation
Which step is the rate-determining step?
Rate = K [A]^2[B]
1. A + B –> AB
2. AB + A –> C
3. C + B –> D
Rate-determining step is step 2 because there are 2 moles of A (2nd order) and 1 mole of B (1st order) in steps 1 and 2 COLLECTIVELY
What is the form of Arrhenius equation which must be learnt?
lnK = - Ea/R x 1/T + lnA
y = m x + c
How is Ea calculated from a lnK-1/T graph?
Gradient = m = - Ea/R so…
Ea = gradient x -R
How do you find A from an Arrhenius plot?
y-intercept = lnA so…
A = e^lnA
What should you do if the x-axis doesn’t start at 0?
- Rearrange the Arrhenius equation:
y = mx + c —–> c = y - mx
- Plug in x and y from ONE point on the best fit line and the gradient (m)
