5.1.1 How Fast

Question 1

how do you measure the rate of a reaction

Answer 1

via changes in concentration

Question 2

what is the equation for the rate of a reaction, and what units

Answer 2

rate= change in conc./change in time

• moldm-3s-1 units (mols per dm3 per second)

Question 3

what symbol is used to demonstrate the concentration of something

Answer 3

[] - square brackets around the species

Question 4

what is the rate of a reaction proportional to

Answer 4

the concentration of a particular reactant raised to a power

• the power is said to be the order of reaction for that reactant
• e.g. rate ∝ [A]ⁿ

Question 5

what does a reactant with 0 order mean

Answer 5

[A]⁰ :
the concentration of the reactant has no affect on the rate of reaction (as anything to power 0 just gives 1)

Question 6

what does a reactant with first order mean

Answer 6

[A]¹ :
if the concentration of the reactant doubles, so does the rate of reaction (x2¹)
if the concentration of the reactant triples, so does the rate of reaction (x3¹)

Question 7

what does a reactant with second order mean

Answer 7

[A]² :
if the concentration doubles, the rate of reaction times by 4 (x2²)
if the concentration triples, the rate of reaction times by 9 (x3²)

Question 8

what does the rate equation show

Answer 8

the mathematical relationship between the rate of reaction, and the concentration of reactant

Question 9

what is the rate equation

Answer 9

rate = k[A]ᵐ[B]ⁿ

• where k is the rate constant
• [A] and [B] is the concentration of reactants A and B
• m and n are the orders of reaction with respect to A and B

Question 10

what does the overall order of a rate equation give you

Answer 10

the overall effect of all of the concentrations of all the reactants on the rate of reaction
- found just by adding together all the orders of all the reactants together

Question 11

how are the units of the rate constant k established for an equation, and do example of: rate = k[A][B]²

Answer 11

1) rearrange the rate equation to make k the subject : k = rate/[A][B]²
2) substitute all the units in place, with rate being moldm-3s-1 and any concentration being moldm-3 : k = moldm-3s-1/(moldm-3)³
3) cancel out common units and show final units : one moldm-3 cancels out above and below, leaving s-1/mol2dm-6, which then goes to dm 6 mol-1s-1 when you bring the denominator to the top

Question 12

what is initial rate

Answer 12

the instantaneous rate at the beginning of an experiment when t=0

Question 13

what is used when you are comparing the rates of different equations

Answer 13

always use the initial rate ideally

Question 14

how do you figure out the rate constant of a reaction using experimental results

Answer 14

• will be given grid, with each experiment and the relative concentrations
  1) compare the first 2 experiments, and determine which reactant has changed in concentration
  2) see how this compares with how the rate of reaction has changes, and figure out the order
  3) repeat for all experiments, until orders of all reactants has been found
  4) form your rate equation will all orders present
  5) input in the rate and [] values from the first equation into your rate equation, leaving you with a way to find out value of k
  6) work out the units too

Question 15

what is the wording for finding the rate orders from an experiment table

Answer 15

• from experiment 1 to 2
• the conc. of A has doubled
• the rate has also doubles
• so the reaction is first order with respect to A

Question 16

what is the continuous monitoring of rate

Answer 16

where continuous measurements are taken throughout the course of a reaction, giving you a concentration-time graph

Question 17

how can you carry out continuous monitoring of rate

Answer 17

1) monitoring gas collection (gas syringe/inverted measuring cylinder)
2) monitoring mass lost (the gas produced with escapes, measured on a balance)
- both suitable when gases are produced

3) colorimeter: measures the amount of light absorbed by a solution (used by using a calibration curve at set concentrations that you can read back from during an experiment)

Question 18

what information can be deduced from a concentration time graph

Answer 18

• the gradient of the graph is the rate of reaction (conc/time)
• can also figure out the order with respect to that reactant being measures, whether first or second (ONLY if the concentrations of all other reactants stays effectively unchanged)

Question 19

how does a zero order graph look on a concentration time graph

Answer 19

• straight line with a negative gradient
• as the rate of reaction does not change at all over the course of the reaction (dy/dx is the same throughout, so changing the concentration of this reactant has no correlation with the rate)
• the gradient will give you the value of k

Question 20

how does a first order relationship look like on a concentration time graph

Answer 20

• a downward curve with a decreasing gradient overtime (so steeper at the start)
• the gradient decreases with time, so the ROR slows as the concentration of reactant decreases

Question 21

what is true about the concentration time graphs of both first and second order relationships

Answer 21

• both produce a downward curve
• but second order is steeper at the start
• and tails off more slowly

Question 22

how do you check if the concentration time graph shows first or second order, as both a downwards curve

Answer 22

• for FIRST ORDER: the time for the concentration of reactant to halve is constant
• so use HALF-LIFE to determine (must be consistent)

Question 23

what is half-life

Answer 23

the time taken for half of a reactant to be used up

Question 24

what is the relationship between first order reactions and half life

Answer 24

• first order reactions have a constant half life
• so the concentration halves ever half life
• a form of EXPONENTIAL DECAY
• used to confirm first order, as if successive half-lives are the same as the first, then the reaction is first order with respect to that reactant

Question 25

where is the first half life of a reaction measured from

Answer 25

when the concentration halves from the initial concentration

Question 26

how can you find out the rate constant from a concentration time graph of first order

Answer 26

1) confirm it is first order via seeing downwards curve and calculating several half-lives 2) write out the rate equation : would be rate = k[A] as first order 3) draw a tangent at a specific concentration on the graph, and figure out the gradient (gives you the rate) 4) input this concentration value and rate into the rate equation, and rearrange to give the rate constant + units

Question 27

what is an alternative way of determining the rate constant from a first order concentration time graph

Answer 27

- due to the exponential relationship of the constant half life - gives you k = ln2/t1/2 - so ln2 divided by half life - much more ACCURATE! than drawing a tangent

Question 28

what is a rate concentration graph

Answer 28

plots the rate of a reaction against different concentrations of a reactant

Question 29

what does the rate concentration graph look like for a zero order relationship

Answer 29

- a straight, horizontal line with 0 gradient - as the rate = k[A]⁰, so [A]⁰ goes to 1, leaving rate = k (the y-intercept) - the rate is always k - and DOES NOT increase with concentration

Question 30

what does the rate concentration graph look like for a first order relationship

Answer 30

- a straight line going through the origin - as the rate = k[A] - so the rate is DIRECTLY PROPORTIONAL to the concentration - k would be the gradient of the graph

Question 31

what does the rate-concentration graph look like for a second order relationship

Answer 31

- an upward curve with an increasing gradient, so like an exponential graph - as rate = k[A]² - the value of k cannot be directly obtained from curve - but you would need to plot another curve of rate against concentration squared - which would then give you a straight line through the origin, with gradient k

Question 32

how do you find out the initial rate

Answer 32

- as it is the instantaneous rate at the start of a reaction when t=0 - would measure the gradient of the tangent at 0 of a concentration time graph

Question 33

what is a better way of obtaining the initial rate of a reaction, rather than just drawing a tangent at t=0

Answer 33

using a clock reaction

Question 34

what happens during a clock reaction

Answer 34

the time from the start of an experiment is measured until a visual change takes place, e.g. colour change or formation of a precipitate - provided that there is no significant change in rate during this time, you can assume that the rate of reaction over this time is the same as the initial rate

Question 35

how do you find the initial rate from a clock reaction

Answer 35

initial rate = 1/t

Question 36

is a clock reaction repeated

Answer 36

yes, repeated at several concentrations - and the initial rate 1/t is found at each

Question 37

PAG: what is a common clock reaction

Answer 37

iodine-clock

Question 38

PAG: what does an iodine clock reaction dependent on

Answer 38

- the colour change taking place when iodine is formed - I2(aq) is orange-brown, so can time how long it takes for this colour to appear - to make even better, often add starch, as when I2 is present, turns dark blue-black colour

Question 39

PAG: how do you carry out an iodine-clock reaction

Answer 39

- carry out several experiments, each time changing the concentration of the same reactant, and keeping all others constant - you can delay the colour change slightly, by adding another chemical (like Na2S2O3(aq) which actually removes iodine as it forms), and when this chemical runs out, the iodine will show up - in each experiment, begins colourless, and then forms a blue-black colour due to the starch-iodine complex - the time is taken for the colour to appear at each concentration - and turned into initial rate via 1/t - you can now plot a rate-concentration graph - (can repeat for different reactants too)

Question 40

PAG: what graph does a clock reaction allow you to plot

Answer 40

- a rate-concentration graph - as the concentration is of the reactant -and the rate is 1/t

Question 41

PAG: how do you figure out the rate equation and rate constant from a clock-reaction

Answer 41

- analyse the rate-concentration graphs of each reactant - analyse the shapes to figure out which order each reactant is - figure out the rate equation via this, adding in the correct orders - can work out rate constant from this, as you know the concentration of each reactant at a specific rate

Question 42

what are the drawbacks of using clock reactions to determine initial rates

Answer 42

we are assuming that the average rate over the starting period of a reaction before the visual change is equal to the initial rate - so the shorter the period of time over which the initial rate is measured - the less the rate actually changes over this time - so the more accurate it is

Question 43

when are clock reactions most accurate

Answer 43

can be reasonably accurate, as long as less than 15% of the whole reaction has taken place - the longer it takes, the more the rate changes, and goes away from initial (imagine the curve steering more away from the tangent as time goes on)

Question 44

what is stoichiometry

Answer 44

the relative amounts of the species in a reaction

Question 45

how does a chemical reaction take place

Answer 45

when particles collide

Question 46

how would the reaction H2O2 + 2I- + 2H+ → I1 + 2H2O take place

Answer 46

- one mole of H2O2, 2 moles one I- and 2 moles of H+ would need to collide simultaneously - VERY UNLIKELY (as unlikely that more than 2 particles collide at the same time) - much more likely to take place in a SERIES of steps

Question 47

what is a reaction mechanism

Answer 47

the series of steps that make up an overall reaction

Question 48

what is true about the different steps in a multi-step reaction

Answer 48

they will take place at different rates

Question 49

what is the rate determining step

Answer 49

the slowest step in a mechanism sequence

Question 50

what is the link between the rate equation and the rate determining step

Answer 50

- the rate equation only includes the reacting species involved in the rate-determining step - the orders the rate equation match the number of species involved in the rate-determining step (the number of moles)

Question 51

if the rate equation if k[A][B]², what must the rate determining step include

Answer 51

A + 2B must be present!!

Question 52

how do you figure out the possible reaction mechanisms and steps

Answer 52

- you know the rate determining step from the rate equation - then, can suggest potential products and reactants next - as must all cancel out either side to form the overall equation (so anything formed not in the final equation must be used next)

Question 53

what would a potential reaction mechanism of the hydrolysis of haloalkanes by hot, aqueous alkali: overall: (CH3)3CBr + OH- → (CH3)3COH + Br- rate: k[(CH3)3CBr]

Answer 53

- (CH3)3CBr alone forms a step, as is rate determining step 1) (CH3)3CBr → (CH3)3C+ + Br- 2) (CH3)3C+ + OH- → (CH3)3COH - (CH3)3C+ acts as intermediate

Question 54

what is the only thing that affects the rate of a reaction, (other than catalyst) as determined during Boltzmann distribution

Answer 54

temperature

Question 55

what does Boltzmann distribution say about effect of temperature on rate of reaction

Answer 55

- as temperature increases, so does ROR, and so the value of rate constant k - rule of thumb, as temperature increases by 10 degrees celsius, the ROR doubles and so does value of k

Question 56

how does increasing temperature affect the rate of a reaction

Answer 56

1) increasing temperature shifts the Boltzmann distribution to the right, increasing the proportion of molecules exceeding the Ea - MAIN FACTOR 2) as the temperature increases, the particles gain more kinetic energy, so move faster and collide more frequently (but also need to collide with correct orientation, so mostly the other factor)

Question 57

what is the relationship between rate of reaction and temperature

Answer 57

exponential, as seen on graph

Question 58

what does the Arrhenius equation show

Answer 58

the exponential relationship between the rate constant k and temperature

Question 59

what is the Arrhenius equation

Answer 59

k = Ae⁻ᴱᵃ/ᴿᵀ - where k is the rate constant - Ea is the activation energy - R is the gas constant, 8.314Jmol-1K-1 - T is the temperature on Kelvin (+273)

Question 60

what are the two parts of the Arrhenius equation

Answer 60

A: the pre-exponential factor e⁻ᴱᵃ/ᴿᵀ: the exponential factor

Question 61

what does A in the Arrhenius equation show

Answer 61

shows the frequency of collisions with the correct orientation, so essentially just the rate of reaction if there was no activation energy - does slightly increase with temperature, but generally stays the same over a small range

Question 62

what does the e⁻ᴱᵃ/ᴿᵀ bit of Arrhenius equation show

Answer 62

- links the activation energy and temperature - gives the proportion of molecules that exceed Ea and have sufficient energy for a reaction to take place

Question 63

what is the logarithmic way of expressing Arrhenius equation

Answer 63

lnk = -Ea/RT + lnA - via just taking logs of both sides - useful graphically, as you can find Ea and A

Question 64

how does the Arrhenius equation correlate with the temperature-rate constant graph

Answer 64

lnk = (-Ea/R)x(1/T) + lnA - where lnk is your y axis - -Ea/R is your gradient - 1/T is your x value - and lnA is your +C, so y intercept

Question 65

what axis do you use when making your temperature-rate constant graph

Answer 65

- your y-axis includes the values of lnk, so first need to find these from the k values given - your x-axis is given in 1/T, so need to find these from the T values given

Question 66

how do you find Ea and A from data given

Answer 66

1) find values of lnk and 1/T, and plot on graph (usually get graph with +ve x-axis, but -ve y axis) 2) find the gradient of your graph, which gives you -Ea/R 3) work back to find Ea 4) lnA is your y-intercept 5) so to find A, just do eˡⁿᴬ