topic 16 Flashcards
what is meant by rate of reaction
the change in the amount of reactants or products per unit time
recall the continuous monitoring methods that can be use to measure rate of reaction
measuring gas volume
change in mass
colour change
change in pH
titration
electrical conductivity
what is meant by a continuous monitoring method
measurements are taken over the duration of the reaction
how can measuring gas volume be used to measure rate of reaction
- if gas is given off in the reaction→ can collect it in a gas syringe
- record how much you’ve got at regular time intervals
- this method can e.g be used for a reaction between an acid and a carbonate → CO2 given off
- find conc. of reactant at each time point → use ideal gas equation to find moles and then use molar ratio to work out conc.
how can measuring change in mass be used to measure rate of reaction
- if gas is given off → system will lose mass
- you can measure the loss off mass at regular time intervals using a balance
- work out moles to work out how much gas has been lost and how many moles of reactant is left
how can tracking colour changes be used to measure rate of reaction
- track colour change of a reaction using colorimeter which measures the absorbance → how much light is being absorbed by the solution
- the more concentrated the colour of the solution → higher absorbance
- plot a calibration curve → known conc. of solution against absorbance
- during the experiment take a small sample from reaction solution at regular intervals and read absorbance
- use calibration curve to convert the absorbance into a conc at each time point
how can changes in pH be used to measure rate of reaction
- if the reaction produces or uses H+ ions → pH of solution will change
- if this is the case → you can measure the pH of the solution at regular intervals + calculate conc. of H+
how can a titration be used to measure rate of reaction
- take small samples of a reaction at regular time intervals and titrate them using a standard solution
- rate can be found from measuring the change in conc. of reactants/ products overtime
how can electrical conductivity be used to measure rate of reaction
if the number of ions changes so will electrical conductivity
how can a conc. time graph be drawn
- by repeatedly taking measurements (continuous monitoring) → can draw a conc.-time graph
- amount of reactant or product- y axis
- time- x axis
how can the rate of reaction be determined from a conc. time graph
- rate at any point in the reaction is given by the gradient at that point on the graph
- if the graph is a curve → draw tangent to the curve and find gradient of that
- draw tangent for the part of the curve that you want to find the rate of reaction from
- gradient= change in y/change in x
- or rise/run
what are the units for rate of reaction
conc. per unit time
why is there a negative gradient when measuring reactant concentration
this is because conc. of reactant falls to make product
what do orders tell you
tells you how a reactants conc. affects the rate
what does it mean if the order of reaction is 0 with respect to X
[X] changes and the rate stays the same
- so if [X] doubles → rate stays the same
- (so change in conc. of reactant doesnt affect the rate)
what does it mean if the order of a reaction is 1 with respect to X
rate is proportional to [X]
- if [X] doubles → rate will double
- whatever happens to the conc. of reactant, the same will happen to rate
what does it mean if the order of a reaction is 2 with respect to X
rate is proportional to[X]2
e.g - if [X] doubles → rate will be 2 to the power of 2 → 4 times faster
- if [X] triples → 3 to the power of 2→ rate is 9 times faster
how do you find the overall order of a reaction
overall order of reaction is the sum of the orders of all the reactants
how do you use a conc.-time graph to construct a rate-conc. graph
- find gradient at various points on the graph
- this will give the rate at that particular conc.
- need to draw tangents and find their gradients
- plot each point on a new graph with the axes rate and conc.
- rate. - Y axis
- conc.- X axis
- draw a smooth line or curve through the points
- the shape of the line will tell you the order of the reaction with respect to that reactant
what can a rate-conc. graph tell you
the order of a reaction
what does a horizontal line on rate-conc. graph mean
- order is 0
- changing the conc. doesnt change the rate
what does a rate-conc. graph with an order of 1 look like
if its a straight line through the origin → rate is proportional to [X]
what does a rate-conc. graph with an order of 2 look like
rate will be proportional to [X]2
what is half life + its symbol
the time it takes for half of the reactant to be used up
how can you work out half life
- to work out half life →plot a conc. time graph
- then draw lines across from the y-axis at points where the conc was halved
- then read off the time taken from the x axis
how can half life be used to identify a first order reaction
half-lives are useful for identifying a first order reaction (without needed to draw a rate-conc. graph)
- half life is always constant for a first order reaction
- always takes the same amount of time to halve
- half life is independent of conc. → first order
what is half life with respect to volume
- volume time graphs can also be used to identify first order reactions
- e.g the half-life is the time it takes for the volume of reactant to half
what is initial rate of reaction
the initial rate of a reaction is the rate at the start of the reaction
how is the initial rates method carried out
- carry out separate experiments using different initial concentrations of one reactant
- keep the rest f the conc. of reactants constant - see how changing initial conc. affects initial rate and figure out the order for each reactant
- can do this by carrying out experiments using continuous monitoring and drawing conc.-time graphs or volume-time graphs
- calculate gradient of tangent at time=0 →gives initial rate
what is the clock reaction an example of
is an example of the initial rates method
the quicker the clock reaction finished the faster the rate of reaction
what do you do in a clock reaction
measure how the time taken for a set amount of product to form changes as you vary the conc. of one of the reactants
why will there be a sudden inc in the conc. of a certain product
there will be a sudden increase in the conc. of a certain product as a limiting reactant is used up
how do you know when the end point occurs in the clock reaction
usually an easily observable end point e.g colour change → tells you when the desired amount of product has formed
what are the assumptions when carrying out a clock reaction
- the conc. of each reactant doesnt change significantly over the time period of your clock reaction
- temp stays constant
- when end point is seen the reaction hasnt proceeded too far
what is the equation for the iodine clock reaction and what is it also known as
- is a clock reaction
- also known as he Harcourt-Esson reaction
how do you carry out the clock reaction
1) a small amount of sodium thiosulphate solution and starch are added to excess hydrogen peroxide and iodine ions in acid solution
- starch used as an indicator → turns blue-black in the presence of iodine
2) sodium thiosulphate that is added to the reaction mixture reacts instantaneously with any iodine that forms
3) all iodine that forms in the first reaction is used up straight away in the second reaction.
once all of the sodium thiosulphate is used up → any more iodine that forms will stay in solution
- so starch indicator will suddenly turn the solution blue-black → end of clock reaction
4)varying iodide or hydrogen peroxide conc. while keeping others constant → gives different times for the colour change
5) the time it takes for the reaction to occur along with the conc. of reactants allows you to calculate the initial rate with respect to iodide or hydrogen peroxide
what is the equation for iodination of propanone and how can it be carried out
can be found by titrating
how can the initial rate of iodination of propanone be found
- monitor reaction by taking samples at regular intervals
- first stop the reaction in each sample by adding sodium hydrogen carbonate → neutralise the acid
- then titrate each sample against sodium thiosulphate and starch to work out the conc. of iodine
- repeat experiment and change the conc. of just one reactant
recall the general rate equation and its units
units moldm-3 s-1
explain the rate equation
- K- rate constant
- the larger the rate constant, the faster the reaction
- m and n are the orders of the reaction with respect to reactant A and B
- m→ tells you how the conc. of reactant A affects the rate
- n→ tells you how the conc. of reactant B affects the rate
- overall order of the reaction is m+n
how can you use the initial rates method to work out orders of reaction
- by comparing initial rates of a reaction with varying conc. of reactants from e.g a table of data → can find the orders of reaction for reactants
- once you know chemical equation for a reaction and orders of reaction → can write rate equation
- so find the order for each reactant → look at the effect of increasing one reactant on the rate to find the order
e.g see if its directly proportional, or no change
how can you calculate the rate constant from orders and the rate of reaction
- write rate equation
- insert values that you have e.g rate and conc
- rate goes on the left hand side and conc goes in the square brackets and the order outside the square brackets
- if order of a reactant equals 0 → not included in equation
- this is because [X] to the power of 0 is 1
- rearrange the equation and calculate K
- find units by inserting the units rather than values into the equation which is used to find K
what is the effect of temperature on the rate constant
- rate constant is always the same for a certain reaction at a particular temperature
- if you increase temp, rate constant will inc aswell
what is the rate determining step
rate determining step → slowest step in a multi-step reaction
in a reaction with a series of steps → each step can have a different rate
what can the rate equation be used to work out
- the rate equation can be used to work out the mechanism of a chemical reaction
how to determine which reactants are involved in the rate determining step from the chemical equation
- if a reactant appears in the rate equation→ it must affect the rate
- so this reactant or something derived from it must be in the rate determining step
- if reactant doesnt appear in rate equation → not involved in rate determining step
- the rate determining step doesnt have to be the first step in the mechanism
- the reaction mechanism cant usually be predicted from just the chemical equation
how can you predict the rate equation from the rate determining step
- the order of a reaction with respect to a reactant shows the number of molecules of that reactant which are involved in or before the rate determining step
- e.g if the reaction is second order with respect to X → two molecules of X in the rate determining step
- so look at the number of molecules of reactants in the rate determining step to determine the order of each reactant
- then write the equation
how can you predict the mechanism for a reaction from the rate equation
- rate equation gives you the reactants that determine the rate of the reaction
- gives reactants that are in the rate determining step
if a reactant from the chemical equation isnt in the rate equation then this means it cant be involved in the reaction until after the rate determining step
- gives reactants that are in the rate determining step
how can you find the mechanism for the reaction between propanone and iodine using the rate reaction- recall the rate equation and chemical equation as well
- propanone and H+ are in the rate equation so they (or something derived from them) must be in the rate determining step
- iodine → not in rate equation isnt involved until after the rate determining step
- order of reaction for both propanone ad H+ is 1 → so one molecule of each
- H+ is a catalyst - regenerated in another step
recall the three different types of halogenoalkanes
primary
secondary
tertiary
what is a primary halogenoalkane
halogen is joined to a carbon with just one alkyl group attached
what is a secondary halogenoalkane
halogen is joined to a carbon with two alkyl groups attached
what is a tertiary halogenoalkane
the halogen is attached to a carbon with 3 alkyl groups attached
how can halogenoalkanes be hydrolysed
all of these halogenoalkanes can be hydrolysed/split by heating them with sodium hydroxide but they react using different mechanisms
why are halogenoalkanes able to undergo nucleophilic substitution
- nucleophile attacks and swaps with the halogen
- C-halogen bond is polar as most halogens are more electronegative than carbon → so they draw e- to themselves
- carbon is partially positive so is easily attacked by nucleophiles
OH- is the nucleophile → provides pair of e- for the partially positive carbon
what are the two types of mechanisms for nucleophilic substitution
SN1 and SN2
what do SN1 reactions involve
SN1 reactions only involve 1 molecule/ion in the rate-determining step
what do SN2 reactions involve
SN2 reactions involve 2 molecules, 1 molecule and 1 ion, or 2 ions in the rate-determining step
what do primary halogenoalkanes only react by
only react by SN2 mechanism
what can secondary halogenoalkanes react by
can react by both the SNS1 and SN2 mechanisms
what can tertiary halogenoalkanes only react by
only react by the SN1 mechanism
what are the characteristics of a halogenoalkane undergoing the SN2 mechanism
will include both reactants
- has a single rate determining step
- because of this a transition state is formed
- the rate equation shows that the rate is dependent on the conc. of both reactants
- order with respect to each reactant is 1
- one molecule of each reactant
- both reactants must be involved in or before the rate determining step
- fits the SN2 mechanism
what are the characteristics of a halogenoalkane undergoing the SN1 mechanism
- rate is only dependent on the conc. of the halogenoalkane
- hydroxide ion is only involved after the rate determining step as its not in the rate equation
- reaction happens in two steps
- first step- halogen leaves the haloalkane
- second step- nucleophile is then able to attack in the second step
why does the SN1 mechanism occur the way it does
the reaction happens this way because → very little space around carbon as its surrounded by alkyl groups
how can you calculate activation energy
you can use the Arrhenius equation to calculate the activation energy
links rate constant with activation energy and temp
what is meant by activation energy
the minimum amount of kinetic energy particles need to react
recall the Arrhenius equation and what each part of the equation is
A- arrhenius constant
what is the link between activation energy and the rate constant(k)
- as the activation energy increases, k gets smaller
- if the activation energy increases this means that less particles have enough energy to react when they collide
- a large activation energy will mean a slow rate
what is the link between temp and the rate constant (k)
- the equation also shows that as temp inc, k also inc
- when temp inc→ particles have greater kinetic energy so are mere likely to collide
how can you simplify the arrhenius equation to find Ea
- can simplify the equation by getting rid of e part → take natural log (ln) of both sides. this gives:
- lnK= lnA - Ea/RT
- then rearrange: Ea/RT= lnA - lnK
- multiply both sides by RT to get Ea on its own: Ea= (lnA - lnK) x RT
- then substitute numbers in
how can you use arrhenius plots to find out the activation energy
- can use Arrhenius equation lnk= lnA - Ea/RT and plot a graph of lnk against 1/T (just add x10-3 to the end of temp)
- then draw a line of best fit
- this wil produce a graph with a gradient of -Ea/R
- once you know this gradient → can find activation energy
- change in y/change in x gives -Ea/R
- rearrange this equation: to get Ea on one side of the equation
how can you use an arrhenius plot to find the arrhenius constant
- substitute value of the gradient and any point on the line into the equation lnk= -Ea/ RT + lnA
- lnA = lnk + (-Ea/RT)
- rearrange the equation to get lnA on one side of the equation
- to work out A → press e button and calc and plug in the value of lnA
what is the units for a reaction with an order of 0
mol dm-3 s-1
what is the units for a reaction with an order of 1
s-1
what is the units for a reaction with an order of 2
mol-1 dm3 s-1
what is the units for a reaction with an order of 3
mol-2 dm-6 s-1
