rates of reaction Flashcards
How are reaction rates measured
- by observing the changes in the quantaties of reactants or products over time
What are the equations for rate of reaction
- quantity reacted or produced/time
- change in concentration/time
what is the unit for rates
moldm-3s-1
what does the power represent for each reactant
order of reaction
what are the common orders
- zero
- first
- second
What is zero order
when the concentration of a reactant has no effect on the rate
what happens in a zero order reaction
- any number raised to the power zero is 1
- concentration does not influence the rate
what is the first order
a reaction is first order respect to a reactant when the rate depends o its concentration when raised to the power of 1
What is a first order reaction
- if concentration is doubled the reaction rate increases by a factor of 2
- if concentration is triples the reaction rate increases by a factor of 3
What is second order
a reaction is second order with respect to a reactant when the rate depends on its concentration raised to the power of 2
What happends in a second order reaction
- if concentration is doubles the reaction rate increases by a factor of 4
- if concentration is triples the reaction rate increases by a factor of 9
What is the rate equation
the rate equation fives the mathematical relationship between the concentrations of the reactants and the reaction rate
What is the rate constant k
- is the proportionality constant it is the umner that mathematically converts the rate of reaction and concentration and orders
What is the overall order
- the overall order of a reaction gives the overall effect of the concentrations of all reactats on the rate of reaction
- sum of orders with respect to each reactant
How do you solve the units of the rate constant k
- rearranging the equation to make k the subject
- substitute units into the expression for k
- cancel common unit and show the final units on a single line
What is the initial rate
the initial rate is the instantaneous rate at the beginning of an experiment t=0
How can concentration time graph be plotted
- from continuous measurement taken during the ccourse of a reaction - continuous monitoring
How can you monitor reactions that produce a gas
- monitoring by gas collection
- monitoring by mass loss
How can you monitor a reaction that does not produce gas and how
- colour change
- by eye or monitored with a colorimeter
How is a colorimeter used to detect rate of reaction
- measures the wavelengthh of the light passing through a coloured solution is using a filter. the amount of light is absorbed by a solution is measured
How can you tell the rate of reaction from a concentration time graph
measure the gradient
Describe the concetration time graph for zero order
- produces a straight line with a negative gradient
- reaction rate does not change at all during the course of the reaction
- value of the gradient is equal to the rate constant k
Describe the concentration time graph for first order
- a first order reaction produces a downward curve with a decreasing gradient over time
- as gradient decreases with time the reactopm gradually slows down
- the time or the concentration of the reactant to halve is constant
- this is called half life
- rate constant can be determined using it value
Describe the concentration time graph for second order
- downward curve
- steeper at the start
- tailing off more slowly
What is half life
time taken for half of a reactant to be used up
What is the first order concentration time graph pattern referred to
exponential decay
how are rate concentration graphs plotted
- measurements of the rate of reaction at different concentrations
Why are rate concentration graphs important
- offer a route into direct link between rate and concentration in the rate reaction
describe the rate concentration graph for zero order
- produces a horizontal straight line with zero gradient
- intercept on y axis gives the rate constant k
- reaction rate does not change with increasing concentration
describe the rate concentration graphs for first order
- reaction produces a straight line graph through the origin
- rate is directly proportional to concentration for first order relationship the rate constant can be determined by measuring the gradient of the straight line of this graph
describe the rate concentration graphs of second order
- second order reactant produce an upward curve with increasing gradient
- as this rate concentration graph is a curve the rate constant cannot be obtained directly from this graph
- by plotting second graph of rate against concentration squared result is a straight line through the origin
- gradient of this straight line graph is equal to the rate constant k
what is a clock reaction
- a convenient way of obtaining the intial rate of reaction by taking a single measurement
- time t from the start of an experiment is measured for a visual change to be observed often a colour or precipitate
What is the initial rate proportional to
1/T
What are iodine clocks
- relies on formation of iodine
- as aqueous iodine is coloured orange brown the time from the start of the reaction and appearance of iodine coloured can be measured
- starch is usually added since it forms a complex with iodine which is an intense blue black colour
How accurate are clock reactions
- in clock reactions you are measuring the average rate during the first past of the reaction
- over this time you can assume that the average rate of reaction is constant and is the same a the initial rate
- you are measuring an average rate of a change in reactant over time
- the shorter the period time over which an average rate is measured the less the rate changes over that period of time
- initial rate measured during clock reaction is approximation but it is sill reasonably accurate
Describe multi step reactions give an example
- a reaction can only take place when particles collide
- for example one molecule of H2O2 and two I- ions and two H+ ions would have to collide togetherr simultaneously which is an unlikely event
- so it is more likely to take place in a series of steps and it is unlikely more than two particles will collide together at the same time
What is a reaction mechanism
the series of steps tthat make up an overall reaction
What is the rate determining step
- slowest step in the sequence
How do we know wether a reaction mecahnism is likely to be correct?
- the rate equation only includes reacting species involved in the rate determining step
- the orrders in the rate equation match the number of species involved in the rate determining step
Describe the hydrolysis of haloalkane and what it is used to show
- they are hydrolysed by hot aqueous alkali
- RBr+Oh–>ROH+Br-
- hydrolysis reactions of haloalkanes can be investigated experimentally to determine the overall order of reaction adn the rate equation as well as a possible mechanism for the reaction
Describe what happens to the rate constant when temperature increases
- as temperature increases, the rate increases and the value of the rate constant k will also increase
- as 10 degrees celcius rise the rate constant will double as well as the rate of reaction
What two factors affect the rate constant when temperature is increases
- increasing temperature shifts the boltzmann distribution to the right increasing amount of particles that exceed activation energy
- temperature increases particles move faster and collide more frequently
What is the exponential form of the arrhenious equation
k=Ae^Ea/RT
What is the logarithmic form of the arrhenious equation
lnk=-Ea/Rx1/T+ln A
y=mx+c
How to calculate the activation energy from a graph
- calculate the gradient
- Ea=- Rx gradient
