rate of reaction chp 18 Flashcards
How are rates of reactions measured
draw the formulas
- changes in quantity of reactant/product over time
- for consistency we measure rates as the change in concentration
formula for rate of reaction and units envolved
- △concentration/time=rate
- conc in mol dm^-3
- time most often in seconds (but whatever is stated in the question)
- rate in mol dm^-3 s^-1
what is meant by square brackets in chemistry
used to illustrate concentration
what is the rate of reaction proportional to
write formula
the rate of reaction is proportional to the concentration
features of order of reaction
- For each reactant the exponent is the order of reaction
- different reactants may have different orders of reaction
- cant work out the order of a reactant from the equation (has to be done experimentally)
what are the common orders of reaction
- zero order (0),
- first order (1),
- second order (2)
what is zero order
- when the concentration of a reactant has no effect on the rate
- any number raised to the power of zero is 1
- concentration does not effect the rate
what is meant by the order of reaction
the relationship between the rate of a chemical reaction and the concentration of the species taking part in it
features of first order
- when the rate depends on its concentration raised to the power of one
- if the concentration of A is doubled (x2), the reaction rate increases by a factor of 2^1 = 2
- in a first order reaction, any change in concentration gives the same change to the rate
features of second order
- when the rate depends on its concentration raised to the power of 2
- if the concentration of A is doubled (x2), the reaction rate increases by a factor of 2^2 = 4
- any change in concentration changes the rate by the square of the change
draw the rate equation
annotated
why is the rate constant important in the rate equation
- it converts between the rate of reaction, concentration and orders
what is the overall order
- its the sum of the orders with respect to each reactant
How are orders of reaction determined in reality
- orders of reaction must be determined experimentally by monitoring changes in quantity over time
- Orders cannot be found directly from the chemical equation
what is the initial rate
- the rate of the reaction at zero seconds
what is continuous monitoring
collecting experimental data throughout the course of a reaction to plot a concentration-time graph.
what are some examples of continuous monitoring of reactions that produce a gas as one of the products
- monitoring gas collection
- monitoring mass loss
for reactions that do not produce gases and so there would be no mass loss what can be used to monitor the rate of the reaction
- Colour changes
^which can be estimated by eye or monitored using a colorimeter - changes in concentration of product/reactant
explain briefly how a colorimeter works
- zero the colorimeter using solution of known value
- plot absorbance vs concentration
- measure absorbance of unknown sample
- compare absorbance of sample with calibration graph
- use appropriate filter given solution
what does the gradient of a concentration time graph represent
the rate of reaction
what can be deduced from the gradient of a concentration-time graph
The order with respect to a reactant can also be deduced from the shape of a concentration-time graph for zero and first order reactions
what condition needs to be met for the order with respect to a reactant to be obtained from a concentration-time graph
The order with respect to a reactant can only be obtained if all other reactant concentrations remain effectively constant.
what concentration-time graph does a zero order reaction produce
what is the gradient = to
- producers a straight line with a negative gradient.
- ^rate does not change at all during the course of the reaction.
- The value of the gradient is = to the rate constant K.
what concentration-time graph does a first order reaction produce, what is unique about it
- time for the concentration of the reactant to halve is constant
- can be used to calc rate constant
what concentration-time graph does a second order reaction produce
- more dramatic curve than 1st order
what is meant by a half-life (t / 1/2)
- time taken for half of a reactant to be used up.
what is exponential decay
- a process in which a quantity decreases over time, with the rate of decrease becoming proportionally smaller as the quantity gets smaller.
- first order reactions have exponential decay
as the concentration halves every half life
What is a way (not finding the gradient) to find out if a reaction is first order from a concentration-time graph
A first order relationship can be confirmed from a concentration-time graph by measuring successive half-lives. If they are the same, the reaction is first order with respect to the reactant
What are the 2 methods to determine the rate constant K from a concentration-time graph
- calculating the rate constant from the rate
- calculating the rate constant from the half-life
How can you calculate the rate constant from the rate (on a concentration-time graph)
A tangent to the curve on the concentration-time graph is drawn at a particular concentration. The gradient of the tangent is calculated giving the rate of reaction
The rate constant is calculated by rearranging the rate equation and substituting the value of rate (the gradient of the tangent) and the concentration at the position where the tangent has been drawn.
how can you calculate the rate constant from the half-life
a way of calculating the value of the rate constant is to make use of the exponential relationship for a constant half-life:
k=ln2/ (t/ 1/2)
This method is also much more accurate than drawing a tangent
what can be the downsides of using a tangent to calculate rate and therefore the rate constant
difficult to judge how steep the tangent should be
explain why a zero order reaction gives the shape it does on a rate-concentration graph
draw a zero order reaction rate-concentration graph
- the intercept on the y-axis gives the rate constant K
- the reaction rate does not change with increasing concentration
why does a first order reaction producer the shape it does on a rate-concentration graph
draw a first order reaction rate concentration graph
- rate is directly proportional to concentration for a first order relationship
how can you find the rate constant from a rate-concentration graph
- the rate constant can be determined by measuring the gradient of the straight line of this graph (only for first order reactions)
- as a second order reaction produces a curve that rate cannot be obtained directly
what shape does a second order reaction produce on a rate-concentration graph why is this
rate = k[A]^2
how can you find the rate constant from a second order reaction from the rate and concentration
by plotting a second graph of the rate against the concentration squared, the result is a straight line through the origin. The gradient of this straight line graph is equal to the rate constant K
how can you find both the order and rate constant from one graph
- graph of log(rate) against log(concentration) can be used
- calculate gradient of tangent to find order
- find out rate constant from calculating, using conc and rate at a specific point
How can the initial rate be found from a concentration-time graph
The initial rate can be found by measuring the gradient of a tangent drawn at t=0 on a concentration-time graph
what is a clock reaction
A clock reaction is a more convenient way of obtaining the initial rate of a reaction by taking a single measurement.
what is the initial rate proportional to
when is the initial rate proportional to this
- initial rate is proportional to 1/t
- when there is no change in rate during an experiment
what is a common type of clock reaction
iodine clocks
what is the function of an iodine clock reaction
- by varying concentrations or conditions, we can determine orders of reaction with respect to each reactant
- abrupt colour change makes it easy to measure reaction rate
What are you measuring in a clock reaction
- In a clock reaction you are measuring the average rate during the first part of the reaction.
- In a clock reaction, you are measuring an average rate of a change in reactant over time
within what percentage of a reaction is calculating the initial rate with its values still valid
clock reaction is an approximation but it is still reasonably accurate provided that less that 15% of the reaction has taken place.
which step is the rate-determining step
Within a multi-step reaction the rate determining step is always the slowest
what is a reaction mechanism
series of steps that make up an overall reaction
How do chemists know that a proposed reaction mechanism is correct for a reaction
- rate equation only includes reacting species involved in the rate determining step
- the orders in the rate equation match the number of species involved in the rate determining step
What are haloalkanes hydrolyses by
write reaction
Haloalkanes are hydrolyses by hot aqueous alkali
How will increases in temperature affect K (the rate constant)
- temp↑, rate↑, K↑
- (For many reactions each 10°C rise in temperature doubles the rate constant and the rate of the reaction.)
what factors affect the rate constant
- temperature
^increases particles with Ea, particles move faster so collide more frequently
Draw the Arrhenius equation
annotated
How can the rate constant be determined experimentally
By carrying out the same experiment at different temperatures, rate constants can be calculated using arrhenius equations
what does the exponential factor represent
- the proportion of molecules that have at least minimum activation energy
What does the pre-exponential term A (frequency factor) take into account
it takes into account the frequency of collisions with the correct orientation.
How does the pre-exponential term react to an increases in temperature
- increases slightly with temperature as frequency of collisions increases
- is essentially constant over a small temperature range.
The Arrhenius equation expressed as a logarithmic relationship, why is this Arrhenius equation useful
It is very useful as it enables both activation energy and the frequency factor to be determined graphically
what can the logirithmic arrhenius equation be interpreted as
draw it
M= gradient
If a ln k and the 1/t / k^-1 are plotted against each other what Is produced
Depending on wether ln k is positive or negative, you my find that your graph comes either side of the origin (it may even straddle the x-axis). However, the principle is the same. The gradient - Ea/R and intercept is ln A
