18 - Rates Of Reactions Flashcards
Rate of reaction definition
The change in the amount of reactants and products per unit time
Rate concentration equation
Rate (moldm-3s-1)= concentration (moldm-3) / time (s)
Order definition
The power to which the concentration of a reactant is raised in the rate equation
Overall order definition
The sum of the individual orders of reactants in the rate equation
Rate constant definition
Links the rate of reaction with the concentration of reactants raised to powers of their orders in the rate equation
The bigger the constant the faster the reaction
What does the order of a reactant tell you
The order of reaction with respect to a reactant tells you how the reactants concentration affects the rate
What is the relationship between rate of reaction and concentration
The rate of reaction is proportional to the concentration of a reactant raised to a power
When is a reaction zero order with respect to a reactant
When concentration of a reactant has no effect on the rate
The rate stays the same as concentration changes
When is a reaction first order with respect to a reactant
When the rate depends on its concentration
And is directly proportional
When is a reaction second order with respect to a reactant
When rate depends on its concentration raised by a power of 2
Different methods of measuring rate of reaction
5
Measure volume of gas produced Measure mass lost Measure change in pH Measure change in conductivity Use colorimetry to measure colour change
What does the colorimeter measure
Measures the intensity of light passing through a sample
The filter is complementary to the absorbed colour
Absorbance is recorded - links to concentration
Coloromitry practical
To get concentration time graph
1) prepare standard solutions = known concentrations of the coloured chemical
2) select a complimentary filter
3) zero the colorimeter
4) measure absorbante readings for each solution
5) plot a calibration curve = absorbance/concentration
6) carry out the reaction - take absorbance reading at timed intervals
7) use calibration curve to find concentration of iodine at each absorbance Reading
8) plot concentration time graph
What can you tell from a concentration time graph
The orders of the reaction
Only if other reactants concentration remain constant
Concentration time graph
Zero order details
Straight negative line
Gradient = rate constant
Concentration time graph
first order details
Negative curve
Reaction slows
The time taken for concentration of reactant is halved
Half life is constant - used to find rate constant
Concentration time graph
Second order details
Same pattern as first
Steeper at the start
And tails off slower
Continuous monitoring definition
Repeatedly taking measurements during a reaction
How to determine the rate constant from a first order concentration time graph
1) using a tangent, to find the rate, then using the rate = k() equation to rearrange and find k
2) use K= ln2 / half life
Rate - Concentration graph
Zero order details
Straight horizontal line
Zero gradient
K= Y intercept
Rate - Concentration graph
First order details
Straight line from origin
Rate is directly proportional to concentration
K = gradient
Rate - Concentration graph
Second order details
Upward increasing curve
K = plotting second graph of rate against concentration2
Will be straight line through origin = gradient
What is the initial rate definition
Instantaneous rate of the start of a reaction when t = 0
How to measure initial rates practical
1) continuously monitor one reactant
2) plot a concentration time graph
3) repeat experiment using different initial concentrations of the reactants - keep the other reactants concentrations the same
4) use graphs to calculate initial rate for each experiment
5) repeat process for the other reactants
Why is finding the clock reaction better than finding initial rates
More convenient
Only taking one measurement
Less chance of human error
What are clock reactions
They estimate the initial rate of reactions by measuring the time taken for a set amount of product to form changes as you vary the concentration of the reactants. The end is usually indicated by a observational change.
Clock reactions
What does the speed of the reaction to finish indicate
Faster end of reaction
Faster initial rate
What are the assumptions you need to make for clock reactions
(4)
The temperature is constant
When the endpoint is seen the reaction has not proceeded too far
The concentration of each reactant does change significantly
-The rate of reaction stays constant during measurements
Iodine clock reaction practical
1) sodium thiosulfate and starch added to excess of hydrogen peroxide and iodine ions in acid solution
2) sodium thiosulfate added to reaction reacts instantaneously with Iodine that forms
3) when thiosulfate is used up, solution turns blue black
How accurate are clock reactions
Is accurate
Assuming rate of reaction is constant = initial rate
Providing that less than 15% of reaction has taken place
What do clock reactions measure
Measures the average rate of a change in reactant during first part of reaction
Reaction mechanism definition
The sequence of bond forming and bond breaking steps that shows the oath taken by electrons during a reaction
Rate determining step definition
The slowest step in the reaction mechanism of a multi step reaction
2 features used to predict reaction mechanisms
- the rate equation only includes the reacting species involved in the rate determining step
- the order of reaction with respect to a reactant match the number of species (molecules) involved in the rate determining step
What is the effect of temperature on the rate of reaction and rate constant
An increase is temperature = increases in KE, increases speed of particles, leads to more frequent collisions
Also mean more particles would have an energy equal or greater than the Ea
As a result the rate of reaction would increase whilst the concentration of the reactants stayed the same
So the rate constant would increase exponentially
What does the Arrhenius equation do
Links the rate constant and activation energy
What is the Arrhenius equation
K = A e^ -Ea/ RT
K = A e^ -Ea/ RT
What does each thing stand for and units
K= rate constant
A = pre exponential factor = frequency of collisions with correct orientation
Exponential factor = proportion of molecules that exceed Ea
Ea = activation energy (jmol-1)
R = gas constant = 8.314 (jk-1mol-1)
T = température (K)
How to plot a suitable graph to calculate Ea and A
1) calculate rate - 1/temp
2) calculate ln k
3) plot lnk against rate
4) Gradient = - Ea / R
5) rearrange to find activation energy
6) find ln A = y intercept
7) use logs to find A
Arrhenius equation sin Y=mx+c
lnK = lnA - Ea/R x 1/T
