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Module 5 - Chapter 18 - Rates of reaction - Coggle Diagram
Module 5 - Chapter 18 - Rates of reaction
Rate-concentration graphs
Orders from shapes
zero order
Horizontal straight-line with a gradient of 0
The y intercept gives the rate constant k
First order
Straight line through the origin
Rate is directly proportional to concentration for a first order relationship
Rate constant can be determined by measuring the gradient
Second order
Upward curve with increasing gradient
Rate constant can't be directly obtained from this graph
Plot a graph of rate against concentration squared, this produces a straight line and the gradient equals k
Initial rates method
Measure the gradient of a tangent drawn at t=0 on a concentration time graph
Clock eaction is a more convenient way of obtaining intial rate of reaction from a single measurement
The average rate of reaction over this time will be the same as the initial rate
Initial rate is proportional to
Repeat several times with difference concentrations
Iodine clocks
Time for the appearance of iodine colour can be measured
Different concentrations are used and all other concentrations are kept constant
Colour change is kept by including a small amount of another chemical (sodium thiosulfate) which removes iodine as it forms
Solution is colourless at the start and the time is measured for the blue black colour of starch-iondine
Graph of
against concentration is plotted
Accuracy
You measure teh average rate of change in reactant over time
Shorter the time over which the rate is measured, the less the rate changes
When the measurement is small, the rate is virtually the same as the initial rate
When the time measurement is large, there is a large differene from initial rate
Initial rate measuring during a lock reaction is an approximation but it is still reasonably accurate provided less than 15% of the reaciton has taken place
Rate determining step
Multi-step reaction
For a reaction with multiple reactants to take place, the particles would need to collide together simultaneiously which is unlikely
Such reactions are more likely if a series of steps are taken (reaction mechanism)
Rate-determining step
Slowest step in the step is the rate-determining step
Predicting reaction mechanisms
Rate equation only indicates reacting species involved in the rate-determining step
Order in the rate equation match the number of species involved in rate-determining step
Temperature
What factors affect the rate constant
Increasing temperature shifts Boltzmann distribution rihgt and increasing propotion of particles that exceed activation energy
Increasing temperature means particles move faster and collide more frequently
Increased frequency of collisions is comparatively small compared with the increase in the propotion of molecules that exceed activation energy
Change in rate is mainly determined by activation energy
Arrhenious equation
Exponential relationship between rate constant and temperature T
Exponential factor is the proportion of molecules that exceed activation energy
Frequency factr takes into account frequency of collisions with correct orientation. It increases slightly with temperature
Rate constant = frequency factor X exponential factor
Logarithmic form
Shows relatiohsip between rate constant and temperature
