Please enable JavaScript.
Coggle requires JavaScript to display documents.
Module 5 Chapter 18 (18.1 Orders, Rate Equations and Rate Constants (Rate…
Module 5
Chapter 18
18.1 Orders, Rate Equations and Rate Constants
Rate of Reaction
Definition
1)
Quantity reacted/ produced with respect to time
2)
Change in concentration with respect to time
Rate has units of moldm-3 s-1
Shorthand for concentration of A=
[A]
Order
rate∝[A]^n
Zero Order (0)
Rate∝[A]^0
Rate is proportional to 1
Concentration doesn't influence rate
First Order (1)
Rate∝[A]
If conc. doubles rate doubles
Second Order (2)
Rate∝[A]^2
If [A] is doubled rate increases by 4x
Rate Equation
Where
r=rate
k= rate constant
x=order with respect to A
y=order with respect to y
Rate Constant
= proportionality constant
Overall order
= sum of orders= x+y
Must work out units of rate constant from the rate equation
18.2 Concentration-time graphs
Continuous monitoring of rate
Can plot conc-time graphs from it
Types
Mass loss
Gas collection
Colorimeter
Control wavelength of light passing using a filter
Measure amount of light absorbed by filter
Order from conc-time graph
Zero
Straight line with negative gradient
Rate is constant
Gradient= rate constant (k)
First
Downward curve with decreasing gradient over time
Constant half-life
Second
Downward curve but starts steeper and tails off more slowly
Half-life
Time taken for concentration of reactant to half
First order- constant half-life=
exponential decay
Can calculate k from half life=
k=ln2/half-life
18.3 Rate-Concentration Graphs
Order from rate-concentration graphs
Zero
Horizontal straight line
y-intercept gives k
Rate doesn't increase with increasing conc.
rate=k
First
Straight line through origin
rate=k[A]
k=gradient
rate directly proportional to [A]
Second
Upward curve with increasing gradient
rate= [A]^2
Can't calc. k from graph
Rate against concentration squared gives straight line through origin- gradient=k
Initial rate
Can find rate at t=0 by drawing tangent to curve and calculating gradient
Clock reaction
More accurate way to get initial rate
Measure time (t) from start of experiment to visual change
So long as no significant change of rate in this time,
initial rate is proportional to 1/t
Repeat several times with different conc.
Common clock relies on formation of iodine- add starch so it produces a dark blue-black colour
Accuracy
You assume average rate is constant and same as initial
Measure at steepest point on curve
Rate virtually same as initial rate
To be accurate should have colour change when between 10% and 15% of products formed
18.5 Rate Constants and Temperature
Rate constant and temperature
Increase temp. = increase rate = increase k
Many reactions if you increase in 10℃ doubles rate constant and rate
Why
Boltzmann distribution shifts to right increasing proportion exceeding Ea-
main reason
Particles move faster and collide more frequently
Particles must collide with correct orientation
Arrhenius
Exponential factor of it represent the proportion of molecules exceeding Ea (sufficient energy for reaction to take place)
Frequency factor takes into account frequency of collisions with correct orientation
Logarithmic form
Means Ea and A to be determined graphically
Plot ln k against 1/T gives equation in form y=mx+c when m is -Ea/R and c is ln A
18.4 Rate-determining step
Multi-step reactions
Unlikely for reactions where 3+ reactants must collide with correct energy to take place in 1 step
Slowest step=
rate determining step
Predicting
Only species in rate-determining step in rate equation
Order in rate equation match number of species involved in rds