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Chapter 18 - Rates of reaction
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Terms in this set (24)
Rate equation
Rate = k[A]^m[B]^n
Overall order = m+n
Units for rate and concentration
rate = moldm-3s-1
concentration = moldm-3
Zero order reaction
The concentration of a reactant has no effect on the rate of reaction:
rate ∝ k[A]^0 = k
(any number to the power of 0 is 1)
First order reaction
The rate of reaction is directly proportional to the concentration of A to the power of 1:
r = k[A]^1
If [A] is doubled => reaction rate increases by a factor of 2^1=2.
Second order reaction
The rate of reaction is proportional to the concentration of A to the power of 2:
r = k[A]^2
If [A] os doubled => reaction rate increases by a factor of 2^2=4.
Zero order reaction in a concentration-time graph
Reaction rate doesn't change at all during the course of the reaction.
Gradient = k
First order reaction in a concentration-time graph
Decreasing gradient over time.
Time for concentration to halve is constant => half-life (t1/2) is constant.
How can reaction rates be calculated from graphs?
By drawing a tangent to the curve (at different times) and calculating the gradient of the tangent.
Working out orders from experimental initial rate data
If conc is doubled and rate stays the same: order= 0
If conc is doubled and rate doubles: order= 1
If conc is doubled and rate quadruples: order= 2
Rate constant k
k is the same for all experiments done at the same temperature.
Increasing the temperature increases the value of the rate constant k
Calculate the rate constant k from the half-life
k = ln2/t1/2
Much more accurate thank drawing a tangent.
Calculate the rate constant from rate
k = rate/[A]
Clock Reaction
A method used to find the initial rate of a reaction.
Time from the start of the reaction until a visible change is measured.
Reaction repeated several times at the same temperature using different initial concentrations of one reactant.
All other concentrations are kept constant.
Initial rate can be expressed as 1/t.
Longer the time period, the less accurate the initial rate because rate varies during reaction.
clock reaction assumptions
- Assumed that the rate is constant during this period and so average rate is same as initial rate.
Exam question:
Hydrogen peroxide reactions with iodide ions in presence of acid as shown by the equation below:
H2O2(aq) + 2I-(aq) + 2H+ -> I2(aq) +2H2O(l)
Describe an experiment that could be used to show that the reaction is first order with respect to hydrogen peroxide, H2O2.
1. Make solutions of different concentrations of H2O2 eg 1.0 moldm-3, 0.8 moldm-3, 0.6 moldm-3, 0.4 moldm-3 and 0.2 moldm-3.
2. Measure out I- (aq) and H+ (aq) and add to a beaker/conical flask.
3. Add small amounts of starch indicator.
4. Add 1.00moldm-3 H2O2 + measure time taken for black/blue colour to form (from colourless).
5. Repeat for the other H2O2 solutions using the same volumes and concentration of I- and H+ as before.
6. Rate = 1/t
7. Plot graph of 1/t against concentration.
8. It is first order with respect to H2O2 if straight-line graph through the origin.
Zero order reaction in a rate-concentration graph
rate = k[A]^0 so rate = k
First order reaction in a rate-concentration graph
rate = k[A]^1 so rate =k[A]
Rate is ∝ to concentration for the first order
Rate constant can be determined by measuring the gradient of a straight line.
Second order reaction in a rate-concentration graph
rate = k[A]^2
Cannot obtain k from second order graph => must draw a second graph of the rate against the concentration^2 = straight line through origin => gradient of line = k
The Arrhenius equation
K = Ae^(-Eₐ/RT)
k: rate constant of a reaction
A: Pre-exponential factor
Eₐ: activation energy
R: ideal gas constant (8.314Jmol-1K-1)
T: temperature (kelvin)
Pre-exponential factor (Arrhenius equation)
= A.
It takes into account the frequency of collisions with the correct orientation.
Gives the rate if there was no activation energy.
Exponential factor (Arrhenius equation)
= e^-Ea/RT
Represents the proportion of molecules that exceed Ea and that have sufficient energy for a reaction to take place.
Logarithmic form of Arrhenius equation
ln k = -Ea/RT + ln A
y = mx + c
y = ln k
m = -Ea/R => gradient
x = 1/T (1/T was taken out of equation)
c = ln A
Rate Determining Step (RDS)
the slowest step in a reaction mechanism
rate constant k increases with temperature
-> grater proportiion of particles have energy that exceeds Ea.
-> The particles move faster and collide more.
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