Chapter 18 Rates Of Reaction Flashcards
What is the equation for the rate of reaction?
Rate= change in concentration / time
What is the shorthand representation of concentration?
Square brackets enclosing the reagent
Eg, [A] means the concentration of chemical A
How does the order of reaction affect rate?
The rate of reaction is proportional to the concentration of a particular reactant raised to a power.
Rate ∝[A]^n
What is zero order?
When changing the concentration of a reactant has no affect on the rate of reaction
Rate ∝ [A]^0
What is first order?
When concentration of a reactant doubles, rate increases by a factor of x 2
Rate ∝ [A]^1
What is second order?
The rate is proportional to the concentration of a reactant to the power of 2
Rate ∝[A]^2
What is the rate equation?
Rate= k[A]^m [B]^n
K= rate constant
[A]= concentration of A
[B]= concentration of B
M= order of reaction with respect to A
N= order of reaction with respect to B
What is the equation for rate constant?
K= rate / [A] [B]
What is overall order?
The overall effect of the concentrations of all reactants have on the rate of reaction
How is the overall order calculated?
Sum of all order with respect to each reactant
What is continuous monitoring?
Taking continuous measurement of data during the course of the reaction
What type of graph can be formed by using continuous monitoring?
Concentration- time graphs
Describe the different method of continuous monitoring to create a concentration time graph
-monitoring gas volume
-monitoring mass loss
-monitoring colour change using a colorimeter which measures the amount of light absorbed by a solution
What is the shape of a zero order concentration- time graph?
Straight line with negative gradient
(Rate of reaction unchanged)
How is k calculated from a zero order concentration time graph?
K= gradient of straight line
Describe the shape of a first order concentration time graph
Downward curve with decreasing gradient over time
(Rate of reaction gradually slows down)
How can k be calculated from a first order concentration time graph?
K calculated from half life
K= ln2 / t1/2
K calculated from rate and substituted into equation
-Tangent drawn at particular concentration
-gradient calculated = rate of reaction
-rate equation rearranged and concentration at position of the tangent and rate is substituted into equation
Describe the shape of a second order concentration time graph
Downward curve which is steeper at the start and then tails off more slowly
What is half life?
The time taken for the concentration of a reactant to halve its original value
Describe how a first order relationship can be determined from a concentration time graph
If successive half lives are constant then the reaction is first order with respect to the reactant.
What are the units for rate?
Moldm^-3s-1
How does the rate of reaction change when the concentration is multiplied by 1/2 for zero order?
1/2^0 = x 1
How does the rate of reaction change when the concentration is multiplied by 1/2 for first order?
1/2^1 = x 1/2
How does the rate of reaction change when the concentration is multiplied by 1/2 for second order?
1/2^2 = x 1/4
Describe the shape of a zero order rate concentration graph
Horizontal straight line with zero gradient
(Rate unchanged)
How is k calculated from a zero order rate-concentration graph?
K= the intercept on y axis
Describe the shape of a first order rate concentration graph
Straight line through the origin
How is k calculated from a first order rate concentration graph?
K= gradient of straight line
Describe the shape of a second order rate concentration graph
Upward curve with increasing gradient
How is k calculated from a second order rate concentration graph?
The graph is a curve so k cannot be directly obtained.
A second graph needs plotting with rate against concentration squared resulting in a straight line through origin.
K= gradient of new line
What is initial rate?
The instantaneous rate at the beginning of an experiment
How can the initial rate of a reaction be measured?
Drawing a tangent when t=0 and measuring gradient
Clock reaction- changing the concentration of a reactant while keeping the concentration of other reactants constant.
At each concentration the time taken from the beginning of an experiment is measured until a visual change is observed.
Initial rate is proportional to 1/t
Values of 1/t are calculated for each time at different concentrations.
What is the Rate determining step?
The slowest step in a multi step mechanism reaction
What equation is used to identify the rate determining step?
Rate equation
What does the rate equation help to identify in the rate determining step?
The reactants involved in the RDS
The number of each reactant involved from the order
When predicting the two step mechanisms what conditions needs to be satisfied?
The RDS and second step add up to overall equation
The RDS is consistent with the rate equation
Any charged must balance out on reactants and products
Why does rate constant increase when temperature increases?
Increasing temperature shifts the Boltzmann distribution curve to the right, increasing the proportion of particles that exceed activation energy
Particles have more kinetic energy so more effective collisions occur
Determine the components of the Arrhenius equation
K=Ae^-Ea/RT
K= rate constant (same units as A)
A= pre exponential factor (same units as k)
-Ea= activation energy in Jmol^-1
May need to change into KJmol^-1
R= gas constant 8.31
T= temperature in Kelvin
What does the Arrhenius equation predict?
How rate constant changes with activation energy or temperature
Using the Arrhenius equation, how does k change when activation energy increases?
Increased activation energy= lower k
Using the Arrhenius equation, how does k change when activation energy decreases?
Decreasing activation energy= higher k
Using the Arrhenius equation, how does k change when temperature increases?
Increasing temperature = higher k
Using the Arrhenius equation, how does k change when temperature decreases?
Decreasing temperature = lower k
Using the Arrhenius equation, how does k change when the pre exponential factor increases?
Increasing A= higher k
Using the Arrhenius equation, how does k change when the pre exponential factor decreases?
Decreasing A= lower k
What is the intermediate of a two step mechanism?
A product formed in one of the steps but is used up again later
Explain why it is unlikely for a reaction to proceed by one step only
An effective collision is unlikely to occur between more than two species
What is the logarithmic form of the Arrhenius equation used to predict?
ln k = -Ea/ RT +lnA
The activation energy and pre exponential factor can be determined graphically
ln k = -Ea/ R x 1/ T +lnA
Y = M X + C
Gradient = -Ea/ R
-Ea= gradient x m
lnA- pick a particular concentration point and isolate Ln A by substituting in the value of ln k, -Ea/ R and 1/ T
Describe how the activation energy and frequency factor can be found graphically
A graph is first plotted of ln k against 1/t
Activation energy can then be calculated by measuring the gradient of the line
Ln A is isolated by substituting in the value of ln k, -Ea/ R and 1/ T
A is measured independently by doing the inverse of ln= e on calculator
Describe the shape of the logarithmic Arrhenius equation graph?
LnA against 1/T
Straight line with negative gradient
What is the equation for half life?
T 1/2 = ln2 / k
