Physical 9: Rate Equations Flashcards
Give the general rate equation
Where k is the rate constant
M and n are orders of reaction
What is the Arrhenius equation?
k = rate constant
A = Arrhenius constant
Ea = activation energy (kJmol-1)
R = gas constant
T = temperature (K)
Describe what zero order, first order, and second order mean in terms of rate.
Zero order: The reaction rate is proportional to [A]^0, so the rate is not affected
First order: The reaction rate is proportional to [A], so the reaction increases as [A] increases
Second order: The reaction rate is proportional; to [A]^2,
What is the order of reaction with respect to a given reactant?
The order of reaction with respect to a given reactant is the power to which the concentration of the reactant is raised in the rate equation
What is the overall order of reaction?
The sum of the powers of the concentrations in the rate equations
What is the only factor that affects rate constant?
Temperature (k increases with temperature)
How do you work out the units of the rate constant?
Rearrange the rate constant so k is the subject
Set the concentrations as equal to moldm-3
Simplify to work out rate constant units
i.e. moldm-3s-1
What is the rate determining step?
The slowest step in the reaction
The rate of reaction is dependent on the rate of this step
Species that are involved in steps after the RDS do not appear in the rate equation
Do catalysts appear in the rate equation?
Yes
A + 2B -> C + D
Rate equation = k[A][B]
Possible mechanism
Step 1: A + B –> P
Step 2: P + B –> C + D
Which is the rate determining step?
Step 1, because A and B are the only reactants that appear in the rate equation, and both appear in step 1, so step 1 is the RDS.
What is the natural log form of the Arrhenius equation?
lnk = lnA - Ea/RT
What equation links gradient, activation energy, and gas constant from a 1/t to lnk graph?
Gradient = -Ea/R
What does the concentration-time graph for a zero order reaction look like?
A decreasing straight line
What does the concentration-time graph for a first order reaction look like?
A downwards curved line
What does the concentration-time graph for a second order reaction look like?
A steeper curve than for first order
What does the rate-concentration graph for a zero order reaction look like?
A straight horizontal line
What does the rate-concentration graph for a first order reaction look like?
A rising straight line
What does the rate-concentration graph for a second order reaction look like?
An upwards curve
What is the initial rates method?
Can be calculated from taking the gradient of a continuous monitoring graph when t = 0
Can also be calculated from clock reactions where the time taken to reach a fixed concentration is measured
E.g. iodine clock
What is the continuous rate method?
Measure the rate at several points during one reaction to see how it changes
