The Rate Constant | A-Level Chemistry | AQA, OCR, Edexcel
How Rate Constant is Related to Rate of Reaction
The rate constant, k, is a number that connects the concentration of reactants in a reaction to the rate of that reaction.
The rate constant is different for every reaction.
The larger the value of k, the faster the rate of reaction.
The value of k can be determined by substituting in the values of rate and concentration of reactants into the rate equation, and then rearranging.
The Units of the Rate Constant, k
The units of k depend on the overall order of the reaction.
Zero Order Reaction
A zero order reaction, rate = k, the units of rate are mol dm−3s−1
First Order Reaction
First order - A first order reaction, rate = k[A].
Rearrange the equation above, so we have k = rate / [A]
We know the units of rate are mol dm−3s−1 and the units of concentration are mol dm−3.
We now can cancel down to find the units of k.
k = mol dm−3s−1 / mol dm−3
Therefore the units of k for a first order reaction are s−1.
Second Order Reaction
A second order reaction, rate = k[A][B]
Rearrange the equation above so we have k = rate / [A][B]
Again, cancel down to find the units of k.
k = moldm−3s−1 / (moldm−3 x moldm−3)
Therefore the units of k for a second order reaction are mol−1dm3s−1
Third Order Reaction
A third order reaction, rate = k[A][B]2
Rearrange the equation above so we have k = rate / [A][B]2
Again, cancel down to find the units of k.
k = moldm−3s−1 / (moldm−3 x moldm−3 x moldm−3)
Therefore the units of k for a third order reaction are mol−2dm6s−1
Worked example:
Results to show the initial rate of reaction with initial concentration of
CH3CHO
Experiment [CH3CHO] mol dm-3 Initial rate (mol dm-3 s-1)
1 0.25 2.90 x 10-6
Calculate the value of the rate constant and its units.
The rate equation is: Rate = k [CH3CHO]2
Answer:
Substitute the value of rate and concentration into the rate equation.
k = rate/ [CH3CHO]2
k= (2.9x10-6)/(0.25)2 = 4.64 x 10-5
To find the units, write the expression for k without any numbers, just
with the units. Cancel common terms.
units of k = mol dm-3 s-1 / (mol dm-3) (mol dm-3)
So k = 4.64 x 10-5 mol-1dm3 s-1
🔓 Unlock Full Course: https://courses.medicmind.co.uk/p/ale...
🔖 Past Papers: https://www.studymind.co.uk/resources/
👩🏻🏫 1-1 Tutoring: https://www.studymind.co.uk/subject/a...
#ALevelChemistry #Chemistry #A-LevelChemistry #StudyMind