# How far How fast, OCR- Unit 5, module 1

How far How fast, OCR- Unit 5, module 1

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• Created on: 13-05-12 18:27

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Sameer Biology
Jahabarali
Unit 5 Modul
e 1 How Far
How Fast
How Fast
Rate of reaction is the calculated through `change in
concentration of products or reactants per unit time'. It is
determined by finding the gradient of the curve/ line in a
concentration against time graph at any given time. This can be
done through:
Drawing the tangent to the curve at a given time
Finding the gradient of that tangent (change in y divided by the change in x)
The units or rate is: mol dm-3 s-1
Order respect to a reactant is the power to which the concentration of reactant will be proportional to the
rate of reaction. The overall order is the sum of all individual orders. Orders vary from zero and above, which
can be determined using initial rate methods, that requires separate experiments using different
concentrations of one of the reactants.
Order 0 ­ has half-life that's decreasing and is a straight line
Order 1- has a constant half life
Order 1< - has increasing half life
The rate constant is the constant that links the rate of reaction to the reactants raised
to their powers of their orders in the rate equation. It is the rate divided by product of all
concentrations of reactants. The units for the rate constant are done by substituting the
units of the rate and concentration of reactants in to the equation.
The rate constant will increase with increasing temperature. This is because molecules
have more kinetic energy, hence more successful collision take place. Therefore if rate
of reaction is increased with the same concentration, the rate constant must increase
as well.
Half-lives of a reactant are the time it takes for the concentration
of reactant to reduce by half. It is identified through a
concentration against time graph of a reactant. The half-life of:
Zero order- decreases with time
First order- constant
Second order / < - increases with time
Rate-determining step is the slowest stage of a multi-step
reaction (reaction that has multiple stages) which dominates the
overall rate, as the whole reaction is slowed down due to that
slow process. It can be determined through:
Finding the rate equation through experimenting
The all reactants that appears in the rate equation are involved in the rate determining step
The order of the reactant determines the number of moles involved in the rate determining step
The step may include decomposition or two molecules that react (unlikely to be more than three)

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Sameer Biology
Jahabarali
Unit 5 Modul
e 1 How Far
How Fast
Overall equation is the sum of each step of the reaction
How Far
Le Chatleir's principle: "equilibrium system under change will shift position of equilibrium in order to
minimise change".
Equilibrium constant Kc, determines the proportion of reactants to product present
in solution according to the relative concentration within equilibrium.…read more

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Sameer Biology
Jahabarali
Unit 5 Modul
e 1 How Far
How Fast
Acids and bases
Acids are species that can donate a proton (H+), if a compound willing to accept it; whereas bases are
species that can accept a proton.…read more

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Sameer Biology
Jahabarali
Unit 5 Modul
e 1 How Far
How Fast
The acid dissociation constant (Ka) measures to the extent dissociation in a weak
acid (position of equilibrium/ Kc for acid), where:
A large Ka value indicates a stronger (weak) acid
A small Ka value indicates a weak acid
pKa values are Ka expressed in a logarithmic scale, to make it more manageable (as
values can get quiet large), however the magnitude of pKa indicates:
Stronger acid if values are lower

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Sameer Biology
Jahabarali
Unit 5 Modul
e 1 How Far
How Fast
Calculation of pH is carried out if [H+(aq)] is known. Therefor [H+(aq)] can be calculated (through rearranging
formulas) for:
strong monobasic acids, using concentration of acid
weak monobasic acids, using concentration of acid and
Ka
strong bases,using Kw and concentration of [OH-(aq)]
Buffer solutions
Buffer solutions is a mixture that minimises pH changes if small amounts of acid or bases are added.…read more

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Sameer Biology
Jahabarali
Unit 5 Modul
e 1 How Far
How Fast
A sharp rise in pH, the vertical section when the centre is the equivalence point (the solution in the
conical flask has exactly reacted with the solution in burette)
A slight rise in pH, when the solution of burette (acid or base) in the