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What is a Buffer?
Firstly, note that buffer is just short for buffer solution. So a
buffer/buffer solution is a solution that comprises a weak acid and its
conjugate base or a weak base and its conjugate acid.
So the Ph of the solution changes very little when acids or bases are added to
it. This is because Buffers were designed to keep a solution’s ph at a
constant number.
Within buffer solutions, there is a balance or equilibrium between the weak
acid and its conjugate base or the weak base and its conjugate acid.
Acids are represented with
HA
Bases are represented with
A-
So you see, when you add hydrogen ions (H+) to the solution, the equilibrium
will move RIGHT!
BUT! When you add hydroxide ions (OH-) to the solution, the equilibrium will
move LEFT!
So think of it as, when you see a + sign, the goes with right hand, and the –
sign goes with the left hand.
The Henderson-Hasselbalch equation is ph = pKa + log ([A-] / [HA])
This equation is used to find the ph of a BUFFER SOLUTION.
EXAMPLE: Calculate the pH of a 0.20M CH3COOH and 0.30M CH3COONa solution.
Ka = 1.8 x 10-5
Answer: 4.9
Ph = pKa + log ([A-] / [HA])
First find pka
Pka = -log(ka)
= 4.74
Now insert values in to formula
Ph = 4.74 + log (0.20 / 0.30)
Ph = 4.56
2ND EXAMPLE: What would be the pH of a 0.20M CH3COOH solution?
pH = 2.72
So you wonder how to make a buffer solution as a specific ph?
Well it depends of two things:
The pka of the weak acid being used,
And the RATIO of [] of the acid and the salt you add to the solution
IT DOES NOT DEPEND OF THE CONCENTRATION OF THE BUFFER BUT RATHER THE TWO
PARTS.
NOTE: If the ph is LESS THAN 7, it is an acid buffer.
If it is MORE THAN 7 it is a base buffer
Electrochemistry
So electrochemistry is the study of the electric charges that chemical
reactions bring out.
There are two electrochemical cells:
Galvanic cells
Electrolytic cells
In the galvanic or voltaic cell, electricity is produced due to the chemical
reaction. While in electrolytic cells, the electricity is provided to drive
the chemical reaction.
Electrochemistry involves the transfer of electrons and this is where redox
reactions come in. We know that reduction is the gain of electrons and
oxidation is the loss of electrons.
Cell Potential
This is just the difference between the positive metal electrode and the
negative metal electrode.
Okay so, when a stripe of metal, which is called an electrode is dipped in a
solution of its own ions, the electrode will undergo oxidation and lose
electrons, becoming an electrode cation.
This will be the forward reaction.
When the solution already has cations of the electrode, those cations will
fill the space of the electrons. Thus, reduction is taking place and the metal
is formed again.
This is the reverse reaction.
To know what is the cell potential difference of an electrode we use the
Standard Hydrogen Electrode (SHE).
When a half reaction has a + E value, that means it can lose electrons easily,
however, when it have a – value, it cannot.
Electrochemical Cells
The most common type of Galvanic Cell is the Daniell Cell.
To make a quick cell structure, you can write the shorthand cell notation.
The anode is written first, then the salt bridge with two lines and the
cathode next.
Cell Potential and Spontaneity (Gibb’s Free Energy)
The formula is:
When you have a + cell potential, G.F.E is – and the reaction is spontaneous.
When you have a – cell potential, G.F.E is + and the reaction is nonspontaneous.
How to Calculate Gibb Free Energy from the Cell Potential
First you have to find E0 and know how many moles (n) you are using. Then
multiply all your values with F.
The Nernst Equation
G.F.E equation is for cells under standard conditions, but for those under
non-standard conditions, we use the Nernst equation.
If the concentrations of two electrodes are not the same that means it is
under non-standard conditions and you need to use the Nernst equation.
To fin Q, it is [products] / [reactions] with the numbers raised to the
concentration.
Once you have found everything (E, n and Q) then substitute your values and
fine the non-standard cell potential of the reaction
So now, if you see a question asking about calculating the equilibrium
constant, this is what you do:
Electrolysis
In an electrolytic cell, electric current is used to drive the nonspontaneous chemical reaction.
So electrolysis is when an electrode is decomposing because of the electric
current passing through it.
So let’s talk about the electrolysis of NaCl
Electrons flow into the + part of the battery and leave through the negative
part. And electrons always flow from the Anode to the Cathode.
So because electrons flow from anode to cathode, the left electrode is the
anode and the right electrode is the cathode.
Now, the cathode is – and the anode is +. And there is an energy force that
is flowing from anode to cathode. However, anions like Cl- will flow in the
opposite direction of that energy force (towards the cathode) and cations like
Na+ and H+ will flow towards the cathode and with the energy force.
Keep in mind that chlorine gas will form at the anode because H2o has a lower
cell potential. So if you have a solution of NaCl, the cl with occur first
because of that higher cell potential, but if you dilute that solution with
water, water will occur first because of its abundance.
So now at the cathode, when Na and H+ are pulled to the negatively charged
cathode, the H+ will form first because it has a higher cell potential (0)
than Na (-2,71).
When you’re done you have the find the net equation.
So it’s all about finding which reaction is more spontaneous (or has the
higher cell potential) within the cathode and the anode. Because each has two
reactions happening and you have find which of those two of both anode and
cathode has the higher cell potential.
When a substance undergoes electrolysis
Hydrolysis
So hydrolysis is when the cation and/or anion reacts with water.
Salt hydrolysis usually affects the Ph of a solution.
Nature of Salts
Now when an acid neutralizes a base, the salt is formed. Depending on the
strength of the acid/base, that will affect the ph of the solution (whether is
it more acidic or basic)
A neutral solution is STRONG ACID + STRONG BASE
An acidic solution is STRONG ACID + WEAK BASE
A basic solution is STRONG BASE + WEAK ACID
NaCl is an example of a neutral salt. Neutral salts have a ph of 7 so they
have equal [] of H+ and OH- ions.
There are some ions that do not react well with water to form OH- and H30
ions.
Examples of these are, metal cations of group one (Li, Na, K) and cations of
earth metal of group 2 (Mg, Ca, Sr, Ba) exempt Be. These are cations of strong
bases.
Anions could be those that are monoprotic acids such as: Cl-, Br-, NO3- and
CLO4-.
Basic Salts
Sodium Ethanoate is the product of the reaction of a weak acid and a strong
base.
Kh is the hydrolysis constant. Which is the product over the reactants except
water.
Kh is related to Kb to Kb=Kh
Remember, Kw = Ka * Kb
So if Kb is = to Kh, then Kh (Kb) = Kw/Ka
Acidic Salts
NH4CL is an example of an acidic salt.
Kh is the hydrolysis constant. Which is the product over the reactants except
water.
Kh is related to Ka to Ka=Kh
Remember, Kw = Ka * Kb
So if Ka is = to Kh, then Kh (Ka) = Kw/Kb
Acid-Base Titration
So when an acid and base are mixed it is called a neutralization reaction.
Indicators are used to make sure that there are equal amounts of acid and
base. The end point, which is after the equivalence point, is when the
indicator changes colour.
The equivalence point will be the point when the acid and base are equal.
Now we will look at Titration curves and describe the different scenarios.
If you are going to
solution will be at
of the strong acid)
base are equal, and
add a strong acid to a strong base solution, the base
14 (because it is strong) and will then drop to 0 (because
The equivalence point once again, is when the acid and
that be at 7.
On the right of the photo the opposite occurs with the equivalence point being
the same.
In the case of a strong acid and weak bas, when adding a strong acid to a weak
base solution, the weak base solution will be at 14 and drop rapidly due to
that strong acid. Thus the equivalence point will be below 7 because of that
strong acid.
While on the right of the photo, we are adding a weak base to a strong acid
solution. The weak base will barely cause the solution to go above 7 because
it’s too weak.
Now in the case of a weak acid and a strong base, the weak acid being added
with barely cause the basic level of the solution to go below 7. The solution
will still be more basic. The equivalence point will be above seven.
While in the case on the right, when adding a strong base to an acid solution,
that solution will significantly rise in terms of its basic value. So it is no
long belong a ph of 7, but above that due to the strong base.
Indicators
Indicators’ colours change over a range of 1.5 to 2 Ph units.
You must choose an indicator that will change colour at the steep part of a
titration curve.