This piece of work is to help with the P4, M4 and D4 Unit 26 Applied Science 2010 - My teacher has signed this assignment off which means that it does meet the grading criteria. Hope it helps, all the best.
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P4 –explain the strength of acids and bases in terms of the acid dissociaton constant:
M4 –use acid dissociaton constant in a range of pH calculatons:
D4 – Explain how the PH of a bufer soluton is afected by the additon of small quanttes of acid
and of alkali:
Acids:
An acid is a substance which is able to donate protons (Brønsted-Lowry) or it could also be the
ones in accepting the valence electrons which would form a bond. Valence electrons are any
electrons which are located in the outermost shell of an atom and it’s capable of forming
different bonds with other molecules. A Lewis acid is a compound which has the ability to
accept electrons and it would be able to form a covalent bond. Acids tend to have a sour
taste; they are also capable of changing colour of litmus from a blue to red pigment. Some
acids are able to react with metals which would be able to produce a gas H2. Some acids are
able to react with bases in order to form a salt and also water. In terms of the PH of the acid
solutions, the PH is lower than PH 7. The higher is the level of acidity in the acid the lower is
the PH of the substance. There are some examples of acids being acetic acid (common
vinegar), there are also some of the other acids such as the sulphuric acid which can be
found in batteries. When talking about the definitions for acids, there are three different
definitions for acids. Arrhenius acid represents the acid, it’s any substances that increases
the concentration of hydronium ions (H3O+) in a certain solution/mixture. According to
Brønsted-Lowry acid, it’s any substances which are able to act as a proton donor. The last
definition of the acid is the Lewis acid; it is any substances which are able to accept
electrons. Each acid would have different strengths and by that I mean how readily an acid is
able to lose or give protons. It tends to go that the stronger the acid the more protons are
able to dissociate comparing to a weaker acid.
Some of the strongest acids are, hydrochloric acid, sulphuric acid and nitric acid. A strong
acid is the acid which is able to fully dissociate in water. It tends to be that one mole of the
generic strong acid (HA) would be able to yield one mole of H+ and it would be able to
conjugate one mole of the base (A-), this would result with no acid (HA) which has no
protons. Acids which tend to be weaker, they seem to be less likely to give protons which
would result with the solution being only partially dissociated. During an equilibrium, both
acids and the conjugate base would be present as well as some specific amount of the HA
which have not been dissociated. There are a few factors which tend to contribute to the
strength of the acid, these being strength of the H-A bonds and also the molecular polarity. It
usually goes that the more polar molecules there are, the more electron density which would
cause the protons to be repelled from the molecule. It also tends to be that the larger the
partial positive charge on the proton it would form a weaker H-A bond; this would result with
more proton dissociation. Also stronger acids would tend to have a larger Ka and they would
have a more negative pKa compared to the weaker acids.
Calculating the strong acid dissociation constant, Ka:
The position of the equilibrium could be determined by the display of the equilibrium
constant for the reaction. The lower is the value of the constant the more equilibrium would
shift to the left. The dissociation of an acid is an examples of the homogeneous reaction.
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