Download Analysing Acids and Bases

Analysing Acids and
Bases
Chapter 4
Back to Basics – A little revision

Many simple acid base reactions are
neutralisation reactions represented by the
general equation
– Acid + base → salt + water

The reaction between aqueous solutions
of HCl and NaOH is a neutralisation
reaction.
Brønsted-Lowry Theory
The acid – is a proton (H+) donor
 The base – is a proton (H+) acceptor
 An acid-base reaction involves two
conjugate acid-base pairs.


Lets look at the reaction of HCl and
water!!
Brønsted-Lowry Theory
Acids according to theory must contain
hydrogen.
 However OH- contains a hydrogen and is almost
always a base.
 Acids must be ready to donate a proton.
 Bases must clearly be capable of accepting
protons (H+ ions).

– To do this they must have a significant region of
negative charge, such as δ- on NH3 or H2O molecules
or full negative charges as on OH- or CO32- ions.
pH
The concentration of H3O+ ions in a
solution is referred to as the solutions
acidity.
 The definition of pH is:

– pH = -log[H3O+]
– Where [H3O+] is the concentration in mol L-1
Neutral solutions pH = 7 at 25°C
 Acidic solutions pH < 7
 Basic solutions pH > 7

Indicators
An indicator is used during acid-base
titration to identify the equivalence point
of the reaction.
 An acid-base indicator is a substance
whose colour depends on the
concentration of H3O+ ions in a solution.
 Indicators are weak acids with their acid
form being one colour and their conjugate
base being another.

Common Indicators

The colours of common indicators and the
pH range over which they change colour
are given
Indicators

The indicator must be chosen carefully to
ensure that the point during the titration
where the indicator changes colour, the
end point, closely matches the
equivalence point.
Indicators
a
b
Figure 4.4 pH curves showing change of pH during a titration of a a
strong base with a strong acid, and b a weak base with a strong acid.
Phenolphthalein, which changes colour in the pH range 8.2–10, gives
a sharp end point in a but a broad end point in b. Methyl orange, which
changes colour between pH 3.1 and 4.5, would be a more suitable
indicator for the second titration.
Examples
a
c
b
d
Dilution





Some analyses using titration involve dilution of the
solution being analysed.
The need to dilute is generally determined during the
design of a particular chemical analysis.
There are many reasons for deciding to dilute a sample
prior to analysis.
When the sample is not a colourless solution, dilution of
the coloured solution may be necessary to enable
effective identification of the endpoint of the titration.
Also dilution may be necessary to ensure that the titre
does not exceed the volume of the burette.
Worked Example


A student analysing a household ‘cleaning agent’ for
ammonia content followed the following procedure. A
20.0mL sample of the cleaning agent was weighed then
added to a clean 250mL volumetric flask. The flask was
then filled to the calibration mark with distilled water and
the dilution thoroughly mixed. 20.0mL aliquots of this
diluted cleaning agent solution were then titrated with
0.120 M HCl using methyl orange indicator.
The following experimental data was recorded mass of
cleaning agent used = 21.392 g
average V(HCl) used = 25.25 ml
Determine
n(HCl) used
 n(NH3) in 20.0 mL of diluted cleaning
agent
 n(NH3) in 20.0 mL of cleaning agent
 The molarity of the cleaning agnet with
respect to NH3
 The percentage, by mass of NH3 in the
cleaning agent

Your Turn
Have a look at this worked example on
page 41
 Try page 42
 Question 6
 Question 8

Back Titration
Some acids and bases are so weak that
they do not produce a sharp colour
change at the end point of a titration.
 A technique known as back titration is
used to overcome this problem.

Steps of Back Titration
1.
2.
The substance to be analysed acts as a weak acid and
is added to an excess of a strong base. The original
amount of the base is known. All of the weak acid
reacts leaving some unused strong base.
The unused strong base is titrated as normal with a
standard solution of a strong acid. Knowing the original
amount of strong base and the amount of strong base
that was unused it is possible to work back and find the
amount of weak acid present.
Determining the concentration of
ammonium ions in lawn fertiliser

Worked Example 4.4 page 43
Your Turn
Page 44
 Question 9
 Page 46
 Question 26 and 28
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