NEUTRALIZATION,
INDICATORS, AND
TITRATIONS
NEUTRALIZATION REACTIONS
• So far we have only looked at acid and base reactions with
water
• Ka and Kb reactions are only looking at what happens
when we put an acid or base in water. Nothing else.
NEUTRALIZATION REACTIONS
• When an acid and base are both placed in the same
aqueous system, they react.
• These are a special type of double replacement reaction
called a neutralization reaction
NEUTRALIZATION REACTIONS
• In a neutralization reaction, an acid and base react to make
water and salt
–Salt is just a name for an ionic compound
• Example:
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
NEUTRALIZATION REACTIONS
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
• Let’s look at the ionic equation for this reaction.
H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) → Na+(aq) + Cl-(aq) + H2O(l)
• Now if we look at the net ionic equation
H+(aq) + OH-(aq) → H2O(l)
NEUTRALIZATION REACTIONS
• We will ONLY look at STRONG ACID AND STRONG
BASES reacting
• This is because when we have equal moles of acid and base, we will
have a neutral solution
– A pH of 7
• If you did not have a BOTH strong acids and bases, the pH when
the moles are equal would not be exactly 7.
– It would depend on the Ka and Kb of the acids and bases
TITRATIONS
• Neutralization reactions are used in a lab procedure called a
Titration
• In a titration, an acid and base are mixed until their moles are
equal.
– When they are neutralized
• This can be used to find the concentration of either substance
• When the moles are equal this is called the equivalency
point
TITRATIONS
• Typically the substance with the
known concentration (standard)
is added to the substance with
the unknown concentration
• The substance being added from
the burette is called the Titrant
• The substance in the flask being
added to is called the Analyte
INDICATORS
• During a titration, the equivalency point is shown using an
indicator
• Indicators change colour in acidic or basic conditions
• Every indicator has it’s own equilibrium that allows it to change
colour
INDICATORS
• Indicators change colour based on their own equilibrium
In-(aq) + H+(aq)
⇌
HIn(aq)
Phenophthalein
Basic Form
Acidic Form
Colour 1
Colour 2
INDICATORS
• Because indicators work using equilibrium, they have a range that
they will change colours over.
Indicator
pH range
Colour change
methyl orange
3.2 - 4.4
red to yellow
litmus
5.8 - 8.0
red to blue
phenolphthalein
8.2 - 10.0
colourless to pink
• This means that when an indicator changes colour, it is not an EXACT
pH that they do this at.
• Because of this, different titrations may need different indicators.
INDICATORS
• The moment the colour changes in a titration is called the
Endpoint
–The moles may not be equal at the endpoint, but the colour
has changed indicating that you should stop adding your
substance.
TITRATION EXAMPLES
1) During a titration 75.8 mL of a 0.100 M standard solution of HCl
is titrated to end point with 100.0 mL of a NaOH solution with
an unknown concentration. What is the concentration of the
NaOH solution?
2) A 20.0 mL solution of strontium hydroxide, Sr(OH)2, is placed
in a flask and a drop of indicator is added. The solution turns
colour after 25.0 mL of a standard 0.0500 M HCl solution is
added. What was the original concentration of the Sr(OH)2
solution?