Acids and bases
Common acids/ bases
Acids
• Most acids have the general formula HA, where A- is
an anion.
• HA --- H+ + A - in solution.
• In aqueous solutions acids increase the hydrogen ion
(H+) concentration.
• Acids are proton donors = H+
• Acids are chemical compounds that have a sharp
taste, a corrosive action on metals, and turn blue
Litmus paper red.
Properties of Acids
• taste sour (don't taste them!)... the word 'acid'
comes from the Latin acere, which means 'sour'
• acids change litmus (a blue vegetable dye) from blue
to red
• their aqueous (water) solutions conduct electric
current (are electrolytes)
• react with bases to form salts and water
• evolve hydrogen gas (H2) upon reaction with an
active metal (such as alkali metals, alkaline earth
metals, zinc, aluminum)
Examples of Common Acids
• citric acid (from certain fruits and veggies,
notably citrus fruits)
• ascorbic acid (vitamin C, as from certain fruits)
• vinegar (5% acetic acid)
• carbonic acid (for carbonation of soft drinks)
• lactic acid (in buttermilk
Acid = proton donor
Bases
• Most bases have the form BOH, where B+ is
an appropriate cation.
• BOH -- B+ + OH- in solution
• Bases are proton acceptors = H+
• H+ + OH- ---- H2O
• Bases are chemical compounds that, in
solution, are soapy to the touch and turn red
vegetable dyes blue.
Properties of Bases
• taste bitter (don't taste them!)
• feel slippery or soapy (don't arbitrarily touch
them!)
• bases don't change the color of litmus; they
can turn red (acidified) litmus back to blue
• their aqueous (water) solutions conduct and
electric current (are electrolytes)
• react with acids to form salts and water
Examples of Common Bases
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•
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detergents
soap
lye (NaOH)
household ammonia (aqueous)
Examples of acids and bases
Strong Acids
HCl (hydrochloric acid)
HNO3 (nitric acid)
HClO4 (perchloric acid)
H2SO4 (sulfuric acid)
Strong Bases
NaOH (sodium hydroxide)
KOH (potassium hydroxide)
Ca(OH)2 (calcium hydroxide)
Acid – base reactions
• Greek words meaning "acid" (from the Greek
οξυς (oxys) meaning "acid“
• Neutralization reactions
• acid+ + base- → salt + water
• 2NaOH + H2SO4 → 2 H2O + Na2SO4
Acids and bases neutralize
• Acids and bases can be grouped into two general
types: strong and weak acids and bases.
• When mixed, acids and bases neutralize one another
and produce salts. These are substances with a salty
taste and none of the characteristic properties of
either acids or bases.
Acids and bases neutralize
• Acid-base reactions are universal.
• When an acid and a base react in an aqueous
solution the H+ and OH- ions combine to form
water. These ions thus "neutralize" one
another.
• HCl + NaOH - NaCl + H2O
pH scale
pH scale
pH scale : definition
• The pH scale measures how acidic or basic a
substance is. The pH scale ranges from 0 to 14. A
pH of 7 is neutral. A pH less than 7 is acidic. A pH
greater than 7 is basic.
• The pH scale is logarithmic and as a result, each
whole pH value below 7 is ten times more acidic
than the next higher value. For example, pH 4 is
ten times more acidic than pH 5 and 100 times
(10 times 10) more acidic than pH 6.
pH scale
pH indicators
Indicators
The exact values for the three
indicators we've looked at are:
indicator
pKind
pH range
litmus
6.5
5-8
methyl orange
3.7
3.1 - 4.4
phenolphthale
in
9.3
8.3 - 10.0
Diagram of indicators
What is an acid-base indicator?
• An acid-base indicator is a weak acid or a
weak base.
• The undissociated form of the indicator is a
different color than the iogenic form of the
indicator.
• An Indicator changes colour depending on the
hydrogen ion concentration.
Adding
hydroxide
ions:
Adding hydrogen ions:
Methyl orange
You have the same sort of equilibrium
between the two forms of methyl orange as
in the litmus case - but the colours are
different.
Volumetric analysis
Titration experiment
• An acid-base titration is a method in
chemistry that allows quantitative analysis of
the concentration of an unknown acid or base
solution. It makes use of the neutralization
reaction that occurs between acids and bases
and the knowledge of how acids and bases
will react if their formulas are known.
Method
• Title : To standardize an unknown solution of
NaOH using a standard solution of HCl.
• Apparatus (Equipment).
Apparatus / Equipment
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•
•
•
•
•
•
•
•
The key equipment used in a titration are:
Burette
White Tile - used to see a colour change in the solution
Pipette
Acid/Base Indicator (the one used varies depending on the
reactants)
Erlenmeyer flask (conical flask)
Standard Solution (a solution of known concentration, a common
one is aqueous Na2CO3)
Solution of unknown concentration
It is also recommended that a control is used in order to see the
difference in colour between the initial and final colour
Procedure
• Rinse all glass ware ,the Burette, pipette and
flask with distilled water.
• Fill the burette with the standard solution of HCl.
Open tap and note start reading (0.0 cm3)
• Using the pippette measure 25 cm3 of the
unknown NaOH into the conical flask.
• Place 2-3 drops of the indicator into the flask.
• Place flask below burette and add acid dropwise
untill the colour of the indicator changes.
Procedure...
• Note final reading on the burette.
• Difference between final reading and first
reading will give amount of HCl used.
• Wash and rinse flask and repeat titration 4-5
times till your readings are very close, not
more than 0.1 cm3.
• Calculate the average amount of HCl used.
Calculation
First run
Final
reading
Start
reading
Difference
in cm3
Second run
Third run
Fourth run
Fifth run
Calculation...
• The equation of the reaction is :
• HCl + NaOH -- NaOH + H2O so 1mole of
HCl reacts with 1 mole of NaOH.
• Volume of acid (Va)x concentration of acid
(Ca) = Volume of base (Vb) x conc of base (Cb)
• Ca = 0.1M, Va = average reading from
titration.
• Vb = 25 cm3 , Cb = unknown
Calculation.....
• Cb = Va x Ca / Vb
• So Cb is = the concentration of NaOH.