4.
Important Reactions of Common Mineral Acids
(Part 4 includes
redox half equations for the metal-acid reaction and how to work out the formula
of a salt given the constituent ions)
so take
care!
needs sub-index
Index of all GCSE level (~US grades 8-10) notes on acids, bases
and salts
What next?
Associated Pages
Part 4.
Some important REACTIONS of MINERAL ACIDS
Acids react with a wide range of metals, oxides,
hydroxides and carbonates to form salts in neutralisation reactions. The reactions of acids with metals, oxides,
hydroxides, carbonates and hydrogencarbonates are described and lots of examples
of word and symbol equations. Part 4 Describes and explains the reactions of
common acids like hydrochloric acid, sulfuric acid and nitric acid with
moderately reactive metals, metal oxides, metal hydroxides, metal carbonates and
aqueous ammonia solution. What is formed in these reactions? Are the products of
these reactions of any use? These revision notes on chemical reactions of acids
e.g. sulfuric, hydrochloric and nitric acids, should prove useful for the new
AQA chemistry, Edexcel chemistry & OCR chemistry GCSE (9–1, 9-5 & 5-1) science
courses. EQUATION
NOTE: The equations are often written
three times: (i) word equation, (ii) balanced symbol equation without state
symbols, and, (iii) with the state symbols (g), (l), (s) or (aq) to give the
complete balanced symbol equation.
4.
Some
important reactions of Acids
-
Acids
are neutralised by reaction with metals, oxides, hydroxides or carbonates
to form salts and other products.
-
Apart from metals (which is
an electron loss/gain redox reaction), the other reactants
listed above are considered as bases, meaning they react by
accepting a proton from an acid in forming the salt.
-
The reaction between acids and bases/alkalis like oxides,
hydroxides and carbonates are classified as neutralisation
reactions.
-
The first part of the salt
name is derived from the positive ion in the base e.g. sodium,
magnesium, ammonium etc.
-
The second part of the salt
name is derived from the acid e.g. chloride (from hydrochloric
acid), sulfate (from sulfuric acid), nitrate (from nitric acid)
etc.
-
You need to be able to
predict the products of these reactions and know
how to work out the formula of a salt
given common ions.
-
The
ionic theory of neutralisation
is described and explained in section 2.
REACTION OF ACIDS WITH METALS
-
General word equation:
metal + acid
==>
a
salt
+ hydrogen
-
The salt, and its name,
depends on the metal and acid used in the reaction and the acid is
neutralised in the process.
-
e.g.
the grey-silvery solid zinc dissolves in hydrochloric acid with effervescence to evolve
hydrogen gas and leave a colourless solution of the salt zinc chloride.
-
zinc +
hydrochloric acid ==> zinc chloride + hydrogen
-
Zn + 2HCl
==> ZnCl2
+ H2 -
Zn(s) + 2HCl(aq)
==> ZnCl2(aq)
+ H2(g)
-
(equation with
state symbols)
-
Its the same equation for
many other Group 2 and Transition metals
e.g. Mg, Ca and Fe, Co, Ni
instead of zinc/Zn in the word/symbol equations-
Note that hydrochloric
acid gives a chloride salt.
-
Similarly
-
magnesium + hydrochloric
acid ==> magnesium chloride + hydrogen
-
Mg + 2HCl ==> MgCl2
+ H2
-
in pictures!
-
iron + hydrochloric
acid ==> iron(II) chloride + hydrogen
-
aluminium + hydrochloric
acid ==> aluminium chloride + hydrogen
-
(you can add the state symbols
as for the Zn + HCl reaction above)
-
-
Illustrated above are
two ways in which the zinc - hydrochloric reaction can be used
to prepare a sample of hydrogen gas.
A strip of magnesium ribbon
dissolves with effervescence to evolve hydrogen gas and leave a
colourless solution of the salt magnesium sulfate.
Mg +
H2SO4
==> MgSO4
+ H2
Mg(s) +
H2SO4(aq)
==> MgSO4(aq)
+ H2(g) (with
state symbols)
Note that
sulfuric/sulfuric gives a sulfate/sulfate salt
Similarly ...
zinc + sulfuric acid
==>
zinc sulfate + hydrogen
Zn + H2SO4
==>
ZnSO4 + H2
Zn(s) + H2SO4(aq)
==>
ZnSO4(aq) + H2(g)
Instead of Mg or Zn, you can
have Fe or Al e.g.
However some metals give
little or no reaction e.g. copper, therefore to make copper salts you
need to react the acid with copper oxide or copper carbonate (see below
in the following sections for the details of these reactions).
You
can test the gas given off with lit splint and the sound of a squeaky
pop confirms it to be hydrogen gas AND therefore the original solid was
a metal.
-
2H2
+ O2 ==> 2H2O + heat energy!
-
2H2(g)
+ O2(g) ==> 2H2O(l)
-
You might see the condensed
water on side of test tube when doing this test.
Note that nitric
acid (HNO3) doesn't usually form hydrogen with a
metal, instead you get nasty
brown fumes of nitrogen dioxide! but you still get
the metal nitrate salt
....
The reaction of metals with
acids is a REDOX reaction and NOT an acid-base reaction.
See also the
REACTIVITY SERIES OF METALS page for
the relative rate of reaction of metals with hydrochloric acid and
sulfuric acid.
Naming salts reminder -
hydrochloric acid makes chloride salts, sulfuric/sulfuric
acid makes sulfate/sulfate salts and nitric acid makes
nitrate salts. This is the second part of the name, and the first
part of the name (in most cases) is simply the metal name from
the metal compound that reacted with the acid. However, sometimes you
may need to add a Roman numeral in brackets to indicate the valency of
that metal in a particular compound where a metal has a variable valency
e.g. copper(II) .., iron(II) ... or an iron(III) salt ... etc.
Advanced REDOX
theory of the metal - acid reaction
(this theory does NOT apply to any other reaction on this page because
the reactions of acids with oxides, hydroxides or carbonates does NOT
involve oxidation and reduction)
Introduction to
oxidation and
reduction theory and application to REDOX reactions
The reaction between a metal and an
acid is technically what is called a REDOX reaction.
This means the reaction takes place
in two parts, an oxidation involving electron loss and a reduction involving electron gain.
The metal atoms lose electrons to
form positive ions (an oxidation - electron loss - the metal atoms
are oxidised).
The hydrogen ions gain electrons forming hydrogen gas molecules (reduction - electron gain
- hydrogen ions are reduced).
The two
simultaneous changes occur on the surface of the metal where the aqueous
positive hydrogen ions hit the surface and pinch electrons from the
metal atoms and then metal ions pass into solution.
These changes can be written as
half equations and then combined to give the full redox ionic
equation.
e.g. for the reaction of magnesium
with sulfuric acid, you can write
(i)
Mg(s) +
H2SO4(aq)
==> MgSO4(aq)
+ H2(g)
(ii)
Mg(s) ===> Mg2+(aq)
+ 2e- (oxidation half equation, 2 electron loss
from Mg atom)
(iii)
2H+(aq) +
2e- ===> H2(g) (reduction half
equation, 1 electron gain by 2 H+ ions)
adding (ii) + (iii) gives the full
(iv) the redox ionic equation ...
(iv)
Mg(s) + 2H+(aq)
===> Mg2+(aq) + H2(g)
... because the two electrons cancel
each other out you don't see them in either the 'normal' equation or the
ionic equation. The sulfate ion from the acid is the spectator ion,
takes no part in the reaction and is not written into the ionic
equation.
You can write exactly the same sort
of equations, whatever the acid and can also substitute Mg with e.g. Zn
or Fe.
Note that the chloride ion (Cl-)
or sulfate ions (SO42-) don't figure here, they
are spectator ions, and don't take part in the reaction, its just the
hydrogen ions that are involved from the acid - their formation is shown
below.
You can think of hydrochloric acid
behaving as: HCl(aq) ===> H+(aq) + Cl-(aq)
and for sulfuric acid: H2SO4(aq)
===> 2H+(aq) + SO42-(aq)
because that is exactly what happens
when you dissolve hydrogen chloride and sulfuric acid in water - they
ionise and its the hydrogen ion that reacts with the metal surface. The
chloride and sulfate ions simply become part of the salt solution formed
in the reaction, but didn't actually chemically change like the metal
atoms and hydrogen ions do.
See also
REDOX reactions and oxidation and reduction
for more examples
REACTION OF ACIDS WITH BASES - basic oxides and hydroxides
These may be
alkalis (soluble bases) or water insoluble bases
-
Metal oxides and metal
hydroxides are typical bases, they may be insoluble in
water, or soluble in water to give an alkaline solution (a soluble
base is called an alkali).
-
The general word equation
for this classic 'neutralisation' reaction is
-
alkali (soluble/insoluble base)
+ acid ==> salt + water
-
i.e. the acid is neutralised in
the process.
Reactions of acids with soluble bases
(alkalis, usually soluble metal hydroxides)
-
metal hydroxide + acid
==>
a salt + water
-
The acid is neutralised in the
process e.g.
-
sodium hydroxide + hydrochloric acid
==>
sodium chloride + water
-
NaOH + HCl
==>
NaCl + H2O
-
Other examples of
neutralising alkalis (soluble bases)
-
potassium hydroxide +
hydrochloric acid ==> potassium chloride + water
-
potassium hydroxide + hydrobromic acid
==> potassium bromide + water
(equation with state symbols)
-
Its the same for any Group 1
Alkali Metal hydroxide e.g. LiOH, NaOH, RbOH etc. and any other Group 7 halogen
acids e.g. HCl hydrochloric acid.
-
calcium hydroxide +
hydrochloric acid ==> calcium chloride + water
-
All solutions involved are
colourless but the reaction can be monitored using universal indicator
or a pH meter.
-
Soluble bases i.e. here,
soluble oxides or hydroxides, are called alkalis.
-
-
This neutralisation reaction is used in
...
or
sodium hydroxide +
sulfuric acid ==> sodium sulfate +
water
(equation with state symbols)
Its the same equation
for any Group 1 Alkali Metal hydroxide e.g. LiOH, KOH etc.
e.g.
sodium hydroxide + nitric acid ==>
sodium nitrate +
water
NaOH + HNO3
==> NaNO3 + 2H2O
NaOH(aq) + HNO3(aq)
==> NaNO3(aq) + H2O(l)
potassium hydroxide + nitric acid ==> potassium nitrate +
water
KOH + HNO3
==> KNO3 + H2O
KOH(aq) + HNO3(aq)
==> KNO3(aq) + H2O(l)
Another soluble base is
ammonia, and here are the equations for its neutralisation
with the three most common mineral acids you come across in the
school/college laboratory.
-
ammonia + nitric acid
==>
ammonium nitrate
-
ammonia + hydrochloric acid
==> ammonium chloride
-
ammonia + sulfuric acid ==> ammonium
sulfate
You should have noticed that
with hydrochloric acid (HCl) and nitric acid (HNO3),
there is one hydrogen (H) that is replaced by the metal ion, and in the
case of sulfuric acid (H2SO4), there are
two protons replaced by a metal ion or ions. This is because each HCl or
HNO3 molecule produces one hydrogen ion (H+)
in water, and each sulfuric acid molecule produces two hydrogen ions in
water.
-
There is one common
mineral acid that potentially can produces three protons per acid
molecule.
-
This is phosphoric(V)
acid, H3PO4, commonly known as just
phosphoric acid.
-
Therefore, phosphoric
acid, will react with three times as much sodium hydroxide when the
solutions are mixed to form the salt trisodium phosphate(V) (or
simply 'sodium phosphate').
-
phosphoric acid +
sodium hydroxide ==> sodium phosphate + water
Reactions of acids with insoluble bases
(metal oxides and metal hydroxides)
-
insoluble base + acid ==> salt + water
-
(note:
oxides that react with acids to form salts are known as 'basic
oxides') -
The acid is neutralised in the
process e.g.
-
e.g.
metal oxide +
acid ==> salt* + water
-
Black copper (ii)
oxide dissolves in colourless sulfuric acid to give a blue
solution of copper(II) sulfate from which the salt can be
crystallised.
-
copper(II) oxide +
sulfuric acid ==> copper(II) sulfate
+
water
-
CuO +
H2SO4 ==> CuSO4 +
H2O -
CuO(s) +
H2SO4(aq) ==> CuSO4(aq) +
H2O(l)
-
Note that insoluble
bases, like these insoluble oxides, although reacting with acids
to form salts, are NOT alkalis. -
Instead of copper/Cu,
you can have magnesium/Mg, zinc/Zn or nickel/Ni in the
word/symbol equations.-
-
This neutralisation reaction is used in
salt preparations method b
for details.
-
Apart from copper compounds,
all solutions involved here are colourless and all the salts form
colourless crystal if the solution is carefully evaporated to cause
crystallisation.
calcium
hydroxide + hydrochloric acid ==> calcium chloride + water
calcium
hydroxide + sulfuric acid ==> calcium sulfate + water
This reaction
soon slows down because calcium sulfate isn't very soluble
in water.
Other examples ...
-
sulfuric acid + zinc oxide
==> zinc sulfate + water
-
magnesium oxide + nitric
acid ==> magnesium nitrate + water
-
calcium oxide + nitric acid
==> calcium nitrate + water
-
magnesium hydroxide + hydrochloric acid
==>
magnesium chloride + water
-
Mg(OH)2(s) + 2HCl(aq)
==>
MgCl2(aq) + 2H2O(l)
-
and ...
-
magnesium hydroxide +
nitric acid ===> magnesium nitrate + water
-
Mg(OH)2(s) + 2HNO3(aq)
==>
Mg(NO3)2(aq) + 2H2O(l)
-
-
The above equation
in pictures
-
Similar for
many other Group 2 and Transition metal hydroxides e.g. Ca, Sr, Ba and Fe,
Co, Ni, Cu, Zn instead of Mg
-
zinc oxide + hydrochloric acid
==>
zinc chloride +
water
-
ZnO(s) + 2HCl(aq)
==>
ZnCl2(aq) +
H2O(l)
-
Similar for
many other Group 2 and Transition metal oxides e.g. Mg, Ca, Ba and Co, Ni,
Cu instead of Zn
-
copper(II) oxide + hydrochloric acid
==>
copper(II) chloride +
water
-
zinc oxide +
sulfuric acid ==> zinc sulfate
+
water
and finally, a bit more tricky!
aluminium
oxide + sulfuric acid ===> aluminium sulfate + water
Al2O3 + 3H2SO4
===> Al2(SO4)3 + 3H2O
(and in pictures below)
Apart from copper compounds,
all solutions involved here are colourless and all the salts form
colourless crystal if the solution is carefully evaporated to cause
crystallisation.
Naming salts reminder -
hydrochloric
acid makes chloride
salts, sulfuric/sulfuric
acid makes sulfate/sulfate
salts and nitric
acid makes nitrate
salts.
Reactions of acids with soluble/insoluble carbonates and
hydrogencarbonates
(also bases, most insoluble)
-
Contrary to what some
textbooks may say, but often do not point out, all carbonates are
bases and react with acids to form salts, the difference in reaction
with alkalis or insoluble bases is that carbon dioxide gas is
evolved.
-
On adding a solid
carbonate or hydrogencarbonate to an acid you see effervescence from
carbon dioxide gas and the general word equation is ...
carbonate/hydrogencarbonate + acid ==> a salt +
water + carbon dioxide
-
The obvious extra observation
compared to the reaction with oxides or hydroxides is the production of a
gas, but the acid is still neutralised in the
process to form a salt e.g.
-
The white solid calcium
carbonate (limestone) dissolves in dilute hydrochloric acid to form a
colourless solution of calcium chloride and colourless carbon dioxide
gas.
-
This is a typical reaction between an
insoluble carbonate and a dilute acid solution.-
calcium carbonate +
hydrochloric acid ==> calcium chloride + water + carbon dioxide
-
(equation with state symbols)
-
-
Illustrated above are
two ways in which the limestone chips (calcium carbonate) -
hydrochloric acid reaction can be used to
prepare a
sample of carbon dioxide gas.
This neutralisation reaction is used in
salt preparations (see method b
for details).
Apart from copper compounds,
all solutions involved here are colourless and all the salts form
colourless crystal if the solution is carefully evaporated to cause
crystallisation.
The dark turquoise-green
solid copper(II) carbonate dissolves in hydrochloric acid to form a
greeny-blue solution of copper(II) chloride and effervescence from the
carbon dioxide formed.
copper(II)
carbonate + hydrochloric acid ==> Copper(II) chloride + water +
carbon dioxide
and with sulfuric acid
a blue solution of copper(II) sulfate is formed and with nitric acid
a blue solution of copper(II) nitrate - word and full symbol
equations below.
copper(II)
carbonate + nitric acid ==> Copper(II) nitrate + water +
carbon dioxide
Similar equations for
other carbonates
-
calcium carbonate
CaCO3, to make three salts - calcium
chloride/nitrate (calcium sulfate is not very soluble)
-
iron(II) carbonate
FeCO3, to make three salts - iron(II)
chloride/sulfate/nitrate
-
magnesium carbonate
MgCO3, to make three salts - magnesium
chloride/sulfate/nitrate
-
manganese(II)
carbonate MnCO3, to make three salts - manganese(II)
chloride/sulfate/nitrate
-
zinc carbonate ZnCO3,
to make three salts - zinc chloride/sulfate/nitrate
-
lead(II) carbonate
PbCO3, only nitric acid to make lead(II) nitrate
Similarly, but
forming colourless solutions from other white insoluble solid carbonates
...
magnesium
carbonate + hydrochloric acid ==> magnesium chloride + water +
carbon dioxide
Note:
Using sulfuric acid and calcium carbonate you don't
get much of a fizz! because the calcium sulfate salt
formed, is not very soluble, and coats the remaining
calcium carbonate inhibiting the reaction! This
will happen with any reaction between an acid and a water
insoluble reactant which forms an insoluble solid product!
magnesium
carbonate + sulfuric acid ==> magnesium sulfate + water +
carbon dioxide
AND six equations for sodium
carbonate and sodium hydrogencarbonate. Both are actually soluble in
water, but in each case you are likely to add the white solid directly
into the acid where it dissolves to give a colourless solution of the
colourless salt with the evolution of carbon dioxide gas (fizzing -
effervescence) e.g....
-
sodium hydrogencarbonate +
hydrochloric acid
==> sodium chloride + water
+ carbon dioxide
-
Note
that hydrogencarbonate is commonly
known as 'bicarbonate'.
-
Its the same for any Group 1
Alkali Metal hydrogencarbonate e.g.
for Li, K etc. instead of Na
sodium hydrogencarbonate + nitric acid
==> sodium nitrate + water
+ carbon dioxide
sodium carbonate + hydrochloric acid
==>
sodium chloride + water + carbon dioxide
sodium hydrogencarbonate +
sulfuric acid
==> sodium sulfate + water
+ carbon dioxide
sodium carbonate + sulfuric acid
==>
sodium sulfate + water + carbon dioxide
sodium carbonate + nitric acid
==>
sodium nitrate + water + carbon dioxide
phosphoric acid + sodium
carbonate ==> sodium phosphate + water + carbon dioxide
That's a really tricky
equation to balance!!!
REACTIONS OF ACIDS WITH AMMONIA
Ammonia gas is very
soluble in water to form an alkaline solution that can be neutralised by
acids to form ammonium salts. All
solutions involved here are colourless and all the salts form colourless
crystal if the solution is carefully evaporated to cause
crystallisation.-
ammonia + acid
==> ammonium salt -
e.g.
(i) ammonia + hydrochloric acid
==>
ammonium chloride
or
(ii)
ammonia + nitric acid
==>
ammonium nitrate
or
(iii)
ammonia +
sulfuric acid ==> ammonium sulfate
NOTE that
(b) All these ammonium
salts are colourless crystalline solids - formed if the water is
carefully evaporated salt preparation method
(a) for details and on
ammonium salts page
for details.
(c) Reactions (ii) and
(iii) are used to make fertiliser salts - see
ammonia chemistry and uses.
Naming salts reminder -
hydrochloric
acid makes
chloride
salts, sulfuric
acid makes sulfate
salts and
nitric
acid makes
nitrate
salts.
NOTE (a):
As already mentioned, and to summarise, the name of the
particular salt formed depends on (i) the metal
name, which becomes the first part of salt name, and (ii) the acid e.g. H2SO4
sulfuric acid on neutralisation makes a ... sulfate; HCl hydrochloric
acid makes a ... chloride;
HNO3
nitric
acid makes a ... nitrate etc.
NOTE (b):
There is a
list of
compound formulae and their solubility
in section 8. The first
part of the salt name is
ammonium derived from ammonia (with metals or their compounds the
metal retains its original name), but the second part of the salt name
is always derived from the acid as in NOTE (a) above.
NOTE (c):
Ammonia
is an alkaline gas that is very soluble in water. It is a weak alkali
or soluble base and is readily neutralised by acids in solution to form ammonium
salts which can be crystallised on evaporating the resulting solution. Sometimes the equations are written with
the 'fictitious' 'ammonium hydroxide'
e.g.
NH4OH(aq) + HCl(aq) ==>
NH4Cl(aq)
+ H2O(l)
but this is not considered to be a
correct representation of this neutralisation reaction these days!
NOTE (d):
An extensive
structured question on acid reaction equations.
Appendix
How to work out the
formula of a salt given the ions
In the formula of the salt the total positive charge must
equal the negative charge i.e. the salt must be overall electrically
neutral.
A list of common positive ions (cations e.g. from the
base) and negative ions (e.g. from the acid) is given on the right.
In the examples below of salt formulae, the derived
formula are shown in 'molecular' formula style, but they are actually ionic
compounds, so the ionic formula is also shown.
For potassium chloride: 1
of K+ balances
1 of Cl- because 1
x 1 = 1 x 1
gives
KCl or K+Cl-
For magnesium chloride: 1
of Mg2+ balances
2 of Cl- because 1 x 2 = 2 x 1
gives
MgCl2 or Mg2+(Cl-)2
For iron(III) chloride: 1
of Fe3+ balances
3 of Cl- because 1 x 3 = 3 x 1
gives
FeCl3 or Fe3+(Cl-)3
For sodium sulfate: 2 of Na+
balances 1 of SO42- because 2 x 1 = 1 x 2
gives
Na2SO4 or (Na+)2SO42-
For calcium nitrate: 1
of Ca2+ balances
2 of NO3- because 1 x 2 = 2 x 1
gives
Ca(NO3)2
or Ca2+(NO3-)2
For iron(III) sulfate: 2
of Fe3+ balances
3 of SO42- because 2 x 3 = 3 x 2
gives
Fe2(SO4)3 or (Fe3+)2(SO42-)3
For more on equations and formulae
see:
How to write
equations, work out formula and name compounds
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Associated Pages
Index of all my GCSE level (~US grades
810) notes on acids, bases
and salts
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GCSE/IGCSE Acids & Alkalis revision notes sub–index:
Index of all pH, Acids, Alkalis, Salts Notes 1.
Examples of everyday acids, alkalis, salts, pH of
solution, hazard warning signs : 2.
pH scale, indicators, ionic theory of acids–alkali neutralisation : 4.
Reactions of acids with
metals/oxides/hydroxides/carbonates, neutralisation reactions : 5.
Reactions of bases–alkalis
like ammonia & sodium hydroxide : 6. Four methods
of making salts : 7. Changes in pH in a
neutralisation, choice and use of indicators : 8. Important formulae
of compounds, salt solubility and water of crystallisation :
10.
More on Acid–Base Theory and Weak and Strong Acids
See also
Advanced Level Chemistry Students Acid-Base Revision
Notes - use index
Multiple choice revision quizzes and other worksheets
-
GCSE/IGCSE foundation-easier multiple choice quiz on pH, Indicators, Acids,
Bases, Neutralisation and Salts
-
GCSE/IGCSE higher-harder multiple choice quiz on pH, Indicators, Acids,
Bases, Neutralisation and Salts
-
GCSE/IGCSE Structured question worksheet on Acid
Reaction word equations and
symbol
equation questions
-
Word
equation answers and
symbol
equation answers)
-
GCSE/IGCSE word-fill worksheet on Acids,
Bases, Neutralisation and Salts
-
GCSE/IGCSE
matching pair quiz on Acids, Bases, Salts and pH
-
See also
Advanced Level Chemistry Students Acid-Base Revision
Notes - use index
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