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Chemical reactions Chemical reactions change substances into other substances.A chemical equation is used to represent a reaction. This has the form:
reactants
products
Word equations
use the
names
of the reactants and products.
e.g.
hydrogen + oxygen
water
Balanced
symbol equations
use the
formulae
of the reactants and products.
e.g.
2 H
+ O 2
2 H
O 2
What happens in a chemical reactions: • Chemical bonds in the
reactants
are broken – this requires energy
- The atoms are rearranged to make new substance(s)• New bonds are formed to hold the atoms together in the
products
Balancing Equations No atoms are lost or gained during a chemical reaction – all the atoms in the reactants arethere in the products. This is why equations have to be
balanced.
This means that mass does not change during a chemical reaction –
mass is conserved
We use numbers in front of the chemical formulae so that the rule about all the sameatoms being present before and after is true.
We never change the formulae themselves
- that would make it a different chemical reaction.e.g.
H^2
+^
O^2
H^2
O^
NOT BALANCED
2H
2
+^
O^2
2H
O 2
BALANCED
State Symbols We use these to show the physical state of the substances in equations:
(s) for solid
(l) for liquid
(g) for gas
(aq) for aqueous solution (i.e. dissolved in water)
e.g.
Ca
(s)^
+ 2H
O 2
(l)^
Ca(OH)
2(aq)
+ H
2(g)
“Solid calcium will react when placed in water, producing a solution of calcium hydroxideand bubbles of hydrogen gas”
What makes an alkali alkaline? All alkalis form hydroxide ions,
OH
, when dissolved in water, making the solution
alkaline.
H^2
O
e.g.
NaOH
(s)^
Na
+(aq)
+ OH
Some bases are soluble in water. We call these alkalis
. Alkalis, especially concentrated ones, are also
corrosive
Why acids are neutralised by alkalis:Acids produce H
ions
Alkalis produce OH
ions
+H
(aq)
OH
H^2
O(l)
Water, which is neutral with a
pH of 7
, is formed when acidic
+ H
ions and alkaline
OH
-^ ions
react with each other. This is what we mean by a neutralisation reaction.
pH and indicatorsIndicators
change colour, telling us if a solution is acidic or alkaline. They can’t measure pH.
The
pH scale
is a measure of the acidity or alkalinity of a solution.
Universal indicator
is SPECIAL because it turns different colours at different pH values, so it
CAN be used to give the approximate pH (or a pH probe can be used).
Colour of universal indicator at
different pH values
Making soluble salts: The following four methods each react an acid with a different type of substance toproduce a solution of a salt in water (i.e. the salts produced are soluble). Once anyunreacted material has been removed to leave a pure solution of the salt, a dry sample canbe prepared by evaporating the water. 1)^
Making salts by reacting acids with metals Used for reactive metals, but DANGEROUS if the metal is too reactive (e.g. Na or K) becausereaction is
exothermic
General equation:
METAL +
ACID
SALT
+ HYDROGEN
e.g.
Zn
(s)^
+ H
SO 2
4(aq)
ZnSO
4(aq)
+^
H2(g)
Method: 1.Add metal to acid until no more bubbling is seen (all the acid hasreacted).2.Filter to remove unreacted metal, leaving salt solution.3.Heat the solution in an evaporating dish to remove some of the water,then allow to cool to form crystals.4.Filter to remove the crystals and dry them (warm oven), or leave inwarm place for all water to evaporate. Observations: Metal dissolves, a gas is produced (bubbling), heat is produced
- Making salts by reacting acids with insoluble bases (metal oxides) Can be used with any metal oxides. Useful for making salts from unreactive metals e.g.copper or lead.General equation:
METAL OXIDE
ACID
^
SALT
+ WATER
e.g.
CuO
(s)^
+ H
SO 2
4(aq)
CuSO
4(aq)
+^
HO^2
(l)
Method: 1.^
Add metal oxide to acid until no more will dissolve (all the acid has reacted). Warmingis often necessary to get the metal oxide to react at a reasonable rate.
2.^
Filter to remove unreacted metal oxide, leaving the salt solution.
3.^
Heat the solution in an evaporating dish to remove some of the water, then allow tocool to form crystals.
4.^
Filter to remove the crystals and dry them (warm oven), or leave in warm place for allwater to evaporate. Observations: Metal oxide dissolves
- Making salts by reacting acids with alkalis (soluble metal hydroxides) This method works for all alkalis and is useful for making salts of reactive metals such assodium or potassium, and for making ammonium salts.
Note: this method can also be used
when reacting soluble metal carbonates with acids to make salts. General equation:
METAL HYDROXIDE + ACID
SALT
+ WATER
e.g.
NaOH
(aq)
(aq)
NaCl
(aq)
H
O 2
(l)
Method: 1.Add acid progressively to the alkali or soluble metalcarbonate, testing the pH periodically using a pH probeor removing a drop and testing with universal indicator.2.Stop when the pH = 7 (neutral).3.Heat the solution in an evaporating dish to removesome of the water, then allow to cool to form crystals.4.Filter to remove the crystals and dry them (warm oven),or leave in warm place for all water to evaporate. Observations: None – although a small temperature rise may bedetected.
Ammonium salts The ammonium ion plays the same role as a metal in reactions with acids to form salts.Know the difference:
AMMONIA
is a molecule,
NH
3
AMMONIUM
an ion formed from ammonia,
NH
When ammonia dissolves in water it forms an ammonium hydroxide solution. This solutioncontains ammonium ions (which can behave like metal ions in forming salts) and hydroxideions (which make the solution alkaline).
NH
3(g)
+ H
O 2
(l)^
NH
OH 4
(aq)
Ammonium hydroxide reacts with acids because it is an alkali:e.g.
ammonium hydroxide + nitric acid
ammonium nitrate +
water
NH
OH 4
(aq)
+^
HNO
3(aq)
NH
NO 4
3(aq)
+^
H^2
O^ (l)
Ammonium nitrate and ammonium sulphate are
important fertilisers
, because they are rich
in nitrogen.
SOLUBILITY RULES Before we can mix two soluble salt solutions, we need to know what is soluble: •^
Sodium, potassium and ammonium salts (usually)
-^
All nitrate salts
-^
Most chloride salts, except silver chloride
-^
Most sulphate salts, except calcium sulphate and barium sulphate
-^
Most carbonates are INSOLUBLE except sodium carbonate, potassium carbonate andammonium carbonate. Choosing what soluble substances to react to make your salt: Write the name of the substance you want to makee.g.
barium sulphate
Choose a soluble salt containing themetal ion you want. All metal nitrates are soluble so…
use BARIUM NITRATE
Choose a soluble saltcontaining the other ion you need. All sodium or potassium salts are soluble so…
use SODIUM SULPHATE
… mix these two solutions together to get the precipitate
