TOPIC 1: CHEMICAL EQUATIONS | CHEMISTRY FORM 3
A chemical equation is a representation of a chemical reaction with the help of symbols and formulae of the substances involved in the reaction.
It is a chemical shorthand for representing the reacting substance or substances combining (the reactants) and the substance or substances formed as a result of the reaction (the products).
Molecular Equations
A Molecular equation is the one which shows the reactants combining and the products formed, in their elemental or molecular forms in a chemical reaction. An example of a molecular equation is the reaction between sodium and water to produce sodium hydroxide solution and hydrogen gas:
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
In this context, sodium (in elemental form) reacts with water (in molecular form) to produce sodium hydroxide (in molecular form) and hydrogen gas (in molecular form).
Word Equations for given Chemical Reactions
Write word equations for given chemical reactions
A word equation is a short form of expressing a chemical reaction by word. Chemical reactions can be summarized by word equations that show all the reactants and the products. This type of equation links together the names of the reactants and the products. For examples, the burning of magnesium in air to produce magnesium oxide can be represented by the following word equation:
Magnesium + Oxygen → Magnesium oxide
Another example is the reaction between sodium and chlorine to give sodium chloride (common salt)
Sodium + Chlorine → Sodium chloride
when different substances are reacted together. But equations can be
made even more useful by writing them using chemical symbols and
formulae.
the
chemical nature of the reactants as well as those of the products must
be clear. The reactants can be in solid, gaseous, liquid or aqueous
forms.
the mole ratios in which the products are combined and
the products are formed must be deducible. This means that atoms of the
reactants and the products must be balanced.
the direction of
the reaction must be established. This means that it should be clearly
shown which substances are the reactants and which ones are the
products. This is normally done by separating the reactants from the
products by an arrow. The arrow normally points from the reactants to
the products.
The
chemical nature of the reactants [potassium (solid); water (liquid)]
and the products [potassium hydroxide (aqueous); hydrogen (gas)] has
been shown.
The mole ratios of the reactants and products are
clearly shown: 2 moles of potassium combines with 2 moles of2water to
produce 2 moles of potassium hydroxide and one mole of hydrogen gas.
The
reactants (potassium and water) and the products (potassium hydroxide
and hydrogen) are separated by an arrow (→) which also indicates the
direction of the reaction.
predict the reaction products precisely, one needs to take into account
the type of reaction occurring. Once you identify the type of reaction
that is going to take place, then you will be in a position of telling
what possible products of reaction would be. A chemical reaction is said
to have taken place when two or more chemical substances called
reactants are converted into very different chemical substances called
products.
are a few ways to predict the reaction products. Firstly, when the
reactants are mixed and then isolated, products can be identified.
Prediction can also be made when elements from the same group in the
Periodic Table show similar reactions. Finally, chemical reactions can
be classified into different categories such as combination (or
synthesis), decomposition, displacement, precipitation, and redox
reactions as described in details below:
a chemical reaction occurs, it is obvious that the changes have taken
place. However, under ordinary conditions it is not easy to see how a
reaction goes on. The neutralization of an acid solution with an alkali
produces no change that you can see. However, reaction has happened. The
temperature of the mixture increases and the new substances have formed
which can be separated and purified. Ideally, we can tell whether a
reaction has taken place if one or more of the following changes are
observed:
heat change has taken place and can be detected by the change in
temperature of the products; (b) a precipitate is formed; (c) there is a
change in state of the reactants, i.e. gas, liquid; solid; (d) a colour
change has occurred; or (e) a gas is evolved and can be identified by
its colour, smell or by effervescence.
heat change has taken place and can be detected by the change in temperature of the products;
a precipitate is formed;
there is a change in state of the reactants, i.e. gas, liquid; solid;
a colour change has occurred; or
a gas is evolved and can be identified by its colour, smell or by effervescence.
are very many different chemical reactions. To make it easy to study
about these reactions, it is useful to try to group certain types of
reactions together. They may be grouped according to certain types of
phenomena which accompany them. They can further be subdivided into
categories of reactions, each of which has its unique characteristics.
Some types of chemical reactions are discussed below:
reaction occurs when two or more simple substances (elements or
compounds) are combined to form one new and more complex substance. The
general form of a synthesis reaction is:
reaction between iron and sulphur to form iron (II) sulphide is the
best example for this kind of reaction. Iron combines directly with
sulphur to form iron (II) sulphide:
occurs when one compound breaks down into simpler substances. All
decomposition reactions have one thing in common: There is only one
reactant and it breaks down into two or more simpler products.
Decomposition can be brought about by heat, light, electricity and even
enzymes or catalysts.
caused by heat is termed as thermal decomposition. An example is the
decomposition of calcium carbonate (limestone) which breaks down into
calcium oxide(quicklime) and carbon dioxide gas when heated.
chemical reactions can be expressed in two forms. The chemical reaction
can be expressed either as a word equation or as a formula (or
symbolic) equation. We have already seen how chemical equations can be
represented by words (word equation). The formula equation makes use of
chemical symbols and formulae to represent a chemical reaction. An
example is the reaction between iron and sulphur to form iron (II)
sulphide: Fe + S → FeS
State the reaction equation in words, for example, carbon reacts with oxygen to form carbon dioxide.
Write
the complete word equation using an arrow to separate the reactants
from the products: Carbon + Oxygen → Carbon dioxide. Conventionally,
the reactants are placed on the left-hand side and the products on the
right-hand side of the equation. An arrow from left to right indicates
that the reaction proceeds from reactants to products as shown.
Change the words into the correct symbols and formulae of the reactants and products: C + O2 → CO2
Balance
the number of each type of atoms on each side of the equation.It is
important to make sure that there is equal number of each kind of atom
on the left of a chemical equation as on the right in order for your
equation to comply with the Law of Conservation of Mass (or
Indestructibility of Matter): Matter can neither be created nor
destroyed in the course of a chemical reaction. This means that the
total mass of all products of a chemical reaction is equal to the total
mass of all reactants. All atoms appearing on the left-hand side must
also be presented on the right-hand side. No atom should appear as a
product if it is not present as a reactant.
Add the state
symbols: Reactants and products may be solids, liquids, gases or
solutions. You show their state by adding state symbols to the equation.
The state symbol are, (s) for solid, (l) for liquid, (g) for gas and
(aq) for aqueous solution (solution in water). For the two reactions
above, the equations with the state symbols are: Fe(s) + S(s) → FeS(s); C(s) + O2(g) → CO2(g) All state symbols must be bracketed and placed as subscripts after the reactant(s) and product(s).
balanced chemical equation has an equal number of atoms of different
elements of the reactants and the products on both sides of the
equation. A balanced equation gives us more information about a reaction
than we get from a simple word equation.
Write the chemical equation for the reaction with the correct symbols and formulae of the reactant(s) and the product(s).
Identify different atoms of the different elements of the reactant(s) and the product(s).
Check whether these different atoms are equal on both sides of the equation. Some atoms may balance each other directly.
Balance the atoms on each sides of the equation by Hit and Trial Method.
Add state symbols.
atoms involved in the reaction are hydrogen and oxygen. It is these
atoms that we are going to balance. The atoms must be equal on both
sides of the reaction equation. There are two hydrogen atoms on each
side of the equation. But, as you can see there are two oxygen atoms on
the left-hand side (LHS) of the equation and only one oxygen atom on the
right-hand side (RHS). To balance oxygen atoms, we write 2 before
water.
introducing 2 before water, another problem has been created. Now we
have 4 hydrogen atoms on the RHS but only 2 hydrogen atoms on the LHS.
To equalize the number of hydrogen atoms we write 2 before hydrogen on
the LHS.
can still check to find out whether the atoms are balanced or not. Now
look at the number of atoms on each side of the equation:
the number of hydrogen and oxygen atoms is the same on both sides of
the equation. This is because the atoms do not disappear during a
reaction. They are neither created nor destroyed. They obey the Law of
Conservation of Mass. When the numbers of different atoms are the same
on the both sides, an equation is said to be balanced. Once the equation
is balanced you can now add the state symbols.
equation which is not balanced is not correct. An unbalanced equation
implies that the atoms have been created or destroyed. It is therefore,
wrong and calculations based on it are certainly unreliable.
that we cannot change the formulae of the substances involved in the
reaction. These are fixed by the bonding in the substance itself. For
instance, in attempt to balance the number of oxygen in water, H2O, we cannot write H2O2. We can only put a multiplying numbers before symbols and formulae, e.g. 2H2O.
How much oxygen is needed to burn 1g of hydrogen?
How much water is formed when 5g of hydrogen is completely burned in oxygen? (Atomic weights: H = 1, O = 16)
equations are equations in which the reacting substances are
represented in ionic forms after the elimination of spectator ions. In
other words, ionic equations are those equations represented in such a
way that spectator ions are not included in the final equation.
Spectator ions refer to those ions, which do not change during the
reaction i.e. they do not take part in a chemical reaction.
order to be able to derive an ionic equation from a molecular equation,
one must be acquainted with the solubility rules as outlined below:
All sodium, potassium and ammonium salts are soluble.
All nitrates, chlorates and acetates are soluble.
All
binary compounds of the halogens (other than F) with metals are
soluble, except those of silver, copper, lead and mercury (lead halides
are soluble in hot water).
All sulphates are soluble except those of silver, lead, mercury (I), barium, strontium and calcium.
All carbonates, sulphites and phosphates are insoluble except those of ammonium and alkali metal (Group I) cations.
All hydroxides are insoluble except those of ammonium, barium and alkali metal (Group I) cations.
All sulphides are insoluble except those of ammonium, alkali metal (Group I) cations and alkali earth metal (Group II) cations.
All
oxides are insoluble except those of calcium, barium and alkali metal
(Group 1) cations; these soluble ones actually react with the water
(hydrolyse) to form hydroxides.
Write a balanced formula equation for the reaction.
Split
all soluble reactants and products into individual ions, clearly
indicating their state symbols. Remember that substances that exists as
molecules such as water, gasses and concentrated mineral acids,
precipitates and neutral atoms do not consist of ions and hence do not
ionize in water.
Cancel out all the ions which appear on both
sides of the equation (spectator ions). These are the ions which remain
unchanged in the reaction.
Re-write the remaining ions. This is the net ionic equation for that reaction.
Step 1: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
Step 2:H+(aq)+Cl–(aq)+Na+(aq)+OH–(aq)→ Na+(aq)+Cl–(aq) + H2O(l)
Step 3: :H+(aq)+Cl–(aq)+Na+(aq)+OH– (aq) → Na+(aq)+Cl–(aq) + H2O(l)
Step 4: H+(aq)+ OH–(aq) → H2O(l)
