Hello, my name is Daniel Ngogo. I'm a blogger of this site. 
READ >> | Ionic equations
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
Equations like these sometimes give us some information
about the products formed when different substances are reacted together. But
equations can be made even more useful by writing them using chemical symbols
and formulae.
Any method for representing a chemical reaction must meet
basic certain requirements. These are:
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.
Consider the reaction between potassium and water:
2K(s) + 2H2O (l) → 2KOH (aq) + H2 (g).
In this reaction, the three requirements have been met:
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.
HOW TO PREDICT REACTION PRODUCTS
To 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.
There 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:
Types of Chemical Reactions
When 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:
(a) 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.
There 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:
Combination or synthesis (A + B → C)
Synthesis 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:
element or compound + element or compound→ compound.
The 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:
Fe(s) + S(s) → FeS(s)
Another example is the reaction between hydrogen and oxygen
to form water:
Hydrogen + Oxygen → Water
Decomposition (A →B + C)
Decomposition 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.
Decomposition by heat
Decomposition 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.
Calcium carbonate → Calcium oxide + Carbon dioxide
Formula Equations Using Chemical Symbols
Write formula equations using chemical symbols
Essentially, 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
Steps for writing a chemical equation
These are the steps to follows when writing a chemical
equation:
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).
Balancing Chemical Equations
Balance chemical equations
A 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.
Below is a step-by-step approach to working out the balanced
equation for the reaction:
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.
Example 1
The reaction between hydrogen and oxygen to produce water:
Hydrogen + Oxygen → Water
H2 + O2 → H2O (not balanced)
The 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.
H2 + O2 → 2H2O (not balanced yet)
By 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.
2H2 + O2 → 2H2O (balanced).
You 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:
Now, 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.
2H2(g) + O2(g) → 2H2O(l)
This gives a standard and an acceptable chemical equation.
An 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.
Remember 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.
Example 2
Hydrogen burns in oxygen to form water. The equation for the
reaction is:
2H2(g) + O2(g) →2H2O(l)
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)
Solution:
a. Reaction equation: 2H2(g)+ O2(g)→2H2O(l)
Atoms present: H : O
Molecular weights: 4 : 32
Reacting weights: 1g : Xg
The weight, X, of oxygen = 1×32⁄4= 8g
So, 1g of hydrogen needs 8g of oxygen
NEXT >> | Ionic equations
