NCERT Class 8 Chemistry Chapter 5 The Language Of Chemistry Notes

Chapter 5 The Language Of Chemistry

The language of chemistry is different from 3. One or two letters of the Latin name of an element in ordinary language. Instead of using words and sentences, we use symbols, formulae (formulas) and chemical equations to represent substances and the changes they undergo.

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Berzelius laid the foundation of this language in the early nineteenth century. And this language gradually developed into its present form.

Symbols

Symbols of different elements have been derived in three ways.

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1. The first letter (in capital) of the English name of an element:

2. The first letter, along with one more letter of the English name of an element:

3. One or two letters of the Latin name of an element:

What Does a Symbol Represent?

The symbol of an element represents the following.

1. An element in particular:

As you know, the symbol of sodium is Na, and that of chlorine is Cl. Instead of saying that the compound common salt is made up of the elements sodium and chlorine, you can say that it is made up of Na and Cl. You can also say that Cu is red-brown, whereas Au is yellow, and that Ca is a metal, whereas Cl is a nonmetal

2. An atom of an element:

In formulae and equations, a symbol represents one atom of an element. A numerical subscript shows more than one atom in a molecule. This is explained in the next section

Formulae

The formula of a molecule gives the number(s) of atoms of the same or different elements present in the molecule. In other words, it tells us how many atoms of which elements have combined to form the molecule.

NCERT Class 8 Chemistry Chapter 5 Language of Chemistry

Formulae of Elements

When an atom of an element combines with another atom(s) of the same element, a molecule of the element is formed. The number of atoms contained in a molecule is called the atomicity of the molecule

Molecules of nitrogen, oxygen, fluorine and chlorine contain two atoms of the element, so they are represented as N₂, O₂, F₂ and Cl_2, respectively and are said to be diatomic. A common example of a triatomic gas is ozone(O₃).

An atom of a noble-gas element,

Example:

Helium (He), neon (Ne), argon (Ar), etc., are highly inactive and do not combine with other atoms.

Hence, a molecule of a noble gas contains only one atom of the element. In other words, noble gases are monoatomic. The atomicity of phosphorus is 4(P₄) and that of sulphur is 8(S₈).

The Valency of an Element

The combining capacity of an element with other elements is known as its valency.

• It is given by the number of hydrogen atoms that one atom of the element combines with or displaces from a compound.
• One atom of Cl combines with one atom of H to form one molecule of hydrogen chloride. So, the valency of Cl is 1.
• But one atom each of O, N and C combines with two, three and four atoms of H to form a molecule of water, ammonia and methane, respectively.
• Hence, the valencies of O, N and C are 2, 3 and 4, respectively. On the other hand, the anion of Na, Mg and Al displaces one, two and three atoms of H, respectively, from an acid.
• So, the valencies of Na, Mg and Al are 1, 2, and 3, respectively.
• Elements with valencies 1, 2, 3, etc., are said to be monovalent, divalent, trivalent, and so on.
• The valencies of the first twenty elements, i.e., those having atomic numbers 1 to 20, are given in the Table.

Valencies of the first twenty elements:

When elements are arranged in increasing order of atomic number, we find that their valencies are also arranged in order,

The Valency of an Element below:

• The valency gradually rises from 1 to 4 and then falls to 1 and finally to 0.
• The elements in a column have the same valence. For example, Li, Na, and K, as well as F and Cl, have the valency
• Similarly, Be, Mg and Ca, as well as O and ,S have the valency
• The elements He, Ne and Ar do not combine with other elements and are, therefore, assigned the valency 0 (zero). They are called noble-gas elements.

You will later learn that the above kind of trend in a property is known as the periodic nature or the periodicity of the property. The term periodic means appearing at certain intervals. Don’t you find that valency has a periodic nature?

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Formulae of Compounds

You have learnt earlier that the formula of a binary compound (i.e., a compound formed by only two elements) is obtained by transposing the valencies. Thus, the formula of the compound formed by the elements A (valency y) and B (valency x) is A_xB_y

The numeral subscripts are divided by a common factor, if any.

For example:

1. \(\stackrel{4}{\mathrm{C}} \stackrel{2}{\mathrm{O}} \Rightarrow \mathrm{C}_2 \mathrm{O}_4 \Rightarrow \mathrm{CO}_2\)

2.  \(\stackrel{2}{\mathrm{Ca}}{\stackrel{2}{\mathrm{O}} \Rightarrow \mathrm{Ca}_2 \mathrm{O}_2 \Rightarrow \mathrm{CaO}}\)

3. \(\stackrel{3}{\mathrm{Al}} \stackrel{3}{\mathrm{~N}} \Rightarrow \mathrm{Al}_3 \mathrm{~N}_3 \Rightarrow \mathrm{AlN}\)

There are some exceptions

Example:

H₂O (hydrogen peroxide), C₂H₂ (acetylene) and C₄H₁₀ (butane).

In which the numeral subscripts are not divided by a common factor. You will learn the reason in higher classes.

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Variable valency

Some elements show variable valency,

Example:

Cu (1, 2), iron (2, 3), phosphorus (3, 5) and sulphur (2, 4, 6).

The valency of such an element in a compound is often indicated in Roman numerals in the name of the compound, as shown below

1. \(\stackrel{1}{\mathrm{Cu}}{\stackrel{2}{\mathrm{O}} \Rightarrow \mathrm{Cu}_2 \mathrm{O} \text { copper(I) oxide }}\)

2. \(\stackrel{2}{\mathrm{Cu}} \stackrel{2}{\mathrm{O}} \Rightarrow \mathrm{CuO} \text { copper(II) oxide }\)

3. \(\stackrel{2}{\mathrm{Fe}} \stackrel{2}{\mathrm{O}} \Rightarrow \mathrm{FeO} \text { iron(II) oxide }\)

4. \(\stackrel{3}{\mathrm{Fe}} \stackrel{2}{\mathrm{O}} \Rightarrow \mathrm{Fe}_2 \mathrm{O}_3 \text { iron(III) oxide }\)

As an exercise, you can guess the valencies of P in PCl₃ and PCl₅, and those of S in H₂S, SO₃ and SO₃

Compounds containing radicals

You remember that a radical is a kind of entity that can be an atom with a charge on it or a group of atoms behaving as a single atom with a charge on the group.

It has a valency which is the same as its charge (without sign).

Positive radicals Examples:

Na⁺, K⁺, NH⁺₄, Mg²⁺, Ca²⁺, Cu²⁺, Fe²⁺, Fe³⁺and Al³⁺)

Combine with Negative radicals. Examples:

Cl^–, OH^– , NO^–₃ , HCO^–₃ , CO^2-₃, SO^2-₄ , O^2- , S^2- and PO^3-₄) to form compounds.

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The formula of such a compound can again be obtained by transposing the valencies (i.e., charges without sign) of the radicals and dividing the numbers of radicals by a common factor, if any.

Some examples are given below:

Radicals carry a charge over them, but the compounds they form do not. The compounds are electrically neutral. Hence, the positive and negative radicals must be present in a compound in such numbers that the opposite charges cancel each other.

For example:

In Al₂ (SO₄)₃, the total positive charge is 2 × 3 = 6 for two Al³⁺ ions, and the total negative charge is 3 × 2 = 6 for three SO^2-₄ ions. You can understand this by using valency cards, also given in below.

Chemical Equation

A chemical change, i.e., a chemical reaction, is represented by a chemical equation. You know that in a chemical reaction, the substances we start with are called reactants, and those we end up with are called products. In an equation, we mention the reactants on the left-hand side and the products on the right-hand side, with an arrow in between.

Reactants → Products

In the previous class, you have learnt about the word equations in which we mention the reactants and products by name. Here, we will learn to write equations using symbols and formulae instead of words

Equations Using Symbols and Formulae

Such equations are quantitative and much more informative than word equations. They are written in the following three steps.

1. Writing the skeleton:

The skeleton of an equation is first written by noting the symbols and formulae of the reactants on the left side and those of the products on the right side, with an arrow in between.

For example:

Carbon, when burnt in a sufficient supply of air, forms carbon dioxide. The skeleton of the equation is written as follows.

C + O₂ → CO₂

But carbon, when burnt in an insufficient supply of air, forms carbon monoxide. And the skeleton for the equation is

2C + O₂ → 2CO

Some more examples are given below:

2. Balancing a chemical equation:

According to the law of conservation of matter, matter can neither be created nor destroyed.

• Thus, nor can be atoms.
• As no atoms are lost or gained in a chemical reaction, the number of atoms of each element on the reactant side must be equal to that on the product side.
• A chemical equation in which the number of atoms of each element on the reactant side is the same as that on the product side is called a balanced chemical equation.
• Balanced chemical equations are of great importance in chemistry.

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After writing the skeleton, you should proceed as follows:

Step 1:

Count the atoms of each element on both sides of the arrow. If they tally, the equation is balanced. If not, proceed to step 2.

Step 2:

Make the number of atoms of each element equal on the both sides by using proper coefficients. (If O or N appears in the equation, balancing it first makes the task easier.)

From the examples discussed below, it will be clear that some of the skeletons written above are already balanced chemical equations and some others are not.

Example 1: Is thefollowing equation balanced? C + O₂ → CO₂
Solution:

Yes, because the number of atoms of C and O is equal on both sides of it.

Example 2: Is thefollowing equation balanced? CO₂+H₂O →H₂CO₃
Solution:

Yes, because the atom counts of C, H and O are the same on both sides of the equation

Example 3: Balance the equation C + O₂ → CO
Solution:

Follow the following steps.

Step 1: 

The atom count shows that the equation is not balanced.

Step 2:

To balance O, place the coefficient 2 before the product

C + O₂ → 2CO

Step 3:

Now, the atom count is as follows.

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Step 4:

Balance C by placing the coefficient 2 before C on the LHS.

2C + O₂ → 2CO

Step 5:

The atom count is now as follows

As the atom count of the elements on both sides is the same, the balanced equation is

2C + O₂ → 2CO

Example 4: Is the following equation balanced? If it is not, balance it.
H₂ + Cl₂ → 2HCl
Solution:

Follow the following steps.

Step 1:

The atom count shows that the equation is not balanced.

Step 2:

Place the coefficient 2 before the product

H₂ + Cl₂ → 2HCl

Step 3:

The number of atoms on the two sides is now as follows.

The numbers tally.

Therefore, the balanced chemical equation for the given reaction is

H₂ + Cl₂ → 2HCl

Example 5: Balance the following equation. H₂ +O₂ → H₂O
Solution:

Follow the following steps

Step 1:

Count the number of atoms of each element on both sides.

The atom count shows that the equation is not balanced.

Step 2:

To balance the number of oxygen atoms on both sides, place the coefficient 2 before H₂O.

H₂ +O₂ → 2H₂O

Step 3:

Again, count the atoms ofeach element on both sides.

The atom count shows that the equation is not balanced.

Step 4:

To balance the number of oxygen atoms on both sides, place the coefficient 2 before H₂O.

2H₂ +O₂ → 2H₂O

Step 5:

Tally the number of atoms of each element on both sides.

The numbers tally. Therefore, the balanced chemical equation for the reaction is

2H₂ +O₂ → 2H₂O

Example 6: The numbers tally. Therefore, the balanced chemical equation for the reaction is.
Solution:

Let us now see if we can balance a chemical equation without writing the steps so elaborately. The reactants and the product may be written as follows.

Mg +O₂ → MgO

Balance O: Mg + O₂ → 2MgO

Balance Mg: 2Mg + O₂ → 2MgO

Therefore, the balanced chemical equation for the reaction is

2Mg + O₂ → 2MgO

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Example 7: On being strongly heated, potassium chlorate (KClO₃ ) gives potassium chloride and oxygen. Write a balanced chemical equation for the reaction
Solution:

The reactant and the products can be written as follows.

KCIO₃→ KCI+O₂

Balance O:

2KCIO₃→ KCI + 3O₂

Balance K and Cl:

2KClO₃ → 2KCl + 3O₂

Hence, the balanced chemical equation is

2KClO₃ → 2KCl + 3O₂

Example: 8 Balance the equation

N₂ +H₂ →  NH₃

Solution:

Balance N: N₂ + H₂ → 2 NH₃

Balance H: N₂ + 3H₂→2NH₃

Thus, the balanced equation is

N₂ + 3H₂ → 2NH₃

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Example 9: Is the following equation balanced? If not, balance it
Na₂CO₃ + HCl → NaCl + H₂O + CO₂
Solution:

The equation is not balanced as the atom counts of Na, H and Cl on the two sides do not tally.

Balance Na: Na₂CO₃ + HCl→ 2NaCl + H₂O + C₂O

Balance H and Cl: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂

Hence, the balanced chemical equation is

Na₂CO₃+ 2HCl → 2NaCl + H₂O+ CO₂

Making a Chemical Equation More Informative

A balanced chemical equation tells us how many atoms and molecules of which reactants give how many atoms of which products. Had you known the masses of the atoms of different elements, you could have calculated the quantities of these substances. Keeping such calculations aside for higher classes, let us learn here how to make a chemical equation more informative.

Mentioning the conditions and catalysts:

The conditions under which a reaction takes place and the catalysts needed, if any, are mentioned at the arrow —generally, the conditions above and the catalyst below the arrow.

• You have learnt earlier that
• A catalyst is a substance that generally speeds up a reaction without itself undergoing any change.
• Sometimes, the symbol or formula is mentioned in square brackets at the arrow to indicate a catalyst.

Mentioning the states of the reactants and products:

The state of each reactant and product is mentioned along with it, using the following symbols:

(s) for the solid state

(l) for the liquid state

(g) for the gaseous state, and

(aq) for an aqueous solution

When these symbols are used, a downward arrow (↓) for a precipitate and an upward arrow ( ↑ ) for a gas on the product side are not used. Instead, we use (s) for (↓) and (g) for ( ↑ ).

Mentioning the name and colour of a substance, if needed:

The name and/or colour of a substance is mentioned, if needed, below the symbol or formula of the substance in the equation—the name outside and the colour within brackets

Examples

The following examples will show how informative a chemical equation becomes when we include the points mentioned above.

1. Hydrogen reacts with chlorine in the presence of light to form hydrogen chloride gas.

2. When ignited, a mixture of hydrogen and oxygen (in the volume ratio 2:1) explodes to form water vapour

3. Solid potassium chlorate, when heated at 200-300 °C in the presence of manganese dioxide as a catalyst, gives oxygen, leaving behind a residue of potassium chloride.

4. Carbon dioxide turns limewater milky.

(Lime water) Ca(OH)₂(aq) + CO₂(g) → CaCO₃ (s) (Milky)+ H₂O (I)

5. A burning piece of magnesium continues to burn in a jar of carbon dioxide, forming white, smoky magnesium oxide with some black carbon particles.