Chemical Equations

 when calcium oxide reacts with water a new substance isformed which is unlike to either calcium oxide or water. The descriptionof chemical reactions in a sentence in activity-1 is quite long. It can bewritten in shorter form as a word equation.The word equation of the above reaction is,

calcium oxide + water $$\rightarrow$$ calcium hydroxide ......................... (1)

(Reactants)                                                            (Product)

The substances which undergo chemical change in the reaction arecalled reactants and the new substances formed are called products.

If there is more than one reactant or product involved in the reaction,

they are indicated with a plus (+) sign between them.

Writing a Chemical Equation

• Can you write a chemical reaction in any other shorter way other than the way we discussed above?

    Chemical equations can be made more precise and useful if we use chemical formulae instead of words.

 Generally, a compound is written by giving its chemical formula, which lists the symbols of the constituent elements and uses the subscript to indicate the number of atoms of each element present in the compound. If no subscript is written the number 1 is understood. Thus we can write calcium oxide as $$CaO_{2}$$, water as $$H_{2}O$$ and the compound formed by the reaction of these two compounds is calcium hydroxide $$CaOH_{2}$$

Now the reaction of calcium oxide with water can be written as:

CaO + $$H_{2}O \rightarrow $$CaOH_{2}$$ ............................ (2)

In the above chemical equation, count the number of atoms of each element on left side and right side of arrow.

• Is the number of atoms of each element are equal on both sides?

Observe the following reactions and their chemical equations. Zinc metal reacts with dilute HCl to yield ZnCl2 and liberates Hydrogen gas.

Zn + HCl $$\rightarrow ZnCl_{2}+H_{2}$$ ............................ (3)

Sodium sulphate reacts with barium chloride to give white precipitate, barium sulphate and sodium chloride.

 $$Na_{2}SO_{4} + BaCl_{2}\rightarrow BaSO_{4}$$ + NaCl ............................. (4)

Balancing Chemical Equations

 According to the law of conservation of mass, the total mass of the products formed in chemical reaction must be equal to the total mass of reactants consumed. It is the atom which accountsfor the mass of any substance. The number of atoms of each element beforeand after reaction must be the same.

All the chemical equations must be balanced, because atoms are neither created nor destroyed in chemical reactions. A chemical equation in which the number of atoms of different elements on the reactant side (left side) are same as those on product side (right side) is called a balanced reaction. Now let us balance the chemical equation using a systematic method. For this take the example of hydrogen reacting with oxygen to form water.

Step 1: Write the equation with the correct chemical formulae for each reactant and product.

Eg: In the reaction of hydrogen with oxygen to yield water, you can write chemical equations as follow:

$$H_{2} + O_{2} \rightarrow H_{2}O$$……(5)

Step 2: After writing the molecularformulae of the substances the equationis to bebalanced. For this we shouldnot touch the ratio of atoms in the molecules of the susbtances but wemay assign suitable numbers as the coefficients before the formulae.

In the above equation write ‘2’ before the molecular formula ofhydrogen and also ‘2’ before the molecular formula of water.  They are in the same number on both sides.

Therefore, the equation is balanced.

 $$2H_{2}+O_{2} \rightarrow 2H_{2}O$$ …………..(6)

Step 3: Verify the equation for the balancing of atoms on both sides ofthe equations. The above equation (6) is a balanced equation.

Let us work out some more examples to see how equations are balanced.

Eg-1: Combustion of propane (C3H8)

Propane, C 3 H8 is a colourless, odourless gas often used as a heatingand cooking fuel. Write the chemical equation for the combustion reactionof propane. The reactants are propane and oxygen and the products arecarbon dioxide and water. Write the reaction in terms of symbols and formulae of the substances involved and follow the four steps described in previous discussion.

Step 1: Write the unbalanced equation using correct chemical formulae for all substances.

$$C_{3}H_{8}+O_{2}\rightarrowCO_{2}+H_{2}O$$............(7),(Skeleton equation)

Note: Unbalanced chemical equation containing molecular formulae of the substances is known as skeleton equation.

Step 2: Compare number of atomsof each element on both sides.

 Find the coefficients to balance theequation. It is better to start with themost complex substance – in this caseC3H8. Look at the skeleton equation, andnote that there are 3 carbon atoms on theleft side of the equation but only 1 onthe right side. If we add a coefficient of 3 to CO2 on the right side thecarbon atoms get balanced.

$$C_{3}H_{8}+O_{2}\rightarrow3CO_{2}+H_{2}O$$ ................... (8)

Now, look at the number of hydrogen atoms. There are 8 hydrogens on the left but only 2 on the right side. By adding a coefficient of 4 to the H2O on the right, the hydrogen atoms get balanced.

$$C_{3}H_{8}+O_{2}\rightarrow3CO_{2}+4H_{2}O$$ .................(9)

Finally, look at the number of oxygen atoms. There are 2 on the left side but 10 on the right side, by adding a coefficient of 5 to the O2 on the left, the oxygen atoms get balanced.

$$C_{3}H_{8}+5O_{2}\rightarrow3CO_{2}+4H_{2}O$$ .....................(10)

Step 3: Make sure the coefficients are reduced to their smallest wholenumber values. In fact, the equation (10) is already with the coefficients in smallest whole number. There is no need to reduce its coefficients, but this might not be achieved in each chemical reaction.

Let us assume that you have got chemical equation as shown below:

$$2C_{3}H_{8}+10O_{2}\rightarrow6CO_{2}+8H_{2}O$$ ................... (11)

Though the equation (11) is balanced, the coefficients are not the smallest whole numbers. It would be necessary to divide all coefficients of equation (11) by 2 to reach the final equation.

$$C_{3}H_{8}+5O_{2}\rightarrow3CO_{2}+4H_{2}O$$ ................... (12)

Step 4: Check the answer. Count the numbers and kinds of atoms on both sides of the equation to make sure they are the same.

Making Chemical Equations more informative:

Chemical equations can be made more informative by expressing followingvcharacteristics of the reactants and products.

i)Physical state

ii)Heat changes (exothermic or endothermic change)

iii. Gas evolved (if any)

iv)Precipitate formed (if any)

i)Expressing the physical state: To make the chemical equation more informative, the physical states of the substances may be mentioned along with their chemical formulae.If the substance is present as a solution in water, the word ‘aqueous’ is written. In the short form it is written as (aq). The balanced equation (16) is written along with the physical states as:

$$Fe_{2}O_{3}+2Al\overset{\Delta }{\rightarrow}2Fe+Al_{2}O_{3}$$

ii)Expressing the heat changes: Heat is liberated in exothermicm reactions and heat is absorbed in endothermic reactions. See the following

examples.

Q’ is heat energy which is shown with plus (+) sign on product side for exothermic reactions and minus (–) sign on product side for endothermic reactions.

iii)Expressing the gas evolved: If a gas is evolved in a reaction, it is

denoted by an upward arrow ‘ or g

$$Zn+H_{2}SO_{4}\rightarrow ZnSO_{4}+H_{2}$$

iv)Expressing precipitate formed: If a precipitate is formed in the

reactions it is denoted by a downward arrow.

Eg:  $$AgNO_{3}+NaCl\rightarrow AgCl+NaNO_{3(aq)}$$

Sometimes the reaction conditions such as temperature, pressure, catalyst etc., are indicated above and/or below the arrow in the equation.

For example,

Interpreting a balanced chemical equation

i)A chemical equation gives information about the reactants and products through their symbols and formulae.

ii)It gives the ratio of molecules of reactants and products.

iii.As molecular masses are expressed in ‘Unified Masses’ (U), the relative masses of reactants and products are known from the equation.

iv)Using molar mass and Avagadro’s number we can calculate the number of molecules and atoms of different substances from the equation. It gives information about relative masses of reactants and products.

from the equation we get,

a) mass - mass relationship

b) mass - volume relationship

c) volume - volume relationship

d) mass - volume - number of molecules relationship etc.,

Suppose that you are asked to calculate the amount of aluminium, required to get 1120 kg of iron by the above reaction.

Solution: As per the balanced equation

$$\therefore$$ to get 1120 kg of iron we have to use 540 kg of aluminum.