Chemical Reactions and EquationsChemical Equations

Describing Activity 1.1 in a sentence — "when a magnesium ribbon is burnt in oxygen, it changes into magnesium oxide" — gets tiring if you have to do it for every reaction. The shortest way to say the same thing in words is a word-equation:

Magnesium + Oxygen → Magnesium oxide    (1.1)

The substances that take part in a reaction and change, here magnesium and oxygen, are the reactants. The new substance formed, magnesium oxide, is the product.

The reactants are written on the leftright-hand side (LHS), with a plus sign between them. The products go on the rightleft-hand side (RHS), again joined by plus signs. The arrow between them points towards the productsreactants and shows the direction of the reaction.

Writing a Chemical Equation

Is there an even shorter way? Yes: write each substance as its chemical formula instead of its name. A reaction written with formulae is a chemical equation. Magnesium is Mg, oxygen gas is O₂ and magnesium oxide is MgO, so the word-equation (1.1) becomes

   (1.2)

Now count the atoms of each element on the two sides of the arrow. Magnesium: atom on the left and on the right. Oxygen: atoms on the left, but only on the right.

The oxygen atoms don't match. As written, Eq. (1.2) says that an oxygen atom simply disappeared, so the mass on the two sides would not be the same. Such an equation is unbalanced. An unbalanced equation like (1.2), with the right formulae but not yet the right numbers, is called a skeletal chemical equation.

Balanced Chemical Equations

Recall the law of conservation of mass from Class IX: mass can neither be created nor destroyed in a chemical reaction. The total mass of the elements in the products must equal the total mass of the elements in the reactants.

In other words, the number of atoms of each element stays the samelargersmaller before and after a reaction. Atoms are only rearranged. So every skeletal equation has to be balanced.

Here is one that is balanced already. The zinc and dilute sulphuric acid of Activity 1.3 react like this:

Zinc + Sulphuric acid → Zinc sulphate + Hydrogen

   (1.3)

The Balancing Lab

Now a harder one. Iron reacts with steam to form an iron oxide and hydrogen:

   (1.4)

There is one rule you may never break while balancing: a formula is locked inside its box. You cannot turn H₂O into H₂O₄ — that would be a different substance. All you may change is the number in front of each formula, its coefficient.

Tap a purple coefficient to change it (1, 2, 3 … 6, then back to 1). The table counts the atoms as you go, and a row turns green when its element balances. Balance all three elements.

You found 3Fe + 4H₂O → Fe₃O₄ + 4H₂. Now let's see how to reach it by reasoning instead of tapping.

Hit and Trial, Step by Step

Step I. Draw a box around each formula. Nothing inside a box may change while you balance.

   (1.5)

Step II. List the number of atoms of each element in the unbalanced equation (1.5).

Step III. It is usually easiest to start with the compound that has the most atoms — here Fe₃O₄ — and, inside it, the element with the most atoms: oxygen. There are 4 oxygen atoms on the right but only on the left. To balance them, put the coefficient in front of H₂O. Write it as 4H₂O — never as H₂O₄ or (H₂O)₄.

   (1.6) — partly balanced

Step IV. Iron and hydrogen are still not balanced. Take hydrogen next. The left side now has 4 × 2 = hydrogen atoms and the right side only 2, so we need molecules of H₂ on the right.

   (1.7) — partly balanced

Step V. Only one element is left: iron. There is 1 atom on the left and in Fe₃O₄ on the right, so we take three atoms of Fe on the left.

   (1.8)

Step VII: Show the States

Look at the balanced Eq. (1.9) again. Does it tell you whether the iron is a solid, or whether the water is a liquid or steam? It does not. To make an equation more informative, we write the physical state of each substance in brackets after its formula:

So the complete equation for iron and steam is

   (1.10)

Here, the (g) written after H₂O tells you that the water is used as steamicea solution. State symbols are usually left out unless they are needed.

An equation can also show the conditions of a reaction — temperature, pressure, a catalyst and so on — written above or below the arrow:

   (1.11)

   (1.12)

The second one is photosynthesis, and C₆H₁₂O₆ is glucose. The words written above and below its arrow say that the reaction needs sunlightheatelectricity and chlorophyll.

Balancing Gym

NCERT ends this section with a challenge: can you now balance Eq. (1.2), the burning of magnesium? Here it is, with two more reactions you will meet again in this chapter.

Balance all three. The list ticks off each reaction as you finish it, and you can tap a reaction in the list to jump to it.

Your three balanced equations: 2Mg + O₂ → 2MgO, then 2H₂ + O₂ → 2H₂O, and CH₄ + 2O₂ → CO₂ + 2H₂O.

Speed Round: Balanced or Not?

No pen, no paper: count the atoms in your head. After each answer you will see which element gives an unbalanced equation away.

Get five right to move on.