Reaction equations

Learn with an AI tutor

An AI tutor explains this page to you and helps you with the exercises. Choose your assistant:

Open with Claude Open with ChatGPT Open with Mistral Vibe

You use your own account with the provider. The assistant is not offered by lernapps.net: lernapps only provides the content and the exercises. The choice holds for all pages, on this device only.

See what your tutor gets for this page

Why

With a reaction equation you describe a reaction briefly and precisely: which substances go in, which come out and in which ratio.

Rule

Reaction equation

A reaction equation describes a reaction with formulas. The starting substances are on the left of the arrow, the products on the right.

Coefficient

A coefficient is the big number in front of a formula. It tells how often the formula occurs.

The atoms are kept in a reaction. So every kind of atom must occur equally often on the left and on the right. You achieve that with coefficients. You never change the indices in the formulas, otherwise it would be a different substance.

2H2+O2→2H2O

Example

Writing the equation for water

  1. Write the formulas: H2+O2→H2O. On the left there are 2 oxygen atoms, on the right only 1.
  2. Put the 2 in front of H2O: H2+O2→2H2O. Now there are 4 hydrogen atoms on the right, only 2 on the left.
  3. Put the 2 in front of H2: 2H2+O2→2H2O. On the left and on the right there are 4 H and 2 O.

Practice

Exercise

Which number belongs in place of x? x H₂ + O₂ → 2H₂O
Solution

Right answer: 2

On the right there are 2 · 2 = 4 hydrogen atoms. So you need 2 particles of H₂ on the left.

More on it:

Exercise

Which number belongs in place of x? 2Mg + O₂ → x MgO
Solution

Right answer: 2

On the left there are 2 magnesium atoms and 2 oxygen atoms. On the right you need 2 particles of MgO for both to be right.

More on it:

Exercise

Which number belongs in place of x? N₂ + x H₂ → 2NH₃
Solution

Right answer: 3

On the right there are 2 · 3 = 6 hydrogen atoms. With 3 particles of H₂ there are also 6 on the left.

More on it:

Exercise

Which number belongs in place of x? CH₄ + x O₂ → CO₂ + 2H₂O
Solution

Right answer: 2

On the right there are 2 + 2 = 4 oxygen atoms. With 2 particles of O₂ there are also 4 on the left.

More on it:

Exercise

Put the steps for writing a reaction equation in the right order.

Give each entry its place: 1 for the first.

Solution

Right answer: 1. Write down the formulas of the starting substances and products, 2. Count the atoms on the left and on the right, 3. Put coefficients in front of the formulas, 4. Count again and check

First come the formulas, then you count, then you balance with coefficients, and finally you check.

More on it: