LEARNATHON
III

Competition for grade 6 to 10 students! Learn, solve tests and earn prizes!

### Theory:

What happens during a chemical reaction?
• The atoms of reacting molecules or elements are rearranged to produce new molecules in a chemical reaction.
• Between atoms, old chemical bonds are broken, and new chemical bonds are formed.
• Bond formation releases energy, whereas bond-breaking absorbs energy.
Representation of chemical reactions:

Consider the reaction of methane with oxygen.

When methane reacts with oxygen, it forms carbon dioxide and water. This reaction can be written in words as follows:

$$Methane+Oxygen→$$ $$Carbon$$ $$dioxide+Water$$

However, the chemical composition of the reactants and products is not given by this equation. A chemical equation is used to learn the properties of a chemical process. In a chemical equation, chemical formulas are used to represent the chemicals involved in a reaction. The reactants (compounds or elements that undergo reactions) are represented to the left of an arrow, while the products (compounds formed) are shown to the right. The reaction's direction is indicated by the arrow as shown below:

$$CH_4+O_2→CO_2+H_2O$$

However, this is an incomplete chemical equation because matter is neither created nor destroyed, according to the law of conservation of matter. Chemical reactions do not produce new atoms but rearrange them in various ways to generate a new compound. Thus, in a chemical equation, the number of atoms of the reactants and the products must be equal.

In the given equation, the number of hydrogen and oxygen atoms in the reactants and the products are not equal. The following equation can be found by balancing the number of atoms:

$$CH_4+2O_2→CO_2+2H_2O$$

In addition, the chemical equation gives information about the physical state of the compounds and the reaction condition.

$\begin{array}{l}{\mathit{CH}}_{4\left(g\right)}+2{O}_{2\left(g\right)}\phantom{\rule{0.294em}{0ex}}\stackrel{}{⟶}{\phantom{\rule{0.294em}{0ex}}\phantom{\rule{0.147em}{0ex}}\mathit{CO}}_{2\left(g\right)}\phantom{\rule{0.588em}{0ex}}+{\phantom{\rule{0.441em}{0ex}}2H}_{2}{O}_{\left(g\right)}\\ \mathit{Methane}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\mathit{Oxygen}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.441em}{0ex}}\mathit{Carbon}\phantom{\rule{0.147em}{0ex}}\mathit{dioxide}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\mathit{Water}\end{array}$

Note: A balanced chemical equation is a simplified representation of a chemical reaction that shows the chemical composition, physical states of the reactants, products, and reaction conditions.

In a chemical reaction, the phases or physical states of the substances are denoted in abbreviated form within a bracket as the subscript of the formula of the corresponding components.

 Symbol Physical state or phase (s) Solid (l) Liquid (g) Gas (aq) Aqueous solution

For example, when solid potassium reacts with water, it produces hydrogen gas and potassium hydroxide solution. The reaction is shown below:

$$2K_{(s)}+2H_2O_{(l)}→2KOH_{(aq)}+H_{2(g)}$$