Describing Chemical Reactions and Equations

Example: If methane burns in oxygen to produce water and carbon dioxide, the word equation is: $\text{methane} + \text{oxygen} \rightarrow \text{water} + \text{carbon dioxide}$

2. Skeleton Chemical Equations

To provide more scientific precision, we use chemical formulas instead of names. This type of equation is known as a skeleton equation.

When writing skeleton equations, it is crucial to include the states of matter for each substance involved to provide a complete picture of the reaction environment.

State Symbols Key:

• $(s)$: solid
• $(l)$: liquid
• $(g)$: gas
• $(aq)$: aqueous (a substance dissolved in water)

Example of a Skeleton Equation: $CH_4(g) + O_2(g) \rightarrow H_2O(g) + CO_2(g)$

3. Balanced Chemical Equations

A skeleton equation often does not obey the Law of Conservation of Mass (the principle that matter is neither created nor destroyed). To fix this, we must create a balanced chemical equation.

Balancing is achieved by using coefficients. A coefficient is a number placed in front of a chemical formula to ensure there are the same number of atoms of each element on both sides of the equation.

Worked Example: Balancing the Combustion of Methane Starting with our skeleton equation: $CH_4 + O_2 \rightarrow H_2O + CO_2$

We notice we have 4 Hydrogens on the left but only 2 on the right. We add a coefficient of $2$ to $H_2O$: $CH_4 + O_2 \rightarrow 2H_2O + CO_2$

Now we have 4 Oxygens on the right (2 from water and 2 from carbon dioxide), but only 2 on the left. We add a coefficient of $2$ to $O_2$: $CH_4 + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)$

Energy Changes in Reactions

Chemical reactions do not occur in isolation; they are always accompanied by the release or absorption of energy. We categorize these reactions into two main types:

Exothermic Reactions

In an exothermic reaction, energy is released into the surroundings. Because energy is a product of the process, it is often conceptually placed on the product side of the reaction.

Endothermic Reactions

In an endothermic reaction, energy is absorbed from the surroundings. In these cases, energy is required for the reaction to occur, so it is conceptually placed on the reactant side.