chemical equations, chemical reactions

A chemical reaction is a process where a set of substances undergo a chemical change to form a different substance.

Examples:

• iron and oxygen combining to make rust
• vinegar and baking soda combined to make sodium acetate, carbon dioxide, and water
• things burning or exploding
• many reactions that happen inside living things
• electrochemical reactions when discharging or recharging batteries

Nuclear reactions are not chemical reactions. Chemical reactions involve only the electrons of atoms; nuclear reactions involve the protons and neutrons in the atomic nuclei.

Law of Conservation of Mass

The Law of Conservation of Mass states that in a chemical reaction, the mass of the products equals the mass of the reactants. This principle ensures that chemical equations must be balanced, meaning the number of each type of atom on the reactants side must equal the number on the products side.

Where do chemical reactions occur?

You may think that chemical reactions only happen in science labs, but they are actually happening all the time in the everyday world. Every time you eat, your body uses chemical reactions to break down your food into energy. Other examples include metal rusting, wood-burning, batteries producing electricity and photosynthesis in plants.

What are reagents, reactants, and products?

Reactants and reagents are the substances that are used to bring about a chemical reaction. A reactant is any substance that is consumed or used up during the reaction. The reactants that are not used up when the reaction is finished are called reagents. The substance that is produced by a chemical reaction is called the product.

A chemical equation consists of reactants and products. Reactants are the substances that react with each other, found on the left side of the equation. Products are the new substances formed, located on the right side. An arrow (→) separates these two sides, indicating the direction of the reaction.

For example, the combustion of methane can be represented as:

\(CH_4 + 2O_2 → CO_2 + 2H_2O\)

This equation shows that one molecule of methane reacts with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water.

Reaction rate

Not all chemical reactions occur at the same rate. Some happen very quickly like explosions, while others can take a long time like metal rusting. The speed that the reactants turn into products is called the reaction rate.

The reaction rate can be changed by adding energy such as heat, sunlight, or electricity. Adding energy to the reaction can increase the reaction rate significantly. Also, increasing the concentration or pressure of the reactants can speed up the reaction rate.

Some reactions give out energy. These are exothermic reactions. In other reactions, energy is taken in. These are endothermic reactions.

Types of reactions

There are many types of chemical reactions. Here are a few examples:

1. Synthesis reaction - A synthesis reaction is one where two substances combine to make a new substance. It can be shown in an equation such that
   A + B → AB.  Example: \(2Na + Cl_2 → 2NaCl\).
2. Decomposition reaction - A decomposition reaction is where a complex substance breaks down to form two separate substances. It can be shown in an equation such that AB → A + B.  Example: \(2H_2O → 2H_2 + O_2\).
3. Combustion – A combustion reaction occurs when oxygen combines with another compound to form water and carbon dioxide. Combustion reactions produce energy in the form of heat.  Example: \(CH_4 + 2O_2 → CO_2 + 2H_2O\).
4. Single displacement – A single displacement reaction is also called a substitute reaction. You can think of it as a reaction where one compound takes a substance from another compound. Its equation is A + BC → AC + B.  Example: \(Zn + 2HCl → ZnCl_2 + H_2\).
5. Double displacement – A double displacement reaction is also called a metathesis reaction. You can think of it as two compounds trading substances. Its equation is AB + CD → AD + CB.  Example: \(AgNO_3 + NaCl → AgCl + NaNO_3\).
6. Photochemical reaction – A photochemical reaction is one involving photons from light. Photosynthesis is an example of this kind of chemical reaction. 

Catalysts and inhibitors

Sometimes a third substance is used in a chemical reaction to speed up or slow down the reaction. A catalyst helps to speed up the rate of reaction. Unlike other reagents in the reaction, a catalyst is not consumed by the reaction. An inhibitor is used to slow down the reaction.