Chemical Reactions

A chemical reaction is a process where a set of substances undergo a chemical change to form a different substance.
You may think that chemical reactions only happen in science labs, but they are actually happening all the time in the everyday world. Chemical reactions are important in both non-living and living systems. The reactions involve the transfer, transformation, and conservation of energy.
The substances in a chemical reaction may be chemical elements, molecules, or compounds. The substances present at the start of a chemical reaction are called the reactants; the substances formed by the reaction are called the products.
Chemical reactions are a part of technology, of culture, and of life itself. Burning fuels, smelting iron, and baking bread are just some human activities incorporating chemical reactions. Chemical reactions abound in the natural world, changing the composition of matter within Earth, on its surface, and in the atmosphere, and driving a vast array of processes in living systems. Chemical reactions in the digestive system help break down food into glucose and other constituent molecules that can be absorbed by cells. The glucose is then further broken down through cellular respiration, a chemical reaction that releases chemical energy that is used to fuel body processes.
Perhaps the most important of all chemical reactions is photosynthesis, which directly and indirectly supports almost every living thing on Earth. In photosynthesis, plants, algae, and certain microorganisms transform light energy from the Sun into the chemical energy of food.

Physical Versus Chemical Changes

Chemical reactions must be distinguished from physical changes. Physical changes include changes of state, such as ice melting to liquid water and liquid water evaporating to water vapour. If a physical change occurs, the physical properties of a substance will change, but its chemical identity will remain the same. No matter what its physical state, water (H2O) is the same compound. In contrast, chemical reactions result from the breaking and re-forming of chemical substances to make a new product. The different chemical composition and properties of the products indicate that a chemical reaction has happened.

Evidence of Chemical Reactions

Some chemical changes cannot be directly observed and must be detected through specialized tests. However, some chemical changes are readily apparent, producing changes in colour or other observable signs.

For example, heating a mixture of iron, which is grey, and sulphur, which is yellow, causes a reaction that produces iron sulphide, which is black. The corrosion of certain metals forms rust, a brownish powdery or flaky material known as a metal oxide.

Many chemical reactions produce a temperature change. This is easily observed in some reactions, as when certain metals exposed to oxygen produce a characteristic flame. Many temperature changes are less apparent, generating heat but no flames. An example is the reaction of water with calcium oxide, which forms calcium hydroxide and releases a great deal of heat in the process.

The appearance of bubbles is evidence of a gas-forming chemical reaction. Baking provides an example of this: cake batter rises when an acid in the batter reacts with baking soda, or sodium bicarbonate. The reaction produces carbon dioxide, which causes the batter to rise.

The appearance of a precipitate is evidence that a chemical change has occurred. A precipitate is a solid material that forms when certain liquids are mixed together. Combining solutions of calcium chloride and sodium bicarbonate produces a precipitate of calcium carbonate, which appears as a white solid material in the solution.

Types of Reactions / Experiments

  1. Disappearing water

  2. Mentos fizz

  3. Colour change

  4. Acid and base

  5. Volcano

  6. Aluminium & Iodine

  7. Glycerin & potassium pomegranate

Energy Changes in Reactions

Energy plays a key role in chemical processes. In a chemical reaction, chemical bonds are broken in the reactants and new chemical bonds are formed in the products. Breaking bonds requires energy, whereas forming bonds allows energy to be released.

In some reactions the energy required to break bonds is greater than the energy released as new bonds form. For such a reaction to proceed, energy must be added to the process. The net result of such a reaction is the absorption of energy. A reaction in which energy is absorbed is called an endothermic reaction.

An example of an endothermic reaction is the decomposition of limestone, or calcium carbonate, to make calcium oxide. Energy in the form of heat is applied to limestone at a high temperature, causing formation of the solid calcium oxide and releasing carbon dioxide gas.

Not all endothermic reactions absorb energy as heat. In the electrolysis of water, the energy absorbed is electrical energy generated by an electrical current passed through the water.

In contrast to endothermic reactions are exothermic reactions; these are chemical reactions in which there is a net release of energy. Exothermic reactions often produce an increase in temperature when heat is the energy form involved. Among widely recognizable exothermic reactions is the combustion of fuels (such as the reaction of methane with oxygen mentioned previously), which releases energy as heat. The formation of water from molecular hydrogen and oxygen and the synthesis of a metal oxide, such as calcium oxide from calcium metal and oxygen gas, are other examples of exothermic reactions.