Chemical-Reactions make new substances. How else can you tell if it is a Chemical-Reaction, rather than a physical change? Chemistry is the study of Chemical-Reactions.



  • Chemical-Reactions make new substances


  • Chemical-Reactions involve large amounts of energy, usually as heat given out


  • Chemical-Reactions are irreversible


Are Chemical-Reactions Useful?


Chemical-Reactions are essential to life. They happen in our bodies all the time. It is the Chemical-Reaction that occurs between oxygen and our brain cells that keeps us alive. It is the Chemical-Reactions in our digestive systems that provide the glucose that our bodies' cells need for respiration.



Chemical-Reactions occur when anything burns and when food is cooked. It is Chemical-Reactions that give us iron, steel and aluminium from ores. Power stations produce heat by burning fuels (Chemical-Reactions). The heat boils water into steam and the steam turns turbines to give us the electricity we all need.



Chemical-Reactions give us dyes or polymers, fertilizers and medicines, metals and paints.


Different Types of Chemical-Reaction


Reduction-Oxidation (Redox Reaxtions)


There are at least four different definitions of oxidation and reduction and scientists usually choose the most appropriate one for a particular reaction.



The simplest definition of oxidation is when a substance gains oxygen; reduction is consequently the loss of oxygen.



Reduction occurs when a substance gains electrons; oxidation is when a substance loses electrons. Electrons can only exist as part of atoms and molecules, so redox reactions involve the transfer of electrons from one atom or molecule to another. The particle that loses electrons is oxidised and the one that gains electrons is reduced. Oxidation MUST be accompanied by reduction, because the lost electrons have to go somewhere.



Combustion is a redox reaction. A fuel (eg CH4) gains oxygen and is changed into carbon dioxide (CO2) and water vapor (H2O). Oxygen starts the reaction as oxygen molecules (O2). The O2 molecules lose oxygen, so they are reduced.



The respiration reaction that happens in all cells is another example of a redox reaction. Glucose and oxygen from the blood is reacts to make carbon dioxide and water. The reaction gives out energy and it is this energy that keeps our bodies warm.


Synthesis Reactions


Synthesis reactions make complex substances from more simple ones. Carbon (C) burns in air (a source of oxygen, O2) and makes carbon dioxide (CO2). This reaction is also an example of oxidation. The two elements, carbon and oxygen react together and make a new compound, carbon dioxide.



Ammonia (NH3) is made commercially by reacting nitrogen (N2) with hydrogen (H2). The two elements, nitrogen and hydrogen react together and make the compound, ammonia. Ammonia is essential in fertilizer manufacture.



When we synthesise drugs we are taking simple compounds like methanol and water to make compounds that have molecules many times more complex.


Acid-Base Reactions (Neutralization Reactions)


Acids and bases are opposites, when they react they cancel each other out. Our stomachs contain hydrochloric acid; too much stomach acid can cause indigestion so we take an antacid (a base) to neutralize some of the stomach acid.



Neutralization reactions are another essential part of fertilizer manufacture. Farmers use lime to neutralize the soil if it is too acid for the intended crops.


Decomposition Reactions


When limestone is heated up it will decompose (break up) into two simpler substances, carbon dioxide and calcium oxide (quick lime).



When you make compost you use bacteria to decompose or break up the complex molecules in the waste plant material into more simple ones that plants can take in through their roots.