WBCS Preliminary (Chemistry): Chemical Reactions and Equations

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  • Atoms and Molecules, Elements and Compounds: There are about a hundred different types of atoms in the Universe. Substances made up of a single type of atom are called Elements. Some elements are made up of single atoms: Carbon©, Helium(He), Sodium(Na), Iron(Fe) etc. He, Fe, and Na are the Chemical Symbols of the elements.
  • Some elements are made up of groups of atoms: Oxygen(O2), Ozone(O3 ), Chlorine(Cl3 ) etc. These groups of atoms are called molecules.
  • Molecules can also be made up of combinations of different types of atoms. These substances are called compounds: Common Salt(NaCl), Methane(CH4), Ammonia(NH3) etc. O2, CH4, NH3 are the Chemical Formulas of Oxygen, Methane and Ammonia respectively. CH4 means that a single molecule of methane contains one atom of Carbon and four atoms of Hydrogen. This chemical formula could have been written but the C1 H4 is never written. Similarly, a molecule of Ammonia (NH3) contains one atom of Nitrogen and three atoms of Hydrogen.
  • A change in which one or more new substances are formed is called a chemical change. A chemical change is also called a chemical reaction. The change may conveniently be represented by a chemical equation.
  • Chemical reactions occur when different atoms and molecules combine together and spit apart. For example, if Carbon (C) is burnt in Oxygen (O2) to form Carbon Dioxide, a Chemical Reaction occurs. This reaction can be written: C + O2--> C O2. This is called a Chemical Equation. The substances on the left hand side of the equation are called the Reactants. The substances on the right hand side are called the Products.
  • There is one very important rule with chemical equations: The number of individual atoms on each side of the equation must be the same. On the left had side, there is an atom of Carbon and a molecule of Oxygen (containing two atoms). On the right hand side there is a molecule of carbon dioxide (containing one atom of carbon and two atoms of Oxygen). The number of atoms on the left hand side is equal to the number of atoms on the right hand side. All that has changed is the arrangement of the atoms. In a chemical reaction atoms are re-arranged; no atoms are destroyed or created.
  • Hydrogen gas is mixed with Oxygen gas. If the mixture is sparked, it explodes to form water. This chemical reaction can be expressed as: H2 + O2--> H2O. On the left had side, there is a molecule of Hydrogen (containing two atoms) and a molecule of Oxygen (also containing two atoms). On the right hand side there is a molecule of water (containing two atoms of Hydrogen and one atom of Oxygen). The left hand side has one extra atom of Oxygen. This is not allowed by the Law of Conservation of Matter. Both sides must contain the same number of atoms. To make the equation conform, we must balance the equation. It is not possible to change the chemical formulas of the reactants or products. Water will always be H2O. Balancing the equation is achieved by changing the number of molecules involved. The balanced form of the above equation is: 2H2 + O2--> 2H2O. Now, on the left had side, there are two molecules of Hydrogen (each containing two atoms making four atoms) and a molecule of Oxygen (containing two atoms). On the right hand side there are two molecule of water (each containing two atoms of Hydrogen and one atom of Oxygen making a total of four atoms of Hydrogen and two of Oxygen). The equation is now balanced. In summary, when Hydrogen reacts with Oxygen, two molecules of Hydrogen react with one molecule of Oxygen to give two molecules of water.
  • The reaction goes in both directions. While the Nitrogen and Hydrogen are combining to form Ammonia, Ammonia splits to form Hydrogen and Nitrogen. A mixture of all three substances results. This type of reaction is called an Equilibrium and is represented by arrows going in both directions. N2 + 3H2--> 2NH3.
  • It is possible to push the reaction in one direction by adding a Catalyst. A catalyst is a substance that helps a reaction without being used up. If Ammonia is removed from the equilibrium mixture, the reaction will move to produce more Ammonia so that equilibrium is attained.
  • The total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants. In other words, the number of atoms of each element remains the same, before and after a chemical reaction.
  • During a chemical reaction atoms of one element do not change into those of another element. Nor do atoms disappear from the mixture or appear from elsewhere. Actually, chemical reactions involve the breaking and making of bonds between atoms to produce new substances.
  • In a combination reaction two or more substances combine to form a new single substance.
  • Decomposition reactions are opposite to combination reactions. In a decomposition reaction, a single substance decomposes to give two or more substances.
  • Reactions in which heat is given out along with the products are called exothermic reactions.
  • Reactions in which energy is absorbed are known as endothermic reactions.
  • When an element displaces another element from its compound, a displacement reaction occurs.
  • Two different atoms or groups of atoms (ions) are exchanged in double displacement reactions.
  • Precipitation reactions produce insoluble salts.
  • Reactions also involve the gain or loss of oxygen or hydrogen by substances. Oxidation is the gain of oxygen or loss of hydrogen. Reduction is the loss of oxygen or gain of hydrogen. The substance that brings about oxidation and is itself reduced is termed as oxidizing agent and the substance that brings about reduction and is itself oxidized is referred to as reducing agent.There are a number of oxidation-reduction reactions that are of industrial use. The production of metals from their ores invariably involves these two processes.

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