Bal Bharati Public School, Pitampura CLASS X CHEMISTRY ASSIGNM Chemical Reactions and Equations Science Notes TERM I 2013-2014 What is a Reaction? Reaction is a term used for depicting a change or transformation in which a substance decomposes, combines with other substances, or interchanges constituents with other substances. What is a Chemical Reaction? A chemical change is always accompanied by a chemical reaction. a chemical change or reaction can be confirmed by any or all of the following observations: => change in state => change in colour => change in temperature => evolution of gas. What is a Chemical Equation? A Chemical Equation represents a chemical reaction. A complete chemical equation represents the reactants, products, and their physical states symbolically. A chemical equation also indicates a number of atoms of each element involved in a reaction. A chemical equation can be written as balanced or unbalanced. An unbalanced chemical equation is also known as Skeletal Chemical Equation for a reaction. An equation having an equal number of atoms of each element on both the sides is called a balanced chemical equation. Types of Chemical Reaction Chemical Reactions are primarily of five types as listed below: 1. Combination reaction
2. Decomposition reaction 3. Displacement reaction 4. Double displacement reactions 5. Oxidation and Reduction reactions What are Combination Reactions? When two or more substances (elements or compounds) combine to form a single product, the reactions are called Combination Reactions. Generally, Combination Reactions are exothermic in nature. The following are some examples of Combination Reactions: CaO (s) + H 2 O (l) -----> Ca(OH) 2 (aq) C (s) + O 2 (g) ------> CO 2 (g) What are Decomposition Reactions? Decomposition reactions are opposite to combination reactions. In a Decomposition reaction, a single substance decomposes to give two or more substances. What are Displacement Reactions? When an element displaces another element from its compound, such reaction is called a displacement reaction e.g. - Zn (s) + CuSO 4 (aq) -----> ZnSO 4 (aq) + Cu (s) What are Double Displacement Reactions? A double displacement reaction is a bimolecular process in which parts of two compounds are exchanged to give two new compounds. The general equation used to represent double displacement reactions can be written as - AB + CD -----> AD + BC Examples of double displacement reactions are - Na 2 SO 4 (aq) + BaCl 2 (aq) -----> BaSO 4 (s) + 2NaCl (aq) Double Displacement Reactions have two commoj features - 1. Two compounds exchange their ions resulting in the formation of two new compounds. 2. One of the new compounds formed would be separated from the mixture in some way, commonly as a solid or gas. Double displacement reactions can be further classified as Precipitation, Gas formation, and Acid-base neutralization reactions. Sample Hot Questions Q.1: Which of the following is a Combination and which is a Displacement Reaction? (a) Cl 2 + 2KI -----> 2KCl + I 2 (b) 2K + Cl 2 ------> 2KCl
Ans: (a) Displacement reaction (b) Combination reaction Q.2: What is a catalytic decomposition? Ans: Those reactions in which a compound splits up into two or more simpler substances in the presence of catalyst are called catalytic reactions. For example, digestion of food in human body. Q.3: What is the term used for the symbolic representation of a reaction. Ans: Chemical equation. Q.4: Write two uses of decomposition reactions. Ans: Two uses of decomposition reactions - (i) These reactions are used to extract several metals from their oxides and salts. When the oxide or salt is electrolyzed, metal is obtained as per the following equation: 2Al 2 O 3 ------> 4Al + 3O 2 (ii) Digestion of food in our body is also an example of decomposition reaction. The starch (carbohydrates) present in the body is decomposed to give simple substance (glucose) and the proteins are broken down into amino acids. Q.5: Explain the terms oxidizing agent and reducing agent. Ans: Oxidizing agent The substance, which oxidizes elements or compounds, is known as oxidizing agent e.g. - 4Fe + 3O 2 ------> 2Fe 2 O 3 Hence, oxygen is an oxidizing agent. In this equation it oxidizes iron (Fe II ) to ferric oxide (Fe 2 O 3 ). Reducing agent The substance, which oxidizes elements or compounds, is known as oxidizing agent e.g. - SnCl 2 + 2FeCl 3 -----> SnCl 4 + 2FeCl 2 Here stannous chloride is a reducing agent. It reduces ferric chloride to ferrous chloride.
(e) Redox reaction : If a substance gains oxygen or looses hydrogen during a reaction it is oxidized. On the other hand if a substance loses oxygen or gains hydrogen during a reaction, it is reduced. Both oxidation and reduction take place simultaneously and hence these are called redox reaction. Oxidation ZnO + C Zn + CO Reduction Oxidation Cl2 + H2S 2HCl + S Reduction (f) Exothermic reaction and endothermic reaction : On the basis of energy changes during chemical reaction, they can be classified as (i) Exothermic reaction- A chemical reaction in which heat energy is produced is called exothermic reaction.for e.g. (i) C (s) + O2 CO2 (g) + Heat (ii) Endothermic reaction- A chemical reaction in which heat energy is absorbed is called endothermic reaction. CaCO3 + Heat CaO + CO2 5. Effects of some oxidation reaction in everyday life: (a) Corrosion The process of slow conversion of metals into their undesirable compounds due to their reaction with moisture and other gases present in the atmosphere is called corrosion of metals. Some common examples of corrosion are rusting of iron, formation of green layer on the surface of copper, tarnishing of silver etc. Formation of brown layer on the surface of iron is called rusting of iron. Formula of rust is Fe2O3.xH2o. Rusting of iron is a serious problem. (b) Rancidity- The taste and odour of food materials containing fat and oil changes when they are left exposed to air for a long time. This is known as rancidity. Rancidity is caused due to oxidation of fat and oil present in food materials. Rancidity can be prevented by using various methods such as by adding antioxidants to the food
materials, storing food in air tight container and by flushing out air with some inert gases like nitrogen.