Class 10th Chemistry Past Paper PDF

## CLASS 10TH ### SUBJECT: CHEMISTRY ### CHAPTER NO.1: CHEMICAL REACTIONS AND EQUATIONS **Chemical reactions:** Chemical reactions are those processes in which original substances lose their nature and identity and form new substances with different properties. In other words, chemical reactions are those processes in which chemical change takes place. A chemical reaction involves breaking of bonds present in the reactants and forming of new bonds to give products. In a chemical reaction, the atoms of an element do not undergo any change to form atoms of a new element but rearrangement of atoms takes place. e.g., burning of magnesium ribbon, reaction between zinc and dilute sulphuric acid etc. **Reactants:** The substances which undergo a change are called reactants. In other words, the substances initially present in a chemical reaction that are consumed during the reaction. e.g., C + O2 → CO2 In this equation, C and O2 are reactants **Products:** Products are the substances that are produced in a chemical reaction. In the above equation, CO2 is the product. **Chemical equation**: The short-hand method of representing a chemical reaction in terms of symbols and formulae of the different reactants and products is called a chemical equation. e.g., 2Mg + O2 → 2MgO Zn + dil. H2SO4 → ZnSO4 + H2 **Chemical equations are of two types**: (i) Balanced (ii) Unbalanced chemical equations (skeletal equations) **(i) Balanced chemical equations (see text questions)** **(ii) Unbalanced chemical equation** is that chemical equation in which the number of atoms of each element on the reactant side and the product side are not equal. e.g, N2 + H2 → NH3 ; CaCO3 + HCI → CaCl2 + H2O + CO2 **Characteristics of chemical reactions:** The easily observable changes that take place in a chemical reaction are known as characteristics of chemical reactions. Some important characteristics are given below: (i) Change in color. (ii) Change in state. (iii) Change in temperature. (iv) Evolution of gas. (v) Formation of precipitate(ppt). **Q. What are the limitations of chemical equations? How these can be removed? OR How to make chemical equations more informative?** Ans. Following things are not written in a chemical equation, if these are added to the equation. The equation becomes more informative and the limitations are removed (a) Physical states of reactants and products. The physical states are represented by using the symbols (s) for solids, (l) for liquids, (g) for gases and (aq) for aqueous solution. Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) (b) Concentration of the acid. Zn(s) + dilute H2SO4(aq) → ZnSO4(aq) + H2 (c) Heat changes accompanying the chemical reaction. In exothermic reactions, the sign (+ Heat) is written along with products and in case of endothermic reactions, (+Heat) is written along with reactants. C(s) + O2(g) → CO2(g) + Heat N2(g) + O2(g) + heat → 2NO(g) (d) Conditions under which the reaction takes place. The conditions of temperature, pressure and the presence of catalyst (a catalyst is a substance that increases the speed of a reaction without being consumed by itself and is recovered chemically unchanged at the end of the reaction. These conditions are written above or below the arrow. N2(g) + 3H2(g) 500°C, 200 atm / Fe (catalyst) → 2NH3(g) **TYPES OF CHEMICAL REACTIONS:** Depending upon the type of chemical change(i.e, nature of reaction) taking place, reactions are of following types: 1. **Combination reactions:** Those reactions in which two or more elements or compounds combine together to form a single compound are called combination reactions. Generally these are exothermic in nature. There are three types of combination reactions, these are: (a) Combination between two elements to form a single compound. Following equations are some of the examples of this kind. 2Mg + O2 → 2MgO C(s) + O2(g) → CO2(g) (b) Combination between two compounds to form a new compound. e.g, NH3(g) + HCl(g) → NH4Cl(s) CaO + CO2 → CaCO3 (c) Combination between an element and a compound to form new compound. e.g, 2CO(g) + O2(g) → 2CO2(g) 2NO(g) + O2(g) → 2NO2(g) 2. **Decomposition reactions:** Decomposition reactions are those reactions in which a single compound breaks down to give two or more simpler substances. Thus these are opposite of combinations reactions. These are endothermic in nature. These take place only when the energy in the form of heat, light or electricity is supplied. Thus, these are of following three types: (a) **Thermal decomposition reactions:** these are those reactions which take by applying heat. e.g, 2Pb(NO3)2 + Heat → 2PbO + 4NO2 + O2 CaCO3 + Heat → CaO + CO2 (b) **Photo decomposition reactions (photolysis):** these are those reactions which take place on absorption of light. e.g, 2AgCl → 2Ag + Cl2 2H2O2 → 2H2O + O2 (c) **Electrolytic decomposition reactions (electrolysis):** those decomposition reaction which take place when electric current is passed through the compound in the molten state or aqueous solution. e.g, 2H2O(1) → 2H2(g) + O2(g) 2NaCl → 2Na + Cl2 Decomposition reactions are used in the extraction of metals. The digestion of food take place through a number of decomposition reactions like proteins decompose to form amino acids. **Reactivity series or activity series of metals:** The arrangement of metals in a vertical column in order of their decreasing reactivity from top to bottom is called activity series. | Element | Symbol | |---|---| | Lithium | Li | | Potassium | K | | Barium | Ba | | Calcium | Ca | | Sodium | Na | | Magnesium | Mg | 3. **Displacement reactions:** Displacement reactions are those reactions in which a more reactive element displaces a less reactive element from its compound. Following equations are the examples of displacement reactions. Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s) Mg(s) + H2SO4(aq) → MgSO4(aq) + H2 Displacement reactions are exothermic in nature and are used in the extraction of metals like Ag and Au. 4. **Double displacement reactions:** Double displacement reactions are those reactions in which two different atoms or groups of atoms are exchanged. These generally occur between two ionic compounds in the solution. Hence, they may be defined as those reactions in which two ionic compounds in the solution react by the exchange of their ions to form new compounds. BaCl2(aq) + Na2SO4(aq) → BaSO4(s) {ppt} + 2NaCl(aq) CuSO4(aq) + H2S(g) → CuS(s) + H2SO4(aq) 5. **Neutralization reactions:** Those reactions in which an acid reacts with a base to form salt and water are called neutralization reactions. e.g., HCI + NaOH → NaCl + H2O 6. **Oxidation-Reduction reactions or Redox reactions:** Those reactions which involve gain and loss of oxygen or hydrogen or electron(s) are called redox reactions, e.g., CuO + H2 → Cu + H2O 2H2S + SO2 → 3S + 2H2O **Oxidation:** The process which involves gain of oxygen, or loss of hydrogen or loss of electrons. **Reduction:** The process which involves loss of oxygen or gain of hydrogen or gain of electrons. **Q. What do you mean by balancing of a chemical equation?** Ans. Balancing of a chemical equation means making the number of atoms of each element equal on both sides of the equation. **(Magnesium + Oxygen → Magnesium oxide)** **Steps that are involved in the balancing of chemical equations are given below:** (a) Write the word/skeletal equation. Mg + O2 → MgO (b) Enclose the formulae in boxes. [Mg] + [O2] → [MgO] (c) Enlist the number of atoms of different elements on LHS and RHS. (d) Start balancing the different elements. (e) Check the correctness of the balanced equation. **Q.1. Why should a magnesium ribbon be cleaned before burning in air?** Ans: Magnesium is very reactive metal. When it is stored, it reacts with oxygen to form magnesium oxide on its surface. This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. So, magnesium ribbon is cleaned by sand paper to remove this layer so that the underlying metal gets exposed to air for burning. **Q.2. Write the balanced equation for the following chemical reactions.** (a) Hydrogen + Chlorine → Hydrogen chloride Sol. H2 + Cl2 → 2HCI (b) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride Sol. 3BaCl2 + Al2(SO4)3 → 3BaSO4 + 2AICI3 (c) Sodium + Water → Sodium hydroxide + hydrogen Sol. 2Na + 2H2O → 2NaOH + H2 **Q.3. Write a balanced chemical equation with state symbols for the following reactions.** a. Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride. Sol. BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq) b. Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride and water. Sol. NaOH(aq) + HCI(aq) → NaCl(aq) + H2O(1) **A solution of a substance X is used for white washing.** (i) Name the substance X and write its formula. Ans:- the substance X is calcium oxide and its formula is CaO. (ii) Write the reaction of the substance X named (i) above with water. Ans:- calcium oxide(quick lime) reacts with water to form calcium hydroxide(slaked lime) CaO(s) + H₂O(I) → Ca(OH)2(aq)

Class 10th Chemistry Past Paper PDF

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