Chemical Reactions and Chemical Equations

Transcription

1 Chemical Reactions and Chemical Equations Everything around us changes with time. Some changes can be noticed immediately, some others after some time, while some others go unnoticed. Some common changes which occur in things around us are, Formation of curd from milk Cooking of rice Cooking of chapatti from wheat flour Burning of crackers, wood etc. Growing of plants Ripening of fruits These changes become noticeable due to one or more of the following changes. Position, Shape and Size Colour Physical state Temperature Composition Structure evolution of gas In some of these changes only the physical properties of the substance get changed, and no new substance is formed. Such a change is called a physical change. For example, melting of ice, getting salt from sea water are physical changes. In some other changes given above, the composition and chemical properties of the substance get changed and one or more new substances are formed. The properties of the new substances formed are different from those of the original substance. Such changes are called chemical changes. Chemical changes occur only under certain conditions. For example, when carbon (viz., coal) is burnt, carbon dioxide is produced. The properties of carbon dioxide are entirely different from those of carbon. when magnesium is burnt in air, magnesium oxide (a white powdery substance) is formed. The chemical changes are the result of chemical reactions that take place. What is a chemical reaction? The change of one or more substances into other substances having different composition and different properties is called a chemical reaction. The substances which take part in a chemical reaction are called reactants and the substances formed in a chemical reaction are called products. In a chemical reaction, the amount of reactants decreases whereas that of products increases with time, until the reaction is over. In the reaction, Hydrogen + Oxygen > Water Hydrogen and Oxygen are reactants and Water is the product.

2 The rate of a chemical reaction is affected by the following factors: Physical state of the reactants.( l, g, s ) Temperature, pressure concentration of the reactants. Catalyst. surface area Chemical reactions show one or more of the following characteristics. 1.Evolution of gas Some chemical reactions take place with the evolution of a gas. Some reactions in which a gas is evolved are described below: Reaction between a metal such as zinc, magnesium, or iron and dilute sulphuric acid produces hydrogen gas. Reaction between iron sulphide and dilute sulphuric acid produces hydrogen sulphide gas. Heating a mixture of potassium chlorate (KC103) and manganese dioxide (Mn02) gives oxygen gas. Change of colour There are some reactions in which there is a colour change. For example, When red lead oxide is heated, yellow lead monoxide is formed. Formation of precipitate In certain reactions, when solutions of two reagents are mixed, one of the products formed gets precipitated immediately. Colour of the precipitate depends upon the reagents used in the reaction. Energy change During a chemical change, energy is either evolved or absorbed. The energy evolved or absorbed may be in the form of heat, light, electricity, sound etc. When coal (or carbon) is burnt, heat and light are produced. In this reaction, heat and light are evolved.

3 Heat and light are evolved in the following reactions also: (i) During the burning of a candle. (ii) During the burning of LPG. When a small quantity of water is added to quicklime, a large amount of heat is evolved. CaO(s) + H20(Z) ---> Ca(OH)2(s) + Heat This reaction is an endothermic reaction. Quicklime e water slaked lime This reaction takes place when lime is added to water for preparing the lime suspension for whtewashing. Limestone (CaC03) is burnt to obtain lime (or quicklime). In this reaction, heat is absorbed. Change of physical state In certain reactions, the physical state of products is different from that of the reactants. That is, there is a change of state during a chemical reaction. When a mixture of hydrogen and oxygen is ignited with an electric spark at room temperature, liquid water is formed. electric spark 2H2(g) + 02(g) > 2H20(I) hydrogen gas oxygen gas room temp. water (liquid) When ammonia (NH3) gas is allowed to come in contact with hydrogen chloride (HC1) gas, solid ammonium chloride is obtained. Home work 1. How are the following indicated in a chemical equation? (a) evolution of a gas (b) formation of precipitate (c) evolution or absorption of heat 2. Write one reaction for each of the following: (a) evolution of gas (b) formation of precipitate (c) change in colour (il) evolution of heat CHEMICAL EQUATION What is a chemical equation During a chemical reaction, certain substances react together to form some new substances. A reaction is described in terms of the names and quantities of the reactants and products. For example, the chemical reaction during the burning of carbon (or coal) may be described by the statement, "carbon is burnt in excess of air (or oxygen) to produce carbon dioxide." This reaction may also be described by a worded equation, viz., Carbon + Oxygen (excess) > Carbon dioxide Such worded equations are long, and not convenient for describing chemical reactions. These long worded equations can be shortened by using symbols and formulae of the substances involved in the reaction. For example, the reaction of zinc metal with dilute sulphuric acid to produce zinc sulphate and hydrogen may be written as, Zinc (metal) + dil. Sulphuric acid > Zinc sulphate + Hydrogen This worded equation may be written in terms of symbols and formulae as, Zn + dil. H2S04 -> ZnS04 + H2 Thus, Zn + dil. H2S04 > ZnS04 + H2 is the chemical equation for the reaction between zinc and sulphuric acid. A shorthand representation of a chemical reaction in terms of symbols and formulae of the substances involved is called a chemical equation. How is a chemical equation written Chemical equation for a chemical reaction is written as follows: Step 1. Identify the reactants and the products of the chemical reaction.

4 Step 2. Write down the formulae or symbols of the reactants on the left-hand side with a sign of plus (+) between them. The formulae or symbols of the products formed in the reaction are written on the right-hand side with a sign of plus (+) between them. The reactants and products are separated by > or =. Such a chemical equation is called a skeleton equation. Step 3. Count the number of atoms of each element on both the sides. If the number of atoms of each element on both the sides are equal, then the equation is called a balanced chemical equation. If the number of atoms of any one or more of the elements on both the sides are not equal, then these are made equal by adjusting the coefficients before the symbols and formulae of the reactants and products. The process by which the number of atoms of each element on both sides are made equal, is called balancing of chemical equation. Step 4. In the end, the chemical equation is made molecular, if required. How can a chemical equation be made more informative A chemical equation can be made more informative by adding additional information to the chemical equation. This is done as follows: Reaction conditions. The information regarding temperature, pressure and catalyst etc., is provided above the arrow (-> or =) separating the reactants and products. A reaction taking place at t C and p atm pressure, and in the presence of a catalyst can be described as follows. Reactants tem /p atm/ catalyst Products For example, nitrogen and hydrogen react to form ammonia under the conditions; temperature = 450 C, pressure = atm, and in the presence of a catalyst (a mixture of iron and molybdenum). Physical states of reactants and products. Information regarding the physical states of the reactants and products, can be provided by using the letters (s), (I), (g) and (aq) for solid, liquid, gas and a solution in water, respectively, at the end of the formula of the substance involved. For example, solid sodium metal reacts with water at room temperature to produce hydrogen gas, and a solution of sodium hydroxide in water. Then, the complete chemical equation is, 2Na(s) + 2H20(Z) > 2NaOH(aq) + H2(g) Heat absorbed or evolved. Chemical reactions proceed with the evolution or absorption of heat. The reactions in which heat is absorbed are called endothermic reactions. The reactions in which heat is given out are called exothermic reactions. This information is provided by adding a heat term on the product-side (right-hand side) of the chemical equation. For example, When carbon is burnt in air (or oxygen) heat is evolved. Then, the chemical equation is written as, C(g) + 02(g) > C02(g) + Heat (393 kj) The reaction between carbon (C) and sulphur (S) to produce carbon disulphide (CS2) proceeds with the absorption of heat, i.e., it is an endothermic reaction. The chemical equation of this reaction is written as, C(s) + 2S(g) > CS2(g) - Heat (92 kj) The reactions with + Heat term on the product side are called exothermic reactions, while those with - Heat term on the product side are called endothermic reactions. Concentration of the reactants and products. This information is added to the chemical equation by adding the word dil. (for dilute) or cone, (for concentrated) before the formulae of the reactants and products. For example, in the reaction between zinc and dilute sulphuric acid, the term dil. is added before the formula of sulphuric acid. Zn(s) + dil. H2S04(l) > ZnSO(aq) + H2(g)

5 Rate of reaction. This information is not commonly added to the chemical equation. Sometimes, however, the term fast or slow may be added over the arrow, if the reaction is fast or slow. For example, the reaction between HC1 and NaOH in solution is a fast reaction. So, HCl(aq) + NaOH(aq) > NaCl(s) + H20(l) What information does a balanced chemical equation convey A chemical equation gives the following two types of information. 1. Qualitative information. A chemical equation provides the following qualitative information about the reaction. It tells us the (1) names of the reactants which take part in the reaction, (2) names of the products formed in the reaction. 2. Quantitative information. A chemical equation gives the following quantitative information. It tells us about, (1) the number of molecules or atoms of reactants and products taking part in the reaction. (2) the number of moles of each substance involved in the reaction, (3) the mass of each substance involved in the reaction. (4) mass-mass, mass-volume, volume-volume relationships between the reactants and products. Home work 1. Define the term chemical equation. 2. How does a skeleton chemical equation differ from a balanced chemical equation? 3. Name the law which requires balancing of the chemical equations. 4. How will you provide the following information in a chemical equation? (a) an aqueous solution (b) absorption of heat (c) catalyst (d) formation of precipitate