CHEMICAL REACTIONS & BALANCING OF CHEMICAL. vikasana

Transcription

1 CHEMICAL REACTIONS & BALANCING OF CHEMICAL EQUATIONS

2 CHEMICALREACTION Matter can combine or break apart to produce new types of matter. When this occurs it is said that matter has under gone a chemical reaction.

3 Chemical Reactions -continued A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions are associated with chemical change.

4 Chemical reactions-continued Whenever a chemical change occurs, chemical reaction is said to takes place. A chemical change is a permanent change in which the chemical composition of the substance changes.

5 Evidence for chemical reactions If any one of these four things occur a chemical reaction has occurred. * Change in Color. * Evolution of Gas. * Change in Temperature. * Change in State.

6 CHEMICAL EQUATION A chemical equation is a short scientific representation of a chemical reaction by using the Symbols or Formulae of the substances involved in the reaction. Chemical equation is the simplest form of description of a chemical reaction.

7 Representation of chemical equations Reactants are written on the left side and products are written on the right side.

8 * Reactants are the substances present before the chemical reaction. * Products are the substances formed after the chemical reaction.

9 An arrow ( ) is placed between them to show the direction of reaction. Coefficients indicates how many molecules participate in the reaction.

10 Example Hydrogen reacts with oxygen to form water. Coefficients 2H 2 + O 2 2H 2 O Reactants Product

11 COMMON SYMBOLS USED IN CHEMICAL EQUATIONS

12 SYMBOLS + MEANING Read plus or and. Used between two formulas to indicate reactants combined or products formed. Read yields or produces. Used to separate reactants from products. (s) Read solid. Written after the symbol or formula to indicate that physical state of the substance is Solid.

13 SYMBOLS (l) (g) (aq) MEANING Read liquid. Written after the symbol or formula to indicate that physical state of the substance is liquid. Read gas. Written after the symbol or formula to indicate that physical state of the substance is gaseous. Read aqueous. Written after the symbol or formula to indicate that the substance is dissolved in water.

14 Example Magnesium reacts with oxygen to form Magnesium Oxide. 2Mg (s) + O 2 (g) 2MgO (s)

15 Burning of Magnesium ribbon in air

16 Symbols used * The gaseous products are indicated by writing a vertical arrow mark pointing upward ( ) after its formula. * If a precipitate is formed in the reaction then it is indicated by writing a vertical arrow mark pointing downward ( ) after its formula.

17 Example Limestone decomposes on heating to give Calcium oxide and Carbon dioxide gas. CaCO 3 (s) CaO (s) + CO 2 (g) Reactant Products

18 Example When silver nitrate solution is added to sodium chloride solution, an insoluble silver chloride separates out. AgNO 3 (aq)+nacl(aq) AgCl (s)+nano 3 (aq) Reactants Products

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20 Balanced Chemical Equation According to the law of conservation of mass, in a chemical reaction, the mass of the products equals to the mass of the reactants. Total mass must be equal on both sides of the chemical equation. That type of equation is known as a Balanced equation.

21 UNBALANCED EQUATION H=2 O=1 O=2 BALANCED EQUATION H=2 2H 2 +O 2 2H2 2 O H=4 H=4 O=2 O=2

22 Balancing of Chemical Equation The process of equalizing the number of atoms present in the molecules of reactants & products is called Balancing of chemical equation. Different methods of balancing the equation: 1. Trial & error method. 2. Oxidation number method.

23 Balancing of chemmicalequations by Trial & Error method Write the unbalanced chemical equation. Count the number of atoms of the elements in the molecules of reactants and products.

24 Start by balancing an element by writing a suitable number as coefficient of the formula of molecule, containing that element in reactants or products. Once one element is balanced, proceed to balance other elements with a suitable coefficient to the formula, until all the elements are balanced.

25 Example 1 Burningof Methanegasinairproduce waterandcarbondioxide. Chemical equation for this reaction is as follows: CH 4 + O 2 CO 2 + H 2 O

26 Count up the atoms in the reactants & products CH 4 + O 2 CO 2 + H 2 O C=1 C=1 H=4 H=2 O=2 O=3

27 Double the number of water molecules in the product. CH 4 + O 2 CO 2 + 2H 2 O C=1 C=1 H=4 H=4. O=2 O=4

28 Balance oxygen atoms by doubling the number of oxygen molecules in reactants. CH 4 + 2O 2 CO 2 + 2H 2 O C=1 C=1 H=4 = H=4 O=4 O=4 It is a Balanced chemical equation.

29 Example 2 Potassium chlorate decomposes on heating to give potassium chloride and oxygen. Chemical equation for this reaction is: KClO 3 KCl + O 2

30 Count up the atoms in the reactants & products KClO 3 KCl + O 2 K=1 K=1 Cl=1 Cl=1 O=3 O=2

31 Double the KClO 3 in the reactant and triple the oxygen molecule in the product. 2KClO 3 KCl + 3O 2 K=2 K=1 Cl=2 Cl=1 O=6 O=6

32 Double the KCl molecule in the product. 2KClO 3 2KCl + 3O 2 K=2 K=2 Cl=2 = Cl=2 O=6 O=6 It is a balanced chemical equation

33 Example 3 Zinc reacts with Conc. Sulphuric acid to form Zinc sulphate and water liberating sulphur dioxide gas. Chemical equation for this reaction is: Zn + H 2 SO 4 ZnSO 4 + SO 2 + H 2 O

34 Count up the atoms in the reactants & products. Zn + H 2 SO 4 ZnSO 4 + SO 2 + H 2 O Zn=1 Zn=1 H=2 H=2 S=1 S=2 O=4 O=7

35 Write 2H 2 SO 4 on the reactant side Zn + 2H 2 SO 4 ZnSO 4 + SO 2 + H 2 O Zn=1 Zn=1 H=4 H=2 S=2 S=2 O=8 O=7

36 Double the water molecules on the product side Zn +2H 2 SO 4 ZnSO 4 + SO 2 + 2H 2 O Zn=1 Zn=1 H=4 H=4 S=2 = S=2 O=8 O=8

37 TYPES OF CHEMICAL REACTIONS

38 COMBINATION REACTION: In a combination reaction two or more substances combine to form new single substance. Example: When Aluminium reacts with Oxygen, Aluminium Oxide is formed. 4Al(s) + 3O 2 (g) 2Al 2 O 3 (s )

39 DECOMPOSITION REACTION: In a decomposition reaction, a single substance breaks down to give two or more simpler substances. Example: Magnesium carbonate decomposes on heating to give Magnesum Oxide and Carbon dioxide. MgCO 3 (s) MgO(s) + CO 2 (g)

40 DISPLACEMENT REACTION: A reaction in which one element displaces another element from its compound. Example: Zinc displaces copper from cupric sulphate solution. Zn(s) +CuSO 4 (aq) ZnSO 4 (aq) + Cu (s)

41 DOUBLE DISPPLACEMENT REACTION: A reaction in which two different atoms or groups of atoms (ions) are exchanged. Example: Sodium sulphate solution reacts with Barium chloride solution to form a precipitate of Barium sulphate and sodium chloride. Na 2 SO 4 (aq)+bacl 2 (aq) BaSO 4 (s)+2nacl(aq)

42 REDOX REACTIONS: A reaction in which oxidation and reduction takes place simultaneously OXIDATION: A reaction in which a substance gains oxygen or loses hydrogen. S (s) + O 2 (g) SO 2 (g) H 2 S (g) + Cl 2 (g) 2HCl (g) + S (s)

43 REDUCTION: A reaction in which substance gains hydrogen or loses oxygen. H 2 (g) + Cl 2 (g) 2HCl(g) ZnO(s) + C (s) Zn (s) + CO (g)

44 Examples Concentrated Sulphuric acid oxidizes carbon to carbon dioxide, getting itself reduced to Sulphur dioxide. 2H 2 SO 4 + C 2SO 2 + CO 2 + 2H 2 O

45 When copper metal is dipped in Silver nitrate solution, copper gets oxidized to copper nitrate and silver nitrate gets reduced to silver. Cu(s)+2AgNO 3 (aq) Cu(NO 3 ) 2 (aq)+2ag (s)

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47 Thank You