L. R. & S. M. VISSANJI ACADEMY SECONDARY SECTION CHEMISTRY - GRADE: VIII

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1 L. R. & S. M. VISSANJI ACADEMY SECONDARY SECTION CHEMISTRY - GRADE: VIII Basic Chemistry Terminology and Reactions SYMBOLS A symbol is the short form that stands for the atom of a specific element. RADICALS A radical is a group of atoms of the same or different elements that behaves as a single unit with a positive or negative charge. VALENCY The valency of an atom or ion is the number of electrons it shares, loses or gains in a chemical reaction to become stable i.e. the number of bonds it forms with other atoms. OR Valency is the combining capacity of an atom or of a radical. VARIABLE VALENCY Certain elements exhibit more than one valency i.e. they show variable valency. Reason for variable valency: An atom of an element can sometimes lose more electrons than are present in its valence shell i.e. there is loss of electrons from the penultimate shell too. Therefore, such an element is said to exhibit variable valency. MOLECULAR FORMULA Molecular Formula denotes the number of atoms of each element present in a compound. CHEMICAL EQUATIONS 1. WORD EQUATIONS A shorthand way of showing what goes on in a chemical reaction is to write a WORD EQUATION. Substances you start with are called REACTANTS. These appear on the left hand side of the equation. Substances that are formed are called PRODUCTS. These appear on the right hand side of the equation. In a word equation + means and and the means produces. e.g. magnesium burns in oxygen to produce magnesium oxide. magnesium + oxygen magnesium oxide (Reactants) (Product) 2. CHEMICAL EQUATIONS A chemical equation is when the words in an equation are replaced by the chemical formula. e.g. magnesium + oxygen magnesium oxide So the chemical equation is Mg + O 2 MgO Formula for magnesium is Mg as it is an element Formula for oxygen is O 2 as it is a diatomic element Formula for magnesium oxide is MgO (use valency rules) BALANCING AN EQUATION When we write chemical equations many of them will not have the same number of each atom at the start of the reaction as we have at the end.

2 We call this an unbalanced equation. e.g. Unbalanced Equation :- CH 4 + O 2 H 2 O + CO 2 If we count how many atoms of each type are on each side of the equation you will see they are not the same. Reactants side Products side 2H 2O 3O In order to balance an equation we have to follow these steps. Step 1: Start by finding out how many atoms of each type are on each side of the equation. Step 2: Next, look for an element which is in only one chemical on the left and in only one on the right of the equation. Step 3: Balance that element by multiplying the chemical species on the side which doesn't have enough atoms of that type by the number required to bring it up to the same as the other side. The number must go in FRONT of the formula. Step 4: Now look for the next element or species that is not balanced and do the same thing. Example 1: How to balance the equation, CH 4 + O 2 H 2 O + CO 2 Reactants side Products side 2H 2O 3O Equation is NOT balanced 2. Balance the H atoms by putting 2 in FRONT of the H 2 O so we have 4 H atoms on each side. Note: This will also increase the number of O atoms. CH 4 + O 2 2H 2 O + CO 2 Reactants side Products side 2O 4O 3. Balance the O atoms by putting 2 in FRONT of the O 2 so we have 4 O atoms on each side. CH 4 + 2O 2 2H 2 O + CO 2 Reactants side Products side 4O 4O The equation is now balanced CHEMICAL REACTIONS I. DIRECT COMBINATION REACTION A reaction in which two or more substances combine to form a single substance is called Direct Combination Reaction. Word equation: Potassium + Chlorine Potassium Chloride Chemical Equation: K(s) + Cl 2 (g) 2 KCl(s)

3 Examples: II. DECOMPOSITION REACTION The breaking up of a compound either into elements or simpler compounds is called as Decomposition Reaction. 1. Word equation: Calcium carbonate Calcium Oxide + Carbon dioxide Chemical equation: CaCO 3 (s) CaO(s) + CO 2 (g) 2. Word equation: Mercury oxide Mercury + Oxygen Chemical equation: 2 HgO(s) 2 Hg(l) + O 2 (g) Decomposition of metal carbonates 1) CaCO 3 CaO + CO 2 2) MgCO 3 MgO + CO 2 3) Al 2 (CO 3 ) 3 Al2O3 + 3CO2 4) ZnCO 3(s) ZnO(s) + CO 2(g) 5) FeCO 3 FeO + CO 2 6) Fe 2 (CO 3 ) 3 Fe 2 O 3 + 3CO 2 Decomposition of metal hydroxides 1) Ca(OH) 2 CaO + H 2 O 2) Mg(OH) 2 MgO + H 2 O 3) 2Al(OH) 3 Al 2 O 3 + 3H 2 O 4) Zn(OH) 2 ZnO + H 2 O 7) PbCO 3 PbO + CO 2 8) H 2 CO 3 CO 2 + H 2 O 9) CuCO 3 CuO + CO 2 10) Cu 2 CO 3 Cu 2 O + CO 2 11) 2HgCO 3 2Hg + 2CO 2 + O 2 12) 2AgCO 3 2Ag + 2CO 2 + O 2 5) Fe(OH) 2 FeO + H 2 O 6) 2Fe(OH) 3 Fe 2 O 3 + 3H 2 O 7) Pb(OH) 2 PbO + H 2 O 8) Cu(OH) 2 CuO + H 2 O Decomposition of Metal Nitrates 1) KNO 3 KNO 2 + O 2 2) NaNO 3 NaNO 2 + O 2 3) Ca(NO 3 ) 2 2CaO + 4NO 2 + O 2 4) 2Mg(NO 3 ) 2 2MgO + 4NO 2 + O 2 5) 4Al(NO 3 ) 3 2Al 2 O 3 +12NO 2 + 3O 2 6) 2Zn(No 3 ) 2 2ZnO + 4NO 2 + O 2 7) 2Pb(NO 3 ) 2 2PbO + 4NO 2 + O 2 8) 2Cu(NO 3 ) 2 2CuO + 4NO 2 + O 2 9) 2Hg(NO 3 ) 2 Hg + 2NO 2 + O 2 10) 2AgNO 3 2Ag + 2NO 2 + O 2

4 III. SIMPLE DISPLACEMENT REACTION The chemical change in which a more active element displaces a less active element from its salt solution is called Simple Displacement Reaction. Occur between a metal element and a compound. The metal element replaces an element in the compound. Common elements that swap are two metals or a metal and hydrogen 1. Word equation: Magnesium + Zinc Magnesium + Zinc nitrate nitrate Chemical equation: Mg(s) + Zn(NO3) 2 (aq) Mg(NO3) 2 (aq) + Zn(s) The element magnesium, Mg is replaced by the element zinc, Zn in the compound zinc nitrate. Zinc becomes the product. 2. Word equation : Magnesium + Hydrochloric Magnesium + Hydrogen acid chloride Chemical equation: Mg (s) + 2 HCl (aq) MgCl 2 (aq) + H 2 (g) In this reaction the element magnesium is replaced by the element hydrogen, H in hydrochloric acid. Hydrogen gas, H 2 becomes the product. ACTIVITY SERIES An activity series is a list of substances ranked in order of relative reactivity. For example, magnesium metal can knock hydrogen ions out of solution, so it is considered more reactive than elemental hydrogen: Mg (s) + 2 HCl (aq) MgCl 2 (aq) + H 2 (g) Zinc can also displace hydrogen ions from solution: Examples: Zn (s) + 2 HCl (aq) ZnCl 2 (aq) + H 2 (g) IV. DOUBLE DECOMPOSITION REACTION A type of chemical change in which two compounds react to form two new compounds by mutual exchange of radicals ( or ions ) is called Double Decomposition Reaction. Occur between two compounds. The two positive metal ions in each reactant swap pl aces with one another. 1.Word equation: Potassium Carbonate + Barium Chloride Potassium + Barium Chloride carbonate Chemical equation: K 2 CO 3 (aq) + BaCl 2 (aq) 2 KCl (aq) + BaCO 3 (s) Ions: K +, CO3 2 Ba 2+, Cl K +, Cl Ba 2+, CO3 2

5 Examples: V. IRREVERSIBLE REACTION In an irreversible reaction, the reactants react to form the products, which cannot revert back into reactants. H 2 O 2 2H 2 O + O 2 VI. REVERSIBLE REACTION In reversible reactions, as the reactants react with other reactants to form products, the products react to form reactants. Examples: 2SO 2 + O 2 2SO 3 3Fe + 2 O Fe 3 O VII. CATALYTICAL REACTION The chemical reactions that involve the use of a catalyst are called as Catalytic Reactions. CATALYST: Catalyst is a compound which alters the rate of reaction but does not take part in it. Example: H 2 O 2 2H 2 O + O 2 VIII. EXOTHERMIC REACTION The reactions which proceed with the evolution of heat energy are called as Exothermic Reaction. Examples: 2SO 2 + O 2 2SO 3 N 2 + 3H 2 2NH 3

6 IX. ENDOTHERMIC REACTION The reactions which proceed with the absorption of heat energy are called as Endothermic Reaction. Examples: N 2 + O 2 2NO C + H 2 O CO + H 2 X. OXIDATION REACTION Definition 1: Oxidation is defined as a chemical process that involves addition of oxygen. Example: Mg + O2 2MgO Definition 2: Oxidation is defined as a chemical process that involves removal of hydrogen. Example: H 2 S + Cl 2 2HCl + S Definition 3: In the electronic concept, oxidation is defined as a process in which an atom or an ion loses electron(s). Example: Zn Zn e - XI. REDUCTION REACTION Definition 1: Reduction is defined as a chemical process that involves addition of hydrogen. Example: CuO + H 2 Cu + H 2 O Definition 2: Reduction is defined as a chemical process that involves removal of oxygen. Example: Cl 2 + H 2 S 2HCl + S Definition 3: In the electronic concept, reduction is defined as a process in which an atom or an ion gains electron(s). Example:Cu e - Cu XII. REDOX REACTION A reaction in which oxidation and reduction take place simultaneously is known as Redox Reaction. 1. OXIDIZING AGENT An oxidizing agent is one that oxidizes other substances either by accepting electrons, or by providing oxygen or by removing hydrogen. 2. REDUCING AGENT A reducing agent is one that reduces other substances either by providing electrons, or by providing hydrogen or by removing oxygen. Addition of Hydrogen [ Reduction ] H 2 S + Cl 2 2HCl + S Removal of Hydrogen [ Oxidation ] Reducing agent Oxidising agent