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L. R. & S. M. VISSANJI ACADEMY SECONDARY SECTION - 2016-17 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 + O2 MgO Formula for magnesium is Mg as it is an element Formula for oxygen is O2 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. We call this an unbalanced equation. e.g. Unbalanced Equation :- CH4 + O2 → H2O + CO2 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 1C 1C 4H 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, CH4 + O2 → H2O + CO2 Reactants side Products side 1C 1C 4H 2H 2O 3O Equation is NOT balanced 2. Balance the H atoms by putting 2 in FRONT of the H2O so we have 4 H atoms on each side. Note: This will also increase the number of O atoms. CH 4 + O2 → 2H2O + CO2 Reactants side Products side 1C 1C 4H 4H 2O 4O 3. Balance the O atoms by putting 2 in FRONT of the O2 so we have 4 O atoms on each side. CH4 + 2O2 → 2H2O + CO2 Reactants side Products side 1C 1C 4H 4H 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. For example: Word equation: Potassium + Chlorine Potassium Chloride Chemical Equation: K(s) + Cl2(g) 2 KCl(s) Examples: II. DECOMPOSITION REACTION The breaking up of a compound either into elements or simpler compounds is called as Decomposition Reaction. For example: 1. Word equation: Calcium carbonate Chemical equation: 2. Word equation: Calcium Oxide + Carbon dioxide CaCO3(s) Mercury oxide Chemical equation: CaO(s) + CO2(g) Mercury + Oxygen 2 HgO(s) 2 Hg(l) + O2(g) Decomposition of metal carbonates 1) CaCO3 → CaO + CO2 2) MgCO3 → MgO + CO2 3) Al2(CO3)3 → Al2O3 + 3CO2 4) ZnCO3(s) → ZnO(s) + CO2(g) 5) FeCO3 → FeO + CO2 6) Fe2(CO3)3 → Fe2O3 + 3CO2 Decomposition of metal hydroxides 1) 2) 3) 4) Ca(OH)2 → CaO + H2O Mg(OH)2 → MgO + H2O 2Al(OH)3 →Al2O3 + 3H2O Zn(OH)2 → ZnO + H2O 7) PbCO3 → PbO + CO2 8) H2CO3 → CO2 + H2O 9) CuCO3 → CuO + CO2 10) Cu2CO3 → Cu2O + CO2 11) 2HgCO3 → 2Hg + 2CO2 + O2 12) 2AgCO3 → 2Ag + 2CO2 + O2 5) 6) 7) 8) Fe(OH)2 → FeO + H2O 2Fe(OH)3 → Fe2O3 + 3H2O Pb(OH)2 → PbO + H2O Cu(OH)2 → CuO + H2O Decomposition of Metal Nitrates 1) 2) 3) 4) 5) KNO3 → KNO2 + O2 NaNO3 → NaNO2 + O2 Ca(NO3)2 → 2CaO + 4NO2 + O2 2Mg(NO3)2 → 2MgO + 4NO2 + O2 4Al(NO3)3 →2Al2O3 +12NO2 + 3O2 6) 2Zn(No3)2 → 2ZnO + 4NO2 + O2 7) 2Pb(NO3)2 → 2PbO + 4NO2 + O2 8) 2Cu(NO3)2 → 2CuO + 4NO2 + O2 9) 2Hg(NO3)2 → Hg + 2NO2 + O2 10) 2AgNO3 → 2Ag + 2NO2 + O2 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 For example: 1. Word equation: Magnesium + Zinc nitrate Chemical equation: Mg(s) + Zn(NO3)2(aq) Magnesium + Zinc nitrate 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 acid Chemical equation: Mg (s) + 2 HCl (aq) Magnesium + Hydrogen chloride MgCl2 (aq) + H2(g) In this reaction the element magnesium is replaced by the element hydrogen, H in hydrochloric acid. Hydrogen gas, H2 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: Zn (s) + 2 HCl (aq) ZnCl 2 (aq) + H 2 (g) Examples: 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. For example: 1.Word equation: Potassium Carbonate + Barium Chloride Potassium + Barium Chloride carbonate Chemical equation: K2CO3(aq) + BaCl2(aq) 2 KCl (aq) + BaCO3(s) + 2‐ 2+ ‐ Ions: K , CO3 Ba , Cl K+ , Cl‐ Ba2+, CO32‐ Examples: V. IRREVERSIBLE REACTION In an irreversible reaction, the reactants react to form the products, which cannot revert back into reactants. H2O2 2H2O + O2 VI. REVERSIBLE REACTION In reversible reactions, as the reactants react with other reactants to form products, the products react to form reactants. Examples: 2SO2 + O2 2SO3 3Fe + 4H2O Fe3O4 + 4H2 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: H2O2 2H2O + O2 VIII. EXOTHERMIC REACTION The reactions which proceed with the evolution of heat energy are called as Exothermic Reaction. Examples: 2SO2 + O2 2SO3 N2 + 3H2 2NH3 IX. ENDOTHERMIC REACTION The reactions which proceed with the absorption of heat energy are called as Endothermic Reaction. Examples: N2 + O2 2NO C + H2O CO + H2 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: H2S + Cl2 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 Zn2+ + 2eXI. REDUCTION REACTION Definition 1: Reduction is defined as a chemical process that involves addition of hydrogen. Example: CuO + H2 Cu + H2O Definition 2: Reduction is defined as a chemical process that involves removal of oxygen. Example: Cl2 + H2S 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:Cu2+ + 2eCu 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 H2S + Cl2 [ Reduction ] 2HCl + S Removal of Hydrogen [ Oxidation ] Reducing agent Oxidising agent