Chapter -10 CHEMICAL REACTIONS Chemical reactions occur when substances undergo fundamental changes in identity. The substances that participate in Chemical reaction are called reactants. The new compounds formed in the reaction are referred as Products. The reactants are represented by their formulae on the left side of the chemical equation. The formula of products are reported on the right side of the equation. The arrow implies “yield or produced “. Carbohydrates are broken down into fundamental units called Glucose. When glucose is being metabolized, carbon-dioxide, water are formed as products of the reaction with the release of energy used to run other physiological processes. The reactants are glucose that combines with oxygen. The unbalanced chemical equation for this reaction is represented as follows: Glucose + O2 _____________>Water + Carbon-dioxide + energy C6H12O6 + O2______________> H20 + CO2 (s) (g) (g) (g) Why should you balance the chemical Equation? In chemical reactions the matter is conserved. The atoms of individual elements that make up the chemical compounds are neither gained nor lost. This creates a need to balance the chemical equation. By trial and error the balanced chemical equation for the metabolism of glucose happens to be : C6H12O6 + 6O2 (g)__________> 6CO2 (g) + 6 H2O (g) If the substance added in a chemical reaction, speeds up the chemical reaction , the added substance serves as catalyst which is reported above the arrow. While balancing the chemical equation, report the physical states of reactants and products formed in the reaction . (g) stands for gaseous state, (s ) stands for solid state, (l) for liquid state and (aq) for substance dissolved in water to form an aqueous solution) The # of atoms of every element that comes from the participating reactants reported on the left side of the equation must be counterbalanced with the # of atoms of elements that make up the products formed on the right side of the equation. While balancing the chemical equation, you are not allowed to change the subcripts of reactants and products formed in the chemical reaction. The whole numbers reported in front of the formula of reactants or products in chemical equation are referred as Co-efficient. A chemical equation is not complete until it is balanced. Review the balanced chemical equation for the following ractions: a) formation of Magnesium Oxide, pg: 278-279, b) formation of water, pg: 280 -281, c) Synthesis of Ammonia, pg: 282 d) Formation of Iron Oxide, pg: 283. During a chemical reaction, the matter is conserved and there is no mass change taking place. Classifying Reactions: Thre are several types of chemical reaction: 1. Combustion reaction: organic compound (compound containing chain of carbon atoms) burns with Oxygen to form CO2 and water as the products of reaction. Eg: CH4 + 2O2_____________> CO2 + 2H20 2. Combination (synthetic ) reaction: When one element combines with another element to synthesize a new compound, the reaction is referred as Synthetic or Combination reaction. A + B_____________> AB N2(g) + 3H2(g) _________________> 2 NH3(g) When H2 gas and O2 gas are ignited by a spark, they combine to produce water which is an explosive reaction. 2H2(g) + O2(g)__spark___________> 2H2O (l) Decomposition reaction: A single compound is broken down into 2 or more simpler substances. Ab___________________> A + B Electrolysis of water generates H2 and O2 gases. Decomposition of heavy metal oxides such as HgO and PbO2 generates the metal element and O2 gas. 2HgO( s)___________________> 2Hg(l) + O2(g) Single Replacement reactions: The Higher metal element in activity series reacts with salt of lower metal to produce new element and forms a new salt. Eg: A + BC___________> AC + B Cu(s) + 2AgNO3 (aq)________________> Cu(NO3)2 (aq) +2Ag(s) A + BC_________________> BA + C (when A is a nonmetal) Double replacement reactions: The Cations and anions of participating reactants exchange or swap their partners to form new ionic compounds. AB + CD ____________> AD + CB. Where A and C are cations and Band D are anions. Case by case analysis of different types of Chemical reaction: Combustion: hydrocarbon + O2 ______________> CO2 + H2O Steps to be followed to balance a combustion reaction; First balance the carbon atoms first, then balance the H atoms and balance the O atoms last. Eg: C2H5OH + O2 ________________> CO2 + H2O (not balanced) C2H5OH + O2 ________________> 2CO2 + H2O 2C C2H5OH + O2 ________________> 2CO2 + 3H2O 6H C2H5OH + O2 ________________> 2 CO2 +3 H2O 4O + 3O C2H5OH(l) +3O2 (g)________________> 2CO2 (g)+ 3 H2O (g) (Balanced Equation) Decomposition Reactions: 2KClO3 (s)___MnO2__________> 2KCl + 3O2(g) Heat Compounds containing nitrates when heated decompose to give a nitrite compound with the release of O2 gas. 2NaNO3(s) ________Heated______________> 2NaNO2 + O2 2H2O2______MnO2 or I- ion_____________> 2H2O + O 2 (g) Decomposition reaction Reactions of Metals: Grp. I metals such as Na are very reactive in their elemental state. Hence they are stored under kerosene or mineral oil. Grp.I metal elements react rapidly with O2 present in air to form metal oxides. Metal + oxygen ________> Metal Oxide. 4Na + O2 (g) ____________> 2Na2O (s) Review the reaction of Mg + O2 and that of Fe + O2 described on page 293. Group I and Group II Metal oxides react with water to form the corresponding Metal Hydroxides which are basic in nature. Metal Oxide + Water _______________> Metal Hydroxide MgO + H2O___________> Mg(OH)2 Oxidation-Reduction reaction: The metal looses or gives up its valance electrons to form Metal cation in an oxidation reaction. Thus the metal is oxidized to metal cation as its oxidation # increases form zero to what ever is the charge carried by the metal cation depending upon the # of electrons lost during oxidation reaction. M(s) gives up its valence electrons > M+ncation + ne electron Oxidation reaction (Zero oxidn. #) oxidation # increases (oxidation # = charge of cation) M stands for metal, n stands for charge of cation and n stands for # of electrons lost during oxidation reaction. The substance that looses electrons serves as Reducing agent. Oxidation reaction is accompanied by Reduction reaction. The substance that gains electrons is getting reduced . The substance that gains elctrons servesc as Oxidizing agent. X + ne___________> X n- (During reduction electrons are gained) Review Example 10.8 Activity Series Some metals are more reactive than others. Certain metal elements have tendency to loose their valence electrons more readily than others eg: while The other metal cations have tendency to gain electrons more readily than others. Eg: Cu +2 ion gains electrons to form cu while Na prefers to loose electron to form Na+ cation . The reactivity of various metals are compiled in a table called activity series. The different metals are arranged in order of decreasing reactivity as you descend the series from top. As you walk down the series from top, the tendency of metal to loose electron decreases. in other words the tendency of metals to undergo oxidation reaction decreases. The activity series is used to predict whether a chemical reaction between metal and acid or with other compound occurs spontaneously or not. A higher metal in the activity series will react with the salt of lower metal spontaneously. The lower metal of the salt is displaced by the higher metal in the reaction. Such reactions are single replacement reactions which also happen to be oxidation – reduction reaction. Zn + CuSO4 _________________> ZnSO4 + Cu (spontaneous reaction) Zn is oxidized to Zn +2cation, Cu +2 cation is reduced to Cu. Higher metal is oxidized to form metal cation while the lower metal is reduced to form a metal. Fe + CaSO4_________________> no reaction. Group I metals are sometimes referred as active metals because they react with cold water to form the corresponding metal hydroxides with the release of H2 gas. They also react with steam and acids to produce H2 gas. Refer to reaction between Na + water, Ca + water on pg: 295. Review examples 10.10 and 10.11. Metals placed above Hydrogen in the activity series also react with acids to form metal salt and produce H2 gas. Reactions of Nonmetals: Nonmetals react with Oxygen present in air to form nonmetal oxides. S8(s) + 8O2(g)_______________> 8 SO2 (g) Carbon forms CO 2, P forms P2O5 Nonmetal oxides react with water to form acids. CO2 + H2O_____________> H2CO3 SO2 + H2O_____________> H2SO3 Review example 10.12 Single replacement reactions of Nonmetals: Most reactive halogen element reacts with compounds containing less reactive halide anions . As you walk down the halogen group, the reactivity of halogen element decreases. The order of reactivity is F> Cl> Br> I . F is most non metallic and most electronegative in character. Cl2 (g) + NaF aq) ________________> no reaction Cl2(g) + 2HI (g) ______________> I2(s) + 2HCl(g) Metathesis or double replacement reactions: When the aqueous solutions containing two different ionic compounds are mixed, the participating cations swap their anions and exchange their partners to form new ionic compound products and one of the product formed happens to be insoluble in water and precipitates out of solution. Such reactions are called as Precipitation reactions. Eg: NaCl(aq) + AgNO3 (aq)__________> NaNO3(aq) + AgCl(s) Review other precipitation reactions listed on bottom of page 300. Alternatively, the aqueous solution of ionic compound reacts with acid to form a stable covalent compound such as water and other common gases. H2SO4 + Na2S_____________> H2S(g) + Na2SO4. Review other reactions listed on page 300. The driving force for metathesis reaction is the formation of stable covalent compound. The chemical equation in which reactants and products are represented in the form of ionic species is referred as Ionic equation. Pb(NO3)2 (aq) + 2KI (aq)______________> PbI2(s) + 2KNO3(aq) Molecular equation Pb+2 (aq)+ 2NO3-(aq) + 2K 1+ + 2I-(aq)_______> PbI2(s) + 2K +1 (aq) + 2NO3- (aq) This equation is called Ionic Equation. The ions that appear as same species on both sides of equation , and do not participate in chemical reaction. These ions are called Spectator ions. Identify the spectator ion. Cancel out the spectator ions from both sides of equation. Ionic equation without spectator ions becomes net ionic equation. NO3- and K+1 serve as spectator ions in the above mentioned chemical Equation for the reaction between Lead Nitrate and Potassium Iodide. So the Net Ionic equation for this precipitation reaction is written as follows: Pb+2 (aq) + 2I-(aq)_______________> PbI2(s) Neutralization reaction is a double replacement reaction: Acid reacts with base to form salt and water. Acid + Base _____________> Salt + Water. HCl(aq) + NaOH (aq) _____________> H2O(l) + NaCl (aq) *******Announcements******* Exam -2 will be administered on March 27th. Review Text,notes and Home work problems on Chapter 7,8, 9, 10 and 11 to score well on the exams. Solutions for home work Questions will be posted on bulletin board across from my office on the 4th floor HW Assignment due Thursday 10.5, 10.7, 10.11, 10.14,10.21,10.27,10.33, 10.35, 10.39, 10.45, 10.47 and 10.55.
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