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Unit 8 Chemical Reactions PAGE 123 Describing Chemical Reactions • A chemical reaction is the process by which one or more substances are changed into one or more different substances. • They are described by chemical equations Describing Chemical Reactions • In a chemical reaction: bonds are broken in the reactants – Bonds are formed in the products – Parts of a Chemical Equation Reactants • original substances in a chemical reaction • written on the left hand side of a chemical equation Products • The resulting substances produced in a chemical reaction. • written on the right hand side of the chemical equation yields Subscripts in (parenthesis) • represents the physical states of the compounds (elements) • Ex: (s)= solid, (l)=liquid, (g)= gas, (aq)= aqueous (dissolved in water) • Words or symbols are placed over/under the arrow(s) to indicate certain conditions under which the reaction is carried out. – : heat is applied – catalyst • substance that speeds up a chemical reaction without becoming part of the reaction. Law of conservation of mass According to the law of conservation of mass, • the total mass of reactants must equal the total mass of products for any given chemical reaction. Law of conservation of mass Balance atoms 2 H2 (g) + O2 (g) 2 H2O Mass must be equal 50 g + 45 g 95g (g) Translate Chemical Equations into Words Chemical Equations • 1) include all of the symbols appearing in the equation (with the exception of the coefficients) Chemical Equations • 3. Diatomic Elements: Elements that combine with each other when found alone in nature Diatomic Elements • 7 diatomic elements (know) H2 O2 F2 Br2 I2 N2 Cl2 Example #2 Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g) Solid magnesium reacts with aqueous hydrochloric acid to produce aqueous magnesium chloride and hydrogen gas Check for Understanding CaCO3(s) CaO(s) + CO2(g) Solid calcium carbonate is heated and produces solid calcium oxide and gaseous carbon dioxide Check for Understanding #2 3KOH(aq) + Fe(NO3)3(aq) Fe(OH)3(s) + 3KNO3(aq) Aqueous potassium hydroxide reacts with aqueous iron (III) nitrate to produce solid iron (III) hydroxide and aqueous potassium nitrate Writing Chemical Equations from a Written Description Example #1 Aluminum metal reacts with oxygen in the air to form solid aluminum oxide. 4 Al (s) + 3 O2 (g) 2 Al2O3 (s) Example #2 When solid mercury(II) sulfide is heated with oxygen gas, liquid mercury metal and gaseous sulfur dioxide are produced. HgS (s) + O2 (g) Hg (l) + SO2 (g) Check for Understanding Oxygen gas can be produced by heating solid potassium chlorate in the presence of the catalyst manganese dioxide. Potassium chloride is a solid residue. 2 KClO3 (s) MnO2 2 KCl (s) + 3O2 (g) Check for Understanding Aqueous potassium nitrate and a precipitate of barium chromate are formed when aqueous solutions of barium nitrate and potassium chromate are mixed. Check for Understanding Aqueous potassium nitrate and a precipitate of barium chromate are formed when aqueous solutions of barium nitrate and potassium chromate are mixed. Ba(NO3)2 (aq) + K2CrO4 (aq) 2 KNO3 (aq) + BaCrO4 (s) Types of Chemical Reactions Synthesis (Combination) Reactions Two or more substances combine to form a new compound. . Formation of only ONE product. Synthesis (Combination) Reactions Examples: 2H2 + CaO + O2 2H2O H2O Ca(OH)2 Decomposition Reactions • A single compound undergoes a reaction that produces two or more simpler substances. Decomposition Reactions Decomposition Reactions Decomposition reactions are the opposite of synthesis reactions. There is only one reactant. Examples • CaCO3 CaO + CO2 • Na2CO3 Na2O + CO2 Single-Replacement Reactions • one element replaces a similar element in a compound. • a single element reacts with a compound. Single-Replacement Reactions Distinguishing Characteristics • Examples: Pb + 2HNO3 F2 + 2NaI Pb(NO3)2 2NaF + + I2 H2 Predict the Products Zn + CuCl2 ZnCl2 + Cu Double Replacement Reactions The ions of two compounds exchange places in an aqueous solution to form two new compounds. Two ionic compounds “swap” ions. Double Replacement Reactions Usually forms a precipitate • precipitate: when two aqueous solutions are mixed and a solid product that is formed AgNO3(aq) + KCl(aq) AgCl(s) + KNO3(aq) Precipitate Examples • PbSO4 + 2KCl K2SO4 + PbCl2 • AgNO3(aq) + KCl(aq) ----> AgCl(s) + KNO3(aq) Predict the Products 2 HOH 2 KOH + H2SO4 K2SO4 + H2O Combustion Reactions A hydrocarbon (CxHy) combines with oxygen, releasing a large amount of energy in the form of light and heat. General Form CxHy + O2 CO2 + H 2O Example • C3H8 + 5O2 3 CO2 • CH4 + O2 CO2 + H2O + 4 H 2O Predict the Products 1 2 6 5 2 C2H6 + 10 O2 4 CO2 + 12 H2O 1) 2 Li + 2 H2O 2 LiOH + H2 single replacement / displacement 2) NH4Cl NH3 + decomposition HCl 3) AgNO3 + NaCl AgCl + NaNO3 double replacement / displacement 4) 2 C6H14 + 19 O2 12 CO2 + 14 H2O combustion 5) N2 + 3 H2 2 NH3 combination / synthesis Oxidation – Reduction Processes Redox Reaction Any chemical reaction that involves the transfer of one or more electrons between atoms. One reactant gains electrons and the other reactant loses electrons. Examples of Redox Reactions • Combustion of gasoline • Burning of wood • Energy from food • Bleaching stains • Iron rusting Oxidation and Reduction • Opposing reactions • Must occur together – Redox (Reduction – Oxidation) Oxidation • Electrons are fully or partially lost Mg0 (s) Mg 2+ + 2e- Electrons lost: written as a product Reduction • Electrons are fully or partially gained S0(s) + 2 e- S2electrons gained: written as a reactant Mnemonic Device • LEO the lion goes GER LEO: Loss of Electrons is Oxidation GER: Gain of Electrons is Reduction Mnemonic Device • OIL RIG – Oxidation Is Loss of electrons – Reduction Is Gain of electrons Complete Redox Reaction • Add together the reduction halfreaction with the oxidation halfreaction to get the complete redox reaction. Complete Redox Reaction + Mg0 (s) Mg 2+ + 2e- 2+ + S2- S0(s) + 2 e- S2- Mg0 (s) + S0(s) Mg Mg + S Mg+2 + S -2 Assigning Oxidation Numbers Oxidation Numbers • Numbers assigned to all of the elements involved in the reaction to determine if electrons have been transferred between atoms in a reaction. Oxidation Numbers • The oxidation number is usually equal to the charge on the ion if it was formed. Rule #1 • Free elements are assigned an oxidation state of 0. – Al = 0 – Na = 0 – H2 = 0 Rule #2 • The oxidation state for any simple one-atom ion is equal to its charge. – Na+ = +1 – Be2+= +2 – F- = -1 Rule #3 • The alkali metals (Li, Na, K, Rb, Cs and Fr) in compounds are always assigned an oxidation state of +1. – LiOH: Li = +1 – Na2SO4: Na= +1 Rule #4 • Fluorine in compounds is always assigned an oxidation state of -1. – HF: F= -1 – MgF2: F= -1 Rule #5 • The alkaline earth metals (Be, Mg, Ca, Sr, Ba, and Ra) and also Zn and Cd in compounds are always assigned an oxidation state of +2. Similarly, Al & Ga are always +3. • MgF2: Mg = +2 • CaO: Ca= +2 • Al2O3: Al=+3 Rule #6 • Hydrogen in compounds is assigned an oxidation state of +1. Exception - Hydrides, ex. LiH (H=-1). • H2SO4: H=+1 • HCl: H=+1 Rule #7 • Oxygen in compounds is assigned an oxidation state of -2. Exception - Peroxide, ex. H2O2 (O = -1). – H3PO4: O=-2 – H2O: O=-2 Rule #8 • The sum of the oxidation numbers of all atoms in a neutral compound is 0. – H2SO4: add up to 0 – CO2: add up to 0 Rule #9 • The sum of the oxidation numbers of all atoms in a polyatomic ion equals the charge on the ion. –SO42-: add up to -2 –NH4+: add up to +1 Examples • Al(s): Rule #1 0 – Al = ____ Examples • CaCl2: +2 Ca – Ca = ____ -1 – Cl=____ Rule #5 Rule #8 1 (+2) + 2 (Cl) = 0 Cl = -1 Examples • HNO3: +1 – H= ____ +5 – N= ____ -2 – O= ____ Rule # 6 Rule # 7 Rule # 8 1(1) + 1(N) + 3(-2) = 0 N = +5 Examples • SO42-: +6 – S= ____ -2 – O= ____ Rule #7 Rule #9 1(S) + 4(-2) = -2 S= +6 Examples • H2O: Rule #6 +1 – H= ____ -2 – O= ____ Rule #7 Examples • (NH4)2CO3 NH4+ CO32- Rule 6 Rule 7 Rule 9 1(N)+4(+1)=+1 N= -3 1(C)+3(-2)=-2 C=+4 -3 N=____ +1 H=____ +4 C=___ -2 O=___ Check for Understanding • S8: 0 – S= ____ • AsO43-: +5 – As= ____ -2 – O= ____ Check for Understanding OH• Cr(OH)3: +3 – Cr= ____ -2 – O= ____ +1 – H= ____ Check for Understanding • (NH4)3PO4: -3 – N= ____ +1 – H=_____ +5 – P= ____ -2 – O= ____ NH4+ PO4-3 Oxidized Element • Atoms that lose electrons in a chemical reaction. – Elements that lose electrons are undergoing oxidation and are said to be oxidized. Oxidized Element • The substance that loses electrons is the oxidized element. • Atoms that are oxidized will have an increase in their oxidation number Oxidized Element Increase in oxidation number Mg0 (s) + S0(s) Mg 2+ Mg is oxidized + S2- Reduced Element • Atoms that gain electrons in a chemical reaction. – Elements that gain electrons are undergoing reduction and are said to be reduced. Reduced Element • The substance that gains electrons is the reduced element. • Atoms that are reduced will have an decrease in their oxidation number Reduced Element Decrease in oxidation number Mg0 (s) + S0(s) Mg 2+ S is reduced + S2- Oxidizing Agent • The element or compound that is reduced. • It is called the oxidizing agent because it oxidizes the other element or compound. Oxidizing Agent Decrease in oxidation number Mg0 (s) + S0(s) Mg 2+ + S2- S is reduced S is the oxidizing agent (it is oxidizing Mg) Reducing Agent • The element or compound that is oxidized. • It is called the reducing agent because it reduces the other element or compound. Reducing Agent Increase in oxidation number Mg0 (s) + S0(s) Mg 2+ + S2- Mg is oxidized Mg is the reducing agent (it is reducing S) Examples • Are the following redox reactions? • If yes: – – – – What What What What element is oxidized? element is reduced? is the oxidizing agent? is the reducing agent? Examples 0 Mg(s) + S(s) 0 +2 -2 MgS Mg What element is oxidized? Step # 2: Do the oxidation numbers Step Oxidation Numbers Yes#1: – ItAssign is a redox change? What element is reduced?S - if is no the itoxidizing is not a redox What agent? S reaction What is the reducing agent?Mg - if yes it is a redox reaction Examples 0 0 +1 +2 2AgNO3(aq) + Cu(s) Cu(NO3)2(aq) + 2Ag(s) NO3- NO3- Step # 2: Do the oxidation What element is oxidized? Cunumbers change? What element is reduced?Ag Yes – It is a redox Step #1: Assign Oxidation Numbers if no it is not a redox What is the oxidizing agent? AgNO3(aq) reaction What is the reducing agent?Cu(s) - if yes it is a redox reaction Check for Understanding • Are the following redox reactions? • If yes: – What element is oxidized? – What element is reduced? – What is the oxidizing agent? – What is the reducing agent? Check for Understanding 0 0 +3 -2 4Fe(s) + 3O2(g) 2Fe2O3(s) What element is oxidized? Fe Yes What element is reduced?O What is the oxidizing agent? O2 What is the reducing agent? Fe Check for Understanding +1 -2 +2 -2+1 0 0 Ca(s) + H2O(l) Ca(OH)2(aq) + H2(g) OHYesis oxidized?Ca What element What element is reduced?H What is the oxidizing agent? H2O What is the reducing agent? Ca Check for Understanding +1 -1 +1 -2 +1 +1 -1 +1 -2 HCl + NaOH NaCl + H2O No – it is not a redox Check for Understanding 0 +3 -2 +2 -2 +4 -2 Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g) What element is oxidized?C Yesis reduced?Fe What element What is the oxidizing agent? Fe2O3 What is the reducing agent? CO Check for Understanding +1 0 +2 +0 2AgNO3(aq) + Cu(s) Cu(NO3)2(aq) + 2Ag NO3- NO3- (s) What element is oxidized? Cu Yes What element is reduced?Ag What is the oxidizing agent? AgNO3 What is the reducing agent? Cu Check for Understanding +1-1 +1 +1 +1 -1 NaCl + AgNO3 NaNO3 + AgCl NO3- NO3- No – it is not a redox Check for Understanding 0 0 +1-1 2 H2(g) + O2(g) 2 H2O(l) What element is oxidized?H Yes is reduced?O What element What is the oxidizing agent? O2 What is the reducing agent? H 2 Check for Understanding -4+1 0 +4 -2 +1 -2 CH4 + 2 O2 CO2 + 2 H2O What element is oxidized?C Yesis reduced?O What element What is the oxidizing agent?O2 What is the reducing agent? CH4