Objectives:1. List three observations that suggest that achemical reaction has taken place.2. List three requirements for a correctlywritten chemical equation.3. Write a word equation and a formulaequation for a given chemical reaction.4. Balance a formula equation by inspection.
Describing Chemical Reactions Chemical reaction Defined as the process by which one or moresubstance are changed into one or more differentsubstances. Reactants: original substances Products: resulting substances According to law of conservation of mass:total mass of reactants must equal total mass of products Chemical equation Defined as represents, with symbols and formulas, theidentities and relative amounts of the reactants andproducts in a chemical reaction. Example: this chemical equation shows the reactantammonium dichromate yields the products nitrogen,chromium(III) oxide, and water(NH4)2Cr2O7(s) N2(g) + Cr2O3(s) + 4H2O(g)
Indications of Chemical Reactions1. Evolution of heat and light2. Production of gas3. Formation of a precipitate4. Color changeChemistrycomes alive!
Characteristics of ChemicalEquations1. Equation MUST represent known facts. Must be actual chemical reaction2. MUST contain the correct formulas for thereactants and products. Sodium Carbonate3. The law of conservation of mass must be satisfied. Same number of atoms on reactant side as on the productside. Coefficients are added where necessaryo Specifies relative number of moles of that substanceH2 + O2 H2OEx: Hydrogen gas reacts with oxygen gas to yield waterNa2CO32 2
H - H O = OH – OHH – OHH - HH2 + O2 H2O2 24 H’s 2 O’s 2 O’s4 H’s Satisfies the law of conservation of mass Same atoms in reactants as the products
Word and Formula Equations Word equation - 1st step in writing chemicalequation Equation in which the reactants and products in achemical reaction are represented by words Only qualitative meaning (doesn’t give quantities) Example: methane + oxygen carbon dioxide + water read as react to yield or yield (produce or form) Equation is read as “methane and oxygen yield carbondioxide and water” formula equation - 2nd step in writing chemicalequation Represents the reactants and products of a chemicalreaction by their symbol Example: (g) represents that it is in gaseous stateCH4 (g) + O2 (g) CO2 (g) + H2O (g)
balance the equation – last step in writing achemical equation Satisfy law of conservation of mass by insertingcoefficients # of atoms of reactant must = # of atoms ofproductsCH4 (g) + O2 (g) CO2 (g) + H2O (g)Reactant ProductCHOCHO142123Step 1:Type &numberStep 2:Addcoefficientstill reactants= products24424Step 3:Whenreactants =products itsbalanced
Additional symbols used inchemical reactionsYieldsReversible reactions(s) or solid state(g) or gaseous state(l) liquid state(aq) aqueous (dissolved in water)reactants are heated2 atm Pressure0⁰ C TemperatureMnO2 CatalystExamples:3Fe(s) + 4H2O(g) Fe3O4(s) + 4H2(g)2HgO(s) 2Hg(l) + O2(g)C2H2(s) + H2(g) pressure C2H6(g)Pt
Most common mistakes!!!!1. Writing incorrect formulas for reactants orproducts2. Trying to balance using subscriptso Can only use coefficientsPractice 11. Write the word and formula equations for the chemical reactionthat occurs when solid sodium oxide is added to water at roomtemperature and forms sodium hydroxide (dissolved in thewater). Include symbols for physical states in the formulaequation. Then balance the formula equation to give a balancedchemical equation.Step 1: write word equationSodium oxide + water sodium hydroxide
Step 2: convert to formula equationNa2O + H2O NaOHStep 3: add all symbolsStep 4: balanceReactant ProductNaHO22111222NaHO2(s) (l) (aq)2
Practice 2The reaction of zinc with aqueous hydrochloric acidproduces a solution of zinc chloride andhydrogen gas. Write the balanced chemicalequation for the reaction.zinc + hydrochloric acid zinc chloride + hydrogenZn(s) + HCl(aq) ZnCl2(aq) + H2(g)Reactant ProductZnHCl11122221ZnHCl2
Practice 3Solid aluminum carbide, Al4C3, reacts with water toproduce methane gas and solid aluminumhydroxide. Write a balanced chemical equationfor this reaction.Aluminum carbide + water + aluminum hydroxidemethaneAl4C3(s) + H2O(l) + Al(OH)3(s)CH4(g)Reactant ProductAlCHAlCHO O4311731521 33 441224122412
Practice 4Aluminum sulfate and calcium hydroxide are used in awater-purification process. When added to water, theydissolve and react to produce two insoluble products,aluminum hydroxide and calcium sulfate. These productssettle out, taking suspended solid impurities with them.Write a balanced chemical equation for the reaction.Aluminum sulfate + Calcium hydroxide + calcium sulfatealuminum hydroxideAl2(SO4)3 (aq) + Ca(OH)2(aq) + CaSO4(s)Al(OH)3(s)Reactant ProductAlSO4CaAlSO4CaOH OH2311112 3226333336