Chapter 8.1 : Describing Chemical ReactionsPresentation Transcript
Chemical equations and reactions Chapter 8.1
Objectives: List three observations that suggest that a chemical reaction has taken place. List three requirements for a correctly written chemical equation. Write a word equation and a formula equation for a given chemical reaction. Balance a formula equation by inspection.
Describing Chemical Reactions Chemical reaction Defined as the process by which one or more substance are changed into one or more different substances. Reactants: original substances Products: resulting substances According to law of conservation of mass: total mass of reactants must equal total mass of products
Defined as represents, with symbols and formulas, the identities and relative amounts of the reactants and products in a chemical reaction.
Example: this chemical equation shows the reactant ammonium dichromate yields the products nitrogen, chromium(III) oxide, and water
(NH4)2Cr2O7(s) N2(g) + Cr2O3(s) + 4H2O(g)
Indications of Chemical Reactions Evolution of heat and light Production of gas Formation of a precipitate Color change Chemistry comes alive!
Characteristics of Chemical Equations Equation MUST represent known facts.
Must be actual chemical reaction
MUST contain the correct formulas for the reactants and products.
Na2CO3 The law of conservation of mass must be satisfied.
Same number of atoms on reactant side as on the product side.
Coefficients are added where necessary
Specifies relative number of moles of that substance
Ex: Hydrogen gas reacts with oxygen gas to yield water H2 + O2 H2O 2 2
2 H2 + O2 H2O 2 H – O H – O H H H - H O = O H - H 4 H’s 2 O’s 2 O’s 4 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 chemical equation Equation in which the reactants and products in a chemical 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 carbon dioxide and water”
formula equation - 2ndstep in writing chemical equation
Represents the reactants and products of a chemical reaction by their symbol
(g) represents that it is in gaseous state
CH4 (g) + O2 (g) CO2 (g) + H2O (g)
balance the equation– last step in writing a chemical equation Satisfy law of conservation of mass by inserting coefficients # of atoms of reactant must = # of atoms of products 2 2 Reactant Product Step 1: Type & number C C 1 1 Step 3: When reactants = products its balanced H H 4 2 4 Step 2: Add coefficients till reactants = products O O 2 3 4 4 CH4 (g) + O2 (g) CO2 (g) + H2O (g)
Additional symbols used in chemical reactions Examples: Yields Reversible reactions (s) or solid state (g) or gaseous state (l) liquid state (aq) aqueous (dissolved in water) reactants are heated 2 atm Pressure 0⁰ C Temperature MnO2 Catalyst 3Fe(s) + 4H2O(g) Fe3O4(s) + 4H2(g) 2HgO(s) 2Hg(l) + O2(g) C2H2(s) + H2(g) pressure C2H6(g) Pt
Significance of a Chemical Equations Coefficients of a chemical reaction indicate relative, not absolute, amount of reactants and products
H2(g) + Cl2(g) 2HCl(g)
1 molecule H2; 1 molecule Cl2; 2 molecules HCl
Shows smallest possible relative amounts
Relative masses of the reactants and products of a chemical reaction can be determined from the reaction’s coefficients = 2.02 g H2 = 70.90 g Cl2 = 72.92 g HCl 1 mol H2 x 2.02 g H2 1 mol Cl2 x 70.90 g Cl2 2 mol HCl x 36.46 g HCl 1 mol H2 1 mol Cl2 1 mol HCl
Reverse reaction for a chemical reaction has the same relative amounts of substances as the forward reaction Balancing Chemical Equations Identify the names of the reactants and the products, and write a word equation. water hydrogen + oxygen Write a formula equation by substituting correct formulas for the names of the reactants and the products. H2O(g) H2(g) + O2(g)
Balance the formula equation according to the law of conservation of mass.
Balance the different types of atoms one at a time.
First balance the atoms of elements that are combined and that appear only once on each side of the equation
Balance polyatomic ions that appear on both sides of the equation as single units.
Balance H atoms and O atoms after atoms of all other elements have been balanced
H2O(g) H2(g) + O2(g) 2 2 Product Reactant H H 2 2 4 4 O O 1 2 2 Count atoms to be sure that the equation is balanced.
Most common mistakes!!!! Writing incorrect formulas for reactants or products Trying to balance using subscripts
Can only use coefficients
Practice 1 Write the word and formula equations for the chemical reaction that occurs when solid sodium oxide is added to water at room temperature and forms sodium hydroxide (dissolved in the water). Include symbols for physical states in the formula equation. Then balance the formula equation to give a balanced chemical equation. Step 1: write word equation Sodium oxide + water sodium hydroxide
Step 2: convert to formula equation Step 3: add all symbols 2 Na2O + H2O NaOH (s) (l) (aq) Step 4: balance Reactant Product Na 2 1 Na 2 2 H 1 2 H 2 O 1 2 O Practice 2 Translate the following chemical reaction PbCl2(aq) + Na2CrO4(aq) PbCrO4(s) + 2 NaCl(aq) Aqueous lead(II) chloride reacts with aqueous sodium chromate to produce a precipitate of lead(II) chromate and aqueous sodium chloride.
Practice 3 The reaction of zinc with aqueous hydrochloric acid produces a solution of zinc chloride and hydrogen gas. Write the balanced chemical equation for the reaction. zinc + hydrochloric acid zinc chloride + hydrogen Zn(s) + HCl(aq) ZnCl2(aq) + H2(g) 2 Reactant Product Zn 1 1 Zn 1 H 2 2 H 2 1 Cl 2 Cl
Practice 4 Solid aluminum carbide, Al4C3, reacts with water to produce methane gas and solid aluminum hydroxide. Write a balanced chemical equation for this reaction. Aluminum carbide + water + aluminum hydroxide methane 3 4 Al4C3(s) + H2O(l) + Al(OH)3(s) CH4(g) 12 Reactant Product 4 4 1 Al Al 3 3 1 C C 24 24 7 15 2 H H 12 1 3 12 O O
Practice 5 Aluminum sulfate and calcium hydroxide are used in a water-purification process. When added to water, they dissolve and react to produce two insoluble products, aluminum hydroxide and calcium sulfate. These products settle out, taking suspended solid impurities with them. Write a balanced chemical equation for the reaction. Aluminum sulfate + Calcium hydroxide + calcium sulfate aluminum hydroxide Al2(SO4)3 (aq) + 2 3 3 Ca(OH)2(aq) + CaSO4(s) Al(OH)3(s) Reactant Product 2 1 2 Al Al 3 1 3 SO4 SO4 3 3 1 1 Ca Ca 6 2 3 6 OH OH