Chapter 8.1 : Describing Chemical Reactions
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Chapter 8.1 : Describing Chemical Reactions

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Chapter 8.1 : Describing Chemical Reactions Chapter 8.1 : Describing Chemical Reactions Presentation 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
    • Chemical equation View slide
    • Defined as represents, with symbols and formulas, the identities and relative amounts of the reactants and products in a chemical reaction. View slide
    • 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.
    • Sodium Carbonate
    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
    • Example:
    • (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