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Balancing Equations
 

Balancing Equations

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    Balancing Equations Balancing Equations Presentation Transcript

    • Chemistry 30S Unit 3 – Chemical Reactions
      • A chemical equation indicates the substances reacting and the substances produced in a chemical reaction. A chemical equation also shows the ratio in which these substances react or are produced.
      • A chemical equation will usually be written with the reactants on the left side of an arrow and the products on the right side of an arrow.
      • REACTANTS PRODUCTS
      CHEMICAL REACTIONS Chemical Equations
      • A word equation can describe a chemical reaction. For example:
      • "Hydrogen gas and oxygen gas react to form (or yield) water vapour."
      • This word equation can be written as a chemical equation by writing chemical formulas rather than words:
      • 2 H 2 ( g ) + O 2 ( g ) 2 H 2 O( g )
      CHEMICAL REACTIONS Chemical Equations
      • The reactants are hydrogen gas and oxygen gas. The product is water vapour. The 2's in front of the H 2 and H 2 O are called coefficients.
      • Coefficients indicate the ratio in which the substances combine or are produced in a chemical reaction.
      • 2 H 2 ( g ) + O 2 ( g ) 2 H 2 O( g )
      • "Two molecules of hydrogen gas react with one molecule of oxygen gas to yield two molecules of water."
      CHEMICAL REACTIONS Chemical Equations
      • Chemists show the states or phases of the reactants and products by using abbreviations in parenthesis following each reactant and product. The abbreviations are as follows:
      • ( s ) or ( c ) means solid or crystalline ( l ) means liquid ( g ) means gas or vapour ( aq ) means the substance is aqueous or dissolved in water (aqua = water)
      • 2 H 2 ( g ) + O 2 ( g ) 2 H 2 O( g )
      CHEMICAL REACTIONS Chemical Equations
      • A chemical equation can also show heat changes that occur. Endothermic reactions cause the reaction vessel to feel cooler because the reaction absorbs energy. The energy is used in the reaction, so energy is a reactant. Exothermic reactions release energy. Consequently, we consider heat or energy to be a product of an exothermic reaction. In our example, the burning of hydrogen is highly exothermic. The reaction can be rewritten as
      • 2 H 2 ( g ) + O 2 ( g ) 2 H 2 O( g )+ energy
      CHEMICAL REACTIONS Chemical Equations
      • Antoine Laurent Lavoisier (1743-1794) is credited with discovering the Law of Conservation of Mass. The Law of Conservation of Mass (or Matter) states:
      CHEMICAL REACTIONS The Law of Conservation of Mass In any chemical reaction matter cannot be created nor destroyed. This means the mass will not change during a chemical reaction.
      • According to the Law of Conservation of Mass (Matter), mass (matter) cannot be created nor destroyed. During a chemical reaction, since atoms are a form of matter, atoms cannot be created nor destroyed. We say that atoms are conserved. This means the number of each kind of atom in the reactants must equal the number of that kind of atom in the product .
      CHEMICAL REACTIONS The Law of Conservation of Mass & Balancing Equations
      • When balancing a chemical equation we do not change the subscripts of the substances in the reaction.
      • To balance atoms, we insert coefficients rather than changing subscripts.
      CHEMICAL REACTIONS Balancing Equations
      • If not already done, write out the formulas of each reactant and product.
      • Use coefficients to balance metals first, ions, and then non-metals. Followed by hydrogen, carbon and finally oxygen.
      • Leave single elements and diatomic molecules to balance last.
      • If possible, reduce the coefficients to the lowest whole number ratio. Multiply fractions, if present, by the denominator to make all coefficients whole numbers.
      CHEMICAL REACTIONS Balancing Equations
    • CHEMICAL REACTIONS Balancing Equations Example 1. Balance the equation C 3 H 8 + O 2 CO 2 + H 2 O
    • CHEMICAL REACTIONS Balancing Equations Example 2. Balance the equation Fe + O 2 Fe 2 O 3 2't'1
    • CHEMICAL REACTIONS Balancing Equations Example 3. Balance the equation Al 2 (SO 4 ) 3 + CaCl 2 AlCl 3 + CaSO 4
    • CHEMICAL REACTIONS Types of Chemical Reactions & Predicting Their Products JIGSAW ACTIVITY Synthesis Decomposition Single Replacement Double Replacement
    • CHEMICAL REACTIONS Types of Chemical Reactions & Predicting Their Products
      • A synthesis reaction starts with two or more simple reactants and results in a single, more complex product.
      • (A + B  AB)
      • A decomposition reaction begins with a single complex reactant and results in two or more simpler products.
      • (AB  A + B)
      • A single replacement reaction usually begins with an element and a compound as reactants. The metals exchange with metals and non-metals exchange with non-metals to produce a new element and compound.
      • (A + BX  B + AX)
      • A double replacement reaction begins with two compounds as reactants. The metals, usually aqueous ions, exchange resulting in two or more new compounds.
      • (AX + BY  AY + BX)
      • The burning of magnesium ribbon to form a white solid
      • The formation of MgO (s) from Mg (s) and O 2(g) solutions.
      CHEMICAL REACTIONS Synthesis Reaction + O 2 (g) 2Mg (s) 2MgO
      • Occurs when the oxygen in the air reacts with the solid magnesium to produce a solid white substance
      • A + B  AB
      • For this demonstration:
      • 2 Mg (s) + O 2(g)  2MgO (s)
      CHEMICAL REACTIONS Synthesis Reaction
      • Sugar (glucose) will decompose or break down, when catalyzed with sulphuric acid, to produce solid carbon and water.
      • The formation of C (s) and H 2 O (g) from the decomposition of glucose.
      CHEMICAL REACTIONS Decomposition Reaction + C 6 H 12 O 6(s) H 2 O (g) C (s)
      • Occur when the elements in a solution of reacting compounds exchange places, or replace one another.
      • AB  A + B
      • For this demonstration:
      • C 6 H 12 O 6(s)  6H 2 O (g) + 6C (s)
      CHEMICAL REACTIONS Decomposition Reaction
      • When silver nitrate solution covers a shiny penny, the penny becomes covered in silver crystals.
      • The formation of Ag (s) from a single replacement with copper.
      CHEMICAL REACTIONS Single Replacement Reaction + Cu (s) Ag + NO 3 - Cu 2+ NO 3 -
      • Occur when the elements in a solution of reacting compounds exchange places, or replace one another.
      • A + BC  AC + B
      • For this demonstration:
      • Cu (s) + AgNO 3(aq)  Ag (s) + Cu(NO 3 ) 2(aq)
      CHEMICAL REACTIONS Single Replacement Reaction +
      • When mixed, two clear solutions produce a dense yellow precipitate
      • The formation of PbI 2(s) from Pb(NO 3 ) 2 and 2KI solutions.
      CHEMICAL REACTIONS Double Replacement Reaction PbI 2(s) + Pb 2+ NO 3 - K + I - K + NO 3 -
      • Occur when the elements in a solution of reacting compounds exchange places, or replace one another.
      • AB + CD  AD + CB
      • For this demonstration:
      • Pb(NO 3 ) 2(aq) + 2KI (aq)  PbI 2(s) + 2KNO 3(aq)
      CHEMICAL REACTIONS Double Replacement Reaction +