Chemical Equations and Reactions Ridgewood High School

Thinking Questions What are some signs that a chemical change may be taking place? Formation of a gas, solid (percipitate), a change in color or odor, and release or absorption of energy. What are the reactants of a reaction? What are the products of a reaction? Reactants are starting materials and products that are substances that form during a reaction. Define the terms synthesis and decomposition. Synthesis is the formation of an item from smaller items while decomposition is the breaking down of an item into smaller items.

Describing Chemical Reactions Chemical Reaction: is the process by which one or more substances change into one or more new substances whose chemical and physical properties differ from those original substances. Hydrogen (g) + Oxygen (g)  Water (l) Reactants Products

Evidence of Chemical Change Changes in EnergyFormation of New Substances Release of energy as heat Formation of a gas Release of energy as light Formation of a precipitate (an insoluble solid) Products of soundChange in color Reduction or increase of temperature Absorption or release of electrical energy

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Reactions and Energy Changes Chemical Reactions will either: Release energy or absorb energy Methane + oxygen  carbon dioxide + water + energy… therefore energy is released Dinitrogen tetroxide + energy  nitrogen dioxide… therefore energy is absorbed

Construction a Chemical Equation Chemical Equation: shows the chemical formulas and relative amounts of all reactants and products. Word Equation: contains the names of the reactants and products seperated by an arrow. Hydrogen gas + Oxygen gas  Water H 2(g) + O 2(g)  H 2 O (l)

State Symbols and Reaction Conditions SymbolMeaning (s), (l), (g)Substance in the solid, liquid, or gaseous state (aq)Substance in aqueous solution (dissolved in water)  “Produces” or “yields” indicating result of reaction  Reversible reaction in which products can reform into reactants  with  or heat on top of arrow Reactants are heated: temperature is not specified.

Balancing Equations Why must an equation be balanced? Tells us the # of molecules of each substance that reacts and is produced Atoms are conserved in a chemical reaction

Balancing Equations Is this equation balanced? Na + Cl 2  NaCl ReactantsProduct Na: 1Na:1 Cl: 2 Cl:1 No this equation is not balanced: there is more Cl on reactants side.

Balancing Equations Coefficients can be used to balance equations 2Na + Cl 2  2NaCl coefficients ReactantsProduct Na: 2(1)=2 Na:2(1)=2 Cl: 1(2)=2Cl:2(1)=2 Multiply coefficient by subscript to get total # of atoms

Balancing Equations Balance this equation:H 2 + O 2  H 2 O 2H 2 + O 2  2H 2 O ReactantsProducts H: 2(2) = 4H: 2(2) =4 O : 1(2) = 2O: 2(1) = 2 Use coefficients to balance equation Never change subscripts!

Diatomic Molecules Certain elements are diatomic (2 atoms) when they are not combined with other elements hydrogenH 2 nitrogenN 2 oxygenO 2 fluorineF 2 chlorineCl 2 bromineBr 2 iodineI 2

Practice Problems Balance the following problems: 1. Fe 2 O 3 + H 2  Fe + H 2 O 2. P 4 + O 2  P 2 O 5 3. Ca 2 Si + Cl 2  CaCl 2 + SiCl 4

Practice Problems: Answers 1. Fe 2 O 3 + 3H 2  2Fe + 3H 2 O 2. P 4 + 5O 2  2P 2 O 5 3. Ca 2 Si + 4Cl 2  2CaCl 2 + SiCl 4 Prove the Law of Conservation of Mass with the answers to the above questions.

Practice Problems II Balance the following Chemical reactions with polyatomic ions. 1. HgCl 2 + AgNO 3  Hg(NO 3 ) 2 + AgCl 2. Al + Hg(C 2 H 3 O 2 ) 2  Al(C 2 H 3 O 2 ) 3 + Hg 3. What are polyatomic ions?

Practice Problems II: Answers 1. HgCl 2 + 2AgNO 3  Hg(NO 3 ) 2 + 2AgCl 2. 2Al + 3Hg(C 2 H 3 O 2 ) 2  2Al(C 2 H 3 O 2 ) 3 + 3Hg 3. What are polyatomic ions? Atoms bonded together that have a positive or negative charge. Ex: C 2 H 3 O 2 -1 or NO 3 -1

Decomposition Reaction a compound breaks down into elements or other (simpler) compounds FeS  Fe + S NH 4 OH  NH 3 + H 2 O Always only one reactant

Synthesis Reaction Two or more elements or compounds combine to form another compound Fe + S  FeS NH 3 + H 2 O  NH 4 OH Always only one product

Single Replacement An element switches with another element of the same type (metal, nonmetal) in a compound NaI + K  KI + Na FeS + Ca  CaS + Fe Always one compound and one element on each side

Double Replacement The cations in two different compounds switch places NaCl + KI  KCl + NaI FeS + CaCO 3  CaS + FeCO 3 Two compounds on either side

Combustion An organic compound reacts with oxygen CH 4 + 2O 2  CO 2 + 2H 2 O A fuel plus an oxidizer (usually oxygen) yields compounds of each element of the fuel and the oxidizer (usually oxides) fuel oxidizer Always produces carbon dioxide and water Always has Oxygen as a Reactant with A Hydrocarbon

Practice Problems III Determine the type of Chemical Reactions 1. H 2 + Cl 2  HCl 2. C 2 H 6 + O 2  CO 2 + H 2 O 3. H 2 CO 3  CO 2 + H 2 O 4. NaCl + BaO  Na 2 O + BaCl 2 5. KI + Br 2  KBr + I 2 6. Mg + Pb(NO 3 ) 2  Pb + Mg(NO 3 ) 2

Practice Problems (III): Answers 1. H 2 + Cl 2  HCl Synthesis 2. C 2 H 6 + O 2  CO 2 + H 2 O Combustion 3. H 2 CO 3  CO 2 + H 2 O Decomposition 4. NaCl + BaO  Na 2 O + BaCl 2 Double Replacement 5. KI + Br 2  KBr + I 2 Single Replacement 6. Mg + Pb(NO 3 ) 2  Pb + Mg(NO 3 ) 2 Single Replacement