Chemical Equations & Reactions

Describing a Chemical Reaction Indications of a Chemical Reaction –Evolution of heat, light, and/or sound –Production of a gas –Formation of a precipitate –Color change

Chemical Equations reactantsproducts Depict the kind of reactants and products and their relative amounts in a reaction. 4 Al(s) + 3 O 2 (g) 2 Al 2 O 3 (s) The numbers in the front are called stoichiometric coefficients stoichiometric coefficients. The letters (s), (g), and (l) are the physical states of compounds.

Chemical Equations This equation means: 4 Al(s) + 3 O 2 (g) 2 Al 2 O 3 (s) 4 Al atoms + 3 O 2 molecules yield 2 molecules of Al 2 O 3 4 Al moles + 3 O 2 moles yield 2 moles of Al 2 O 3 or

Chemical Equations Because the same atoms are present in a reaction at the beginning (reactants) and at the end (reactants), the amount of matter in a system does not change. Law of Conservation of Matter The Law of Conservation of Matter Kotz web

Chemical Equations conservation of matter Because of the principle of the conservation of matter, equation must be balanced An equation must be balanced. It must have the same number of atoms of the same kind on both sides. Lavoisier, 1788

Signs of Chemical Reactions There are five main signs that indicate a chemical reaction has taken place: change in colorchange in odorproduction of new gases or vapor input or release of energy difficult to reverse release input

Characteristics of Chemical Equations The equation must represent known facts. The equation must contain the correct formulas for the reactants and products. The law of conservation of mass must be satisfied.

Chemical Equations Reactants – the substances that exist before a chemical change (or reaction) takes place. Products – the new substance(s) that are formed during the chemical changes. CHEMICAL EQUATION indicates the reactants and products of a reaction. REACTANTS  PRODUCTS

Word Equations A WORD EQUATION describes chemical change using the names of the reactants and products. Write the word equation for the reaction of methane gas with oxygen gas to form carbon dioxide and water. methane + oxygen carbon dioxide + water Reactant Product

Unbalanced and Balanced Equations Cl H H H HH H H H 2 + Cl 2  HCl (unbalanced) H 2 + Cl 2  2 HCl (balanced) reactants products 2 H Cl H

Meaning of Chemical Formula Chemical Symbol Meaning Composition H 2 O One molecule of water: Two H atoms and one O atom 2 H 2 O Two molecules of water: Four H atoms and two O atoms H 2 O 2 One molecule of hydrogen peroxide: Two H atoms and two O atoms

Balancing Chemical Equations Balanced Equation – one in which the number of atoms of each element as a reactant is equal to the number of atoms of that element as a product What is the relationship between conservation of mass and the fact that a balanced equation will always have the same number of atoms of each element on both sides of an equation? Determine whether the following equation is balanced. 2 Na + H 2 O  2 NaOH + H 2 2 Na + 2 H 2 O  2 NaOH + H 2

Balancing Chemical Equations Write a word equation for the reaction. Write the correct formulas for all reactants and products. Determine the coefficients that make the equation balance.

1) Write a word equation for the reaction. Write a balanced equation for the reaction between chlorine and sodium bromide to produce bromine and sodium chloride. 2) Write the correct formulas for all reactants and products. 3) Determine the coefficients that make the equation balance. chlorine + sodium bromide  bromine + sodium chloride Cl 2 + NaBr  Br 2 + NaCl Cl NaBr  Br NaCl

1) Write a word equation for the reaction. 2) Write the correct formulas for all reactants and products. 3) Determine the coefficients that make the equation balance. aluminum sulfate + calcium chloride  calcium sulfate + aluminum chloride Al 2 (SO 4 ) 3 + CaCl 2  CaSO 4 + AlCl 3 Write the balanced equation for the reaction between aluminum sulfate and calcium chloride to form a white precipitate of calcium sulfate. Al 2 (SO 4 ) CaCl 2  3 CaSO AlCl 3

CH O 2  CO H 2 O Reactants Products 1 C atom 1 C atom 4 H atoms 4 H atoms 4 O atoms 4 O atoms

Reactants  Products Unbalanced

Showing Phases in Chemical Equations Solid Phase – the substance is relatively rigid and has a definite volume and shape. NaCl(s) Liquid Phase – the substance has a definite volume, but is able to change shape by flowing. H 2 O(l) Gaseous Phase – the substance has no definite volume or shape, and it shows little response to gravity. Cl 2 (g) H 2 O(s) H 2 O(l) H 2 O(g)

Additional Symbols Used in Chemical Equations “Yields”; indicates result of reaction Used to indicate a reversible reaction A reactant or product in the solid state; also used to indicate a precipitate Alternative to (s), but used only to indicate a precipitate A reactant or product in the liquid state A reactant or product in an aqueous solution (dissolved in water) A reactant or product in the gaseous state (s)(s) (l)(l) (aq) (g)(g)

Additional Symbols Used in Chemical Equations Alternative to (g), but used only to indicate a gaseous product Reactants are heated Pressure at which reaction is carried out, in this case 2 atm Pressure at which reaction is carried out exceeds normal atmospheric pressure Temperature at which reaction is carried out, in this case 0 o C Formula of catalyst, in this case manganese (IV) oxide, used to alter the rate of the reaction 2 atm pressure 0 o C MnO 2 

Types of Chemical Reactions Synthesis (Combination) reaction Decomposition reaction A Single-replacement reaction B Double-replacement reaction Neutralization reaction Combustion reaction (of a hydrocarbon) A + B  AB AB  A + B A + BC  AC + B AB + CD  AD + CB HX + BOH  BX + HOH CH + O 2  CO 2 + H 2 O A use activity series to predict B driving force…water, gas, or precipitate

Chemical Equations

Synthesis Reaction Direct combination reaction (Synthesis) General form: A + B  AB element orelement or compoundcompound Na + Cl - Na ClNa Cl Na + Cl - 2 Na + Cl 2  2 NaCl

Synthesis Reaction Direct combination reaction (Synthesis) General form: A + B  AB element orelement or compoundcompound Na ClNa Cl 2 Na + Cl 2  2 NaCl  ClNa Cl

Formation of a solid: AgCl AgNO 3 (aq) + KCl(aq)  KNO 3 (aq) + AgCl(s)

H H H H Decomposition Reaction H H H H O Decomposition reaction 2 H 2 O 2 H 2 O2O2 General form: ABAB compoundtwo or more elements or compounds OO O

Single and Double Replacement Reactions Double-replacement reaction CaCO HCl  CaCl 2 + H 2 CO 3 General form: AB + CD  AD + CB Single-replacement reaction Mg + CuSO 4  MgSO 4 + Cu General form: A + BC  AC + B

Ca Activity Series Foiled again – Aluminum loses to Calcium Element Reactivity Li Rb K Ba Ca Na Mg Al Mn Zn Cr Fe Ni Sn Pb H 2 Cu Hg Ag Pt Au Halogen Reactivity F 2 Cl 2 Br 2 I 2

Potassium reacts with Water P O W !

Double Replacement Reaction K 2 CO 3 (aq) Potassium carbonate BaCl 2 (aq) Barium chloride 2 KCl (aq) Potassium chloride BaCO 3 (s) Barium carbonate ++

Classes of Reactions Chemical reactions Precipitation reactions Acid-Base Reactions Oxidation-Reduction Reactions Combustion Reactions

Summary of Classes of Reactions Chemical reactions Precipitation reactions Acid-Base Reactions Oxidation-Reduction Reactions Combustion Reactions Decomposition reactions (Products are elements.) Synthesis reactions (Reactants are elements.)

Summary of Classes of Reactions Chemical reactions Precipitation reactions Acid-Base Reactions Oxidation-Reduction Reactions Combustion Reactions Decomposition reactions Synthesis reactions

IONIC BONDING: Formation of Magnesium Chloride MgMg 2+ Cl Loses 2e- Each gains 1e- One magnesium ion Two chloride ions Mg 2+ Cl 1- [(2+) 2 (1-) = 0] MgCl 2 magnesium chloride

Mg 2+ IONIC BONDING: Formation of Magnesium Chloride MgMg 2+ Cl Loses 2e- Each gains 1e- One magnesium ion Two chloride ions Mg 2+ Cl 1- [(2+) 2 (1-) = 0] MgCl 2 magnesium chloride

Production of Methanol

A Mixture of Molecules