Unit 4 Chemical Reactions

Unit 4 : Chemical Reactions INTRODUCTION FOR REACTIONS ● Physical changes ○ When a substance undergoes a physical process, the composition of that substance does not change ○ Physical properties such as shape or solubility, may change, but there do not result in a change in composition of the substance ○ Ex: separation of mixtures, phase changes ● Chemical changes ○ When a substance undergoes a reaction and a new substance is formed ○ Chemical properties change, such as reactivity with acids or flammability, resulting in a new composition ○ Evidence of chemical changes: ■ Production of heat/light ■ Formation of bubbles/gas ■ Change in color ■ Formation of a precipitate NET IONIC EQUATIONS ● Balanced molecular equations show all species participating in a reaction ○ Indicate that mass is conserved ● Complete ionic equations show ions in aqueous solution as separate charge particles ○ Indicate the spectator ions in solution ● Net ionic equations do not include spectator ions ○ Indicate the substances that are undergoing a chemical change ● Example: ○ Balanced chemical equation ■ 2NaOH (aq) + CuSO4 (aq) ⟶ Na2SO4 (aq) + Cu(OH)2 (s) ○ Complete ionic equation ■ 2Na+ (aq) + 2OH- (aq) + Cu2+ (aq) + SO4 2- (aq) ⟶ 2Na+ (aq) + SO4 2-(aq) + Cu(OH)2 (s) ○ Net ionic equation ■ 2OH- (aq) + Cu 2+ (aq) ⟶ Cu(OH)2 (s) REPRESENTATIONS OF REACTIONS ● Chemical formulas ○ N2 (g) + 3H2 (g) ⟶ 2NH3 (g) ● Particulate models PHYSICAL AND CHEMICAL CHANGES ● Physical processes ○ Involve changes in intermolecular attractions ○ Bonds are generally not broken and/or formed ■ But, sometimes do break bonds (ex: dissolution) ■ Reversible processes that involve breaking bonds and forming intermolecular forces ○ Chemical composition is unchanged ● Chemical processes ○ Involve the breaking and/or formation of bonds ○ Chemical composition is changed STOICHIOMETRY ● Stoichiometry: the study of the relationships between the relative quantities of substances taking part in a chemical reaction ○ Illustrated by the balanced chemical equation (shows ratio of particles involved) ● Used to predict amounts of products produced or reactants required for reactions involving: ○ Moles or grams ■ Limiting reactants, excess reactants, theoretical yields ○ Aqueous solutions ○ Gases INTRODUCTION TO TITRATION ● Titration: an experimental technique where a solution of known concentration (standard solution) is combined with a solution of unknown concentration to determine the amount of moles in the unknown ○ Titrant: the solution of known concentration in the buret ○ Analyte: the unknown solution in the flask ○ Equivalence point: when the amount of titrant added from the buret has completely reacted with all the analyte in the flask in stoichiometric ratios ○ End point: the point at which the indicator changes color ● Titration Curves ○ Created when a pH meter monitors the progress of the titration ○ Can be used to determine the equivalence point (when the pH change is the greatest) ● Redox Titrations ○ Not all titrations are acid-base reaction ○ A redox reaction can be used to determine the concentration of an unknown solution ○ Some redox titrations do not require an indicator TYPES OF CHEMICAL REACTIONS ● Acid-Base Reactions ○ When solutions of an acid and a base are mixed, a neutralization reaction occurs ○ Bronsted-Lowry Acids and Bases ■ Involves a transfer of one or more protons (H+) from the acid to the base ■ Bronsted-Lowry acid = proton donor ■ Bronsted-Lowry base = proton acceptor ■ Products of reactions are often water and an ionic compound ○ Conjugate Acid-Base Pairs ■ Every acid has a conjugate base formed by removing an H+ ■ Every base has a conjugate acid forms by adding an H+ ■ In an acid-base reaction, two sets of conjugate acid-base pairs are present ● Oxidation-Reduction (Redox) Reactions ○ Involve the transfer of one or more electrons between reactants ○ Electrons are transferred from the species that is oxidized to the species that is reduced ■ Substance that loses electrons is oxidized ■ Substance that gains electrons is reduced ■ Oxidation Is Loss Reduction Is Gain ○ The oxidation of one substance is always accompanied by the reduction of another substance ○ Oxidation Numbers ■ Assigned to each of the atoms in the reactants and products to identify the oxidized and reduces species in a redox reaction ● Atoms in their elemental form have oxidation numbers of zero ○ Examples: H2, Fe, O2, Ca, Ag, N2, Al ● Monatomic ions have oxidation numbers equal to their charge ○ Examples: Li+, Br-, H+ ● Oxygen in a compound is most often given an oxidation number of -2 ○ EXCEPT in peroxides, where oxygen has an oxidation number of -1 ● Hydrogen usually is +1 when bonded to a nonmetal, and -1 when bonded to a metal ■ The sum of all oxidation numbers in a neutral compound is zero ■ The sum of all oxidation numbers in a polyatomic ion is equal to the charge ○ Half Reactions ■ Redox reactions can be separated into two half reactions: an oxidation half-reaction and a reduction half-reaction ■ Oxidation half reactions show a loss of electrons ● A⟶ A ⁺ + e■ Reduction half reactions show a gain of electrons ● B + e- ⟶ B⁻ ■ The electrons lost by one species must equal the electrons gained by the other species ● In order to do this, half-reactions can be multiplied if needed by the appropriate coefficient and then combined to get the overall balanced net ionic equation ● Precipitation Reactions ○ Frequently involve mixing ions in aqueous solutions to produce an insoluble ionic compound(a solid) called a precipitate ■ All sodium, potassium, ammonium, and nitrate salts are soluble in water