Ch 3 Chemical Reactions 3 1 Introduction to Chemical Equations Interpretation Particulate level 1 molecule formula unit Molar level 1 mole 1 Law of conservation of matter matter can neither be created nor destroyed Basis for balancing chemical equations Atoms are conserved in chemical reactions Same number have to appear on both sides If 20 g of P4 and 20 g of Cl2 then there must be how much of PCl3 Clicker Q 15 g of P4 and 15 g of Cl2 give PCl3 2 3 2 Balancing Chemical Reactions Steps in balancing a chemical reaction using coefficients 1 Write the equation using the formulas of the reactants and products Include the physical states if known s l g aq etc 2 Balance the compound with the most elements in the formula first using integers as coefficients 3 Balance elements on their own last 4 Check to see that the sum of each individual elements are equal on each side of the equation 5 If the coefficients can be simplified by dividing though with a whole number do so 3 Methane reacts with oxygen to produce carbon dioxide and water Write the chemical equation then balance it Iron III oxide reacts with carbon monoxide to yield pure elemental iron and carbon dioxide Clicker Q what is formula for Iron III oxide 4 Magnesium nitride reacts with water to form ammonia and magnesium hydroxide Potassium reacts with potassium nitrate to produce potassium oxide and nitrogen gas 5 Ammonia burns in oxygen to form nitrogen monoxide and water KClO3 C12H22O11 KCl CO2 H2O 6 Clicker Q Magnesium metal is burned in air to produce magnesium oxide Write the balanced chemical equation Input just the numbers in order with no spaces in between put a 1 For example 123 7 3 3 Introduction to Chemical Equilibrium Chemical reactions are reversible and many reactions lead to incomplete conversion of reactants to products The rate of the forward reaction is equal to the rate of the reverse reaction that is equilibrium Dynamic equilibrium 8 If N2 3H2 2 NH3 then the reverse is true also 2 NH3 N2 3H2 9 Fig 3 6 p 118 Chemical reactions always proceed spontaneously toward equilibrium Dynamic equilibrium rate of forward reaction rate of reverse reaction looks like nothing is happening Product favored reactions reactants are completely or largely converted to products reactions proceed to products Reactant favored reactions only a small amount of reactant is converted to product reactions occurs toward the reactants 10 3 4 Aqueous Solutions In aqueous solutions water is the solvent Solution homogeneous mixture of 2 or more substances The one present in the least amount is the solute The one present in the most amount is the solvent 11 Ions and Molecules in Aqueous Solutions When an ionic compound dissolves in water each negative ion becomes surrounded by water molecules with the positive ends of water molecules pointing toward it and each positive ion becomes surrounded by the negative ends of several water molecules See next slide 12 13 Fig 3 7 p 120 Electrolytes compounds whose aqueous solutions conduct electricity All ionic compounds that are soluble in water are electrolytes strong electrolytes 100 dissociates weak electrolyte conducts electricity but doesn t dissociate 100 Nonelectrolyte Compounds whose aqueous solutions do not conduct electricity Most molecular compounds that dissolve in water are nonelectrolytes examples sugar alcohol 14 Species in Solution Electrolytes Strong electrolytes Characterized by ions only cations anions in solution water Conduct electricity well Weak electrolytes Characterized by ions cations anions molecules in solution Conduct electricity poorly Non electrolytes Characterized by molecules in solution Do not conduct electricity 15 16 Fig 3 8 p 120 Strong electrolytes dissociate 100 Table 3 1 page 129 need to know strong electrolytes and solubility guidelines Figure 3 10 page 122 Need to know all strong acids and strong bases and soluble compounds solubility rules Strong acids HCl HBr HI HNO3 H2SO4 HClO4 Strong bases LiOH NaOH KOH Ba OH 2 Sr OH 2 17 Solubility of Ionic Compounds in water Many ionic compounds are soluble in water Like dissolves like Need to know the following guidelines Figure 3 10 on page 122 See next slide then do Calcium carbonate Silver chloride Ammonium phosphate Clicker Q is sodium nitrate A soluble B insoluble 18 19 Fig 3 10 p 122 Types of Reactions in a Solution Precipitation Reactions A reaction where an insoluble solid precipitate forms and drops out of the solution Acid base Neutralization A reaction in which an acid reacts with a base to yield water plus a salt Gas forming Reactions A reaction where an insoluble gas is formed Reduction and Oxidation Reactions RedOx A reaction where electrons are transferred from one reactant to another 20 3 5 Precipitation Reactions Exchange reaction double displacement metathesis double replacement ions of reactants change partners A precipitation reaction produces a waterinsoluble solid product known as a precipitate 21 Net ionic equations Complete ionic equations write the chemical equation as ions if there are any so if they exist as ions write them as such only strong acids and bases can be dissociated for net ionic equations need to know solubility guidelines only soluble compounds can be written as ions Cannot write as ions if s l or g Net ionic equations remove spectator ions from both sides A spectator ion is one that doesn t participate in the reaction They will appear the same on both sides of the equation 22 Predicting the outcome of a precipitation reaction Need to know solubility rules for that Write complete ionic equation and net ionic equation for calcium chloride and sodium phosphate reacting 23 Write complete ionic equation and net ionic equation for calcium nitrate and potassium carbonate reacting 24 Clicker Q what is the sum of the coefficients of the net ionic equation for Ni NO3 2 Na2CO3 NiCO3 NaNO3 25 3 6 Acids and Bases 0 1 M HCl in stomach Drano solid NaOH with some Al chips Organic acids carboxylic acids occur in nature acetic acid vinegar There are 2 most commonly used definitions of acids and bases 1 Arrhenius 2 Bronsted Lowry 26 Acids and Bases The Arrhenius Definition An acid is a substance that when dissolved in water increases the concentration of H ions in solution So if it makes an H then it is an acid HCl H ClA base is a substance that when dissolved in water increases the concentration of OH ions in solution So if it makes an OH it is a base NaOH Na OH27 Strong acids the ones that ionize