CHEM 1211 1st Edition Exam 1 Study Guide Lectures 8 13 Chapter 3 4 3 9 Aqueous solutions Aqueous solutions are solutions in which water is a solvent Water is good at dissolving ionic compounds because each water molecule has a positively charged end and a negatively charged end All ionic compounds that are soluble in water are electrolytes What is the difference between strong electrolytes and weak electrolytes Substances whose solutions are good electrical conductors due to the presence of ions are strong electrolytes for example BaCl2 and HCl Compounds whose aqueous solutions do not conduct electricity are called non electrolytes Most molecular compounds that dissolve in water are nonelectrolytes Weak electrolytes are poor conductors of electricity and when these compounds dissolve in water only a small fraction of the molecules form ions ex acetic acid Takeaway Strong acids and ionic compounds form strong electrolyte ions and weak acids and weak bases form weak electrolyte molecules and ions Most molecular compounds form nonelectrolyte molecules Precipitation Reactions Exchange reactions the ions of the reactants change partners Precipitation reaction produces a water insoluble solid product called a precipate The reactants undergo an exchange reaction Complete Ionic Equations a balanced equation where the number of atoms on both sides of the equation agree with each other In addition the sum of the positive and negative charges on both sides of the equation must equal each other Spectator ions do not participate in the net reaction Recap What happens when a solute dissolves A solute dissolves and a solvent is what a solute dissolves into For an aqueous solution the solvent is water Acids and Bases Acid substance that increases the concentration of hydrogen ions H in a solution Base a substance that increases the concentration of hydroxide ions OH Acid base reactions fit in the category of equilibrium Under the conditions of the reaction the products that you make can easily revert back to the original reactants What are some examples of strong weak acids and strong weak bases HCl and HNO3 are strong acids acetic acid and hydrofluoric acid are weak acids NaOH and KOH are strong bases and ammonia is a weak base Strength of base acid is a property that is inherent in the molecules If you take HCl that molecule wants to disassociate into H and Cl In an equilibrium reaction there is a balance between how much Cl wants to disassociate and H wants to hold on Bronsted Lowry definition an acid is a proton donor a base is a proton acceptor and an acidbase reaction involves the transfer of a proton from an acid to a base to for a new acid and a new base The equilibrium favors the weaker acid and base Acid base reactions usually produce a salt and a water Because Na and Cl ions appear on both sides of the equation the net ionic equation is just the combination of the ions H 2O and OH to give water H3O OH 2 H2O is always the net ionic equation when a strong acid reacts with a strong base Neutralization reactions reactions between a strong acid and a strong base On completion of the reaction the solution is neutral if exactly the same number of moles of the acid and base are mixed Acidic oxides can react with water to produce H3O and basic oxides are oxides of metals Practice question Which is considered a basic oxide based on the product it forms when added to water i CO2 ii SO3 iii CO2 and SO3 iv CaO v None of these ANSWER iv CaO Reasoning Basic oxides are formed mainly by metals especially the alkali metals and alkaline earth metals Gas forming reactions the most common of these are those that produce CO2 Also those that form H2CO3 H2SO3 H2S and NH4 with OH Oxidation reduction reactions in these equations there is a reducing agent and an oxidizing agent The substance that loses electrons is oxidized and is called the reducing agent and the substance that gains electrons is reduced and is called the oxidizing agent What are the rules for oxidation numbers Each atom in a pure element has an oxidation number of 0 For monoatomic ions the oxidation number is equal to the charge on the ion In most compounds the oxidation of H is 1 and the oxidation number of O is 2 When combined with another element fluorine always has an oxidation number of 1 Cl Br and I have oxidation numbers of 1 in compounds except when combined with fluorine and oxygen The algebraic sum of the oxidation numbers for the atoms in a neutral compound must be 0 in a polyatomic ion the sum must be equal to the ion charge You can tell whether a reaction involves oxidation and reduction by assessing the oxidation number of each element and noting whether any of the numbers change over the course of the reaction Practice Question Which elements get oxidized and reduced respectively in the equation 2Fe2O3 s 3 C s 4Fe s 3CO2 g a Iron oxygen b Carbon iron c Carbon oxygen d Iron carbon ANSWER B Oxygen is 2 so iron in the 2Fe2O3 must have an oxidation number of 3 On the other side 4 Fe has an oxidation number of 0 Since it goes from 3 to 0 it is reduced Carbon is oxidized Chapter 4 Steps for a Stoichiometry Equation 1 Write a balanced equation 2 When converting mass to moles or moles to mass use formula weight 3 When finding the ratio of moles use stoichiometric coefficients Practice Problem For the reaction Cl2 g H2 g 2 HCl g how many grams of HCl are produced from 7 09 grams of chlorine gas ANSWER 7 29 grams Reasoning convert 7 09 grams Cl2 to grams HCl by multiplying 7 09g Cl2 x 1 mol Cl2 70 9 g Cl2 x 2 mol HCl 1 mol Cl2 x 36 458 g HCl 1 mol HCl Practice Problem For the reaction 4 NH3 5 O2 4 NO 6 H2O if 17 g of ammonia reacts how many moles of hydrogen atoms are contained in the products ANSWER 3 moles of H REASONING 17g NH3x 1 mol 17g NH3 x 6 mol H2O 4 mol NH3 x 2 mol H 1 mole H2 Limiting Reactants The amount of the limiting reactant determines or limits the amount of the product formed Steps to limiting reactant calculations 1 Find the amount of each reactant in moles 2 Compare the ratio of the amount of reactants available to the ratio of reactants in the balanced equation Determine the limiting reactant 3 Calculate the mass of the product based on the moles of the limiting reactant available 4 Calculate the mass of the excess reactant by a Converting the moles of the limiting reactant to the moles of the excess reactant required b Subtract the moles of the excess reactant required from the moles of the excess reactant