Chapter 4 Flashcards
Limiting Reagent
- When chemical reaction involves 2 or more reactants, often 1 reactant is of limited quantity and the others are in excess
- the limiting reagent determines or limits the amount of product formed in a reaction
- to determine the limiting reagent, calculate the amount of product (in g. or mol.) produced from each reactant -> the lower quantity is the limiting reagent
- amount not reacted = total amount - amount reacted
Theoretical Yield
- amount of product that would be formed if the reaction went to completion (value with the limiting reagent)
- based on stoichiometry of the reaction and the ideal conditions in which starting material is completely consumed and no losses in work-up procedure
- calculated based on balanced chemical equation
Percent Yield
- under experimental conditions, actual yield is almost always less than amount predicted by theory (theoretical yield)
- equation = (actual yield ÷ theoretical yield) x 100
- yields above about 90% = very good
- yields above about 75% = good
- yields above about 60% = modest
- yields above about 30% = poor
*yields can also be above 100% when an extraneous chemical from outside of the reaction has found its way into the yield
Solution
- homogeneous mixture of two or more substances
- solutions can be solids, liquids, gases, or gas/liquid mixtures
examples: gas solution = dry air solution of ions = sea water gas/liquid solution = carbonated water solid solution = brass (homogeneous metal alloy with different metal components)
Solute
- does not react with solvent
ie. ) dissolved table salt (NaCl ions) does not react chemically with water - present in smaller amount
- uniformly dispersed in another substance/ solvent
- aqueous solutions are transparent
Solvent
- present in larger amount
example:
25 g. sugar dissolved in 100 g. water
- sugar is smaller amount (=solute)
- water is larger amount (=solvent)
Aqueous solutions
- solvent = water
- solutes = dissolved ionic or polar substances
- homogeneous mixture of dissolved ions in water
- water binds to both positive and negative ions in an aqueous solution. when ionic substance dissolves in water/becomes solvated, its cation and anion are surrounded by a sheath of water molecules. Dissolved ions can move about freely.
Molarity (M)
- concentration of solution= amount of solute dissolved in a given volume of solution
- Molarity = mol. solute ÷ vol. solution (solute + solvent)
- molarity refers to amount of solute per liter of solution, NOT per liter of solvent
- When making solutions of a given molarity, dissolve the solute in a volume of solvent smaller than the desired volume of solution. Then add solvent until the final volume of solution is reached.
Unimolecular Solutes vs. Ion Solutes
- unimolecular = do not form ions when dissolved in water; number of solute particles they form in a solution remain unchanged before and after the solvation process
example: glucose and urea - ionic solutes: soluble in water; produce ions whose concentrations can be a multiple of the compounds form which they were derived
- to calculate the molarity of a specific ion solute, one must take into account the ionic formula of the compound from which the ion was derived
example:
1 mol, CaCl2 (s) –(H2O)–> 1 mol. CaCl2 (aq)
or
1 mol Ca 2+ (aq)
2 mol Cl - (aq)
= 3 mol. solute particles total
Dilution
- made when a solution of lower concentration is prepared form a more concentration one
- solution concentration inversely proportional to volume of solution
- C1 V1 = C2 V2
example: when water (solvent) is added to a can of orange juice concentrate (solute), the amount of orange juice present does not change BUT its concentration decreased, because total volume of solution increases
Dilution problem:
- ) a stock solution is prepared by dissolving 10.8 g. (NH4)2 SO4 (molar mass: 132.15 g/mol) in enough water to make 100 mL (solution 1). A 10.0-mL sample of this stock solution (solution 1) is then place in a 50-mL volumetric flask and diluted to the mark with water to make solution 2.
a. ) What is the molarity of solution 2?
b. ) (NH4)2 SO4 is an ionic compound which dissolves in water to form an aqueous solution of ions. What is the molarity of the NH4+ ions in solution 2?
a. ) 0.163 M (NH4)2 SO4
b. ) 0.326 M NH4+
Electrolytes
- ionic or molecular substances which, when dissolved in water to form solution, produce ions that can conduct an electric current
- commonly exists as solutions of acids, bases, or salts
Strong Electrolytes
- ionic compounds that dissociate 100% in solution to form ions only
- conduct electricity strongly
example:
HCl (aq) -> H+ (aq) + Cl- (aq)
0% 100% 100%
(strong acid = dissociates completely)
Weak Electrolytes
- Ionic compounds that dissociate only partially (< 100%) in solution to form some ions
- conduct electricity weakly
example:
HF (aq) -> H+ (aq) + F- (aq)
Non-Electrolytes
- polar, covalent compounds that dissolve in aqueous solution BUT do not form ions (i.e. no dissociation)
- DO NOT conduct electricity
- many non electrolyte compounds have Oxygen atoms linked to Hydrogen atoms, forming hydroxyl group (-OH). Presence of PH groups in nonionic substances allow H-bonding with water to occur, thereby increasing the solubility of these substances in aqueous solution.
examples:
Methanol (CH3OH), Ethanol (CH3CH2OH)
Selective Precipitation Problem:
A solution contains one or more of the following ions: Ag+, Ca 2+, and Cu 2+.
a.) When sodium chloride is added to the solution, no precipitate occurs.
b.) When sodium sulfate is added to the solution, a white precipitate occurs, which is then filtered off.
c.) To the remaining solution, sodium carbonate is added, producing a precipitate.
A.) Which ions were present in the original solution?
B.) Write a balanced net ionic equation for the formation of each of the precipitates observed?
C.) Write a balanced molecular equation with NO3- for the formation of each of the precipitates observed.
A.) Ca 2+ and Cu 2+
B.) Ca 2+ (aq) + SO4 2- (aq) -> CaSO4 (s)
Cu 2+ (aq) + CO3 2- (aq) -> CuCO3 (s)
C.) Ca(NO3)2 (aq) + Na2SO4 (aq) -> CaSO4 (s) + 2NaNO3 (aq)
Cu(NO3)2 (aq) + Na2CO3 (aq) -> 2NaNO3 (aq) + CuCO3 (s)
Molecular Equation
- equation that shows the complete neutral formulas for every compound in the reaction
example:
AgNO3 (aq) + KCl (aq) -> AgCl (s) + KNO3 (aq)
Net ionic equation
- equation that shows only the species that actually participate in the reaction
example:
Ag+ (aq) + Cl- (aq) -> AgCl (s)