Exam #2 (Chapters 4-6) Chemistry
Acid/base reactions
(aka neutralization reactions) - an acid reacts with a base and the two neutralize each other producing water (or in some cases a weak electrolyte)
Oxidation-Reduction Reactions (identifying what is oxidized/reduced; what is the oxidizing agent/reducing agent; assigning oxidation states) ******
(aka neutralization reactions) - an acid reacts with a base and the two neutralize each other producing water (or in some cases a weak electrolyte)
Combustion reaction
- particular type of chemical reaction in which a substance combines with oxygen to form one or more oxygen-containing compounds. Also emit heat. CH4(g)+*2*O2(g) -> CO2(g) + *2*H2O(g) LEAVE HYDROGEN AND OXYGEN UNTIL LAST
Solution concentration (molarity)
- the amount of solute in a solution is given by its concentration - molarity = common concentration unit Molarity (M) = amount of solute (in moles)/volume of solution (in L)
Writing and Balancing Chemical Equations
1. Write an unbalanced equation, using the correct formulas for all reactants and products. H2SO4 + NaOH -> Na2SO4 + H2O (unbalanced) 2. Add appropriate coefficients to balance the numbers of atoms of each element. - Begin with elements that appear in only one compound or formula on each side of the equation H2SO4 + *2* NaOH -> Na2SO4 + H2O (Balanced for Na) - If a polyatomic ion appears on both sides of an equation, it is treated as a single unit. H2*SO4* + 2 NaOH -> Na2*SO4* + H2O (Balanced for Na and sulfate) - Leave elements that exist in elemental forms, such as oxygen (O2) and hydrogen (H2), until last *H2*SO4 + 2 Na*OH* -> Na2SO4 + 2 *H2O* (Completely balanced) 3. Check the equation to make sure the numbers and kinds of atoms on both sides of the equation are the same. 4. Make sure the coefficients are reduced to their lowest whole- number values. For example: 2 H2SO4 + 4 NaOH -> 2 Na2SO4 + 4 H2O is completely balanced, but can be simplified by dividing all coefficients by 2: H2SO4 + 2 NaOH -> Na2SO4 + 2 H2O
Calculating the amount of excess reagent leftover
1. Write and balance chemical reaction 2. Take the grams of the one element and convert it into the grams of the element being asked 3. Initial - reacts (the # from step 2) = answer
Ion Concentrations in Solutions
CuCl2 (aq) -> Cu2+ (aq) + 2 Cl- (aq) Example: If [CuCl2] = 0.30 M, what is the concentration of Cu2+ and Cl- in solution? 0.30 molesCuCl2/1 L solution x 1 mol cu2+/1 mol CuCl2 = 0.30 mols Cu2+/1 L solution = 0.30 M Cu2+ 0.30 molesCuCl2/1 L solution x 2 mol cl-/1 mol CuCl2 = 0.60 mols Cl-/1 L solution = 0.60 M Cl- ions
Stoichiometry Steps for Solving
Definition: numerical relationships between chemical amounts in a balanced chemical equation 1. Write and balance the chemical reaction 2. For each reactant calculate a number of moles ex: 3 H2 + 1 N2 à 2 NH3 3 mol H2/1 mol H2 3 mol H2/2 mol NH3 3. Determine which reactant is the limiting reagent •Calculate the number of moles of product produced for each reactant •The reactant that produces the smallest number of moles of product is the limiting reagent 4. Convert moles of product to grams of product to calculate a percent yield: actual yield/theoretical yield * 100%
Theoretical yield
Determined when the reactant is used up, the reaction stops and no more product is made (the amount calculated after all the steps)
No precipitation = no reaction rule
Ex: KI(aq) + NaCl (aq) -> no reaction Based on "double displacement" our possible products are: •KCl and NaI BUT •Both are soluble! •Therefore, no precipitate forms and consequently there's no reaction *both soluble = no reaction*
Writing acid-base reactions
General scheme of an acid-base (neutralization) reaction: *Acid + Base -> Water + Salt* 1)Identify the acid and base in the reaction 2)Double displacement - swap cation from each reactant with the anion from the other reactant 3)Balance the equation (molecular equation) 4)Write the net ionic equation by removing the spectator ions
Solving Reactions Involving Strong Acids
HBr (aq) + LiOH (aq) -> 1. Identify acid/base: HBr (aq) *strong acid* + LiOH (aq) *strong base* -> 2. Switch the cation to form H2O and a salt HBr (aq) + LiOH (aq) -> LiBr (aq) *aq = solubility rules* + H2O (l) 3. Balance the molecular equation HBr (aq) + LiOH (aq) -> LiBr (aq) + H2O (l) 4. Complete ionic equation: H+ (aq) + Br- (aq) + Li+ (aq) + OH- (aq) -> Li+ (aq) + Br- (aq) + H2O (l) *l = stays together* 5. Net ionic equation: H+ (aq) + OH- (aq) -> H2O (l)
Solving Reactions Involving Weak Acids
HCHO2 (aq) + NaOH -> 1. Identify acid/base: HCHO2 (aq) *weak acid* + NaOH *strong base*-> 2. Switch the cations to form H2O and a salt HCHO2 (aq) + NaOH -> NaCHO2 (aq) + H2O (l) 3. Balance the molecular equation HCHO2 (aq) + NaOH -> NaCHO2 (aq) + H2O (l) 4. Complete ionic equation: HCHO2 (aq) *this does not completely dissociate* + Na+ (aq) + OH- (aq) -> Na+ (aq) + CHO2- (aq) + H2O (l) 5. Net ionic equation: HCHO2 (aq) + OH- (aq) -> CHO2- (aq) + H2O (l)
Solution stoichiometry
In aqueous reactions, we can use the volume and concentration of a reactant or product to calculate its amount in moles and then use stoichiometry to convert the amount of another reactant or product into moles Volume A -> Amount A (in moles) -> Amount B (in moles) -> Volume B Type of question: What volume (in mL) of a 0.150 M HNO3 solution will completely react with 25.7 mL of a 0.108 M Na2CO3 solution according to the following balanced chemical reaction: ?
Precipitation Reactions Questions Step-by-Step Approach Example Question of NO REACTION: Write an equation for the precipitation reaction that occurs (if any) when solutions of ammonium chloride and iron (III) nitrate mix
NH4Cl (aq) + Fe(NO3)3 (aq) -> NH4NO3 FeCl3 NH4NO3 - soluble FeCl3 - soluble NH4Cl (aq) + Fe(NO3)3 (aq) à NO REACTION
Solubility
Soluble = compound that dissolves in water Insoluble = compound that doesn't dissolve in water
Precipitation Reactions Questions Step-by-Step Approach Example Question: Write an equation for the precipitation reaction that occurs (if any) when solutions of sodium hydroxide and copper (II) bromide mix
Step 1. Write the formulas for the two compounds being mixed as reactants Step 2. Below the equation write the formula for the possible products that could form. Combine the cation from each reactant with the anion from the other. Step 3. Refer to the solubility rules to determine whether any of the possible products are insoluble. Step 4. If any of the possible products are insoluble, write their formulas as the products of the reaction, using (s) to indicate solid. Include (aq) to indicate aqueous after any soluble products. *aq = soluble* Step 5. Balance the equation. Remember to adjust only coefficients not subscripts!
Solution dilutions (M1V1=M2V2)
Stock solution - concentrated solution of a substance used to prepare solutions of lower concentration Dilution - the process of lowering the concentration of a solution by adding more solvent M1 (molarity stock) x V1 (volume stock) = M2 (molarity dilute) x V2 (volume diluted solution)
Strong acids and bases *KNOW THIS* *MEMORIZE THIS*
Strong acids (everything else is weak): Hydrochloric acid HC l Hydrobromic acid HB r Hydroiodic acid HI Nitric acid HNO3 Sulfuric acid H2SO4 Perchloric acid HCIO4 Strong bases (everything else is weak): Group 1 and 2 hydroxides (except Be & Mg) Lithium hydroxide - LiOH Sodium hydroxide - NaOH Potassium hydroxide - KOH Rubidium hydroxide - RbOH Cesium hydroxide - CsOH Calcium hydroxide - Ca(OH)2 Strontium hydroxide - Sr(OH)2 Barium hydroxide - Ba(OH)2 NH3 and CaCO3 are weak bases
Limiting reagents/reactant
The reactant that limits the amount of product - gets completely consumed - makes the least amount of product How many grams of nitric acid can be made in the reaction of 100.0 g of NO2 with 15.0 g of water After calculating -> 91.3 g HNO3 and 104g HNO3 so NO2 is the limiting reagent
Polyprotic acid
acids that have more than one ionizable proton and release them sequentially (e.g., *H2SO4* - diprotic - strong in its first ionizable proton, but weak in its second) 2 ionizable protons H2SO4 -> H+ + HSO4- -> H+ + SO4^2-
Actual yield
actual yield = the measured amount of produce formed - lab result (the amount said in the word problem)
Percent yield
actual yield/theoretical yield x 100 can never be >100% "efficiency" of a reaction = how good is it?
Net ionic equation
an equation showing only the species that actually change during the reaction Pb2+ (aq) + 2 Cl- (aq) -> PbCl2 (s)
Molecular equation
an equation showing the complete neutral formulas for each compound in the reaction as if they existed as molecules -> BREAK UP ANYTHING THAT'S AQ
Complete Ionic Equation
an equation that lists all of the ions present as either reactants or products in a chemical reaction -> solid stays together -> shows the charge too Pb2+ (aq) + 2*NO3-* (aq) + 2 *K+* (aq) + 2 Cl- (aq) -> PbCl2 (s) + 2 *K+* (aq) + *NO3-* (aq) *Spectator ions* - appear unchanged on both sides of the equation
At the equivalence point ...
an indicator changes color
Strong acid or strong base
completely ionized in aqueous solution = strong electrolyte
Empirical Method (Solubility Rules to Determine When a Salt Dissolves)
conduct tests to determine whether a compound will dissolve in water, and then develop rules based on these experimental results (chart is given on exam) chart: compounds containing the following ions are generally soluble/insoluble
Aqueous Solutions
dissolved in water •Salt water (ionic) - homogeneous mixture of NaCl and H2O •Sugar water (molecular) - homogeneous mixture of C12H22O11 and H2O
molecular compound relationship to ions
doesn't break up into ions
Molar mass to Count Molecules by Weighing
grams compound -> (molar mass of compound g/mol) mol compound -> (Avogadro's number) number of molecules
Equivalence point
in a titration is reached when enough standard solution has been added to completely react with the unknown solution
Aqueous solutions
ionic compound -> electrolyte molecular compound -> non-electrolyte
Vapor pressure
partial pressure of the water vapor; dependent on temperature Ptotal = PH2 + PH2O
Weak acid or weak base
partially ionize in aqueous solution = weak electrolyte
Excess reactant
reactants not completely consumed
Gas-evolution reaction
reaction in which a gas forms, resulting in bubble; many are also acid-base reactions
Precipitation reactions
reactions in which a solid (precipitate, ppt) forms when two solutions are mixed NaOH (aq) + AgNO3 (aq) -> AgOH (*s*) + NaNO3 (aq - depending on solubility rules, precipitates may not form - *only insoluble compounds form precipitates*
Standard solution
solution of known concentration
Homogenous mixtures
solutions are homogenous mixtures of two or more substances
Solvent vs. solute
solvent = substance in the greater amount solute = substance in the lesser amount Example: solvent = water solute = sugar *sugar is put into water*
Titration
volumetric method used to determine the concentration of an unknown solution by reacting it with a standard solution that we do know the concentration of
Acid-base reaction terminology
•Arrhenius acid - substance that produces H+ ions in aqueous solutions •Arrhenius base - substance that produces OH- ions in aqueous solutions •Bronsted-Lowry acids - proton (H+) donors •Bronsted-Lowry bases - proton acceptors *Note: free hydrogen ions (H+) don't exist in water because they strongly associate with water to create hydronium ion (H3O+)* •Amphiprotic - substances that can either accept or donate H+
Real Gases
•Assumptions of the Ideal Gas Law/KMT: 1.Volume of the gas is negligible compared to volume of the container 2.Gas molecules act independently (they don't interact with each other) • •These assumptions are valid at STP (standard temperature and pressure), but they deviate at high P and low T Because real molecules take up space, the molar volume of a real gas is larger than predicted by the ideal gas law at high pressures At the lower T, the gas atoms interact with each other more and collide less with the walls, making the actual P less than that predicted by the ideal gas law.
How can we determine what gets oxidized and what gets reduced?
•Each atom is given an oxidation number (or oxidation state) which is equivalent to the "charge" that atom would have if all shared electrons were assigned to the atom with the greatest attraction for those electrons •Oxidation states are assigned based on a set of rules: 1. Elements in their elemental form have an oxidation state of 2. Oxidation state of a monoatomic ion is equal to its charge. 3. The sum of the oxidation state of all atoms in: a. A neutral compound is 0. b. A polyatomic ion equals its charge. 4. In their compounds, metals have a positive oxidation state: a. Group 1A always is +1. b. Group 2A always is +2. c. For any other metals, the OS will depend on the anion. 5. In their compounds, non-metals are assigned the oxidation states of the table on the top right (Entries at the top take precedence over entries at the bottom of the table).
Electrolyte v. weak v. nonelectrolytes (dissociation)
•Electrolytes - substances that dissolve in water to form solutions that conduct electricity (allows for flow of electricity) •Non-electrolytes - a solution in which no ionization occurs; no conduction of electrical current Ex: aqueous solutions of sugar, ethylene glycol, ethanol C12H22O11(aq) Strong electrolytes - conduct current efficiently •Ionic compounds that dissociate into their component ions when dissolved in water (e.g., NaCl, CuCl2) •Strong acids and bases (molecular compound) - completely ionize (form ions) when dissolved in water Weak electrolytes - •Conduct only a small current - few ions in solution •Examples: •Tap water •Weak acids (molecular compound) - do not completely ionize (form ions) when dissolved in water; vinegar (acetic acid): HC2H3O2 (aq) ⇌ H+ (aq) + C2H3O2- (aq) ⇌ = partial ionization
Kinetic Molecular Theory
•Gas is modeled as a collection of particles (either molecules or atoms) in constant motion •Basic assumptions of this theory: •*Volumes* of individual molecules are *negligible* •Molecules of gas are in *continuous random* motion. Any collisions between particles or with the container walls are perfectly *elastic* (no energy loss) •There are *no attractive/repulsive forces* exerted by gas molecules on other gas molecules •*Average kinetic energy* of gas molecules is proportional to the *temperature* (in Kelvin); i.e., KE ~ T (higher the temperature, higher average speed (motion) of the particles) temp up, kinetic energy up
Graham's Law
•Gas molecules travel only short distances before they collide with another molecule and change directions. •Mean free path - average distance a gas molecule travels between collisions •Diffusion - process by which gas molecules spread out in response to a concentration gradient (heavier molecules (MW) diffuse more slowly) •Effusion - process by which a gas escapes through a tiny hole into an evacuated chamber (heavier molecules (MW) effuse more slowly) •Rate of effusion is inversely proportional to the square root of molar mass of the gas: •Graham's law of effusion: rateA/rateB = tB/tA = sqrt(MWB/MWA) Where rateA and rateB are the effusion rates of gases A and B, tB and tA are time, and MWA and MWB are their molar masses (assuming constant temperature and pressure)
Stoichiometry involving gases
•In reactions involving gaseous reactant or product, we often specify the quantity of gas in terms of a volume at a given temperature and pressure. •Stoichiometry, however, involves relationships between amounts in moles. •For gases, we can use the ideal gas law to find moles and then use these relationships in stoichiometric calculations
Oxidation-Reduction (Redox) Reactions
•Reactions in which electrons transfer from one reactant to the other. •Oxidation and reduction must occur simultaneously •If an atom loses electrons another atom must take them OIL RIG Oxidation is loss Reduction is gain •Atoms that *lose electrons* are being *oxidized* •increase in oxidation state (or oxidation number) •*Reducing Agent* (the reactant) •Atoms that gain electrons are being reduced. •decrease in oxidation state (or oxidation number) •Oxidizing Agent (the reactant)
Molar volume and stoichiometry
•Remember that under STP, *1 mol of an ideal gas occupies 22.4 L* •Therefore if a reaction occurs at STP, we can use the conversion factor: 1 mol/22.4 L •Ex. How many liters of oxygen (at STP) are required to form 10.5 g of H2O? 2 H2 (g) + O2 (g) -> 2 H2O (g)
Corrections to the Ideal Gas Law
•Van der Waals Equation (used to calculate properties of a gas under nonideal conditions): [P + a(n/v)^2] x [v-nb] = nRT (n/v)^2 = Correction for intermolecular forces a - correction term related to attractive/repulsive forces nb = Correction for particle volume b - volume correction term
Molecular Velocities and Molar Mass
•urms is the speed of molecules with an average kinetic energy (KE) where KEavg = 1/2murms^2 m = mass rms = velocity urms = sqrt(3RT/MW) MW = kg/mol where MW = molar mass (kg/mol), T = temperature (K), and R = 8.314 J/K*mol note 1 J = 1 kg*m2/s2 •KMT state that particles of different masses have the same average KE at a given temp. For this to be true, particles of different masses must have different velocities Lighter particles travel faster (on average) than heavier ones!
⇌ meaning
⇌ = not completely ionized WA ⇌ SB
