CHEM103 Exam 2
Molecular Equation
Expresses compounds as molecules; does not explicitly represent the ionic species that are present in solution.
Precipitate
Insoluble solid product formed from a reaction in solution.
Brønsted-Lowry acid
A proton donor; any substance which donates a proton (H⁺) to another substance.
Electrolyte
- A substance that dissolves in water to yield a solution that conducts electricity. - Forms ions in a solution. - Nearly 100% dissociation (separation) of ions. - Readily conduct current (electricity).
Nonelectrolyte
- A substance that dissolves in water to yield a solution that does NOT conduct electricity. - Do NOT produce any ions in a solution. - Typically molecular (covalent) compounds, which contain covalent bonds that can't be broken by dissolving.
Conventions for Writing Chemical Equations
- Standard Format: Reactant Particles → Product Particles - An arrow (→) is used to show the conversion from reactants (consumed) to products (produced). - For reactions involving heat, put a triangle over the arrow. - For reversible reactions, a double arrow is used: ⇄. - The amount of each particle is shown with a number in front of the particle, called a stoichiometric coefficient.
Weak Bases
- Substances capable of accepting hydrogen but do not completely ionize in solution. - One common weak base is ammonia, NH₃. - Since ammonia reacts only to a small extent to form ions when it is dissolved in water, it is a weak electrolyte and a weak base.
Strong Bases
- Substances which contain "-OH" in their formula and dissociate completely in aqueous solution. - Most metal oxides and hydroxides are insoluble and can have complex chemistries. - Any Group 1 or 2 hydroxide.
Tips for Balancing Equations
1. Balance the equation by using coefficients. Do NOT change the formulas of reactants or products by altering subscripts! Only change coefficients. 2. Balance atoms that appear only once on each side first. Save H and O for last. 3. If polyatomic ion atoms "stay together," balance together. 4. This is a trial and error approach; some reactions are easier to balance than others. Try a coefficient of 2 if a 1 doesn't work; try a 3 if a 2 doesn't work, etc. 5. Check to see if there is a lowest common denominator once you're finished with your balanced equation; reduce if possible.
Determining the Empirical and Molecular Formulas of Organic Compounds
1. Calculate the mass of oxygen by computing the masses of carbon and hydrogen: Total mass of compound - mass C - mass H = mass O. 2. Find the number of moles of each element. 3. Divide by the smallest number of moles to find the empirical formula (multiply up to the nearest whole number if needed.) 4. Calculate molecular formula (if asked to solve for this.)
Determining the Molecular Formula from the Empirical Formula
1. Calculate the mass of the empirical formula. 2. Divide the molar mass by the empirical formula to achieve x (the whole number multiplier). x = molar mass/empirical mass. 3. Multiply the subscripts of each element in the empirical formula to arrive at the molecular formula. empirical formula * x = molecular formula.
Determining the Empirical Formula from Percent Composition
1. Change each of the percentages to grams. Assume you have 100 g of the compound.) 2. Calculate the number of moles of each element in the compound. 3. Divide each number of moles by the least number of moles (this is the mole ratio). 4. Multiply up to the nearest whole number only if needed (i.e., if you end up with a fraction of an atom, which isn't possible.)
Determining the Empirical and Molecular Formulas of Hydrocarbons
1. Find the number of moles of each element: Convert mass of CO2 and mass of H2O to moles of each compound. Then convert moles of CO2 to moles of carbon; convert moles of H2O to moles of hydrogen. 2. Divide by the smallest number of moles to find the empirical formula (multiply up to the nearest whole number if needed.) 3. Calculate molecular formula (if asked to solve for this.) Empirical mass * x = molar mass; solve for x; multiply subscripts of each element in the empirical formula.
Rules for Assigning Oxidation Numbers
1. Oxidation numbers of atoms in a neutral compound sum to zero; oxidation numbers of atoms in an ion sum to charge on the ion. 2. The oxidation number of a free element is always 0. 3. The oxidation number of a monatomic ion (ion of only one atom) equals the charge of the ion (e.g., Cl−, Be2+). 4. The oxidation number of H is +1 in most compounds. 5. The oxidation number for F in all compounds is -1. 6. The oxidation number of O in most compounds is -2.
Solubility Rules
1. Soluble Cations: - All Group 1 ions (alkali metals) and NH₄⁺. 2. Soluble Anions: - NO₃⁻, CH₃COO⁻ (acetate), and ClO₃⁻ (chlorate). - Halides (Group 17) except Ag⁺, Pb²⁺, Hg₂²⁺. - Sulfates (SO₄²⁻) except Pb²⁺, Hg₂²⁺, Ca²⁺, Ba²⁺, Sr²⁺.
Calculating Percent Composition from Molecular Formula
1. Use molecular formula and atomic masses to convert the number of atoms of each element to grams of that element in 1 mole of compound (g/mol format). 2. Calculate molar mass of the compound, which is equal to the total mass of 1 mole of that compound. 3. Use equation to find mass percent of element.
Chemical Changes
A change in which one or more substances combine or break apart to form new substances.
Empirical Formula
A chemical formula that shows the composition of a compound given as the simplest whole-number ratio of atoms.
Stock Solution
A concentrated solution of a substance used to prepare solutions of lower concentration.
Solution
A homogeneous mixture of two or more substances.
Combustion Analysis
A method of obtaining empirical formulas for unknown compounds, especially those containing carbon and hydrogen, by burning a sample of the compound in pure oxygen and analyzing the products of the combustion reaction.
Brønsted-Lowry Base
A proton acceptor; any substance which accepts a proton (H⁺) from another substance.
Decomposition Reaction
A reaction in which a single compound breaks down to form two or more simpler substances. General Equation: AX → A + X
Combustion Reaction
A reaction in which a substance combines with oxygen, releasing a large amount of energy in the form of light and heat. The two products of a combustion reaction are always CO₂ and H₂O.
Neutralization Reaction
A reaction in which an acid and a base react in an aqueous solution to produce a salt and water.
Single Replacement Reaction
A reaction in which one element/radical takes the place of another, similar element/radical in a compound. General Equation: A + BX → AX + B
Double Replacement Reaction
A reaction in which the ions of two compounds exchange places in an aqueous solution to form two new compounds. General Equation: AX + BY → AY + BX
Composition (Synthesis/Combination) Reaction
A reaction in which two or more substances combine to form a new compound. General Equation: A + X → AX
Soluble Substance
A substance that dissolves to a significant extent in a solvent.
Insoluble Substance
A substance that does not dissolve to a significant extent in a solvent.
Strong Acids
An acid that is fully ionized in water. Common strong acids include: 1. HCl: Hydrochloric acid 2. HBr: Hydrobromic acid 3. HI: Hydroiodic acid 4. HNO₃: Nitric acid 5. H₂SO₄: Sulfuric acid 6. HClO₄: Perchloric acid
Practice: Write the molecular equation, total ionic equation, and net ionic equation to describe Na₂CO₃ (aq) mixing with CaCl₂ (aq).
Answer: Molecular Equation: Na₂CO₃(aq) + CaCl₂(aq) → Ca₂CO₃(s) + 2NaCl(aq) Total Ionic Equation: 2 Na⁺ (aq) + CO₃²⁻ (aq) + Ca²⁺ (aq) + 2 Cl⁻ (aq) → CaCO₃ (s) + 2 Na⁺ (aq) + 2 Cl⁻ (aq) Net Ionic Equation: Ca²⁺ (aq) + CO₃²⁻ (aq) → CaCO₃ (s)
Practice: How many moles of sugar are contained in a 10-mL sip of a 0.375 M soft drink? (report your answer in the correct number of sig figs).
Answer: 0.004 mol sugar.
Practice: If 40.00 g of NaOH are dissolved into enough water to make a 1.00 L of solution, what is the molarity of the resulting solution?
Answer: 1.00 M.
Practice: What volume of 12.0 M commercial stock HCl should you use to prepare 240.0 L of 0.10 M HCl?
Answer: 2.0 L.
Practice: 2 mL of 100 mM KCl is diluted to a final volume of 10 mL. What is the final molar concentration of a solution?
Answer: 20 mM.
Practice: What is the approximate percent by mass of hydrogen in C₂H₆?
Answer: 20.11%.
Practice: A solution of CaCl₂ in water forms a mixture that is 40.5% CaCl₂ by mass. If the total mass of the mixture is 703.1 g, what masses of CaCl₂ and water were used?
Answer: 285 g CaCl₂ and 418 g H₂O.
Practice: Calculate the weight percent of each element in Ca₃(PO₄)₂.
Answer: 38.76% Ca, 19.97% P, and 41.26% O.
Practice: What mass of P is present in 454 g of Ca₃(PO₄)₂?
Answer: 90.7 g P.
Practice: 1.516 g of a compound containing carbon, hydrogen and oxygen (CXHYOZ) is subjected to combustion analysis. The results show that 2.082 g of CO2 and 1.705 g of H2O were produced. What is the empirical formula for the compound and if the molecular weight of the compound is 160.2 g/mol, what is the molecular formula of the compound?
Answer: CH₄O and C₅H₂₀O₅.
Practice: Calcium oxide reacts with ammonium chloride to produce ammonia (NH3), water, and calcium chloride. Write the balanced equation for this reaction.
Answer: CaO + 2 NH₄Cl → 2 NH₃ + H₂O + CaCl₂.
Practice: Ascorbic acid (vitamin C) contains 40.92% C, 4.58% H, and 54.50% O by mass. What is the empirical formula of ascorbic acid?
Answer: C₃H₄O₃.
Practice: Write a balanced equation for the reaction of propane, C₃H₈, with elemental oxygen to produce carbon dioxide and water.
Answer: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O.
Practice: Upon combustion, a hydrocarbon (compound containing only hydrogen and carbon) produces 1.83 g of CO2 and 0.901 g of H2O. What is the empirical formula for the compound?
Answer: C₅H₁₂.
Practice: The molar mass of ascorbic acid (C3H4O3) is 176.12 g/mol. What is the molecular formula of ascorbic acid?
Answer: C₆H₈O₆.
Practice: Assign oxidation numbers to each element in the compound SF₆.
Answer: Fluorine is -1 and sulfur is +6.
Practice: Assign oxidation numbers to each element in the compound (Cr₄O)²⁻.
Answer: Oxygen is -2 and chromium is +6.
Practice: Identify the Brønsted-Lowry acid and base in this reaction: NH₃ (aq) + H₂O (l) ⇌ NH₄⁺ (aq) + OH⁻ (aq).
Answer: The Brønsted-Lowry Acid is H₂O and the Brønsted-Lowry Base is NH₃.
Spectator Ions
Any ion that appears the same before and after mixing. These ions are not included in the net ionic equation.
Arrhenius Acid
Any substance which, when dissolved in water, increases the concentration of H⁺ (H₃O⁺, hydronium ion).
Arrhenius Base
Any substance which, when dissolved in water, increases the concentration of OH⁻ (hydroxide ion).
Molecular Formula
Formula indicating the composition of a molecule of a compound and giving the actual number of atoms of each element in the molecule of the compound.
Indication of Chemical Reaction
Giving off (evolution) of heat and light, production of a gas, formation of a precipitate (a solid that separates from a solution involving a chemical reaction), and color change.
Mixing Solutions
Many important solutions are mixtures of multiple compounds. Ringer's solution used in IV's for rehydration is made by adding solid NaCl (saline), KCl, and CaCl₂ to water to match body conditions.
Law of Conservation of Mass
Mass and matter are conserved in chemical reactions; matter is neither created nor destroyed.
Oxidation Reduction (Redox) Reactions
Reactions where electrons are transferred from one reactant to another. Results in the generation of an electrical current.
Solubility
The ability of a solute to dissolve in a certain solvent
Solvent
The dissolving medium in a solution (e.g., water, ethanol). Typically is the species present in greater amount.
Physical States in Chemical Reactions
The letters s, l, g, and aq show the physical state of the substance: 1. s indicates that the substance is a solid. 2. l indicates that the substance is a liquid. 3. g indicates that the substance is a gas. 4. aq indicates that the substance is an aqueous solution (substance dissolved in water).
Molarity (M)
The number of moles of solute per liter of solution.
Percent Composition
The percent by mass of each element in a compound.
Dilution
The process of lowering the concentration of a solution by adding more solvent.
Concentration
The relative amount of solute in a solution.
Solute
The substance that is dissolved in a solution (e.g., sugar, salt, CuSO4). Typically is the species present in lesser amount.
Total (Complete) Ionic Equation
Where all of the dissociated ions in a chemical reaction are explicitly written out.
Net Ionic Equation
Where ions that occur on both sides of the arrow are eliminated from the equation (because they essentially don't participate in the reaction of interest).
