CHEM 191 Flashcards
What is stoichiometry
Stoichiometry is the study of the relative amounts of reactants and products in chemical reactions, derived from the Greek words for “element” and “to measure.”
Why must chemical equations be balanced?
Chemical equations must be balanced to obey the law of conservation of mass, ensuring the same number of each type of atom exists on both sides of the equation.
What is a mole?
A mole is the SI unit for the amount of substance, defined as the amount containing as many entities (atoms, molecules, etc.) as there are atoms in exactly 12 g of carbon-12 (6.022 × 10²³ entities).
What is Avogadro’s constant?
Avogadro’s constant is 6.022 × 10²³ mol⁻¹, representing the number of entities in one mole of a substance.
What is molar mass (M)?
Molar mass is the mass of one mole of a substance, calculated as
M=m/n with units of g mol⁻¹.
How do you calculate the amount of substance (n) from mass (m)?
Use the formula n=m/M where M is the molar mass.
How do you calculate the concentration (c) of a solution?
c=n/V, where n is the number of moles of solute and V is the volume in liters. (units are mol L-1.
What are the two key equations in stoichiometry?
n=m/M amount from mass
c=n/V concentration
What makes water an excellent solvent for biological systems?
Water’s polarity and hydrogen bonding enable it to dissolve and transport gases, inorganic materials, and drugs, crucial for biological processes.
Why is water a polar molecule?
Oxygen is more electronegative than hydrogen, creating partial charges (δ⁻ on O, δ⁺ on H), resulting in a dipole.
What is hydrogen bonding?
A strong dipole-dipole interaction between water molecules, where δ⁺ H bonds with δ⁻ O of another molecule.
Name three unusual properties of water due to hydrogen bonding.
High melting/boiling points.
Ice is less dense than liquid water.
High surface tension and heat capacity.
Why does ice float?
Hydrogen bonding in ice creates an open, less dense structure compared to liquid water.
What is solvation? How does it work for NaCl in water?
Solvation (hydration in water) involves water molecules surrounding ions. For NaCl, δ⁻ O attracts Na⁺, and δ⁺ H attracts Cl⁻, dissolving the crystal.
What types of molecules are soluble in water?
Polar molecules (e.g., alcohols, acids) and ionic compounds (e.g., NaCl), due to water’s ability to form hydrogen bonds or ion-dipole interactions.
Why are non-polar gases (e.g., O₂) poorly soluble in water?
They lack polarity but may have weak induced dipole interactions. Solubility increases with molecular size (e.g., CO₂ > O₂).
Define: solution, solute, solvent, electrolyte.
Solution: Homogeneous mixture of solute(s) in solvent.
Solute: Dissolved substance.
Solvent: Dissolving medium (e.g., water).
Electrolyte: Substance producing ions in solution (e.g., NaCl).
What is the difference between strong and weak electrolytes?
Strong: Completely dissociates (e.g., NaCl → Na⁺ + Cl⁻).
Weak: Partially dissociates (e.g., acetic acid ⇌ CH₃COO⁻ + H⁺).
Give an example of a non-electrolyte.
Glucose (C₆H₁₂O₆), which dissolves but does not form ions.
Why is NH₃ highly soluble in water despite being a gas?
NH₃ forms hydrogen bonds with water and reacts chemically (NH₃ + H₂O ⇌ NH₄⁺ + OH⁻).
What is the difference between a reaction that “goes to completion” and one that reaches equilibrium?
Completion: All reactants convert to products.
Equilibrium: Forward and reverse reactions occur at equal rates, with constant concentrations of reactants and products.
Define the reaction quotient Q. How is it calculated for
aA+bB⇌cC+dD?
Q measures relative amounts of reactants/products at any time:
(Concentrations raised to stoichiometric coefficients.)
What does Q=Kc indicate?
The system is at equilibrium; no net change in concentrations.
How does the system respond if
Q<Kc>Kc?</Kc>
Q<Kc: Reaction proceeds forward (more products).
Q>K c: Reaction proceeds in reverse (more reactants).
Why are pure solids/liquids omitted from
Kc expressions?
Their concentrations are constant and do not change during the reaction.
What does a large Kc vs. a small Kc imply?
Large Kc: Equilibrium favors products (reaction nearly complete).
Small Kc: Equilibrium favors reactants (reaction barely proceeds).
What is an ICE table? When is it used?
ICE: Initial, Change, Equilibrium.
Used to calculate equilibrium concentrations from initial conditions and Kc
State Le Châtelier’s Principle.
If a system at equilibrium is disturbed, it shifts to counteract the change (e.g., adding reactant → shifts toward products).
How does increasing pressure affect an equation
Shifts toward fewer gas moles (right, to produce more)
How does a system at equilibrium respond to added reactants or products (Le Châtelier’s Principle)?
Added reactant: Q<K → shifts right (toward products).
Added product: Q>K → shifts left (toward reactants).
(K remains constant!)
How does changing volume affect equilibrium for a solution?
Volume ↑ (pressure ↓)
Volume ↓ (pressure ↑)
Why does adding an inert gas (e.g., He) NOT affect equilibrium position?
Inert gases don’t appear in Q; only changes in volume (affecting concentrations) or reactant/product amounts shift equilibrium.
Define solubility (s) and saturated solution.
Solubility (s): Max moles of solute dissolved per liter of solvent at equilibrium.
Saturated solution: Equilibrium between undissolved solute and dissolved ions.
Relate Ksp and solubility (s) for a 1:1 electrolyte.
Ksp=s^2
(For 1:1 ratio; adjust for other stoichiometries)
How would you predict if a precipitate forms?
compare the reaction quotient (Q) with the solubility product constant (Ksp): if Q > Ksp, a precipitate forms; if Q < Ksp, no precipitate forms.
What is the common ion effect?
an effect that suppresses the ionization of an electrolyte when another electrolyte (which contains an ion which is also present in the first electrolyte, i.e. a common ion) is added
What are the three types of chemical reactions?
Electron transfer (redox)
Precipitation (formation of a solid)
Acid-base (proton transfer)
Define Lewis acid and Lewis base.
Lewis acid: Electron-pair acceptor (e.g., BF₃).
Lewis base: Electron-pair donor (e.g., NH₃).
What is the Brønsted-Lowry definition of acids and bases?
Acid: Proton (H⁺) donor (e.g., HCl).
Base: Proton (H⁺) acceptor (e.g., OH⁻).
What is a conjugate acid-base pair?
Pairs differing by one H⁺.
Why is HCl a strong acid but CH₄ is not acidic?
HCl has a polar H-Cl bond (Cl is electronegative), making H⁺ easily donated.
CH₄ has nonpolar C-H bonds; H⁺ is not readily released.
What is the auto-ionization of water? Write its equilibrium constant (Kw).
Kw = [H3O+][OH-].
Value of Kw:
At 25°C, the value of Kw is approximately 1.0 x 10^-14.
What is the relationship between pH and pOH?
pH+pOH=14(at 25°C)
List 3 strong acids and 2 strong bases.
Strong acids: HCl, H₂SO₄, HNO₃.
Strong bases: NaOH, KOH.
Calculate the pH of 0.16 M HCl.
pH=−log(0.16)≈0.8
Why does pure water at 35°C have pH = 6.66?
Kw increases with temperature, and the new value of Kw is used to calculate the new pH value. =2.2×10 −7 M (still neutral).
How do antacids work?
They neutralize excess acid via reactions
What is the key difference between strong and weak acids?
Strong acids: Completely dissociate in water (e.g., HCl).
Weak acids: Partially dissociate, reaching equilibrium (e.g., CH₃COOH).
Write the Ka expression for a weak acid HA
Ka = [H+][A-]/[HA]
How are Ka and pKa related?
pKa= -log(ka)
What is the relationship between Ka, Kb and Kw?
For conjugate pairs:
Ka x Kb = Kw = 1.4x10-14
pKa + pKb = 14
Why is the conjugate base of a strong acid weak?
Strong acids fully dissociate, leaving their conjugate bases with negligible tendency to accept protons (e.g., Cl⁻ from HCl).
What assumptions simplify weak acid pH calculations?
[A-]e = [H3O+]
[HA]e = [HA] initial (small Ka)
Why do hydrated metal ions act as weak acids?
The metal’s positive charge polarizes bound water, weakening O-H bonds and releasing H+
What is a buffer solution?
A solution of a weak acid and its conjugate base (or weak base and conjugate acid) that resists pH changes when small amounts of
H3O+ or OH-are added.
Write the Henderson-Hasselbalch equation.
pH = pKₐ + log([A⁻]/[HA])
What is the pH of a buffer when [A-]=[HA]
pH = pKa
How does a buffer resist pH changes?
Added acid (H3O+): Consumed by conjugate base (A−).
Added base (OH−): Consumed by weak acid (HA).
How does buffer pH change compare to pure water when acid is added?
Buffer: pH drops slightly (e.g., 4.74 → 4.65).
Water: pH plummets (7.00 → 2.04).
What is buffer capacity?
The amount of acid/base a buffer can neutralize before pH changes significantly. Exhausted when [HA] or [A−] is depleted.
What is the purpose of an acid-base titration?
To determine the concentration of an unknown acid/base solution by reacting it with a solution of known concentration (titrant) and monitoring pH changes.
What are the key regions in a weak acid-strong base titration curve (e.g., CH₃COOH + NaOH)?
Initial pH: Determined by weak acid alone (e.g., pH ≈ 2.89 for 0.1 M CH₃COOH).
Buffer region: pH ≈ pKₐ when [HA] = [A⁻].
Equivalence point: pH > 7 (basic) due to conjugate base hydrolysis (e.g., CH₃COO⁻ + H₂O → OH⁻).
Excess base region: pH dominated by excess OH⁻.
Why is there no buffer region in a strong acid-strong base titration?
No weak acid/conjugate base pair exists to resist pH changes. The pH shifts abruptly near the equivalence point.
What is a zwitterion?
A zwitterion is an ion that contains two functional groups
What is the isoelectric point (pI) of an amino acid?
The pH where the amino acid exists predominantly as a zwitterion with no net charge (e.g., pI = 5.98 for glycine).
How does a diprotic acid (e.g., H₃PO₃) titration curve differ from a monoprotic acid?
It has two equivalence points (one for each proton) and two buffer regions near pKₐ₁ and pKₐ₂.
What happens to glycine at very low pH (<2) and very high pH (>10)?
Low pH: Fully protonated (+H₃N–CH₂–COOH).
High pH: Fully deprotonated (H₂N–CH₂–COO⁻).
Why is the equivalence point pH basic in a weak acid-strong base titration?
The conjugate base (e.g., CH₃COO⁻) reacts with water to produce OH⁻
What distinguishes polar (hydrophilic) molecules from non-polar (hydrophobic) molecules?
Polar: Contain polar bonds/charged regions (e.g., –OH, –COOH). Soluble in water.
Non-polar: Lack polar bonds (e.g., hydrocarbons). Insoluble in water; aggregate in aqueous solutions.
Why is water an effective solvent for polar molecules?
Water’s polarity enables hydrogen bonding and electrostatic interactions with charged/polar solutes (e.g., hydration shells around ions).
How does pH affect the ionization state of functional groups?
pH < pKₐ: Protonated (e.g., –COOH, –NH₃⁺).
pH > pKₐ: Deprotonated (e.g., –COO⁻, –NH₂).
(At pH = pKₐ, 50% ionized.)
What is the significance of pKₐ for amino acid side chains (e.g., histidine vs. arginine)?
Histidine (pKₐ = 6.0): Protonated at physiological pH (7.4), important for enzyme catalysis.
Arginine (pKₐ = 12.5): Always protonated; stabilizes DNA-protein interactions.
How do non-polar molecules behave in water?
They form hydration shells but ultimately aggregate to minimize water disruption (hydrophobic effect).
Describe the structure of a phospholipid bilayer.
Polar heads: Face aqueous environments (hydrophilic).
Non-polar tails: Form the membrane core (hydrophobic
What roles do membranes play in cells?
Barrier: Separate compartments.
Communication: Receptor proteins.
Transport: Channels/pumps for molecules.
Why do proteins like chymotrypsin require specific ionization states for activity?
Catalytic residues (e.g., His⁺, Asp⁻ in chymotrypsin’s triad) must be protonated/deprotonated to stabilize reaction intermediates.
How do buffers maintain pH in biological systems?
Blood: Bicarbonate/carbonic acid system.
Cells: Phosphate/proteins.
(Resist pH changes by absorbing/releasing H⁺.)
What drives protein folding in aqueous environments?
Hydrophobic effect: Non-polar regions fold inward to avoid water; polar regions face outward.
Why is DNA negatively charged, and how do proteins interact with it?
DNA charge: From phosphate groups (pKₐ ~1).
Protein binding: Requires positively charged regions (e.g., lysine/arginine).
What is the role of membrane-bound proteins?
Signaling: Receptors transmit signals.
Transport: Channels/pumps move molecules across membranes.
