CEE 4501 – Environmental Chemical Processes Midterm 1

Flashcards to help review core concepts and applications in Environmental Chemical Processes for Exam 1.

What are the four major cations in freshwater?

Calcium (Ca²⁺), Magnesium (Mg²⁺), Sodium (Na⁺), Potassium (K⁺). They originate mainly from rock and soil weathering.

What are the four major anions in freshwater?

Bicarbonate (HCO₃⁻), Sulfate (SO₄²⁻), Chloride (Cl⁻), Nitrate (NO₃⁻). They come from atmospheric deposition, minerals, and nutrient cycles.

What is the typical pH range of freshwater?

Between 6.5 and 9.0, buffered by the carbonate system (CO₂, HCO₃⁻ , CO₃²⁻).

What is ionic strength, and why is it important?

Ionic strength (I = ½ Σ cᵢzᵢ²) measures the total charge-weighted concentration of ions; it affects activity coefficients, solubility, and conductivity.

What is the difference between activity and concentration?

Concentration measures total quantity of a species; activity represents its 'effective' concentration considering ionic interactions (activity = γ × concentration).

What are standard states?

Reference conditions for comparing energies: 1 bar pressure, 25°C (298 K), 1 M concentrations, pure solids and liquids, used for defining ΔG°, ΔH°, and ΔS°.

What is Gibbs Free Energy (ΔG)?

Energy available to do work at constant temperature and pressure, expressed as ΔG = ΔH – TΔS; negative ΔG means a spontaneous reaction.

What is Enthalpy (ΔH)?

Total heat content of a system: internal energy + pressure-volume work (H = U + pV); indicates if heat is absorbed (endothermic) or released (exothermic).

What is Entropy (ΔS)?

A measure of disorder or randomness in a system; increases when systems become more disordered.

What is an equilibrium constant (K)?

Ratio of product activities to reactant activities at equilibrium, linked to Gibbs energy by ΔG° = –RT ln K.

What is the reaction quotient (Q)?

Same ratio as K but for non-equilibrium conditions; if Q < K, the reaction proceeds forward; if Q > K, it shifts backward.

What is alkalinity?

The water’s acid-neutralizing capacity, calculated as Alk = [HCO₃⁻] + 2[CO₃²⁻] + [OH⁻] – [H⁺]; increases when bicarbonate or carbonate ions form or H⁺ is consumed.

Define buffer intensity.

The ability of a system to resist pH change; it’s highest near a solution’s pKa.

What is the difference between a strong and a weak acid?

Strong acids dissociate completely (e.g., HCl), while weak acids partially dissociate (e.g., HCN), controlled by Ka and pKa.

What is pKa?

The negative log of the acid dissociation constant (Ka); lower pKa = stronger acid.

What are conjugate acid–base pairs?

Two species that differ by one proton, e.g., H₂CO₃ / HCO₃⁻ or NH₄⁺ / NH₃.

What is a Lewis acid versus a Brønsted acid?

Lewis acid: electron-pair acceptor; Brønsted acid: proton (H⁺) donor.

What is the proton condition?

A charge balance expression that relates total positive and negative protonated species in a system, used for solving pH equilibria.

What is the importance of buffer intensity?

Buffer intensity is strongest at the midpoint of a titration curve, where pH = pKa.

What is the equivalence point of a titration?

The point where moles of acid = moles of base; no excess reactant remains.

Define the term 'standard state.'

The physical and chemical condition used as a reference (pure solids/liquids, 1 M solutions, 1 atm gas).

What is the role of ionic strength in activity coefficients?

As ionic strength increases, activity coefficients decrease, making ions less 'effective' in reactions.

What is the ionic strength equation?

I = ½ Σ cᵢzᵢ²; measures total charge-weighted ion concentration and corrects activity coefficients.

What does the Debye-Hückel Equation calculate?

Calculates ion activity coefficients based on ionic strength: –log(γ) = 0.5z²(√I / (1 + √I) – 0.3I).

What is Gibbs Free Energy (nonstandard)?

ΔG = ΔG° + RT ln(Q); determines spontaneity under actual (nonstandard) conditions.

What does the equilibrium constant relationship express?

ΔG° = –RT ln K; links equilibrium constant with thermodynamic energy change.

What is the Ideal Gas Law?

PV = nRT; relates gas pressure, volume, and temperature to moles and the gas constant.

What is the alkalinity formula?

Alk = [HCO₃⁻] + 2[CO₃²⁻] + [OH⁻] – [H⁺]; expresses acid-neutralizing capacity in terms of species that buffer pH.

What does the Van’t Hoff Equation predict?

Predicts how equilibrium constants change with temperature: ln(K₁/K₂) = ΔH/R (1/T₂ – 1/T₁).

What is pX?

pX = –log([X]); converts concentration to log scale; applies to pH, pOH, pKa, etc.

What is alkalinity by charge balance?

Alk = Σ[Cations] – Σ[Anions]; a simplified method for calculating alkalinity using measured ions.

Why is the carbonate system the main buffer in freshwater?

Because CO₂ dissolves in water to form carbonic acid and its conjugate bases (bicarbonate, carbonate), stabilizing pH.

How does photosynthesis affect pH in lakes?

Photosynthesis removes CO₂, reducing carbonic acid and increasing pH (makes water more basic).

How does respiration affect pH in lakes?

Respiration adds CO₂, producing carbonic acid and lowering pH (more acidic).

What is the relationship between conductivity, TDS, and ionic strength?

All increase with higher ion concentrations; conductivity reflects charge mobility, TDS measures mass, and ionic strength weights by charge.

Why is ionic strength important in environmental chemistry?

It determines how strongly ions interact, influencing reaction rates, precipitation, and complex formation.

How are ΔH, ΔG, and ΔS related?

ΔG = ΔH – TΔS; a reaction is spontaneous if ΔG < 0, with temperature affecting spontaneity depending on the signs of ΔH and ΔS.

What happens to equilibrium when temperature increases?

Endothermic reactions (ΔH > 0) shift toward products; exothermic reactions (ΔH < 0) shift toward reactants.

What determines buffer strength?

A buffer is strongest when pH = pKa, as both acid and base forms are present in nearly equal amounts.

Why are titration curves important?

They show how pH changes as acid or base is added, identifying equivalence points, buffer regions, and alkalinity.

What does the proton condition represent in acid–base equilibrium?

It ensures conservation of positive and negative species, allowing pH to be calculated accurately.

How can ΔG° and K predict if a reaction is favorable?

Large K (≫1) or negative ΔG° means products are favored and the reaction proceeds spontaneously.

Example of a strong acid and its environmental use.

HCl (hydrochloric acid) – used for pH control in water treatment.

Example of a weak acid.

Acetic acid (CH₃COOH) – partially dissociates, forms buffer systems.

Example of a strong base.

NaOH (sodium hydroxide) – completely dissociates; used for neutralization and pH adjustment.

Example of a weak base.

NH₃ (ammonia) – partially reacts with water to form NH₄⁺ and OH⁻.

What reactions increase alkalinity in natural waters?

Dissolution of CaCO₃ and sulfate reduction, producing bicarbonate and consuming H⁺.

What is the environmental role of zero-valent iron (Fe⁰)?

It reduces chlorinated hydrocarbons to less harmful compounds like methane, with the reaction being spontaneous due to strongly negative ΔG°.

What controls the pH of rainfall or softwater lakes?

Dissolved CO₂ and weak acids from the atmosphere, with limited buffering capacity due to low alkalinity.

Why does alkalinity matter in environmental engineering?

It determines a system’s ability to neutralize acid pollution and maintain stable pH, crucial for water treatment and aquatic life.