14.4 Chemical Reactions, pH & Equilibrium

14.4 Chemical Reactions, pH & Equilibrium

Quick Answer: Learn to classify combination, decomposition, single replacement, double replacement, and neutralization reactions. Acids and bases control pH (0–14 scale; 7 ≈ neutral). Catalysts speed reactions without being used up. Equilibrium is dynamic balance of forward and reverse rates. Buffers stabilize pH—especially in blood—so this section connects directly to ABGs, antacids, and drug stability on the NEX and in nursing practice.

Chemical reactions rearrange atoms into new substances. For entrance-exam Chemistry, pattern recognition beats memorizing endless equations.

Evidence of a Chemical Reaction

Common signs a chemical change occurred:

• Color change not explained by simple mixing
• Gas production (bubbles) without boiling
• Precipitate (solid) forms from clear solutions
• Heat or light released or absorbed beyond ordinary warming
• Permanent new properties (e.g., rust forming on iron)

Major Reaction Types

Combination (Synthesis)

Two or more reactants form one product.

General: A + B → AB

Example: 2 H₂ + O₂ → 2 H₂O

Decomposition

One compound breaks into two or more simpler substances (often requires heat, light, or electricity).

General: AB → A + B

Example: 2 H₂O₂ → 2 H₂O + O₂ (hydrogen peroxide decomposition—sped by catalase in tissues or by catalysts on a wound surface)

Single Replacement (Single Displacement)

One element replaces another in a compound.

General: A + BC → AC + B

Example: Zn + 2 HCl → ZnCl₂ + H₂ (zinc displaces hydrogen)

Double Replacement (Double Displacement)

Two compounds exchange partners; often forms a precipitate, gas, or molecular compound (like water).

General: AB + CD → AD + CB

Example: AgNO₃ + NaCl → AgCl (s) + NaNO₃

Neutralization (Acid–Base)

A special double-replacement pattern: acid + base → salt + water (typically).

Example: HCl + NaOH → NaCl + H₂O

Antacids neutralizing excess stomach acid are everyday neutralization. IV sodium bicarbonate interacting with acids is the same acid–base idea in a clinical frame.

Quick Classification Table

Acids and Bases

Intro definitions sufficient for NEX:

• Arrhenius acid: produces H⁺ (really H₃O⁺, hydronium) in water
• Arrhenius base: produces OH⁻ in water
• Brønsted–Lowry (useful upgrade): acids donate protons (H⁺); bases accept protons

Properties:

Strong acids/bases dissociate extensively in water; weak ones partially dissociate. Gastric juice is strongly acidic; blood is tightly controlled near pH 7.35–7.45—slightly alkaline.

The pH Scale

pH measures the acidity of aqueous solutions, related to hydrogen-ion concentration. For NEX:

• Scale commonly 0 to 14
• pH 7: neutral (pure water at standard conditions)
• pH < 7: acidic (lower number = more acidic)
• pH > 7: basic/alkaline (higher number = more basic)
• The scale is logarithmic: each whole-number step is a tenfold change in [H⁺]. pH 3 is ten times more acidic (higher [H⁺]) than pH 4.

Nursing applications: ABG interpretation rests on pH; urine pH affects drug excretion and stone risk; topical acids/bases can burn tissue; storage of meds may specify pH-sensitive stability.

Catalysts

A catalyst increases reaction rate by lowering activation energy (the energy barrier to start the reaction). The catalyst is not permanently consumed; it can act repeatedly.

• Enzymes are biological catalysts (proteins)—catalase, amylase, digestive enzymes, clotting-cascade enzymes.
• Inorganic catalysts appear in industry and labs (e.g., manganese dioxide speeding H₂O₂ decomposition).

Catalysts do not change the position of equilibrium ultimately in the simple intro model—they help the system reach equilibrium faster. They do not make an impossible reaction spontaneous by magic; they ease the path.

Chemical Equilibrium

Many reactions are reversible: reactants form products, and products reform reactants.

A + B ⇌ C + D

Chemical equilibrium is reached when:

• Forward rate = reverse rate
• Macroscopic concentrations of reactants and products become constant (not necessarily equal)
• The system is dynamic—reactions continue at the molecular level both ways

Le Chatelier’s principle (intro awareness): if you stress an equilibrium (change concentration, temperature, or pressure for gas systems), the system shifts to partially counteract the stress. Example: removing a product pulls the forward reaction; adding reactant pushes forward.

Clinical/conceptual echo: oxygen binding to hemoglobin is a reversible equilibrium influenced by pH, CO₂, and temperature (Physiology will deepen this)—Chemistry gives you the vocabulary of dynamic balance.

Buffers: pH Stability in the Body

A buffer is a system that resists changes in pH when small amounts of acid or base are added. Typically a buffer contains a weak acid and its conjugate base (or weak base and conjugate acid).

Blood Bicarbonate Buffer (Must-Know Intro)

The major blood buffer pair:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

• Adding acid (H⁺): bicarbonate (HCO₃⁻) binds H⁺ → limits pH drop
• Removing acid / adding base: the equilibrium shifts to release H⁺ → limits pH rise
• Lungs regulate CO₂; kidneys regulate HCO₃⁻—physiology partners with Chemistry

Other body buffers include phosphate systems and proteins (hemoglobin). Without buffers, metabolic acids from normal life would swing blood pH into fatal ranges.

Nursing link: interpreting metabolic vs respiratory acidosis/alkalosis starts with understanding that pH is buffered, not free-floating.

Putting Reactions and pH Together

Exam Traps

• Neutralization products: salt + water—not “more acid.”
• pH 6 vs 8: 6 is acidic; 8 is basic—do not reverse.
• Log scale: One pH unit = tenfold [H⁺] change—not a linear “one notch.”
• Catalyst consumed? No—not if it is truly a catalyst; enzymes can denature, but the definition is not stoichiometric consumption like a reactant.
• Equilibrium means stopped? No—rates equal; dynamic, concentrations constant.
• Buffer = makes solution neutral: No—buffers resist change near their designed pH (blood ~7.4, not necessarily 7.0).
• Combination vs decomposition: Opposite patterns—count whether products or reactants “multiply.”

Study Strategy for Chemistry on the NEX

Chemistry is only ~9% of scored Science (about 5 items), but the topics are dense. Efficient prep:

1. Drill particle identity: proton/neutron/electron → isotope/ion/element.
2. Sort bonding: ionic vs covalent vs polar water story.
3. Label mixture bottles: solution / suspension / emulsion / tincture.
4. Classify five reaction types + map acids/bases onto the pH scale.
5. Tell the buffer story in two sentences tied to HCO₃⁻/CO₂.

When a stem feels wordy, strip it to: particles, bonds, mixture type, or reaction/pH pattern. That reduction is how small Chemistry item counts still reward clear conceptual mastery—and how entrance-exam Chemistry supports later nursing acid–base reasoning.