13.3 Reactions, Acids, and Bases

13.3 Reactions, Acids, and Bases

Why This Matters for the USTET

USTET Science chemistry finishes with change: how substances rearrange, how to balance equations, what acids and bases do in water, and how the mole connects grams to reaction ratios. Items are often quick classification or one-step calculation problems. If you can balance an equation, name a reaction type, interpret pH, and convert grams to moles, you cover most of this slice.

What a Chemical Reaction Is

A chemical reaction produces new substances with new properties. Evidence can include color change, gas formation, precipitate formation, or heat/light — but physical changes (melting ice, dissolving sugar without reaction) are not chemical reactions. In equations:

• Reactants appear on the left; products on the right.
• Coefficients tell mole ratios; subscripts are part of chemical identity and must not be altered to "balance."

Example: 2H₂ + O₂ → 2H₂O. Changing H₂O to H₂O₂ would invent a different product (hydrogen peroxide), not balance the combustion of hydrogen.

Balancing Equations

Law of conservation of mass: atoms are rearranged, not created or destroyed. Count atoms of each element on both sides; adjust coefficients until counts match.

Efficient SHS method:

1. Write correct formulas first.
2. Balance metals, then nonmetals, then hydrogen, then oxygen (a common order — adjust when polyatomics repeat).
3. If a polyatomic ion appears unchanged on both sides, balance it as a unit.
4. Clear fractions by multiplying through; verify all elements.

Major Reaction Types

Precipitation is a double-replacement outcome when an insoluble solid forms (AgCl, BaSO₄). Acid–base neutralization is often written as a double replacement producing water and a salt: HCl + NaOH → NaCl + H₂O.

Activity series tip: in single replacement, a more active metal can displace a less active metal from a compound (or H₂ from an acid). Exact series memorization varies by item bank; know the concept that not every metal/acid pairing reacts.

Acids and Bases

At Grade 11 / USTET depth, use the Arrhenius picture first:

• Acid: substance that increases H⁺ (often described as forming H₃O⁺, hydronium) in water.
• Base: substance that increases OH⁻ in water.

Brønsted–Lowry (also tested at SHS): acids donate protons (H⁺); bases accept protons. NH₃ is a base because it accepts H⁺ to form NH₄⁺, even though it does not contain OH⁻ in its formula.

Strong vs weak (exam meaning): strong acids/bases ionize essentially completely in water (HCl, HNO₃, H₂SO₄ for acids; NaOH, KOH for bases). Weak acids/bases ionize only partially (acetic acid, ammonia). Strength is about extent of ionization, not the same thing as concentration. A dilute strong acid can be less damaging than a concentrated weak acid in everyday language, but exam language separates strength from molarity.

Properties:

• Acids: sour taste (never taste in lab), turn blue litmus red, react with active metals to produce H₂, react with carbonates to produce CO₂.
• Bases: bitter taste / slippery feel (again, not a lab method), turn red litmus blue, neutralize acids.

pH Scale

pH measures acidity related to hydrogen-ion concentration. For USTET:

Each 1-unit pH change is a tenfold change in [H⁺]. pH 3 is ten times more acidic in [H⁺] than pH 4, and one hundred times more than pH 5. Stomach acid (~pH 1–2), pure water (7), soap solutions (often > 8), and oven cleaners (strongly basic) are useful anchors.

Indicators: litmus, phenolphthalein (colorless in acid, pink in base), and universal indicator give qualitative pH information without a meter.

Neutralization and Salts

Neutralization: acid + base → salt + water (Arrhenius). Example: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O. The salt is the ionic compound from the acid's anion and the base's cation.

Titration language may appear: a known concentration of base (or acid) is added until the indicator signals equivalence. You do not need full lab technique for USTET, but you should know neutralization produces salt and water and that indicators mark the endpoint.

The Mole and Simple Stoichiometry

The mole is the chemist's counting unit: 1 mole = 6.022 × 10²³ particles (Avogadro's number). Molar mass (g/mol) equals the formula mass in grams per mole — numerically the same as the sum of atomic masses from the periodic table.

Core conversions:

• moles = mass (g) / molar mass (g/mol)
• mass = moles × molar mass
• From a balanced equation, coefficients are mole ratios. For CH₄ + 2O₂ → CO₂ + 2H₂O, 1 mol CH₄ reacts with 2 mol O₂ and produces 1 mol CO₂.

Example: How many moles in 36 g of water? 36 g / 18 g·mol⁻¹ = 2.0 mol. How many grams of O₂ are needed to react completely with 2 mol CH₄? Ratio 2 mol O₂ per 1 mol CH₄ → 4 mol O₂ → 4 × 32 = 128 g O₂.

Limiting reactant idea: the reactant that runs out first limits product amount. If a stem gives amounts of two reactants, convert both to moles, compare to the required ratio, and identify which is insufficient.

Energy in Reactions (Brief)

• Exothermic: releases heat (combustion; many neutralizations feel warm).
• Endothermic: absorbs heat (some decompositions; cold packs).

Catalysts speed reactions without being consumed; they do not change the balanced stoichiometry of reactants and products.

Exam Strategy

• Never "balance" by changing subscripts inside a formula.
• Classify reaction type from the pattern of reactants and products before looking at options.
• For acid–base items, decide Arrhenius vs Brønsted language from the stem (OH⁻ production vs proton transfer).
• For pH, remember tenfold steps and that lower pH means higher [H⁺].
• For stoichiometry, convert to moles first; use coefficients as ratios; convert back to grams only at the end.

With balancing, acid–base behavior, and mole ratios in hand, you complete the chemistry core of USTET Science and are ready to connect these ideas to physics and Earth science items elsewhere in the Science subtest.