High School Chemistry practice questions and exam guide

Hess's law:

1. Enthalpy change for reaction is same regardless of path
2. Random
3. Path-dependent
4. Has no law

Answer: A — Enthalpy change for reaction is same regardless of path

A) Standard. B/C/D) Each is incorrect.

A buffer solution resists pH change because it contains:

1. Only a strong acid
2. Only water
3. A weak acid and its conjugate base (or weak base and conjugate acid) — neutralizing added acid or base while keeping pH near pKa
4. Only a salt

Answer: C — A weak acid and its conjugate base (or weak base and conjugate acid) — neutralizing added acid or base while keeping pH near pKa

C) Standard buffer composition. A/B/D) Each is incorrect.

Standard conditions (STP): pressure and temperature are:

1. 0 atm, 0 K
2. 1 atm (or 100 kPa), 0°C (273.15 K) (chemistry convention; molar volume = 22.4 L for ideal gas)
3. 1 atm, 25°C
4. 0 atm, 25°C
5. 1 atm, 100°C

Answer: B — 1 atm (or 100 kPa), 0°C (273.15 K) (chemistry convention; molar volume = 22.4 L for ideal gas)

STP (traditional): 1 atm and 0°C (273.15 K). Molar volume = 22.4 L. IUPAC redefined to 100 kPa in 1982 (molar volume 22.7 L). Many courses still use 22.4 L convention. Distinguish from SATP.

Activation energy is:

1. Energy released
2. Equal to entropy
3. Always zero
4. Equal to ΔH
5. Energy barrier that must be overcome for reactants to form products; catalysts lower it

Answer: E — Energy barrier that must be overcome for reactants to form products; catalysts lower it

Activation energy (Eₐ): hill on reaction coordinate diagram. Catalysts lower Eₐ → faster reactions without being consumed. Distinct from ΔH (overall enthalpy change). Arrhenius equation relates rate to Eₐ and T.

In the neutralization HCl + NaOH → NaCl + H₂O, what role does NaOH play?

1. Brønsted-Lowry acid — donates a proton
2. Brønsted-Lowry base — accepts a proton from HCl (OH⁻ + H⁺ → H₂O)
3. Oxidizing agent — gains electrons
4. Catalyst — speeds up reaction without being consumed

Answer: B — Brønsted-Lowry base — accepts a proton from HCl (OH⁻ + H⁺ → H₂O)

B) Standard acid-base description. A/C/D) Each is incorrect.

Buffer capacity:

1. Resistance to pH change depends on concentration of buffer components
2. Random
3. Always infinite
4. Has no capacity

Answer: A — Resistance to pH change depends on concentration of buffer components

A) Standard. B/C/D) Each is incorrect.

Polar vs nonpolar molecule:

1. Polar molecules have net dipole; nonpolar molecules have symmetric arrangement canceling dipoles (e.g., CO₂ nonpolar despite polar bonds)
2. Same
3. Random
4. All molecules nonpolar

Answer: A — Polar molecules have net dipole; nonpolar molecules have symmetric arrangement canceling dipoles (e.g., CO₂ nonpolar despite polar bonds)

A) Standard. B/C/D) Each is incorrect.

Polymers:

1. Large molecules made of repeating monomer units — natural (proteins, DNA, cellulose, starch, rubber) and synthetic (polyethylene, nylon, polystyrene, polyester); formed by addition (vinyl chloride → PVC) or condensation polymerization
2. Are always small
3. Only synthetic
4. Cannot exist

Answer: A — Large molecules made of repeating monomer units — natural (proteins, DNA, cellulose, starch, rubber) and synthetic (polyethylene, nylon, polystyrene, polyester); formed by addition (vinyl chloride → PVC) or condensation polymerization

A) Standard. B/C/D) Each is incorrect.

Lewis structure for CO₂:

1. Bent shape
2. Single bonds only
3. Triple bonds
4. O=C=O with double bonds, linear shape

Answer: D — O=C=O with double bonds, linear shape

D) Standard. A/B/C) Each is incorrect.

Which bond type forms between sodium (Na) and chlorine (Cl)?

1. Covalent
2. Van der Waals
3. Metallic
4. Hydrogen
5. Ionic (metal + nonmetal; Na transfers electron to Cl; Na⁺ Cl⁻)

Answer: E — Ionic (metal + nonmetal; Na transfers electron to Cl; Na⁺ Cl⁻)

Ionic bonds: large electronegativity difference (>1.7 typically) → electron transfer → ions held by electrostatic attraction. Na (1.0 EN) + Cl (3.0 EN) = 2.0 difference → ionic NaCl. Crystal lattice structure.

A phase change from solid to gas without passing through liquid is called:

1. Melting
2. Sublimation — e.g., dry ice (CO₂) sublimating, naphthalene; reverse is deposition
3. Boiling
4. Freezing

Answer: B — Sublimation — e.g., dry ice (CO₂) sublimating, naphthalene; reverse is deposition

B) Standard term. A/C/D) Each is a different phase change.

Which is a STRONG ACID?

1. Acetic acid (CH₃COOH, weak)
2. Carbonic acid (H₂CO₃, weak)
3. Hydrochloric acid (HCl) — fully dissociates in water; also: H₂SO₄, HNO₃, HBr, HI, HClO₄
4. Citric acid (weak)
5. Boric acid (weak)

Answer: C — Hydrochloric acid (HCl) — fully dissociates in water; also: H₂SO₄, HNO₃, HBr, HI, HClO₄

Strong acids fully dissociate: HCl, H₂SO₄, HNO₃, HBr, HI, HClO₄. Weak acids partially dissociate (most others). pKa < 0 for strong; weak acids have measurable Ka. Memorize the 6-7 strong acids for HS chem.

What's the 'octet rule'?

Atoms tend to gain/lose/share electrons to have 8 valence electrons (full outer shell).

What is 'thermochemistry'?

Study of heat changes during chemical reactions.

What's an 'anion'?

A negatively charged ion (gained electrons).

What are Group 18 elements called?

Noble gases. Inert (full valence shell).

Which element has the highest electronegativity?

Fluorine (F).

Examples of common bases?

Soap, ammonia, baking soda, drain cleaner (NaOH).