Redox, Cells & Nuclear Chemistry — Chapter quiz · Physical Science: Chemistry, Grade 11

Track electron transfer, electrical energy, isotope change, and statistical decay.

Problem 1

A 240 g radioactive sample has a half-life of 5 years. How many grams remain after 15 years?
Answer and explanation

30 g

15 ÷ 5 = 3 half-lives, so the sample halves three times: 240 → 120 → 60 → 30 g (240 ÷ 2³ = 30 g).

Problem 2

An alpha particle consists of…
  1. 2 protons and 2 neutrons
  2. a single electron
  3. a single proton
  4. a high-energy gamma ray
Answer and explanation

2 protons and 2 neutrons

An alpha particle is a helium-4 nucleus: 2 protons and 2 neutrons (⁴₂He).

Problem 3

Nuclear reactions release far more energy per atom than chemical reactions. Briefly explain why, referring to what changes in each type of reaction.
Answer and explanation

Chemical reactions only rearrange electrons and chemical bonds, which involves relatively little energy. Nuclear reactions change the nucleus itself and convert a tiny amount of mass directly into energy through E = mc², releasing millions of times more energy per atom.

Problem 4

Find iron oxidation number in Fe₂O₃ with O=−2.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

3

2x−6=0, x=3.

Problem 5

Why balance electron counts between half-reactions?
Answer and explanation

An ordinary closed reaction cannot create or destroy net electric charge; transferred electrons must cancel.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 6

Independent chapter check: For Mg+2Ag⁺→Mg²⁺+2Ag, identify oxidation, reduction, both agents and the electron amount per mole of Mg reacted.
Answer and explanation

Mg goes 0→+2 and is oxidized/reducing agent. Ag⁺ goes +1→0 and is reduced/oxidizing agent. One mole Mg transfers two moles electrons, consumed by two moles Ag⁺ in the selected complete account.

Mg goes 0→+2 and is oxidized/reducing agent. Ag⁺ goes +1→0 and is reduced/oxidizing agent. One mole Mg transfers two moles electrons, consumed by two moles Ag⁺ in the selected complete account.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 7

Using F=96500, 9650 C corresponds to how many electron moles?
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

0.1 mol electrons

Q/F=.100.

Problem 8

Name the process always occurring at a cathode.
Answer and explanation

Reduction, whether the cell is galvanic or electrolytic.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 9

Independent chapter check: A selected Cu²⁺ deposition receives 0.0100 mol electrons with 100% target current efficiency. Find copper moles and mass using 63.55 g/mol. Explain why a measured lower mass need not violate charge conservation.
Answer and explanation

nCu=0.0100/2=0.00500 mol, giving 0.318 g. Competing reactions, incomplete recovery or measurement error can reduce observed target mass while total transferred charge is still accounted for elsewhere.

nCu=0.0100/2=0.00500 mol, giving 0.318 g. Competing reactions, incomplete recovery or measurement error can reduce observed target mass while total transferred charge is still accounted for elsewhere.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 10

Explain the following claim and supply a valid example or counterexample: Gamma emission changes excitation energy, not atomic number.
Answer and explanation

A=206,Z=82, corresponding to lead; the alpha accounts for four nucleons and charge+2. Gamma emission changes excitation energy, not atomic number.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 11

Independent chapter check: Balance the alpha-decay bookkeeping ²³⁸₉₂U → daughter + ⁴₂He. Give daughter A and Z, then explain why the integers alone do not provide released energy.
Answer and explanation

Daughter A=234 and Z=90, corresponding to thorium. Precise consistent masses or binding energies are needed for an energy calculation; mass-number conservation is not equality of exact rest masses.

Daughter A=234 and Z=90, corresponding to thorium. Precise consistent masses or binding energies are needed for an energy calculation; mass-number conservation is not equality of exact rest masses.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 12

After four half-lives, what percent parent remains?
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

6.25 %

100/16=6.25.

Problem 13

Why must an isotope-dating model specify whether the system was closed?
Answer and explanation

Parent or daughter movement can change their ratio independently of radioactive decay.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 14

Independent chapter check: A modeled isotope has t½=8.00 days. Comparable initial net signal is 800 counts/min. Later gross signal is 250 and matched background 50 counts/min. Find remaining fraction and modeled elapsed time, naming one assumption.
Answer and explanation

Later net signal=200, so fraction=200/800=0.250. That is two half-lives, giving 16.0 days. Assume stable geometry/efficiency and a net signal proportional to the same parent population with an appropriate closed account.

Later net signal=200, so fraction=200/800=0.250. That is two half-lives, giving 16.0 days. Assume stable geometry/efficiency and a net signal proportional to the same parent population with an appropriate closed account.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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