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

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

Review the key ideas

Oxidation Numbers & Redox Equations

Redox equations conserve atoms, charge, and electron transfer.

Electrochemical Cells & Electrolysis

Oxidation is always at the anode; electrical charge links current to reaction amount.

Nuclear Chemistry

Nuclear transformations change composition or nuclear energy state. Balance nucleon/charge bookkeeping, then use consistent precise masses for energy. Half-life predicts parent-population expectations; radiation quantities and detector responses require distinct accounts.

Half-Life Models & Nuclear Equations

Half-life describes exponential parent reduction; nuclear equations conserve number and charge bookkeeping.

Problem 1

A 64 g radioactive sample has a half-life of 3 days. How many grams remain after 6 days?
Answer and explanation

16 g

6 ÷ 3 = 2 half-lives, so the sample halves twice: 64 → 32 → 16 g (or 64 ÷ 2² = 16 g).

Problem 2

Polonium-210 (²¹⁰₈₄Po) undergoes alpha decay. Give the mass number and atomic number of the new nucleus that forms.
Answer and explanation

Mass number 206 and atomic number 82 (lead-206)

An alpha particle (⁴₂He) carries away 2 protons and 2 neutrons. Mass number: 210 − 4 = 206. Atomic number: 84 − 2 = 82, which is lead. The product is lead-206, ²⁰⁶₈₂Pb.

Problem 3

A radioactive isotope decays until only 1/16 of the original sample remains after 20 years. What is the isotope's half-life?
Answer and explanation

5 years

1/16 = (1/2)⁴, so 4 half-lives have passed. If 4 half-lives take 20 years, then one half-life is 20 ÷ 4 = 5 years.

Problem 4

Does H⁺+OH⁻→H₂O involve redox?
Answer and explanation

No; H remains +1 and O remains −2 in this reaction.

Response criteria

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

Problem 5

How many electrons are lost when 2 Mg atoms become 2 Mg²⁺ ions?
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

4 electrons

Each loses 2, total 4.

Problem 6

Chapter review: 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

Cathode reduction potential .80 V, anode reduction potential −.40 V. Find E°cell.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

1.2 V

.80−(−.40)=1.20.

Problem 8

Why not multiply voltage by a doubled coefficient?
Answer and explanation

Voltage is energy per charge; doubling the reaction doubles both energy and charge.

Response criteria

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

Problem 9

Chapter review: 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

An A=210,Z=84 nucleus emits alpha. Give daughter numbers.
Answer and explanation

A=206,Z=82, corresponding to lead; the alpha accounts for four nucleons and charge+2.

Response criteria

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

Problem 11

Chapter review: 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

Only 1/8 remains after 18 years. Find half-life.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

6 years

1/8 represents 3 half-lives;18/3=6.

Problem 13

What changes in beta-minus decay?
Answer and explanation

A neutron becomes a proton, an electron and an antineutrino are emitted; Z increases 1 and A stays the same.

Response criteria

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

Problem 14

Chapter review: 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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