Chem 156 Exam 4 – Step-by-Step Study Guidance

Q1. Which of the following represents an alpha particle?

Background

Topic: Nuclear Chemistry – Types of Radiation

This question tests your understanding of the different types of particles emitted during radioactive decay, specifically the identification of an alpha particle.

Key Terms and Formulas:

• Alpha particle: A type of nuclear radiation consisting of 2 protons and 2 neutrons (essentially a helium nucleus).

• Beta particle (β-): An electron emitted from the nucleus.

• Positron (β+): A particle with the same mass as an electron but a positive charge.

• Gamma ray (γ): High-energy electromagnetic radiation with no mass or charge.

Step-by-Step Guidance

1. Recall the notation for an alpha particle: it is represented as $^4_2\mathrm{He}$, indicating 2 protons and 2 neutrons.

2. Compare this to the other options: $^0_{-1}e$ (beta particle), $^0_{+1}e$ (positron), and $^0_0\gamma$ (gamma ray).

3. Identify which symbol matches the definition of an alpha particle.

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Q2. Which of these isotopes would be the least stable?

Background

Topic: Nuclear Stability and Isotopes

This question tests your ability to assess nuclear stability based on the neutron-to-proton ratio and the location of the isotope on the periodic table.

Key Terms and Concepts:

• Isotope: Atoms of the same element with different numbers of neutrons.

• Nuclear stability: Generally, isotopes with a balanced neutron-to-proton ratio are more stable. Very heavy nuclei or those far from the band of stability are less stable.

Step-by-Step Guidance

1. Write out the number of protons and neutrons for each isotope.

2. Calculate the neutron-to-proton ratio for each.

3. Recall that very heavy elements (high atomic number) tend to be less stable.

4. Compare the ratios and atomic numbers to determine which is likely the least stable.

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Q3. What is the electron configuration of Cr+?

Background

Topic: Electron Configuration and Transition Metals

This question tests your understanding of how to write electron configurations for transition metal ions, especially those with exceptions like chromium.

Key Terms and Formulas:

• Electron configuration: The arrangement of electrons in an atom or ion.

• Cr (Z=24): Neutral chromium has a special configuration due to stability of half-filled d subshells.

• When forming cations, electrons are removed first from the 4s orbital, then from 3d.

Step-by-Step Guidance

1. Write the ground-state electron configuration for neutral Cr: $[Ar] 4s^1 3d^5$ (due to the stability of half-filled d orbitals).

2. Remove one electron to form Cr+. Electrons are removed from the 4s orbital first.

3. Write the resulting configuration for Cr+.

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Q4. Which of the following would be expected to release energy via nuclear fission?

Background

Topic: Nuclear Fission and Stability

This question tests your understanding of which nuclei are likely to undergo fission and release energy, based on their size and stability.

Key Terms and Concepts:

• Nuclear fission: The splitting of a heavy nucleus into lighter nuclei, releasing energy.

• Typically, very heavy nuclei (like uranium or thorium) are candidates for fission.

Step-by-Step Guidance

1. Identify which of the given isotopes are heavy nuclei (high atomic number and mass number).

2. Recall that fission is more likely for nuclei heavier than iron (Fe-56).

3. Determine which option fits this criterion.

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Q5. Which of the following ligands is expected to result in the largest crystal field splitting energy?

Background

Topic: Crystal Field Theory and Ligand Field Strength

This question tests your knowledge of the spectrochemical series and how different ligands affect the crystal field splitting energy ($\Delta$).

Key Terms and Concepts:

• Crystal field splitting energy ($\Delta$): The energy difference between d-orbitals in a complex ion due to the presence of ligands.

• Spectrochemical series: An empirical list of ligands ordered by their ability to split d-orbital energies.

Step-by-Step Guidance

1. Recall the order of ligands in the spectrochemical series (provided in your exam info).

2. Identify which ligand among the options is highest in the series (strongest field ligand).

3. The ligand highest in the series will cause the largest $\Delta$.

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Q6. A single electron capture transforms $^{40}_{19}K$ into what nuclide?

Background

Topic: Nuclear Chemistry – Electron Capture

This question tests your understanding of electron capture and how it changes the atomic and mass numbers of a nuclide.

Key Terms and Concepts:

• Electron capture: A process where the nucleus captures an inner electron, converting a proton into a neutron.

• Atomic number decreases by 1; mass number remains unchanged.

Step-by-Step Guidance

1. Write the nuclear equation for electron capture: $^{40}_{19}K + ^0_{-1}e \rightarrow$ ?

2. Decrease the atomic number by 1 (from 19 to 18), mass number stays at 40.

3. Identify the element with atomic number 18.

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Q7. A solution of an unknown metal complex appears blue. What color light is it absorbing?

Background

Topic: Coordination Chemistry – Color and Absorption

This question tests your understanding of complementary colors and how the observed color of a solution relates to the color of light absorbed.

Key Terms and Concepts:

• Complementary colors: The color observed is the complement of the color absorbed.

• Refer to the color wheel or the wavelength chart provided.

Step-by-Step Guidance

1. Recall that a blue solution absorbs light of the complementary color.

2. Use the color wheel or chart to determine which color is complementary to blue.

3. Identify the corresponding color from the options.

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Q8. What is the coordination number for the complex ion [Co(en)3]2+?

Background

Topic: Coordination Chemistry – Coordination Number

This question tests your ability to determine the coordination number of a complex ion, considering the denticity of the ligand.

Key Terms and Concepts:

• Coordination number: The number of ligand donor atoms directly bonded to the central metal ion.

• en (ethylenediamine): A bidentate ligand (binds through two donor atoms).

Step-by-Step Guidance

1. Identify the number of en ligands in the complex (3).

2. Recall that each en ligand is bidentate (binds through 2 sites).

3. Multiply the number of ligands by their denticity to find the coordination number.

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Q9. What is the charge on iron in the complex ion [Fe(en)2(CN)2]F?

Background

Topic: Coordination Chemistry – Oxidation States

This question tests your ability to assign oxidation states to the central metal ion in a coordination complex.

Key Terms and Concepts:

• Oxidation state: The hypothetical charge an atom would have if all bonds were ionic.

• en is a neutral ligand; CN- is a -1 ligand; F- is the counterion.

Step-by-Step Guidance

1. Let x be the charge on Fe.

2. Sum the charges of all ligands and the metal to equal the charge on the complex ion.

3. Set up the equation: $x + (2 \times 0) + (2 \times -1) = +1$ (since F- is the counterion for a +1 complex).

4. Solve for x.

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Q10. The diethylenetriamine is shown below. What type of coordination would you expect for this ligand?

Background

Topic: Coordination Chemistry – Ligand Denticity

This question tests your understanding of how many donor atoms a ligand can use to bind to a metal ion (its denticity).

Key Terms and Concepts:

• Denticity: The number of donor atoms a ligand uses to attach to a central metal ion.

• Diethylenetriamine ("dien") has three nitrogen atoms that can coordinate.

Step-by-Step Guidance

1. Count the number of donor atoms (nitrogens) in diethylenetriamine.

2. Recall the definitions: monodentate (1), bidentate (2), tridentate (3), etc.

3. Assign the correct denticity based on the number of donor atoms.

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Q11. Select the particle that properly completes the following nuclear chemistry reaction: $^{244}_{94}Pu \rightarrow ^{240}_{92}U +$ ?

Background

Topic: Nuclear Chemistry – Balancing Nuclear Equations

This question tests your ability to balance nuclear equations by conserving mass and atomic numbers.

Key Terms and Concepts:

• Conservation of mass number (A) and atomic number (Z) in nuclear reactions.

• Common particles: alpha ($^4_2He$), beta ($^0_{-1}e$), positron ($^0_{+1}e$), gamma ($^0_0\gamma$).

Step-by-Step Guidance

1. Write the equation: $^{244}_{94}Pu \rightarrow ^{240}_{92}U + x$.

2. Set up equations for conservation of mass number and atomic number.

3. Solve for the missing particle's mass and atomic numbers.

4. Match the result to the options provided.

Try solving on your own before revealing the answer!

Q12. Which of the following complex ions could possibly have an octahedral geometry?

Background

Topic: Coordination Chemistry – Molecular Geometry

This question tests your knowledge of the typical coordination numbers and geometries for complex ions.

Key Terms and Concepts:

• Octahedral geometry: Characteristic of complexes with a coordination number of 6.

• Count the number of ligands around the central metal ion in each option.

Step-by-Step Guidance

1. For each complex, count the number of ligands attached to the central metal ion.

2. Recall that octahedral geometry requires 6 ligands.

3. Identify which complex fits this criterion.

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Q13. What is the proper formula for potassium triaquatrichlororhodate(II)?

Background

Topic: Coordination Chemistry – Nomenclature and Formulas

This question tests your ability to write the correct formula for a coordination compound based on its name.

Key Terms and Concepts:

• "Triaqua" = 3 H2O ligands; "trichloro" = 3 Cl- ligands; "rhodate(II)" = Rh with a +2 oxidation state.

• Potassium is the counterion.

Step-by-Step Guidance

1. Write the formula for the complex ion: [Rh(H2O)3Cl3].

2. Determine the overall charge on the complex ion.

3. Balance the charge with the appropriate number of potassium ions.

4. Match your formula to the options provided.

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Q14. What is the correct formula for hexane?

Background

Topic: Organic Chemistry – Alkanes

This question tests your knowledge of the general formula for alkanes and the naming conventions for hydrocarbons.

Key Terms and Concepts:

• Alkane: Saturated hydrocarbon with the general formula CnH2n+2.

• Hexane: "Hex-" indicates 6 carbons.

Step-by-Step Guidance

1. Apply the general formula for alkanes: CnH2n+2.

2. Plug in n = 6 for hexane.

3. Write the resulting molecular formula and compare to the options.

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Q15. What is the name of the following molecule?

Background

Topic: Organic Chemistry – Nomenclature

This question tests your ability to apply IUPAC naming rules to branched hydrocarbons with rings and chains.

Key Terms and Concepts:

• Identify the parent chain and the substituent group.

• Apply the rules for naming cycloalkanes and alkyl substituents.

Step-by-Step Guidance

1. Identify the longest carbon chain and the cycloalkane ring.

2. Determine which group is the parent and which is the substituent.

3. Apply IUPAC rules to assign the correct name.

Try solving on your own before revealing the answer!

Q16. Which of the following would likely be a low spin complex?

Background

Topic: Crystal Field Theory – High Spin vs. Low Spin Complexes

This question tests your understanding of how ligand field strength affects electron configuration in transition metal complexes.

Key Terms and Concepts:

• Low spin complex: Occurs with strong field ligands, leading to paired electrons in lower energy orbitals.

• Refer to the spectrochemical series to identify strong field ligands.

Step-by-Step Guidance

1. Identify the ligands in each complex and their position in the spectrochemical series.

2. Recall that strong field ligands (like CO, CN-) favor low spin configurations.

3. Determine which complex contains a strong field ligand and is likely to be low spin.

Try solving on your own before revealing the answer!

Q17. The half-life of $^{14}C$ is 5370 years. The carbon in an unearthed fossil was determined to have a radioactivity of $3.97 \times 10^6$ Bq. If a fresh sample of the same mass has an activity of $1.58 \times 10^7$ Bq, how old is the sample?

Background

Topic: Nuclear Chemistry – Radioactive Decay and Half-Life Calculations

This question tests your ability to use the radioactive decay law to determine the age of a sample based on its activity.

Key Terms and Formulas:

• Radioactive decay law: $A = A_0 e^{-kt}$, where A is the current activity, $A_0$ is the initial activity, k is the decay constant, and t is time.

• Half-life formula: $t_{1/2} = \frac{\ln 2}{k}$

Step-by-Step Guidance

1. Calculate the decay constant k using the half-life formula: $k = \frac{\ln 2}{t_{1/2}}$.

2. Set up the decay equation: $A = A_0 e^{-kt}$, where $A_0$ is the activity of the fresh sample and A is the activity of the fossil.

3. Take the natural logarithm of both sides to solve for t: $\ln(A/A_0) = -kt$.

4. Rearrange to solve for t, but stop before plugging in the final values.

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Q18. Which of the following would be expected to be diamagnetic?

Background

Topic: Coordination Chemistry – Magnetism

This question tests your understanding of electron configurations and how they relate to magnetic properties (paramagnetic vs. diamagnetic).

Key Terms and Concepts:

• Diamagnetic: All electrons are paired; no unpaired electrons.

• Paramagnetic: One or more unpaired electrons.

• Determine the electron configuration for each complex and count unpaired electrons.

Step-by-Step Guidance

1. Determine the oxidation state and d-electron count for the metal in each complex.

2. Consider the ligand field strength (high spin vs. low spin) for each complex.

3. Assign electrons to the d-orbitals and check for unpaired electrons.

4. Identify which complex has all electrons paired (diamagnetic).

Try solving on your own before revealing the answer!

Q19. What is the name of the following molecule (using the rules we learned in class)?

Background

Topic: Organic Chemistry – Nomenclature of Alkenes and Alkadienes

This question tests your ability to apply IUPAC rules to name molecules with multiple double bonds and substituents.

Key Terms and Concepts:

• Identify the longest chain containing both double bonds.

• Number the chain to give the double bonds the lowest possible numbers.

• Name and number substituents appropriately.

Step-by-Step Guidance

1. Identify the parent chain and the positions of the double bonds.

2. Number the chain to give the double bonds the lowest possible locants.

3. Identify and number the substituents.

4. Assemble the name according to IUPAC rules.

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Q20. Calculate the energy released if 1.50 g of $^{235}_{92}U$ undergoes fission according to the following equation:

$^{235}_{92}U + ^1_0n \rightarrow ^{141}_{56}Ba + ^{92}_{36}Kr + 3 ^1_0n$

Background

Topic: Nuclear Chemistry – Mass Defect and Energy Release

This question tests your ability to calculate the energy released in a nuclear reaction using mass defect and Einstein's equation.

Key Terms and Formulas:

• Mass defect ($\Delta m$): The difference in mass between reactants and products.

• Einstein's equation: $\Delta E = \Delta m c^2$

• 1 u = 931.5 MeV; $c = 3.00 \times 10^8$ m/s

Step-by-Step Guidance

1. Calculate the total mass of reactants and products using the given masses.

2. Find the mass defect: $\Delta m = \text{mass of reactants} - \text{mass of products}$.

3. Convert the mass defect from grams to kilograms (if needed).

4. Use $\Delta E = \Delta m c^2$ to find the energy released per fission event.

5. Calculate the number of moles of $^{235}U$ in 1.50 g, then the total number of fission events.

6. Multiply the energy per event by the total number of events to get the total energy released, but stop before the final multiplication.

Try solving on your own before revealing the answer!