IB Chemistry 3 — Questions and Answers
Question 1: Which of the following best describes a buffer solution?
- A solution with pH = 7
- A solution that resists changes in pH upon addition of small amounts of acid or base (Correct answer)
- A concentrated acid solution
- A solution containing only water
Correct answer: A solution that resists changes in pH upon addition of small amounts of acid or base
A buffer resists pH changes because it contains a weak acid and its conjugate base (or weak base and conjugate acid) that neutralize added acid or base.
Question 2: What is the hybridization of the carbon atoms in ethene (C₂H₄)?
- sp
- sp² (Correct answer)
- sp³
- sp³d
Correct answer: sp²
Each carbon in ethene is sp² hybridized, forming three sigma bonds and one pi bond with the double bond.
Question 3: Which type of radioactive decay results in the atomic number increasing by 1?
- Alpha decay
- Beta-minus decay (Correct answer)
- Gamma decay
- Positron emission
Correct answer: Beta-minus decay
In beta-minus decay, a neutron converts to a proton, emitting an electron, which increases the atomic number by 1.
Question 4: The rate of a first-order reaction depends on:
- The square of the concentration of reactant
- The concentration of reactant raised to the first power (Correct answer)
- The product of two reactant concentrations
- Temperature only
Correct answer: The concentration of reactant raised to the first power
For a first-order reaction, the rate = k[A]¹, meaning the rate is directly proportional to the concentration of the reactant.
Question 5: Which colligative property explains why adding salt lowers the freezing point of water?
- Vapor pressure elevation
- Osmotic pressure
- Boiling point depression
- Freezing point depression (Correct answer)
Correct answer: Freezing point depression
Dissolving a solute in water disrupts the crystalline structure formation, requiring a lower temperature to freeze — this is freezing point depression.
Question 6: In the VSEPR model, which molecular geometry corresponds to a molecule with 4 bonding pairs and 0 lone pairs?
- Trigonal planar
- Bent
- Tetrahedral (Correct answer)
- Linear
Correct answer: Tetrahedral
Four bonding pairs and no lone pairs around a central atom give a tetrahedral molecular geometry with bond angles of ~109.5°.
Question 7: Which of the following is an example of a nucleophilic substitution reaction?
- Combustion of methane
- Reaction of CH₃Br with OH⁻ to form CH₃OH (Correct answer)
- Polymerization of ethene
- Thermal cracking of alkanes
Correct answer: Reaction of CH₃Br with OH⁻ to form CH₃OH
The reaction of CH₃Br with OH⁻ is an SN2 nucleophilic substitution where the hydroxide ion replaces the bromide.
IB Chemistry 3
Which of the following best describes a buffer solution?