OAT General Chemistry Principles Questions and Answers — Questions and Answers
Question 1: A spontaneous chemical reaction is observed to have a positive enthalpy change (ΔH > 0) and a positive entropy change (ΔS > 0). Which of the following statements must be true for this process to be spontaneous?
- The reaction is spontaneous only at low temperatures.
- The reaction is spontaneous at all temperatures.
- The temperature is high enough for the TΔS term to outweigh the ΔH term. (Correct answer)
- The reaction is non-spontaneous at all temperatures.
Correct answer: The temperature is high enough for the TΔS term to outweigh the ΔH term.
The spontaneity of a reaction is determined by the Gibbs free energy change (ΔG = ΔH - TΔS). For a reaction to be spontaneous, ΔG must be negative. Given that both ΔH and ΔS are positive, the term -TΔS will be negative. For ΔG to be negative, the magnitude of TΔS must be greater than the magnitude of ΔH. This condition is met only at sufficiently high temperatures.
Question 2: Which of the following correctly describes the effect of adding a catalyst to a chemical reaction that has reached equilibrium?
- It increases the concentration of the products.
- It increases the rate of both the forward and reverse reactions equally. (Correct answer)
- It shifts the equilibrium position to the right.
- It decreases the enthalpy change (ΔH) of the reaction.
Correct answer: It increases the rate of both the forward and reverse reactions equally.
A catalyst works by providing an alternative reaction pathway with a lower activation energy. This increases the rate of both the forward and reverse reactions to the same extent. While the system reaches equilibrium faster, the catalyst does not change the concentrations of reactants and products at equilibrium, nor does it affect the overall thermodynamics (like ΔH) of the reaction.
Question 3: A researcher has a 0.1 M solution of a weak acid, acetic acid (CH₃COOH), with a Ka of 1.8 x 10⁻⁵. Which of the following species will be present at the lowest concentration in the solution?
- CH₃COOH
- H₃O⁺
- OH⁻ (Correct answer)
- CH₃COO⁻
Correct answer: OH⁻
In a solution of a weak acid, the undissociated acid (CH₃COOH) is the most abundant species. The acid dissociates to form H₃O⁺ and its conjugate base, CH₃COO⁻, in equal but small amounts. The concentration of OH⁻ ions comes from the autoionization of water and is significantly suppressed by the H₃O⁺ from the acid, making it the species with the lowest concentration.
Question 4: According to the Ideal Gas Law, if the volume and the number of moles of a gas are held constant, what is the relationship between its pressure and absolute temperature?
- They are inversely proportional.
- They are directly proportional. (Correct answer)
- They have a logarithmic relationship.
- There is no relationship between them.
Correct answer: They are directly proportional.
The Ideal Gas Law is PV=nRT. If volume (V) and moles (n) are constant, the equation can be rearranged to P/T = nR/V. Since n, R, and V are all constants, P/T is a constant. This shows a direct proportionality between pressure (P) and absolute temperature (T). As temperature increases, pressure increases, which is also known as Gay-Lussac's Law.
Question 5: In the context of quantum mechanics, which quantum number is the primary determinant of the shape of an atomic orbital (e.g., s, p, d, f)?
- Magnetic quantum number (m_l)
- Principal quantum number (n)
- Spin quantum number (m_s)
- Azimuthal quantum number (l) (Correct answer)
Correct answer: Azimuthal quantum number (l)
The azimuthal (or angular momentum) quantum number, l, determines the shape of an atomic orbital. l = 0 corresponds to an s orbital (spherical), l = 1 corresponds to a p orbital (dumbbell-shaped), l = 2 corresponds to a d orbital (more complex shapes), and so on. The principal quantum number (n) determines the energy level and size, the magnetic quantum number (m_l) determines the orbital's orientation in space, and the spin quantum number (m_s) describes the electron's intrinsic angular momentum.
Question 6: A scientist is performing an electroplating experiment which involves the following half-reaction: Cu²⁺(aq) + 2e⁻ → Cu(s). Which of the following statements accurately describes this process?
- Copper is being oxidized at the anode.
- This is an oxidation half-reaction.
- Copper is being reduced at the cathode. (Correct answer)
- This reaction occurs at the anode of a galvanic cell.
Correct answer: Copper is being reduced at the cathode.
The half-reaction shows Cu²⁺ gaining two electrons (2e⁻) to become solid copper (Cu). The gain of electrons is defined as reduction. In electrochemical cells, reduction always occurs at the cathode. Therefore, copper is being reduced at the cathode.
OAT General Chemistry Principles Questions and Answers
A spontaneous chemical reaction is observed to have a positive enthalpy change (ΔH > 0) and a positive entropy change (ΔS > 0).
Which of the following statements must be true for this process to be spontaneous?