EmSAT EmSAT Chemistry 4 — Questions and Answers
Question 1: What does Le Chatelier's principle state?
- Reactions always go to completion
- A system at equilibrium will shift to counteract a stress applied to it (Correct answer)
- Equilibrium means equal concentrations of all species
- Temperature has no effect on equilibrium
Correct answer: A system at equilibrium will shift to counteract a stress applied to it
Le Chatelier's principle states that if a stress (change in concentration, temperature, or pressure) is applied to a system at equilibrium, the system will shift in the direction that partially counteracts that stress.
Question 2: What is the Kc expression for: N₂ + 3H₂ ⇌ 2NH₃?
- Kc = [NH₃]² / [N₂][H₂]³ (Correct answer)
- Kc = [N₂][H₂]³ / [NH₃]²
- Kc = [NH₃] / [N₂][H₂]
- Kc = [N₂][H₂] / [NH₃]
Correct answer: Kc = [NH₃]² / [N₂][H₂]³
Kc = [products]^coefficients / [reactants]^coefficients = [NH₃]² / ([N₂][H₂]³). Each concentration is raised to the power of its stoichiometric coefficient.
Question 3: What is an endothermic reaction?
- A reaction that releases heat
- A reaction that absorbs heat from the surroundings (Correct answer)
- A reaction with no energy change
- A reaction that produces light
Correct answer: A reaction that absorbs heat from the surroundings
An endothermic reaction absorbs heat energy from the surroundings. The products have higher energy than the reactants, and the enthalpy change (ΔH) is positive.
Question 4: What is enthalpy (H) in thermodynamics?
- The temperature of a reaction
- The heat content of a system at constant pressure (Correct answer)
- The rate of a reaction
- The equilibrium constant
Correct answer: The heat content of a system at constant pressure
Enthalpy is the heat content of a thermodynamic system at constant pressure. The change in enthalpy (ΔH) indicates whether a reaction releases (exothermic) or absorbs (endothermic) heat.
Question 5: What is Hess's Law?
- The total enthalpy of a reaction depends on the path taken
- The total enthalpy of a reaction is independent of the pathway (Correct answer)
- Enthalpy is always negative
- Reactions always produce energy
Correct answer: The total enthalpy of a reaction is independent of the pathway
Hess's Law states that the total enthalpy change for a reaction is the same regardless of whether it occurs in one step or multiple steps. Enthalpy is a state function.
Question 6: What is the standard enthalpy of combustion?
- Heat absorbed when 1 mole is formed from elements
- Heat released when 1 mole of substance completely burns in oxygen (Correct answer)
- Heat required to break all bonds
- Enthalpy change for a neutralization reaction
Correct answer: Heat released when 1 mole of substance completely burns in oxygen
Standard enthalpy of combustion (ΔHc°) is the enthalpy change when one mole of a substance completely combusts in excess oxygen under standard conditions. It is always exothermic (negative ΔH).
EmSAT EmSAT Chemistry 4
What does Le Chatelier's principle state?