Unit 6 Practice: Easy
Thermodynamics · 20 questions. Try each question before revealing the answer and worked explanation.
Question 1
What is the system in thermodynamics?
Show answer and explanation
Answer: The part of the universe being studied.
Explanation: The system might be a chemical reaction, a gas in a container, or a substance being heated.
Everything outside the system is called the surroundings.
Question 2
What are the surroundings?
Show answer and explanation
Answer: Everything outside the system.
Explanation: Energy can be transferred between the system and the surroundings.
For example, in a coffee-cup calorimeter, the reacting chemicals may be the system while the solution and calorimeter act as the surroundings.
Question 3
What is heat?
Show answer and explanation
Answer: Energy transferred because of a temperature difference.
Explanation: Heat naturally flows from a hotter object to a colder object until thermal equilibrium is reached.
Question 4
What does temperature measure at the particle level?
Show answer and explanation
Answer: Average kinetic energy of particles.
Explanation: Higher temperature means particles have greater average kinetic energy.
Temperature is not the same thing as total thermal energy because the amount of substance also matters.
Question 5
Write the first-law equation for internal energy.
Show answer and explanation
Answer:
ΔE = q + w
Explanation: Here:
ΔE = change in internal energy
q = heat transferred
w = work done on the system
Energy is conserved.
Question 6
What sign does q have when the system absorbs heat?
Show answer and explanation
Answer: Positive
Explanation: If heat enters the system:
q > 0
This corresponds to an endothermic process.
Question 7
What sign does q have when the system releases heat?
Show answer and explanation
Answer: Negative
Explanation: If heat leaves the system:
q < 0
This corresponds to an exothermic process.
Question 8
What sign does ΔH have for an endothermic reaction?
Show answer and explanation
Answer: Positive
Explanation: An endothermic system absorbs energy from the surroundings.
Therefore:
ΔH > 0
Question 9
What sign does ΔH have for an exothermic reaction?
Show answer and explanation
Answer: Negative
Explanation: An exothermic system releases energy to the surroundings.
Therefore:
ΔH < 0
Question 10
Which is endothermic: melting or freezing?
Show answer and explanation
Answer: Melting
Explanation: Energy must be absorbed to overcome some of the attractive forces holding particles in the solid structure.
Question 11
Which is exothermic: vaporization or condensation?
Show answer and explanation
Answer: Condensation
Explanation: When gas particles come together to form a liquid, intermolecular attractions form and energy is released.
Question 12
Write the calorimetry equation used when temperature changes but no phase change occurs.
Show answer and explanation
Answer:
q = mcΔT
Explanation: Where:
m = mass
c = specific heat
ΔT = Tf − Ti
Question 13
Write the equation for temperature change.
Show answer and explanation
Answer:
ΔT = Tf − Ti
Explanation: Always subtract the initial temperature from the final temperature.
Question 14
What is the specific heat of liquid water approximately?
Show answer and explanation
Answer: 4.18 J/g°C
Explanation: This value means it takes about 4.18 J of energy to raise 1 g of water by 1°C.
Question 15
What principle is used in an insulated calorimetry experiment?
Show answer and explanation
Answer:
qsystem + qsurroundings = 0
Explanation: Energy lost by one part is gained by another.
A common simplified form is:
qhot = −qcold
Question 16
What equation is used to calculate energy during a phase change?
Show answer and explanation
Answer:
q = nΔHphase
Explanation: For example:
q = nΔHvap
for vaporization.
Question 17
State Hess’s law.
Show answer and explanation
Answer: The enthalpy change of an overall reaction equals the sum of the enthalpy changes of the steps used to produce it.
Explanation: Enthalpy is a state function, meaning it depends only on the initial and final states.
Question 18
What is the standard enthalpy of formation of O₂(g) in its standard state?
Show answer and explanation
Answer: 0 kJ/mol
Explanation: An element in its standard state has:
ΔH°f = 0
Question 19
Does breaking a chemical bond require or release energy?
Show answer and explanation
Answer: It requires energy.
Explanation: Attractive forces between bonded atoms must be overcome.
Therefore bond breaking is endothermic.
Question 20
Write the bond-enthalpy equation used to estimate reaction enthalpy.
Show answer and explanation
Answer:
ΔH ≈ Σ(bonds broken) − Σ(bonds formed)
Explanation: Breaking bonds requires energy.
Forming bonds releases energy.