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Quizzes > High School Quizzes > Science

Free Energy Practice Quiz

Ace your exam with concise energy exercises

Difficulty: Moderate
Grade: Grade 12
Study OutcomesCheat Sheet
Paper art representing a high school chemistry quiz on thermodynamics and Gibbs free energy.

What is the formula for Gibbs free energy?
Î"G = TÎ"S - Î"H
Î"G = Î"H/T - Î"S
Î"G = Î"H + TÎ"S
Î"G = Î"H - TÎ"S
The correct formula for Gibbs free energy is Î"G = Î"H - TÎ"S, which directly relates enthalpy change, temperature, and entropy change to predict spontaneity. This equation is foundational in thermodynamics.
What does a negative value of Î"G indicate?
Reaction is non-spontaneous
Reaction is endothermic
Reaction is at equilibrium
Reaction is spontaneous
A negative Gibbs free energy indicates that the reaction can proceed spontaneously under constant temperature and pressure. This property is used to determine the feasibility of reactions.
Which of the following defines an endothermic process?
Absorbs heat from the surroundings
Loses entropy to the surroundings
Reaches equilibrium without heat exchange
Releases heat to the surroundings
Endothermic processes absorb heat from the surroundings, resulting in a positive Î"H. This basic concept differentiates them from exothermic processes, which release heat.
In the Gibbs free energy equation, what does 'T' represent?
Tension
Time
Transformation
Temperature in Kelvin
In thermodynamics, 'T' represents the absolute temperature measured in Kelvin. This is critical because thermodynamic equations require absolute temperature to ensure accuracy.
If Î"H is positive and Î"S is positive, increasing temperature will likely make the process:
Always spontaneous regardless of temperature
Independent of spontaneity
More spontaneous
Less spontaneous
For reactions where both Î"H and Î"S are positive, the TÎ"S term becomes more significant at higher temperatures, making Î"G more negative and the reaction more spontaneous. Temperature plays a pivotal role in shifting the balance of the equation.
A reaction has Î"H = -50 kJ/mol and Î"S = -100 J/(mol·K). At what temperatures might the reaction become non-spontaneous?
Below 500 K
Above 500 K
At exactly 500 K
It remains spontaneous under all temperatures
By setting Î"G = 0, the threshold temperature is calculated as T = Î"H/Î"S after converting Î"H to joules. Since both Î"H and Î"S are negative, at temperatures above 500 K the TÎ"S term makes Î"G positive, rendering the reaction non-spontaneous.
Which statement best describes the role of entropy in determining reaction spontaneity?
Entropy has no effect on spontaneity
An increase in entropy (positive Î"S) favors spontaneity
A decrease in entropy only favors spontaneity at very high temperatures
A decrease in entropy (negative Î"S) favors spontaneity
Entropy is a measure of disorder, and an increase in entropy contributes to making Î"G more negative, thereby promoting spontaneity. This effect can counterbalance an unfavorable enthalpy change under the right conditions.
If Î"G = 0 for a reaction, what does this imply about the system?
The reaction is irreversible
The reaction is endergonic
The reaction is spontaneous
The system is at equilibrium
When Î"G equals zero, there is no net driving force for the reaction to proceed in either direction, indicating equilibrium. In this state, the forward and reverse reaction rates are equal.
Which factor does NOT directly influence the value of Gibbs free energy?
Entropy change
Enthalpy change
Temperature
Volume of the reaction vessel
Gibbs free energy is calculated using enthalpy, temperature, and entropy. The volume of the reaction vessel does not appear in the equation, making it the factor that does not directly influence Î"G.
For an exothermic reaction with a decrease in entropy, what temperature condition would favor spontaneity?
Lower temperatures
Higher temperatures
Temperature has no effect
Both extremes in temperature
In exothermic reactions where Î"S is negative, lowering the temperature minimizes the unfavorable TÎ"S term, resulting in a more negative Î"G. This enhances the spontaneity of the reaction under cooler conditions.
How does an increase in temperature affect the spontaneity of a reaction where both Î"H and Î"S are positive?
It reverses the spontaneity
It has no effect on spontaneity
It makes the reaction less spontaneous
It makes the reaction more spontaneous
For reactions with both positive Î"H and Î"S, a higher temperature increases the influence of the TÎ"S term, making Î"G more negative. This shift results in a more spontaneous reaction at elevated temperatures.
What is the relationship between the equilibrium constant and Gibbs free energy?
Î"G° = K/RT
Î"G° = RT ln K
Î"G° = -K/(RT)
Î"G° = -RT ln K
The standard Gibbs free energy change is directly related to the equilibrium constant by the equation Î"G° = -RT ln K. This relationship connects thermodynamic favorability with the extent of reaction at equilibrium.
A reaction is observed to be spontaneous at high temperatures but not at low temperatures. What does this indicate about Î"H and Î"S?
Î"H > 0 and Î"S < 0
Î"H < 0 and Î"S > 0
Î"H < 0 and Î"S < 0
Î"H > 0 and Î"S > 0
When both Î"H and Î"S are positive, the TÎ"S term must overcome the positive Î"H to yield a negative Î"G. This scenario typically results in a reaction that becomes spontaneous only at higher temperatures.
Which change in conditions is expected to make an exothermic reaction with a decrease in entropy more spontaneous?
Adding a catalyst
Increasing the pressure
Lowering the temperature
Raising the temperature
For an exothermic reaction with a negative entropy change, lowering the temperature reduces the detrimental TÎ"S term, thereby making Î"G more negative. This adjustment favors the spontaneity of the reaction.
The spontaneity of a chemical reaction is determined by which of the following?
The sign of Î"G
The reaction rate
The activation energy
The magnitude of Î"H
It is the sign of the Gibbs free energy (Î"G) that determines the spontaneity of a reaction. A negative Î"G indicates that a reaction is thermodynamically favorable, regardless of the reaction rate or activation barrier.
Which of the following equations correctly relates the standard Gibbs free energy change to the equilibrium constant?
Î"G° = -RT ln K
Î"G° = -R ln K/T
Î"G° = K/RT
Î"G° = RT ln K
The equation Î"G° = -RT ln K is fundamental in thermodynamics as it connects the standard free energy change with the equilibrium constant. This relation allows prediction of reaction equilibria based on thermodynamic data.
A reaction has Î"H = +20 kJ/mol and Î"S = +100 J/(mol·K). At 250 K, what is the sign of Î"G?
Cannot be determined
Negative
Zero
Positive
Converting Î"H to joules gives 20000 J/mol, and calculating Î"G = Î"H - TÎ"S yields 20000 - (250 Ã - 100) = -5000 J/mol. A negative Î"G indicates that the reaction is spontaneous at 250 K.
In a system where both pressure and temperature vary, which of the following statements best describes the use of Gibbs free energy?
It always decreases regardless of external conditions
It can be used to predict reaction rates
It predicts the direction of chemical reactions under constant temperature and pressure
It is independent of pressure
Gibbs free energy is defined for processes occurring under constant temperature and pressure, making it a valuable tool for predicting the spontaneity of reactions. Its application is limited to these conditions, which are common in many chemical processes.
How does the concept of Gibbs free energy help in understanding the performance of fuel cells?
It quantifies the maximum useful work obtainable from a fuel cell under constant conditions
It determines the rate at which fuel is consumed
It describes only the heat transfer in the process
It measures the total energy released during combustion
Gibbs free energy helps quantify the maximum amount of work that can be extracted from a fuel cell operating under constant temperature and pressure. This concept is central to evaluating the efficiency and effectiveness of energy conversion in fuel cells.
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Study Outcomes

  1. Analyze the components of Gibbs free energy and their impact on reaction spontaneity.
  2. Calculate free energy changes using standard thermodynamic equations.
  3. Interpret the relationship between enthalpy, entropy, and Gibbs free energy in chemical reactions.
  4. Apply thermodynamic principles to predict the feasibility of chemical processes.
  5. Evaluate experimental data to assess reaction spontaneity and equilibrium conditions.

Energy Practice Problems Cheat Sheet

  1. Gibbs free energy and spontaneity - Gibbs free energy (ΔG) is like the chemical world's YES/NO meter for reactions. If ΔG drops below zero, the reaction eagerly moves forward; if it's above zero, it simply won't. At the perfect balance point called equilibrium, ΔG sits right at zero, meaning forward and reverse rates are best buddies. Pearson General Chemistry
  2. Master the ΔG equation - The magic formula ΔG = ΔH - TΔS connects enthalpy (ΔH), temperature (T), and entropy (ΔS) to predict if a reaction has a green light. Enthalpy tells you if heat is absorbed or released, while entropy shows the disorder dance of molecules. By plugging in the numbers, you can forecast whether the reaction will happen spontaneously. Pearson Analytical Chemistry
  3. Temperature's twist - Temperature isn't just about feeling hot or cold; it can flip a reaction's spontaneity switch. When ΔH and ΔS are both positive, cranking up the heat makes chaos (entropy) win and the reaction go. If both are negative, cooling things down keeps the order-loving reaction happiest. Pearson General Chemistry
  4. Linking ΔG and equilibrium constant (K) - The equation ΔG° = -RT ln K bridges spontaneity to the equilibrium constant. A big K (greater than 1) means ln K is positive, so ΔG° is negative and spontaneity is on your side. It's like translating chemical talk into a number that shouts "Go for it!" Unacademy JEE Chemistry
  5. Calculate ΔG° from formation data - Grab standard ΔG_f° values for products and reactants and use ΔG° = ΣΔG_f°(products) − ΣΔG_f°(reactants) to see if a reaction is rowdy or reluctant. This systematic approach gives you a step-by-step map to predict spontaneity under standard conditions. Save My Exams
  6. Zero for elemental formation - For pure elements in their natural state, ΔG_f° is pegged at zero to keep calculations breezy. This zeroing hack means you only focus on compounds and makes your life way easier when balancing those Gibbs free energy books. Wikipedia
  7. Hit the practice problems - Flex your brain muscles by tackling real-world Gibbs free energy problems, from simple calculations to equilibrium fun‑fests. Regular drills turn confusion into confidence and help you spot patterns faster than you can say "enthalpy." Pearson Exam Prep
  8. Maximum work potential - ΔG isn't just about spontaneity; it tells you the maximum non‑expansion work a system can do at constant temperature and pressure. Think of it as the ultimate energy budget that a reaction has up its sleeve to power gadgets or fuel cells. Pearson Analytical Chemistry
  9. Standard condition clarity - ΔG° values assume the golden rules of 1 atm pressure, 1 M concentration, and a cozy 25 °C (298 K). Anytime conditions stray, expect ΔG to shift and the spontaneity drama to change. Save My Exams
  10. Mnemonic magic - Use catchy mnemonics like "Goldfish Are Horrible Without Tartar Sauce" (G = H - TS) to lock in the relationship between Gibbs free energy, enthalpy, temperature, and entropy. A fun phrase goes a long way when exam day nerves strike! Pearson General Chemistry
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