Thermodynamics Fundamentals Quiz

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Questions and Answers

Which of the following is true about reactions with positive ΔH and negative ΔS?

They are spontaneous at all temperatures.

They are spontaneous only at high temperatures.

They are spontaneous only at low temperatures.

They are non-spontaneous. (correct)

How can you calculate reaction enthalpy using bond enthalpies?

ΔHreaction = Σ(bond energies of reactants) + Σ(bond energies of products)

ΔHreaction = Σ(bond energies of products) + Σ(bond energies of reactants)

ΔHreaction = Σ(bond energies of reactants) - Σ(bond energies of products) (correct)

ΔHreaction = Σ(bond energies of products) - Σ(bond energies of reactants)

What is the unit of measurement for entropy?

J K mol

J K-1 mol-1 (correct)

K J mol-1

K J mol

At what temperature does ΔG equal 0 given ΔH = 40.63 kJ mol-1 and ΔS = 108.8 J K-1 mol-1?

373.4 K Signup and view all the answers

What does the standard enthalpy of combustion measure?

Enthalpy change during complete burning of one mole of substance in oxygen. Signup and view all the answers

Which of the following describes the enthalpy of atomization?

Enthalpy change when one mole of a substance is dissociated into its atoms in gaseous state. Signup and view all the answers

What is the relationship between ΔG and the equilibrium constant K?

ΔG = -RT ln(K) Signup and view all the answers

Which equation represents the work done in a chemical reaction?

W = -PΔV Signup and view all the answers

What does the first law of thermodynamics state about the energy of a system?

The total energy of a system remains constant in any process. Signup and view all the answers

In the equation ΔU = Q + W, what do the variables represent?

ΔU is the change in internal energy, Q is heat transfer, W is work done. Signup and view all the answers

Which of the following correctly explains the relationship between heat transfer and work done as per the first law of thermodynamics?

Heat transfer can result in a change in internal energy along with work done. Signup and view all the answers

What can be said about the work done during expansion of gas in a vacuum based on thermodynamics?

No work is done by the system during the expansion. Signup and view all the answers

If a system absorbs heat while performing work, what can be determined about its internal energy?

The internal energy must increase. Signup and view all the answers

What does a positive change in internal energy (ΔU) indicate about heat and work for a system?

The system gained more heat than work lost. Signup and view all the answers

Which of the following statements regarding the first law of thermodynamics is correct?

Energy can be transformed from one form to another, but the total remains constant. Signup and view all the answers

Study Notes

Enthalpy Basics

Enthalpy of formation for all elements in their standard states is zero.
The enthalpy of vaporization for ethanol is 38.9 kJ mol⁻¹.
Standard enthalpy of formation of HCl is -93 kJ mol⁻¹.

Thermodynamic Principles

The first law of thermodynamics: total energy of a system remains constant, expressed as ΔU = Q + W.
The second law of thermodynamics: entropy of an isolated system increases over time; spontaneous processes increase total entropy of the universe.
Work in gas expansion against a vacuum is not reversible; although work is done by the system, no work is done in the expansion itself.

Entropy and State Changes

Enthalpy of fusion: heat required to melt one mole of a substance at its melting point.
Standard state: most stable form of a substance at 1 atm and 298 K.
Reaction 2H(g) → H₂(g) leads to a decrease in entropy (ΔS is negative) due to a reduction in gaseous particles.

Calculating Changes

To calculate entropy change of surroundings when given enthalpy change: ΔSsurroundings = -ΔH/T.
For spontaneous reactions, Gibbs free energy change (ΔG) must be negative; ΔG = ΔH - TΔS.
If ΔH is positive and ΔS is negative, the reaction is not spontaneous.

Equilibrium and Temperature Calculations

Relationship between standard Gibbs free energy (ΔG⁰) and equilibrium constant (K): ΔG⁰ = -RTlnK.
Reaction enthalpy can be derived from bond enthalpies: ΔHreaction = Σ(bond energies of reactants) - Σ(bond energies of products.
Temperature at which ΔG = 0 can be calculated with T = ΔH / ΔS.

Enthalpy Definitions and Examples

Standard enthalpy of combustion: enthalpy change when 1 mol of a substance combusts completely in oxygen; example: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l), ΔHₒcombustion = -890 kJ mol⁻¹.
Enthalpy of atomization: enthalpy change when 1 mol of a substance in standard state breaks down into gaseous atoms; example: O₂(g) → 2O(g), ΔHₒatomization = +498 kJ mol⁻¹.

Work Done in Chemical Reactions

Expression for work done in a chemical reaction: W = -PΔV, where P is the external pressure and ΔV is the change in volume.

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Description

Test your knowledge on the basics of thermodynamics, including enthalpy, entropy, and the laws governing energy transformations. This quiz covers key concepts such as enthalpy of formation, vaporization, and the implications of thermodynamic principles on chemical reactions.