Thermodynamics and Energy

Grade 10 · Science · Study Guide

Explore the laws of thermodynamics, enthalpy, entropy, and how energy changes drive or prevent chemical reactions.

Thermodynamics is the study of energy transformations, particularly heat and work. The First Law of Thermodynamics states that energy cannot be created or destroyed, only converted from one form to another: the total energy of the universe is constant. Mathematically: ΔU = q + w, where ΔU is change in internal energy, q is heat, and w is work.

The Second Law states that in any spontaneous process, the total entropy of the universe increases. Entropy (S) is a measure of disorder or randomness in a system. Processes that increase disorder tend to be spontaneous.

Enthalpy (H) is the heat content of a system at constant pressure. Exothermic reactions release heat (ΔH < 0) and endothermic reactions absorb heat (ΔH > 0). Hess's Law allows us to calculate enthalpy changes for reactions by combining known reactions.

Gibbs free energy (G) combines enthalpy and entropy to predict spontaneity: ΔG = ΔH - TΔS. A reaction is spontaneous when ΔG < 0, non-spontaneous when ΔG > 0, and at equilibrium when ΔG = 0. The sign of ΔG depends on both temperature and the magnitudes of ΔH and ΔS.

Key terms

Enthalpy (H)
The total heat content of a system at constant pressure; ΔH < 0 for exothermic, ΔH > 0 for endothermic reactions
Entropy (S)
A measure of disorder or randomness in a system; spontaneous processes tend to increase total entropy
Gibbs Free Energy (G)
ΔG = ΔH - TΔS; predicts spontaneity: ΔG < 0 means spontaneous, ΔG > 0 means non-spontaneous
Exothermic reaction
A reaction that releases heat to the surroundings; ΔH is negative; products have less energy than reactants
Hess's Law
The total enthalpy change of a reaction is the same regardless of the pathway taken; allows indirect calculation of ΔH
Specific heat capacity
The amount of heat required to raise 1 gram of a substance by 1°C; water has a specific heat of 4.184 J/(g·°C)

Flashcards

First Law of Thermodynamics
Energy is conserved: ΔU = q + w; energy can neither be created nor destroyed
Second Law of Thermodynamics
In any spontaneous process, the total entropy of the universe increases
Third Law of Thermodynamics
The entropy of a perfect crystal at absolute zero (0 K) is zero
Hess's Law
ΔH for a reaction = sum of ΔH values of individual steps; path-independent
Heat capacity formula
q = mcΔT; where m = mass, c = specific heat capacity, ΔT = temperature change
Spontaneous process
A process that occurs without continuous external input; ΔG < 0 at constant T and P
Standard enthalpy of formation
ΔHf°; enthalpy change when 1 mole of compound forms from elements in standard states
Calorimetry
Experimental technique to measure heat transfer in reactions using an insulated device called a calorimeter

Questions

  1. A reaction has ΔH = -200 kJ/mol and ΔS = +50 J/(mol·K). At 298 K, is this reaction spontaneous?

    1. No, because ΔH is negative
    2. Yes, because ΔG = ΔH - TΔS is negative
    3. No, because ΔS is positive
    4. Cannot be determined without more information
    Show answer

    Answer: Yes, because ΔG = ΔH - TΔS is negative

    ΔG = ΔH - TΔS = -200,000 J - (298 K)(50 J/K) = -200,000 - 14,900 = -214,900 J. Since ΔG < 0, the reaction is spontaneous

  2. Exothermic reactions always have a negative change in enthalpy (ΔH < 0).

    Show answer

    Answer: True

    By definition, exothermic reactions release heat to the surroundings, so the enthalpy of the system decreases (ΔH < 0)

  3. The law that states energy cannot be created or destroyed, only converted between forms, is the ________ Law of Thermodynamics.

    Show answer

    Answer: first

    The First Law of Thermodynamics is the law of conservation of energy: ΔU = q + w

  4. How much heat is needed to raise 50.0 g of water from 20.0°C to 80.0°C? (specific heat of water = 4.184 J/g·°C)

    1. 4184 J
    2. 12552 J
    3. 209.2 J
    4. 1673.6 J
    Show answer

    Answer: 12552 J

    q = mcΔT = 50.0 g × 4.184 J/(g·°C) × (80.0 - 20.0)°C = 50.0 × 4.184 × 60.0 = 12,552 J

  5. Which scenario represents an increase in entropy?

    1. Water vapor condensing into liquid water
    2. Salt crystallizing from a solution
    3. Ice melting into liquid water
    4. A gas being compressed into a smaller container
    Show answer

    Answer: Ice melting into liquid water

    Ice melting to liquid water increases disorder (entropy) as the rigid crystal lattice breaks down into freely moving molecules

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