Skip to content
Quick Guides

Chemistry - Thermodynamics

Preview 10 of 38 pages

See the first 10 pages exactly as they appear in the document.

  1. Page 1
    Read page 1 as text

    Study Guide Chemistry – Thermodynamics 1. Enthalpy 1.1 What Is Enthalpy? Scientists discovered that almost all chemical reactions either absorb heat or release heat . Because of this, we say that substances contain heat energy . The heat content of a substance is called enthalpy . When a chemical reaction occurs, the heat change of the reaction is the difference between the enthalpy of the products and the reactants . We write this as: Here: • Δ ( delta) means change • H stands for enthalpy So, Δ H tells us how much heat is gained or lost during a reaction . W e can think of enthalpy as chemical energy that usually appears as heat . 1.2 Using Enthalpy to Calculate Heat Change Let’s look at a real example to see how enthalpy calculations work. Example: Decomposition of Ammonium Nitrate The reaction is: To calculate Δ H, we need the enthalpy values of each substance.

  2. Page 2
    Read page 2 as text

    Study Guide 1.3 Calculating Δ H for the Reaction First, add the enthalpy values of the products : • N ₂ O: +82 kJ • 2 H ₂ O: 2 × (−242 kJ) = −484 kJ Total products = 82 + (−484) = −402 kJ Next, subtract the enthalpy of the reactant : Final Answer: This means: • 36 kJ of heat is released • The reaction is exothermic Important Note: The enthalpy of H ₂ O is doubled because the reaction produces 2 moles of water .

  3. Page 3
    Read page 3 as text

    Study Guide 1.4 What Does the Sign of Δ H Mean? • A negative Δ H means heat is released • A positive Δ H means heat is absorbed 1.5 What Happens If the Reaction Is Reversed? If we reverse the reaction: The sign of Δ H also reverses : This means: • Heat must be added • The reaction is now endothermic So, 36 kJ of energy is required to form ammonium nitrate from nitrous oxide and water vapor. 1.6 Standard Enthalpies of Formation Scientists measure enthalpy values under standard conditions: • 25°C • 1 atm pressure These values are called standard enthalpies of formation and are written as:

  4. Page 4
    Read page 4 as text

    Study Guide Important rules: • The standard enthalpy of any element in its natural form is 0 o Example: O ₂ (g), N ₂ (g), Al(s) • Compound values depend on their physical state o Example: H ₂ O(l) and H ₂ O(g) have different values

  5. Page 5
    Read page 5 as text

    Study Guide 1.7 Using Standard Enthalpy Tables To find the enthalpy change of a reaction, use this formula: What Does This Mean? • 1124 kJ of heat is released • The reaction is exothermic Practice Problems Use Table 3 to solve the following: 1. Calculate Δ H and classify the reaction: 2. Calculate the heat released when 100 g of CaO reacts with liquid water to form Ca(OH) ₂ (s) . 2. Quiz: Enthalpy 1. Question The difference in the heat contents of the products and the reactants is called Answer Choices • temperature • heat of a reaction • heat of calorimetry

  6. Page 6
    Read page 6 as text

    Study Guide Correct Answer heat of a reaction Why This Is Correct The heat of a reaction refers to the energy change that occurs when reactants are converted into products. It represents the difference between the heat content (enthalpy) of the products and the reactants, not temperature or the measurement method (calorimetry). 2. Question The heat of a given reaction is called the change in Answer Choices • enthalpy • entropy • free energy Correct Answer enthalpy Why This Is Correct Enthalpy ( Δ H) measures the heat energy absorbed or released during a chemical reaction at constant pressure. Entropy and free energy describe different thermodynamic concepts, not heat content directly. 3. Question The change in enthalpy is the change in heat energy of a reaction and is calculated by

  7. Page 7
    Read page 7 as text

    Study Guide Answer Choices • Σ( heat content of the reactants) − Σ( heat content of the products) • Σ( heat content of the products) − Σ( heat content of the reactants) • heat content of products × heat content of the reactants Correct Answer Σ( heat content of the products) − Σ( heat content of the reactants) Why This Is Correct Δ H is calculated by subtracting the total enthalpy of the reactants from the total enthalpy of the products. This equation shows whether heat is absorbed or released during the reaction. 4. Question If the reaction is endothermic, the sign of Δ H will be Answer Choices • negative • neutral • positive Correct Answer positive Why This Is Correct Endothermic reactions absorb heat from their surroundings. Because energy is gained, the change in enthalpy ( Δ H) has a positive value.

  8. Page 8
    Read page 8 as text

    Study Guide 5. Question Standard enthalpies occur at what set of conditions? Answer Choices • 25°C and 1.0 atm • 100°C and 1.0 atm • 25°C and 2.0 atm Correct Answer 25°C and 1.0 atm Why This Is Correct Standard enthalpy values are defined at standard conditions: 25°C (298 K) and a pressure of 1.0 atmosphere. 6. Question The standard enthalpy for an element in its reference form is defined as Answer Choices • – 2.0 kJ/mol • 0 kJ/mol • 2.0 kJ/mol Correct Answer 0 kJ/mol Why This Is Correct By definition, elements in their most stable (reference) form at standard conditions have a standard enthalpy of zero.

  9. Page 9
    Read page 9 as text

    Study Guide 7. Question When looking up enthalpy values for a set of reaction species, the enthalpy depends on the Answer Choices • physical state of the species • color of the species • concentration of the species Correct Answer physical state of the species Why This Is Correct Enthalpy values differ depending on whether a substance is solid, liquid, gas, or aqueous. Color and concentration do not affect standard enthalpy values. 8. Question If the Δ H of a reaction is negative, the reaction is Answer Choices • endothermic and releasing heat • exothermic and releasing heat • exothermic and absorbing heat Correct Answer exothermic and releasing heat Why This Is Correct A negative Δ H indicates that heat is released to the surroundings. This release of energy defines an exothermic reaction.

  10. Page 10
    Read page 10 as text

    Study Guide 9. Question When 1.0 mole of ZnO(s) decomposes, the Δ H = 348 kJ/mol of heat energy. What does this tell you about the formation of ZnO(s)? Answer Choices • the formation of ZnO(s) is endothermic • the formation of ZnO(s) does not require energy • the formation of ZnO(s) is exothermic Correct Answer the formation of ZnO(s) is exothermic Why This Is Correct If decomposition requires energy (endothermic), then the reverse process — formation — must release energy. Therefore, forming ZnO(s) is an exothermic process. 10. Question What is the Δ H value for O ₂ (g)? Answer Choices • 142 kJ/mol • 0 kJ/mol • – 142 kJ/mol Correct Answer 0 kJ/mol

28 more pages in the full document

Unlock it once and it stays in your library, ready to chat with or turn into flashcards.

Unlock the full document

More Chemistry study materials

See all

Study this document with CramX

Once it is in your library, every tool can work from it.