Comprehensive Study Notes on Energy Changes and Reaction Energetics

Exothermic and Endothermic Reactions

Chemical reactions are classified based on the direction of energy transfer between the reaction system and its surroundings.

  • Exothermic Reactions

    • An exothermic reaction transfers thermal energy (heat) from the reaction system to the surroundings.

    • This transfer of heat causes the temperature of the immediate surroundings to increase.

    • Chemical Examples:

      • Combustion reactions.

      • Many oxidation reactions.

      • Acid-base neutralisation reactions.

    • Everyday Applications:

      • Self-heating cans (e.g., self-heating coffee cans).

      • Hand warmers.

  • Endothermic Reactions

    • An endothermic reaction absorbs thermal energy (heat) from the surroundings into the reaction system.

    • This absorption of heat causes the temperature of the immediate surroundings to decrease.

    • Chemical Examples:

      • Thermal decomposition reactions.

      • The reaction between citric acid and sodium hydrogen carbonate.

    • Everyday Applications:

      • Instant sports injury cold packs.

  • Mnemonic for Distinction:

    • EXOthermic: Energy EXits into the surroundings.

    • ENdothermic: Energy ENters from the surroundings.

Enthalpy Change and Activation Energy

  • Enthalpy Change (ΔH\Delta H)

    • The enthalpy change (ΔH\Delta H) represents the net transfer of thermal energy into or out of a reaction system during a chemical reaction at constant pressure.

    • For exothermic reactions, energy is lost from the system to the surroundings, so the enthalpy change is negative (ΔH<0\Delta H < 0).

    • For endothermic reactions, energy is gained by the system from the surroundings, so the enthalpy change is positive (ΔH>0\Delta H > 0).

  • Activation Energy (EaE_a)

    • The activation energy (EaE_a) is defined as the minimum amount of energy that colliding reactant particles must possess in order to react successfully.

    • Chemical reactions occur when particles collide with sufficient energy (equal to or greater than EaE_a) and correct orientation.

Reaction Pathway Diagrams

Reaction pathway diagrams (also known as energy profile diagrams) visually represent the energy levels of reactants and products as a reaction proceeds.

  • Standard Axes and Conventions:

    • The vertical axis (yy-axis) is labeled Energy.

    • The horizontal axis (xx-axis) is labeled Progress of reaction.

    • Reactants and products are drawn as horizontal energy levels.

    • A curved line connects the reactant energy level to the product energy level, depicting the energy transition over the course of the reaction.

    • The activation energy (EaE_a) is indicated by a vertical arrow starting from the reactant energy level up to the highest peak of the curve.

    • The overall enthalpy change (ΔH\Delta H) is indicated by a vertical arrow extending from the reactant energy level directly to the product energy level.

  • Exothermic Reaction Pathway Diagram:

    • The energy level of the reactants is higher than the energy level of the products.

    • Energy is released to the surroundings as reactants are converted to products.

    • The arrow representing ΔH\Delta H points downwards from the reactant level to the product level, indicating a negative value (ΔH<0\Delta H < 0).

Exothermic Reaction Pathway Diagram

  • Endothermic Reaction Pathway Diagram:

    • The energy level of the reactants is lower than the energy level of the products.

    • Energy is absorbed from the surroundings as reactants are converted to products.

    • The arrow representing ΔH\Delta H points upwards from the reactant level to the product level, indicating a positive value (ΔH>0\Delta H > 0).

Endothermic Reaction Pathway Diagram

Energetics of Bond Breaking and Bond Making

Chemical transformations involve rearrangement of chemical bonds, consisting of bond breaking in reactants and bond formation in products.

  • Bond Breaking:

    • Breaking chemical bonds requires an input of energy from the surroundings.

    • Bond breaking is an endothermic process.

  • Bond Making:

    • Forming new chemical bonds releases energy to the surroundings.

    • Bond making is an exothermic process.

  • Determining Overall Reaction Energetics:

    • Exothermic Reaction (ΔH<0\Delta H < 0):         Energy released making bonds>Energy taken in breaking bonds\text{Energy released making bonds} > \text{Energy taken in breaking bonds}

    • Endothermic Reaction (ΔH>0\Delta H > 0):         Energy taken in breaking bonds>Energy released making bonds\text{Energy taken in breaking bonds} > \text{Energy released making bonds}

Bond Energy Calculations

Enthalpy change (ΔH\Delta H) can be calculated quantitatively using average bond energies (expressed in kJ/molkJ/mol or kJ mol−1kJ\,mol^{-1}).

  • General Formula:     ΔH=Energy In (Bond Breaking)−Energy Out (Bond Making)\Delta H = \text{Energy In (Bond Breaking)} - \text{Energy Out (Bond Making)}

  • Systematic Step-by-Step Procedure:

    1. Draw the full displayed structural formulas for all reactant and product molecules to explicitly reveal every covalent bond.

    2. Identify, tally, and sum the total bond energy required to break all bonds in the reactants (totalling "Energy In").

    3. Identify, tally, and sum the total bond energy released when forming all bonds in the products (totalling "Energy Out").

    4. Apply the formula: ΔH=Energy In−Energy Out\Delta H = \text{Energy In} - \text{Energy Out}.

    5. Interpret the algebraic sign of ΔH\Delta H:

      • Negative sign (−-) indicates an exothermic reaction.

      • Positive sign (++) indicates an endothermic reaction.

Worked Examples of Bond Energy Calculations

  • Example 1: Reaction of Ethane with Chlorine

    • Chemical Equation:         CH3CH3+Cl2→CH3CH2Cl+HClCH_3CH_3 + Cl_2 \rightarrow CH_3CH_2Cl + HCl

    • Given Bond Energies:

      • C−HC-H: 413 kJ mol−1413\,kJ\,mol^{-1}

      • C−CC-C: 346 kJ mol−1346\,kJ\,mol^{-1}

      • Cl−ClCl-Cl: 240 kJ mol−1240\,kJ\,mol^{-1}

      • C−ClC-Cl: 327 kJ mol−1327\,kJ\,mol^{-1}

      • H−ClH-Cl: 428 kJ mol−1428\,kJ\,mol^{-1}

    • Calculation:

      • Reactants (Energy In):

        • 6×(C−H)=6×413=2478 kJ mol−16 \times (C-H) = 6 \times 413 = 2478\,kJ\,mol^{-1}

        • 1×(C−C)=1×346=346 kJ mol−11 \times (C-C) = 1 \times 346 = 346\,kJ\,mol^{-1}

        • 1×(Cl−Cl)=1×240=240 kJ mol−11 \times (Cl-Cl) = 1 \times 240 = 240\,kJ\,mol^{-1}

        • Total Energy In=2478+346+240=3064 kJ mol−1\text{Total Energy In} = 2478 + 346 + 240 = 3064\,kJ\,mol^{-1}

      • Products (Energy Out):

        • 5×(C−H)=5×413=2065 kJ mol−15 \times (C-H) = 5 \times 413 = 2065\,kJ\,mol^{-1}

        • 1×(C−C)=1×346=346 kJ mol−11 \times (C-C) = 1 \times 346 = 346\,kJ\,mol^{-1}

        • 1×(C−Cl)=1×327=327 kJ mol−11 \times (C-Cl) = 1 \times 327 = 327\,kJ\,mol^{-1}

        • 1×(H−Cl)=1×428=428 kJ mol−11 \times (H-Cl) = 1 \times 428 = 428\,kJ\,mol^{-1}

        • Total Energy Out=2065+346+327+428=3166 kJ mol−1\text{Total Energy Out} = 2065 + 346 + 327 + 428 = 3166\,kJ\,mol^{-1}

      • Enthalpy Change (ΔH\Delta H):             ΔH=3064−3166=−102 kJ mol−1\Delta H = 3064 - 3166 = -102\,kJ\,mol^{-1}

    • Conclusion: The enthalpy change is negative (−102 kJ mol−1-102\,kJ\,mol^{-1}), so the reaction is exothermic.

Example 1 Bond Calculations

  • Example 2: Decomposition of Water

    • Chemical Equation:         2H2O→2H2+O22H_2O \rightarrow 2H_2 + O_2

    • Given Bond Energies:

      • O−HO-H: 464 kJ mol−1464\,kJ\,mol^{-1}

      • H−HH-H: 436 kJ mol−1436\,kJ\,mol^{-1}

      • O=OO=O: 498 kJ mol−1498\,kJ\,mol^{-1}

    • Calculation:

      • Reactants (Energy In):

        • 4×(O−H)=4×464=1856 kJ mol−14 \times (O-H) = 4 \times 464 = 1856\,kJ\,mol^{-1}

        • Total Energy In=1856 kJ mol−1\text{Total Energy In} = 1856\,kJ\,mol^{-1}

      • Products (Energy Out):

        • 2×(H−H)=2×436=872 kJ mol−12 \times (H-H) = 2 \times 436 = 872\,kJ\,mol^{-1}

        • 1×(O=O)=1×498=498 kJ mol−11 \times (O=O) = 1 \times 498 = 498\,kJ\,mol^{-1}

        • Total Energy Out=872+498=1370 kJ mol−1\text{Total Energy Out} = 872 + 498 = 1370\,kJ\,mol^{-1}

      • Enthalpy Change (ΔH\Delta H):             ΔH=1856−1370=+486 kJ mol−1\Delta H = 1856 - 1370 = +486\,kJ\,mol^{-1}

    • Conclusion: The enthalpy change is positive (+486 kJ mol−1+486\,kJ\,mol^{-1}), so the reaction is endothermic.

  • Example 3: Determining an Unknown Bond Energy (O−HO-H)

    • Chemical Equation:         CH4+2O2→CO2+2H2OCH_4 + 2O_2 \rightarrow CO_2 + 2H_2O

    • Known Enthalpy Change (ΔH\Delta H): −808 kJ mol−1-808\,kJ\,mol^{-1}

    • Given Bond Energies:

      • C−HC-H: 413 kJ mol−1413\,kJ\,mol^{-1}

      • C=OC=O: 800 kJ mol−1800\,kJ\,mol^{-1}

      • O=OO=O: 498 kJ mol−1498\,kJ\,mol^{-1}

      • O−HO-H: Unknown (xx

    • Calculation:

      • Reactants (Energy In):

        • 4×(C−H)+2×(O=O)=(4×413)+(2×498)4 \times (C-H) + 2 \times (O=O) = (4 \times 413) + (2 \times 498)

        • Total Energy In=1652+996=2648 kJ mol−1\text{Total Energy In} = 1652 + 996 = 2648\,kJ\,mol^{-1}

      • Products (Energy Out):

        • 2×(C=O)+4×(O−H)=(2×800)+4x2 \times (C=O) + 4 \times (O-H) = (2 \times 800) + 4x

        • Total Energy Out=1600+4x\text{Total Energy Out} = 1600 + 4x

      • Setting up Formula:             ΔH=Energy In−Energy Out\Delta H = \text{Energy In} - \text{Energy Out}             −808=2648−(1600+4x)-808 = 2648 - (1600 + 4x)             −808=1048−4x-808 = 1048 - 4x

      • Rearranging to solve for xx:             4x=1048+8084x = 1048 + 808             4x=18564x = 1856             x=18564=464 kJ mol−1x = \frac{1856}{4} = 464\,kJ\,mol^{-1}

    • Conclusion: The bond energy of the O−HO-H bond is 464 kJ mol−1464\,kJ\,mol^{-1}.