Chapter 5.1: The energy of Physical, Chemical, and Nuclear Processes

• Thermodynamics: a study of energy and energy transfer

• Thermochemistry: the study of energy involved in chemical reactions

• Studying Energy Changes:

  • Law of conservation of energy: the total energy of the universe is constant, can’t be destroyed or created ∆Universe = 0

  • System: part of universe being studied

  • Surroundings: everything else in that universe

    • ∆Ssystem = −∆Ssurroundings

• Heat and Temperature

  • Heat, Q: transfer of kinetic energy in joules (J)

  • Temperature, T: a measure of the average kinetic energy of the particles that make up a substance or system in Celsius degrees ( ̊C) or kelvins (K)

    • The temperature in Kelvin degrees = Temperature in Celsius degrees + 273.15

• Enthalpy and Enthalpy Change:

  • Enthalpy, H: total internal energy of a substance at constant pressure

  • Enthalpy change, ∆H: relative enthalpy of the reactants and products in the system

• Enthalpy Changes in Chemical Reaction:

  • Breaking a bond is a process that requires energy. Creating a bond is a process that releases energy.

  • Endothermic reaction: net absorption of energy (+)

  • Exothermic reaction: net release of energy (-)

• Representing Enthalpy Changes:

  • Enthalpy of reaction, ∆Hrxn reaction: enthalpy change of a chemical reaction

  • Standard enthalpy of reaction, ∆H ̊rxn: enthalpy change of a chemical reaction that occurs at SATP

    • Standard Ambient Temperature and Pressure: 25 ̊C and 100 kPa

  • Enthalpy of a reaction is also called heat of reaction

  • Visualizing Exothermic and Endothermic reactions:

    • Thermochemical equation: a balanced chemical equation that indicates the amount of heat that is absorbed or released by the reaction it represents (in kJ)

    • You can also show enthalpy of reaction as a separate expression with ∆H ̊

    • Also can be represented with an enthalpy diagram which represents reactants and products and the enthalpy of the system

      • Enthalpy decreases as energy are released in an exothermic reaction

      • Enthalpy increases as energy are absorbed in an endothermic reaction

• Stoichiometry and Thermochemical Equations:

  • Enthalpy of reaction is linearly dependent on the number of products

    • If the amount of products doubles, enthalpy changes

• Heat Changes and Physical Changes:

  • Enthalpy of vaporization, ∆Hvap: the enthalpy change for the phase change from liquid to gas

  • Enthalpy of condensation, ∆Hcond: the enthalpy change for the phase change of a substance from gas to liquid

  • Enthalpy of melting, ∆Hmelt: the enthalpy change for the phase change of a substance from solid to liquid

  • Enthalpy of freezing, ∆Hfre: the enthalpy change for the phase change of a substance from liquid to solid

  • ∆Hvap = −∆Hcond

  • ∆Hmelt = −∆Hfre

  • Enthalpy of a solution: the enthalpy change when a solute dissolves in a solvent

• Energy and Nuclear Reactions:

  • In nuclear reactions, a significant amount of the mass of the reactants is actually converted into energy

    • C2 = 9.0 × 1016 m2/s2 and E = mc2E is energy in kg • m2/s2 (J)is the mass in kgc2 is the square of the speed of light

  • Mass defect: difference in mass between a nucleus and its nucleons

  • Nuclear binding energy: energy associated with the strong force that holds a nucleus together

    • Using the E = mc2 can be used to find this

    • Higher binding energy means more stable nucleus, most stable is at mass number 60

  • Nuclear fission: A heavy nucleus undergoing split into lighter nuclei which releases energy

  • Nuclear fusion: two smaller nuclei fusing to form a larger nucleus

Chapter 5.2:  Determining Enthalpy of Reaction by Experiment

• Specific Heat Capacity (C): amount of energy needed to raise temperature of one gram of substance 1 celsius or 1 kelvin

  • In units of J/g •˚C

• Heat capacity (C): heat of sample, object, or system to its change in temperature

  • In units of kJ/˚C

• Q = m • c • ∆T

  • Q = heat (J)

  • m = mass (g)

  • c = specific heat capacity (J/g •˚C)

  • ∆T = Tf (final temperature) − Ti (initial temperature)(˚C or K)

• Calorimeter: measure enthalpy changes for chemical and physical reactions

• Qreaction = − Qinsulated system

• Enthalpy changes represent the heat change between products and reactants at a constant temperatureShould be open to atmosphere

• Coffee-cup calorimeter: calorimeter is composed of two nested polystyrene cups

  • Placed in 250 mL for stability

  • Constant-pressure calorimeter: open to atmosphere

Chapter 5.3: Hess’s Law of Heat Summation

• Hess’s law of heat summation: states that the enthalpy change of a physical or chemical process depends only on the beginning conditions (reactants) and the end conditions (products).

  • Enthalpy change is independent of the pathway of the process and the number of intermediate steps in the process

  • Allows algebraically combining chemical reactions and be represented by a enthalpy diagram

  • To manipulate an equation, you can:

    • Reverse equation so products become reactants

    • Multiply coefficients by integer or fraction

• Formation reaction: substance is formed from elements in their standard states

• Standard molar enthalpy of formation, ∆H ̊ f : enthalpy change of a formation reaction in their standard states

  • standard molar enthalpy of formation is the amount of energy absorbed or released when one mole of a compound is formed directly from its elements in their standard states

• The enthalpy of formation of an element in its standard state is zero

• The reactants do not actually break down into their elements and then react to form products

Chapter 5.4: Energy sources

• Energy efficiency: ratio of useful energy produced to energy used in its production, expressed as a percent

  • [Useful energy produced] / [ Energy used] x 100 %

    • Useful energy: work done

    • Energy used: ideal energy output

      • Specify how fuel is used up

        • Ex. natural gas is around 37% efficiency

• Environmental focus on:

  • Non-renewable energy: coal, oil, or natural gas can never be reused

Renewable: solar energy can give a constant source of energy