Chapter 5 - Energy and Change

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