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The Energy of Physical, Chemical and Nuclear Reactions

The Energy of Physical, Chemical and Nuclear Reactions . Thermodynamics - the study of energy and energy transfer Thermochemistry - the study of energy involved in chemical reactions. Energy (symbol: E – unit: J joules) Law of Conservation of Energy .

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The Energy of Physical, Chemical and Nuclear Reactions

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  1. The Energy of Physical, Chemical and Nuclear Reactions • Thermodynamics - the study of energy and energy transfer • Thermochemistry - the study of energy involved in chemical reactions

  2. Energy (symbol: E – unit: J joules) Law of Conservation of Energy - total energy of the universe is constant - energy can neither be created nor destroyed Δ𝐸 universe=0 - energy can be transferred from one substance to another - energy can be converted into various forms

  3. System- part of the universe that is being studied and observed (reactants & products) Surroundings- everything else in the universe - part of the universe that is likely to be affected by energy changes in the system

  4. Universe = System + Surroundings Δ𝐸univeres= Δ𝐸system+Δ𝐸surrounding = 0 Any change in the system is accompanied by an equal and opposite change in the surroundings. Δ𝐸system =−Δ𝐸surrounding

  5. Heat (symbol: Q – unit: J joules) - transfer of kinetic energy (Ek) spontaneously from warmer to cooler objects

  6. Temperature (symbol: T – unit: K Kelvin, °C Celsius degrees) TK = T°C + 273.15 - measure of the average kinetic energy of the particles that make up a substance or system Celsius scale – relative to water (0°C water melting point, 100°C water boiling point) Kelvin scale – absolute scale (O K is temp. when a substance has no kinetic energy)

  7. Enthalpy (symbol: H – unit: J joules) - total internal energy of a substance at a constant pressure - hard to measure; includes (1) energy of physical state, (2) energy in bonds, (3) energy in nucleus; includes: - moving electrons within atoms - vibration of atoms connected by chemical bonds - rotation and translation of molecules - nuclear potential energy of protons and neutrons in atomic nuclei - electronic potential energy of atoms connected by chemical bonds

  8. - easily measure and study change in enthalpy (ΔH) in a reaction - enthalpy change of a process is equivalent to its heat change at constant pressure - chemical bonds are sources of stored energy - breaking a bond is a process that requires energy - creating a bond is a process that releases energy

  9. Endothermic Reaction: - system absorbs E - more energy needed to break bonds than released by creating bonds - change in enthalpy is positive

  10. Exothermic Reaction: - system releases E - more energy released by creating bonds than needed to break bonds - change in enthalpy is negative

  11. Representing Enthalpy Changes • ΔHrxnenthalpy of reaction (depends on temperature and pressure) • ΔH°rxnstandard enthalpy of reaction at SATP (25°C, 100 kPa)

  12. (1) Thermochemical Equation – balanced equation that indicates enthalpy change

  13. (2) Separate Expression

  14. (3) Enthalpy Diagram

  15. 1. Physical changes - Small change in enthalpy (tens of kJ/mol) - Change in intermolecular bonds between molecules as they change phase o ΔHvap enthalpy of vaporization (liquid gas) o ΔHcond enthalpy of condensation (gas liquid) o ΔHmelt enthalpy of melting (solid liquid) o ΔHfre enthalpy of freezing (liquid solid) o ΔHsoln enthalpy of solution (solid aqueous – dissolving)

  16. 2. Chemical changes - Moderate change in enthalpy (hundreds of kJ/mol) - Change in intramolecular bonds within molecules as reactants break apart to form products o ΔHcomb enthalpy of combustion o ΔHneut enthalpy of neutralization o ΔHf enthalpy of formation

  17. 3. Nuclear changes - Enormous change in enthalpy (billions of kJ/mol) - Change in nuclear binding energy holding together the nucleus of an atom o A significant amount of mass of the reactants is actually converted to energy o Einstein: E=mc2, where m=mass and c=speed of light (3.0x108m/s)  so a tiny mass is equivalent to a significant amount of energy

  18. Mass Defect • o difference in mass between a nucleus and its nucleons (particles found in the nucleus) • o caused by the energy associated with the strong force that holds a nucleus together • o the higher the nuclear binding energy, the more stable the nucleus • Nucleus + Nuclear binding energy Nucleons

  19. References • McGraw-Hill Ryerson Chemistry 12 • Nelson Chemistry 12

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