Energetics I

Enthalpy of Reaction: Enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation under standard conditions of 100kPa (1atm) and 298 K

Enthalpy of Formation: Enthalpy change when 1 mole of a compound of formed from its elements in their standard states under standard conditions of 100kPa and 298 K

Enthalpy of Combustion: Enthalpy change when 1 mole of a substance is completely combusted/burned in oxygen under standard conditions of 100kPa and 298 K

Enthalpy of Neutralisation: Enthalpy change when an acid and alkali react together to form 1 Mole of water under standard conditions of 100kPa and 298 K

Hess Cycles:

• ●  Enthalpy of Formation Hess Cycle and Enthalpy of Reaction Hess Cycle arrows go from elements to reactants/products

• ●  Enthalpy of Combustion Hess Cycle go from reactants and products to combustion products of water and carbon dioxide

  Standard Lattice Energy: Enthalpy change when 1 mole of an ionic solid is formed from its gaseous ions under standard conditions of 298 K and 100kPa

  Lattice dissociation: Enthalpy change when one mole of a solid ionic compound is completely dissociated into its gaseous constituent ions under standard conditions (ENDOTHERMIC)

  Lattice formation: Enthalpy change when one mole of solid ionic compounds formed from its gaseous constituent ions under standard conditions

  Enthalpy of Hydration: Enthalpy change when 1 mole of gaseous ions dissolves in water to form one mole of aqueous ions under standard conditions

  Enthalpy of Solution: Enthalpy change when 1 mole of ionic solid is dissolved to infinite dilution so that the ions no longer interact under standard conditions

  Enthalpy of Atomisation: Enthalpy change when 1 mole of gaseous atoms is formed from its constituent elements in its standard state

  Ionic bond strength is related to lattice enthalpy. The higher the enthalpy the stronger the bond strength

• -  A system makes reference only to the materials or mixtures of chemicals being studied

• -  Everything around the given system is the surroundings

• -  A closed system cannot exchange matter with the surroundings but energy can be

  exchanged (open systems exchange both matter and energy)

  

• -  The energy transferred between the surroundings and the system is described as enthalpy change

• -  Exothermic reactions/enthalpy changes occur when the system gives out energy to the surroundings

• -  Energy given out can be displayed using an energy level diagram

• -  Exothermic energy level diagrams show the energy of the products at a lower energy to

  that of the reactants.

• -  Calcium oxide with H2) to form the hydroxide is a good example of an exothermic

  reaction

• -  Endothermic reaction take in energy from the surroundings to the system

• -  This includes reactions such as melting or vaporisation

• -  An energy level diagram would display the system having more energy when as product

  than as reactant

• -  Bond forming is exothermic while bond breaking is endothermic

• -  Measuring enthalpy: Combustion

  • -  We can use the enthalpy of the reaction to work out the energy changes occurring in terms of using temperature changes to achieve a value for the energy change

  • -  Use Q=mc deltaT

  • -  A calorimeter can be used which utilises the temperature change that water in

    the surrounding area undergoes to gauge the temperature change (deltaT)

• -  There are numerous assumptions made and errors that are used in claiming this t be the

  enthalpy of a reaction

- All of the energy given off in the reaction is used to heat the water as opposed to

the vesicle holding the water which is often a metal ca or copper etc. for combustion, of course some of the energy is also lost to the air and other aspects of the surroundings

• -  To fix such an assumption a bomb calorimeter cna be used which is used for combustion to assure that a sample burms completely and that energy losses are avoided

• -  Measuring enthalpy: Solution

  • -  Enthalpy changes in solutions can be measured using insulated plastic

    containers and resources such as plastic containers or polystyrene cups as these materials are insulators and have an almost negligible impact on the specific heat capacity

  • -  For dilute solutions the specific heat capacity is that of water but for more concentrated solutions a different specific heat capacity will be used

  • -  Again a thermometer measures temperature lost or gained

• -  Enthalpy is displayed as standard enthalpy, this is the enthalpy change under standard

  conditions of 100kPa, 298K, substance in their standard state and with a solution of

  concentration 1 mol/dm3

• -  Standard enthalpy is any enthalpy measured under standard conditions