Thermodynamics

• what is thermodynamics ?

  The branch of physical chemistry that deal as with heat, energy, temperature and the physical properties of matter.
• define enthalpy change

  change in heat content during a chemical reaction at a constant pressure
• define enthalpy of formation

  enthalpy change when one mole of substance is formed from its constituent elements it’s all substances in their standard states
• define enthalpy of formation

  Enthalpy change when one mole of a substance is formed from its constituent elements with all substances in their standard states.

  • can be exo or endothermic.
  • example - 2Na(s) + 1/2O2 —> Na2O
• define enthalpy of combustion

  Enthalpy change when one mole of a substance undergoes complete combustion in oxygen with all substances in standard states.

  • Exothermic (-ve)
  • example - H2(g) + 1⁄2 O2(g) —> H2O(g)
• define Enthalpy of neutralisation give an example between sulfuric acid and sodium hydroxide

  Enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions.

  • Exothermic (-ve)
  • 1⁄2H2SO4(aq) + NaOH(aq) —> 1⁄2Na2SO4(aq) + H2O(l)
• define Ionisation enthalpy

  First ionisation energy = enthalpy change when each atom in one mole of gaseous atoms loses one electron to form one mole of gaseous 1+ ions.

  • always endothermic as energy is need to overcome the attraction between he electron and the nucleus.
• Define lattice enthalpy of dissociation

  The enthalpy change When one mole of a solid ionic compound is broken up into its constituent ions in the gas phase.

  • Endothermic (+ve) Bonds are broken
  • MgCl2(s) —> Mg2+(g) + 2 Cl–(g)
• define lattice enthalpy of formation

  Enthalpy change when one mole of a solid ionic compound is formed from its constituent ions in the gas phase

  • Exothermic (-ve)
  • Mg2+(g) + 2 Cl–(g) —> MgCl2(s)
• What does the strength of ionic bonding depend on ?

  1. Size of the ions
  2. charges of ions

  the strength of ionic bonding increases as…

  1. the ions get smaller
  2. the ions increase in charge
• What does the strength of ionic bonding depend on ?

  1. Size of the ions
  2. charges of ions

  the strength of ionic bonding increases as…

  1. the ions get smaller
  2. the ions increase in charge

  The stronger the ionic bonding in an ionic compound…

  • The more exothermic its lattice enthalpy
• why can we not find the lattice enthalpy directly from an experiment?

  1. There isn’t any equipment capable of forming and reacting gaseous ions
  2. heating an ionic compound result in gaseous ion pairs, not individual ions.
• how do we measure the lattice enthalpy?

  using the born-Haber process
• define enthalpy of atmosisiation

  Enthalpy change when one mole of gaseous atoms is produced from an element in its standard state.

  • Endothermic (+ve)
  • 1⁄2 I2(s)  I(g)
• Define enthalpy of electron affinity

  First electron affinity - the enthalpy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1- ions.

  • Exothermic
  • O(g) + e- —> O–(g)
• is the 2nd and higher electron affinities are endothermic or exothermic.

  endothermic as the overall force between an external electron and an ion is repulsive
• what does the perfect ionic model assume is true ?

  Ions are perfectly spherical

  bonding perfectly ionic
• theoretical lattice enthalpy calculations are based on the perfect ionic model which assumes the bonding in salts is 100% (perfectly) ionic. however that’s wrong because most ionic compounds have some degree of covalent character. this increases the strength of the bonding in the compound. which is accounted for in experimental lattice enthalpies. this is why experimental lattice are more exothermic than theoretical ones.
• What does the difference between the theoretical and experiemntal value tell us about the bonding in the lattice enthalpy ?

  The bigger the difference the greater the covalent character of an ionic compound. Due to the distorted ions.
• when an ionic salt dissolves in water…

  when an ionic salt is dissolved in water, the ionic lattice breaks up into ions. Theses ions interact with the polar water molecules to form hydrated ions, which are held together by ion-dipole forces
• define enthalpy of solution

  Enthalpy change when one mole of an ionic solid dissolves in an amount of water large enough so that the dissolved ions are well separated and do not interact with each other.
• Define Entropy

  The measure of disorder of a system

  the more disordered something is the more entropy it has
• what is the entropy change for this reaction?

  S8–>4S2

  The entropy change is positive

  the number of products is higher than the reactants
• What symbol do we use to represent entropy

  S
• What is the entropy of a substance at absolute 0 (0K)?

  Zero (0JK-1)
• in general when will there be a significant increase in entropy?

  • change in state from solid or liquid to gas
  • increase in number of moles (molecules) between products and reactants
• what are the units for entropy ?

  joules per kelvin - $JK^-1 mol^-1$
• at roughly what temperature does solid turn to liquid ?

  273 k
• What is the entropy change of the surroundings for an exo and endothermic reaction ?

  Exothermic reaction - positive entropy change in surroundings

  exothermic reaction. - negative entropy change
• Some reactions are feasible however they dont occur why is that ?

  The activation energy for the reaction is too high

  the rate of th reaction is too slow
• Define the 2nd law of thermodynamics

  For any process to happen it must increase the entropy of the universe
• what is the enthalpy of the surrounding equal to

  DeltaSsurr = -deltaH / T
• a reaction is feasible when the Gibbs free energy is…

  negative
• explain why the following reaction occurs at room temperature even though the reaction is endothermic

  NaHCO3(aq) + HBr(aq) —> NaBr(aq) + H2O(l) + CO2(g)

  the value for deltaG at room temperature must be negative.

  deltaS should be positive because there are more moles of products than reactants and a gas is forming from an aqueous solution.

  since deltaG is negative TdeltaS must be more positive than deltaH
• what would the graph look like for a reaction that has a negative deltaH and a negative deltaS

  Negative intercept - (deltaH)

  positive gradient - (deltaS)
• at what point does a reaction become feasible ?

  and how would you find the temperature that the reaction is feasible ?

  deltaG = 0

  T = deltaH / deltaS