1.8 - Thermodynamics Flashcards
What does Hesses law state?
The enthalpy change for a reaction is independent of the route taken
Define standard enthalpy of formation
The enthalpy change when one molecule of a compound is formed from its constituent elements in standard conditions with all products and reactants in their standard states
What is the standard enthalpy of an element?
Zero
Define standard enthalpy of combustion
The enthalpy change when one mole of a substance is completely burned in excess oxygen
Define standard enthalpy of atomisation
Enthalpy change when one mole of gaseous atoms is formed from a compound in its standard state in standard conditions
Define first ionisation energy
Enthalpy change when one mole of electrons is removed from one mole of gaseous, atoms to form one more gaseous 1+ ions
Define second ionisation energy
Enthalpy change when one mole of electrons is removed from one mole of gaseous 1+ ions from one mole gaseous 2+ ions
Define first electron affinity
Enthalpy change when one mole of gaseous atoms gain one mole of electrons to form one mole of gaseous 1- ions
Define second electron affinity
Enthalpy change when one mole of gaseous 1- ions gains one mole of electrons to form one mole of gaseous 2- ions
Define lattice enthalpy of formation
Enthalpy change when one mole of solid ionic lattice is formed from its constituent gaseous ions
Define lattice enthalpy of dissociation
Enthalpy change when one mole of solid ionic lattice is dissociated into its gaseous ions
Define Enthalpy of hydration
Enthalpy change when one mole of gaseous ions becomes hydrated/dissolved in water to infinite dilution
Define enthalpy of solution
Enthalpy change when one mole of solute dissolves, completely in a solvent to infinite dilution
Define mean bond dissociation enthalpy
Enthalpy change when one mole of a certain type of covalent bond is broken with all species in the gaseous state
What is a born-haber cycle?
Thermochemical cycle, showing all the enthalpy changes involved in the formation of an ionic compound. start with elements in there standard states
What factors affect the lattice enthalpy of an ionic compound?
Size of the ions
Charge on the ions
How can you increase the lattice enthalpy of a compound? Why does it increase?
Smaller ions - send is the charge centres will be closer together
Increased charge - since there will be a greater electrostatic force of attraction between the oppositely charged ions
How can born-haber cycles used to see if compounds could theoretically exist?
Use known data to predict certain values of theoretical compounds, and then see if these compounds would be thermodynamically stable. Was used to predict the existence of the first noble gas containing compound.
What actually happens when a solid is dissolved in terms of interactions of the ions with water molecules?
Break lattice - gaseous ions; dissolve each gaseous ion in water. The aqueous ions are surrounded by water molecules ( which have a permanent dipole due to polar O-H bond)
What is the perfect ionic model?
Assumes that ions are perfect Lisa, very cool and that there is an even charge distribution. Act as point charges.
Why is the perfect ionic model often not accurate?
Ions are not perfectly spherical. polarisation often occurs when small positive ions are large negative ions are involved so the ionic bonds gain covalent character. Some lattices are not regular and the crystal structure can do for
What kind of bonds will be the most Ionic?
Between large positive ions and small negative ions
Define the term, spontaneous and feasible
If a reaction is spontaneous and feasible, it will take place of its own accord, does not take account of rate of reaction
Is a reaction with a positive or negative enthalpy change more likely to be spontaneous?
Negative – exothermic
Define entropy
Randomness/disorder of a system
Higher value for entropy = more disordered
What units is entropy measured in?
JK^-1mol^-1
What is the second law of thermodynamics?
Entropy of an isolated system always increases as it is overwhelmingly, more likely for molecule to be disordered than ordered
Is a reaction with a positive or negative entropy change more likely to be spontaneous
Positive – reactions always try and increase the amount of disorder
How would you calculate the entropy change for a reaction?
Entropy change = sum of products entropy- sum of reactants entropy
Define Gibbs free energy using an equation
G = H - TS
What does the value for Gibbs free energy for a reaction show?
if G < 0 reactions feasible
if G = 0 reaction is just feasible
if G > reaction is not feasible
What is the significance of the temperature which G = 0?
This is the temperature in kelvin, at which the reaction becomes feasible
What are the limitations of using G as an indicator of whether a reaction will occur?
Gibbs free energy only indicates if a reaction is feasible. It does not take into account the rate of reaction. In reality, many reactions that are feasible at certain temperature have a rate of reaction that is so slow that effectively no reaction is occurring
Why is entropy at 0 K ?
No disorder
How is it possible for the temperature of a substance, undergoing an exothermic reaction to stay constant?
The heat that is given out, escapes to the surroundings
