17 Thermodynamics Flashcards
Standard molar enthalpy of formation
Enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions, all reactants and products in their standard states
Standard molar enthalpy change of combustion
Enthalpy change when one mole of substance is completely burnt in oxygen
Standard enthalpy of atomisation
Enthalpy change which accompanies the formation of one mole of gaseous atoms from the element in standard state under standard conditions
First ionisation energy
Standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions each with a single positive charge
Second ionisation energy
Loss of a mole of electrons from a mole of singly positively charge ions
First electron affinity
Standard enthalpy change when one mole of gaseous ions each with a single negative charge
(Always negative as energy is given out when an electron is attracted to positively charged nucleus)
Second electron affinity
Enthalpy change when one mole of electrons is added to mole of gaseous ions each with a single negative charge to from ions each with two negative charges
(Always positive negative electron has to overcome the repulsion between the electron and negative ion)
Lattice enthalpy of formation
Standard enthalpy change when one mole of solid ionic compound is formed from its gaseous ions
Enthalpy of lattice dissociation
Standard enthalpy change when one mole of solid ionic compounds dissociated into its gaseous ions
Enthalpy of hydration
Standard enthalpy change when water molcules surround one mole of gaseous ions
Enthalpy of solution
Standard enthalpy change when one mole of solute dissolves completely in sufficient solvent to form a solution in which the molecules or ions are far enough apart not to interact with each other
Mean bond enthalpy
Enthalpy change when one mole of gaseous molecules each break covalent bonds to form two free radicals averaged over a range of compounds
Hess’s law
The enthaply change accompanying a chemical change is independent of the route by which the chemical change occurs
Trends in lattice enthalpies
Larger ions lead to smaller lattice enthalpies. This is because the opposite charges do not approach each other as closely when the ions are larger
Charge to size ratio increase
Explain why the enthalpy of lattice dissociation of potassium oxide is less endothermic than that of sodium oxide
The potassium ion is bigger to the electrostatic forces of attraction between ions is weak
Suggest why the electron affinity of fluorine has a negative value
The nucleus which is positive attracts the negative electron when an electron is gained energy is given out
Explain why the theoretical enthalpy of lattice dissociation for silver fluoride is different form the experimental value that can be calculated using the born-harbour cycle
Theoretical value is calculated assuming that the bonding is perfecting ionic. Experimental value allows for the bonding to have some covalent character
Why would the lattice dissociation for silver chloride be less than silver fluoride
Chloride ion is larger so the attraction between Cl- and Ag+ is weaker
Define the term enthalpy of lattice dissociation
Enthalpy change when one mole of an compound is broken into its ions in the gaseous state
Explain why the bond enthalpy of Cl-Cl bond is greater than that of a Br-Br bond
Bond pair is closer to the nucleus so attraction to bonding pair is stronger
Explain the meaning of the term perfect ionic model
Ions can be regarded as point charges ( perfect spheres)
Factors that increase polarisation
Positive ion small size and high charge
Negative ion larger size and high charge
Spontaneous
Occurring as a result of a sudden impulse
Entropy
A numerical measure of disorder in a chemical system
Positive 🔺G
Means the reaction isn’t feasible at room tempeture
Determining entropy change
At a boiling point water is equally likely to exist as liquid or vapour
🔺H = T🔺S
Extracting metals
Extracting metals form their oxide ore is to heat them with carbon ( coke is cheap)
Kinetic factors
Neither enthalpy change nor entropy changes tells us anything about how quickly or slowly a reaction is likely to go ( large activation energy)
Partial pressure of A
Mole fraction of a ✖️ total pressure
Mole fraction of A
Number of moles of A / total moles of gas
Effect of pressure on gaseous equilibrium
Pressure only effects in there are a different number of moles
Increasing the pressure on a gas phase reaction will increase the rate at which equilibrium is reached because there would be more collisions
Effect of temperature of a gaseous equilibrium
Increasing the temperature will also increase the rate of equilibrium is reach with a catalyst
what does ∆H mean?
∆H, whilst important, is not suf cient to explain feasible change
equation
∆G = ∆H – T∆S
