energetics

Question 1

enthalpy change

Answer 1

heat change measured at constant pressure

Question 2

enthalpy of formation

Answer 2

enthalpy change when one mole of substance is formed from its constituent elements with all products and reactants in their standard states.

Question 3

enthalpy of combustion

Answer 3

enthalpy change when one mole of substance is completely burned in oxygen with all products and reactants in their standard states.

Question 4

mean bond enthalpy

Answer 4

energy required to break one mole of specified covalent bonds averaged over a range of compounds.

Question 5

hess’s law

Answer 5

enthalpy change is independent of the route taken and depends only on the initial and final states.

Question 6

enthalpy of atomisation

Answer 6

enthalpy change when one mole of gaseous atoms is formed from an element in its standard state

Question 7

first ionisation energy

Answer 7

the standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions each with a single positive charge

Question 8

first electron affinity

Answer 8

the enthalpy change when a mole of gaseous 1- ions is formed from 1 mole of gaseous atoms

Question 9

second electron affinity

Answer 9

the enthalpy change when a mole of gaseous 2- ions is formed from 1 mole of gaseous 1- ions

Question 10

lattice enthalpy of formation

Answer 10

the standard enthalpy change when one mole of solid ionic compound is formed from its gaseous ions under standard conditions

Question 11

lattice enthalpy of dissociation

Answer 11

the standard enthalpy change when one mole of solid ionic compound dissociates into its gaseous ions under standard conditions

Question 12

enthalpy of hydration

Answer 12

the enthalpy change when 1 mole of aqueous ions is formed from 1 mole of gaseous ions

Question 13

enthalpy of solution

Answer 13

the standard enthalpy change when one mole of solute is dissolved in enough solvent that no further enthalpy change occurs on further dilution

Question 14

bond dissociation enthalpy

Answer 14

the enthalpy change when all the bonds of the same type in 1 mole of gaseous molecules are broken

Question 15

what does the perfect ionic model assume

Answer 15

all ions are spherical and have their charge evenly spread around them

Question 16

why might experimental lattice enthalpy be different

Answer 16

most ionic compounds have covalent character. positive ions polarise negative ions. the more polarisation, the more covalent the character is.

Question 17

explain the difference in lattice enthalpies of magnesium and sodium halides

Answer 17

-much bigger for the magnesium halides
-bonding in magnesium halides is stronger than the ionic model predicts as it has strongly polarised bonds and covalent character
-the bonding in sodium halides is similar to the predictions of the ionic model so the compounds are close to being purely ionic

Question 18

what is entropy

Answer 18

a measure of the number of ways that particles can be arranged and the number of ways that the energy can be shared out between the particles

Question 19

what affects entropy

Answer 19

physical state- solids have fixed positions so do not have much disorder- they have the lowest entropy
gas particles move around randomly and have the most disorder so they have the highest entropy

Question 20

when is a reaction feasible

Answer 20

if delta G is negative or equal to zero

Question 21

what is the formula for Gibbs free energy

Answer 21

delta G= delta H - T delta S
delta G= Jmol-1 delta H= Jmol-1
T=K delta S= JK-1mol-1

Question 22

what is delta G if the reaction is exothermic and has a positive entropy

Answer 22

then delta G is negative due to the equation. the reaction is feasible at any temperature

Question 23

what is delta G is the reaction is endothermic and has a negative entropy

Answer 23

delta G is positive and the reaction is not feasible at any temperature