5.2.1&2 Enthalpy & Entropy

Question 1

bond dissociation enthalpy

Answer 1

standard molar enthalpy change when one mole of a covalent bond is broken into two gaseous atoms
Cl2—>2Cl

Question 2

first electron affinity

Answer 2

the enthalpy change that occurs when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of gaseous -1 ion
O + e- —-> O-

Question 3

lattice enthalpy

Answer 3

the standard enthalpy change that occurs when 1 mole of an ionic crystal lattice is formed from its constituent ions in gaseous form
Na+ + Cl- —-> NaCl

Question 4

enthalpy of hydration

Answer 4

one mole of gaseous ions become aqueous ions

Question 5

enthalpy of solution

Answer 5

standard enthalpy change when 1 mole of ionic solid dissolves in a large enough amount of water to ensure the dissolved ions are well separated and do not interact with one and other

Question 6

enthalpy of atomisation

Answer 6

enthalpy change when 1 mole of gaseous atoms are formed from its elements in its standard states

Question 7

enthalpy of neutralisation

Answer 7

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

Question 8

enthalpy of formation

Answer 8

standard enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions

Question 9

enthalpy of combustion

Answer 9

enthalpy when 1 mole of a substance is completely burned in oxygen with all reactants and products in standard states under standard conditions

Question 10

in hess’ law when using formation data which way do the arrows point

Answer 10

upwards

Question 11

in hess’ law when using combustion data which way do the arrows point

Answer 11

downwards

Question 12

give the calculation for delta H

Answer 12

delta H = sum of products - sum of reactants

Question 13

when calculating bond enthalpy what equation is used

Answer 13

enthalpy change = sum of reactants - sum of products

Question 14

give the equation that calculates q

Answer 14

q=mcdeltaT /1000
m= mass of substance heated
c= specific heat capacity (4.18)

Question 15

give the equation to find enthalpy change per mole

Answer 15

enthalpy change = q/n

Question 16

which two factors contribute to a greater lattice enthalpy

Answer 16

• sizes of ions - the larger the ion the less negative the lattice enthalpy, the charges become further apart so have a weaker attractive force between them
• charges of the ions- the bigger the charge the greater the attraction between the ions so the stronger the lattice enthalpy

Question 17

what is the perfect ionic model

Answer 17

where the ions are completely ionic and spherical and the attractions are purely electrostatic

Question 18

when is there tendency towards covalent character in ionic substances

Answer 18

• the positive ion is small
• the positive ion has multiple charges
• the negative ions is large
• the negative ions has multiple negative charges

Question 19

what is polarisation

Answer 19

when the negative ion becomes distorted and more covalent the metal cation is more polarising if it polarises the negative ion

Question 20

what is entropy

Answer 20

the number of ways atoms can share quanta of energy, high entropy means the system is disorders as there are more ways of arranging it

Question 21

what is the substance with the lowest entropy and why

Answer 21

perfect crystal of diamond because there is only 1 type of atom 1 regular giant structure

Question 22

give the order of entropy of solids, liquids and gases

Answer 22

highest
             gas
             liquid 
             solid
lowest

Question 23

what is the effect of moles on entropy

Answer 23

if there is a significant increase in the number of moles from reactants to products then entropy increases

Question 24

what is the unit of entropy

Answer 24

JK^-1 mol^-1

Question 25

what is the equation for calculating deltaS

Answer 25

delta S = sum of products entropy - sum of reactants entropy

Question 26

give the equation for working out delta G

Answer 26

delta G = delta H (K) - T delta S /1000

Question 27

when is a reaction feasible

Answer 27

• reaction is exothermic and delta S is positive
• reaction is exothermic, delta S is negative and delta H > T delta S
• reaction is endothermic, delta S is positive and delta H < T delta S

Question 28

when is a reaction spontaneous

Answer 28

• if delta H is negative and delta S is positive
• if both delta H and delta S are positive the reaction is spontaneous above a certain temperature
• if both delta H and delta S are negative the reaction is spontaneous below a certain temperature

Question 29

what equation calculates delta H of a solution

Answer 29

delta H sol = delta H lattice dissociation + sum of delta H hydration