chemical energetics

Question 1

what is enthalpy change given by

Answer 1

delta H = H(products) - H(reactants)

Question 2

what is the eqn used for determining heat evolved/absorbed

Answer 2

mc (delta T) or Vc (delta T)

Question 3

what is Hess law

Answer 3

enthalpy changes is independent of the pathway in which the reaction takes place provided intial and final states of the system remain the same

sum of clockwise arrows = sum of anti clockwise arrows

Question 4

what is the standard enthalpy change of formation

Answer 4

enthalpy change when one mole of a pure substance is formed from its constituent elements in their standard states under standard conditions

delta H = Hf (products) -Hf (reactants)

Question 5

what is the standard enthalpy change of combustion

Answer 5

enthalpy change when 1 mole of a substance is completely burnt in excess oxygen under SC

delta H = Hc (reactants) -Hc (products)

Question 6

what is the standard enthalpy change of neutralisation

Answer 6

enthalpy change when 1 mole of water is formed when an acid neutralises a base in an infinitely dilute aq solution under standard conditions

• strong acid strong base neutralisation is always constant as they are completely ionised and can be seen as H+ and OH- together to form H2O

Question 7

why is the enthalpy change of HF more exo than -57kJ mol -1

Answer 7

F- is an extremely small anion so alot of energy is released when f- is hydrated due to formation of strong ion-dipole interactions

Question 8

what is the bond dissociation energy or BE

Answer 8

enthalpy change when 1 mole of C bonds between 2 atoms in a gaseous molecule are broken (alw positive as heat is absorbed to break bonds)

delta H = BE of reactants - BE of products

Question 9

what is the standard enthalpy change of atomisation

Answer 9

enthalpy change when 1 mole of isolated gaseous atoms are produced from the element in its standard state under standard conditions

Question 10

what is electron affinity

Answer 10

heat change when 1 mole of gaseous atoms or -ve charged ions gains an e- each

Question 11

what is lattice energy

Answer 11

enthalpy change when 1 mole of an ionic solid is formed from its isolated gaseous ions from an infinite distance apart under SC

delta Hle proportional to (q+ x q-)/(r+ + r-)

Question 12

what is the standard enthalpy change of hydration

Answer 12

enthalpy change when one mole of gaseous ions is dissolved in a large amount of water under SC

delta hyd given by charge density of anion/ion

Question 13

what is the standard enthalpy change of solution

Answer 13

enthalpy change when one mole of a solute is completely dissolved in a solvent to form an infinitely dilute aq solution

delta Hsol = -delta H le + delta H hyd

Question 14

what are the 2 cases where entropy is the greatest

Answer 14

gaseous state - particles possess highest energy and are also moving randomly in all directions (highly disordered)

large complex molecule - more ways for atoms to be arranged

Question 15

what are the 4 factors that cause a change in entropy

Answer 15

1. change in temp - KE of molecules increase, broadening of boltzmann E distribution and more ways of arranging the energy quanta in the hotter species
2. change in phase
3. change in number of particles - no. of ways to arrange greater no. of particles is greater
4. mixing - higher state of disorder