energetics

Question 1

define the term mean bond enthalpy:

Answer 1

the amount of energy required to break one mole of a covalent bond into gaseous atoms, averaged over a range of different compounds

Question 2

define the term enthalpy of combustion:

Answer 2

the enthalpy change when one mole of a substance is completely burnt in excess oxygen, under standard conditions. with all reactants and products under their standard states.

Question 3

define the term enthalpy of formation:

Answer 3

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

Question 4

define Hess law :

Answer 4

that the enthalpy change for chemical reaction is the same whatever route is taken from reactants to products.

Question 5

describe how you would measure an enthalpy change using a cooling a curve (6 marks)

Answer 5

• measure 50cm3 of water using a measuring cylinder then add to a polystyrene cup (instead of glass beaker) as its a better what insulator.
• weigh the solid by using ‘weigh by difference’ method -use mass before and after and then take away to get the difference.
• record the temperature at the start for 3-4 times to establish an initial temperature.
• add solid then record temp for at regular intervals until a trend is seen
• plot a temp/time graph then extrapolate back to the point of addition to get a maximum change in temp then use the equations Q=mcdeltaT and Q/n= enthalpy change