Energetics and thermodynamics

Question 1

Standard conditions for enthalpy change measurements

Answer 1

temperature 298K
pressure 100Kpa
Concentration for all reactants is 1 moldm-3

Question 2

mean bond enthalpy

Answer 2

Standard energy change when one mole of gaseous moles each break a covalent bond to form free radicals averaged over a range of different compounds

Question 3

Standard enthalpy of formation

Answer 3

Standard enthalpy change when one mole of a substance is formed from its consituent elements under standard conditions with all reactants and products being in their standard forms

Question 4

standrad enthalpy of combustion

Answer 4

Standard enthalpy change when one mole of a substance is burnt completely in excess oxygen under standard conditions with all reactants and products being in their standard forms

Question 5

why may enthalpy of combustion of a compound be different from the one that is measured

Answer 5

Incomplete combustion

Question 6

Hess Law

Answer 6

It states that enthalpy change of a chemical reaction stays same whatever route is taken

Question 7

Why might bond enthaly calculated in experiment might not be the same as mean bond enthaly in data booklet?

Answer 7

mean bond enthalpy id averaged over a range of different compounds

Question 8

Colorimetry - combustion reaction

Answer 8

m= mass of water =volume of water

Question 9

Two solution reacting

Answer 9

mass = total volume = v1 + v2

Question 10

adding solid to a solution

Answer 10

m= volume of solution
For moles, use moles of limiting reagent

Question 11

assumption made in colorimetry

Answer 11

Energy transferred to water from reacting chemicals is same as energy released by reaction

Question 12

What are the sources of error in coloremetry

Answer 12

Heat loss
incomplete combustion ( Combustion colorometry)
Some fuel may evaporate

Question 13

improvements to minimise error

Answer 13

Put a lid to prevent heat loss
Use a polystyrene cup
Cover with insulating sleeve
Reduce the distance between flame and beaker (only in combustion)

Question 14

steps to measure enthalpy change using cooling curve

Answer 14

Add 50cm3 of water using a measuring cylinder
add to a polystyrene cup
Add known mass of solid
Mass by difference
Record initial temperature for 4 mins to establish room temperature
Record temperature at regular interval unless trend is seen
Plot a time v|s temperature graph
Extrapolate to where the solid was added
to get delta T
Q=mcDT DH=Q\n

Question 15

Standard enthalpy of atomisation

Answer 15

Enthalpty change when one mole of a gaseous atoms is formed from its elements under standard conditions

Question 16

1st ionisation enthalpy

Answer 16

Standard enthalpy change when one mole of electrons is removed from its gaseous atoms to give one mole of gaseous ions each with a single positive charge.

Question 17

1st electon affinity

Answer 17

standard enthalpy change when one mole of gaseous atoms are converted into one mole of gaseous ions each with a single negative charge under standard conditions

Question 18

lattice formation of enthalpy

Answer 18

standard enthalpy change when one mole of solid ionic compound is formed from gaseous Ions

Question 19

Lattice dissociation enthalpy

Answer 19

Standard enthalpy change when one mole of solid ionic compound dissocaites into its gaseous ions

Question 20

standard enthalpy of solution

Answer 20

Standard enthalpy change when one mole of solute dissolves to form aqueous ions
NaCl(s) + aq = Na+(aq) + Cl-( aq)

Question 21

Standard enthalpy of hydration