Energetics

Question 1

What is enthalpy

Answer 1

enthalpy is the measure of the total energy of a system

Question 2

What is enthalpy change?

Answer 2

amount of heat energy taken in or given out in any system when pressure is constant

Question 3

standard enthalpy conditions

Answer 3

1. Temperature: 298K or 25 degrees celsius
2. Pressure: 1 atm or 100 kPa or 100000 Pa
3. Concentration: 1 mol/dm^-3
4. Normal physical states at standard temperature and pressure

Question 4

Exothermic Reactions

Answer 4

• energy given out to the surroundings
• energy of reactants > energy of products
• ΔH: negative
• high activation energy required

Question 5

Endothermic Reactions

Answer 5

• heat energy gained from the surroundings
• energy of reactants < energy of products
• ΔH: positive
• low activation energy required

Question 6

standard enthalpy of combustion

Answer 6

enthalpy change when 1 mol of substance is completely burned in excess oxygen under standard conditions
* always exothermic

Question 7

errors in experiment for standard enthalpy change of combustion

Answer 7

ERRORS
1. heat loss to the surroundings
2. incomplete combustion of fuel (black soot deposited at the bottom of the beaker)
3. conditions are not standard
4. some heat released heats up air and not the water ( as distance increases = air heat increases = increased heat loss)
5. beaker and thermometer absorbs some of the heat produced

Question 8

assumptions made in experiment for standard enthalpy change of combustion

Answer 8

ASSUMPTIONS
1. 1cm3 of water = 1g
2. no heat loss to the surroundings
3. all heat energy produced is absorbed by the water

same goes for neutralisation experiment

Question 9

improvements to be made in experiment for standard enthalpy change of combustios

Answer 9

1. use a lid: heat loss minimized
2. use a copper can: good absorber AND conductor of heat
3. draught shield: insulation, minimizes heat loss
4. reduce the distanve between the spirit burner and the beaker: reduce heat loss

Question 10

standard enthalpy change of neutralisation

Answer 10

enthalpy change when 1 mol of water is formed from the reaction between acid and alkali under standard conditions

Question 11

errors in neutralisation experiment

Answer 11

1. heat loss
2. polystyrene cup and thermometer absorbs some heat
3. human errors while measuring volume and temperature

Question 12

improvements to be made in neutralisation experiment

Answer 12

1. use a lid
2. use laggings

Question 13

standard enthalpy change of formation

Answer 13

enthalpy change when 1 mol of substance is formed by its elements in their standard states under standard conditions

Question 14

standard enthalpy change of atomisation

Answer 14

enthalpy change when 1 mole of gaseous atom is formed from the element in their standard state under standard conditions

Question 15

bond enthalpy

Answer 15

enthalpy change when 1 mol of a bond in the gaseous state is broken

Question 16

mean bond enthalpy

Answer 16

average quantity of energy required to break one mole of covalent bond in gaseous species (at 1 atm)

Question 17

limitations of using bond enthalpy

Answer 17

1. bond enthalpies are measured in gaseous state
2. bond enthalpies are an average value