Energetics

Question 1

temperature

Answer 1

measure of the average kinetic energy of the particles

Question 2

What is the total energy in chemical reactions?

Answer 2

total energy conserved - energy cannot be created or destroyed

Question 3

Can enthalpy directly be measured?

Answer 3

no but changes in enthalpy can be

Question 4

How are enthalpy changes calculated?

Answer 4

from their effect on the temperature of the surroundings

Question 5

equation for heat change of a substance

Answer 5

q = mc deltaT

Question 6

what are the units of enthalpy changes?

Answer 6

kjmol-1^​

Question 7

what do -ve enthalpy changes represent?

Answer 7

energy lost to surroundings (exothermic)

Question 8

what do +ve enthalpy changes represent?

Answer 8

energy gained from surroundings (endothermic)

Question 9

breaking bonds enthalpy change

Answer 9

endothermic (energy required)

Question 10

making bonds enthalpy change

Answer 10

exothermic (energy released)

Question 11

average bond enthalpy

Answer 11

energy needed to break one mol of a bond in a gaseous molecule

Question 12

enthalpy of formation

Answer 12

enthalpy change when one mol of substance is formed from its elements in standard states

Question 13

enthalpy of combustion

Answer 13

enthalpy change when one mol of substance is completely burnt in oxygen under standard conditions

Question 14

standard enthalpy of reaction

Answer 14

enthalpy change for the given reaction at standard conditions

Question 15

standard conditions

Answer 15

• normal, most pure state
• 100kPa
• 298K

Question 16

calorimetry method

Answer 16

• measure heat transferred to surroundings from reactants
• this is amount of energy that has been lost/gained by reaction
  
  • divide by number mol that reacted to give enthalpy of reaction

Question 17

main sources of error in calorimetry experiments

Answer 17

• heat loss to surroundings
• specific heat capacity of equipment

Question 18

How to construct a Hess cycle?

Answer 18

if 2mol are made multiply enthalpy change by 2

Question 19

how do you calculate the enthalpy change of the reaction from bond enthalpies?

Answer 19

reaction enthalpy = sum bond enthalpies of reactants - sum bond enthalpies of products

Question 20

reasons for differences between true reaction enthalpy and bond calculated reaction enthalpy?

Answer 20

• bonds are not average
• usually reactants and products are not in gaseous state as in definition