Chemical Energetics

Question 1

what is the unit for enthalpy change delta H?

Answer 1

kJ/mol

Question 2

what is an exothermic reaction?

Answer 2

a reaction where heat is released to the surroundings

Question 3

what is the enthalpy change of an exothermic reaction?

Answer 3

negative

Question 4

what is the standard enthalpy change of combustion?

Answer 4

the heat energy released when one mole of the substance is completely burnt in excess oxygen at standard conditions

Question 5

what is the standard enthalpy change of neutralisation?

Answer 5

heat energy released when an acid and base react to form one mole of water at standard conditions

Question 6

what are some other exothermic reactions other than neutralisation and combustion?

Answer 6

• Haber process
• respiration
• freezing
• condensation

Question 7

what is an endothermic reaction?

Answer 7

a reaction where heat is absorbed from the surroundings

Question 8

what is the enthalpy change of an endothermic reaction?

Answer 8

positive

Question 8

what is activation energy?

Answer 8

the minimum amount of energy that molecular collisions must possess for a chemical reaction to occur

Question 8

what are some endothermic reactions?

Answer 8

• thermal decomposition of carbonates
• dissolution of potassium and ammonium salts
• photosynthesis
• melting and boiling

Question 9

is bond breaking endothermic or exothermic?

Answer 9

bond breaking absorbs energy, hence it is endothermic

Question 10

is bond forming endothermic or exothermic?

Answer 10

bond forming gives out energy, hence it is exothermic

Question 11

what is bond energy?

Answer 11

the average energy required to break a mole of covalent bond in the gas phase into constituent gaseous atoms under standard conditions

Question 12

how to use calorimetric to determine enthalpy change?

Answer 12

1. using a measuring cylinder, place 25cm3 of X in a polystyrene cup and note the temperature
2. using another measuring cylinder, measure 25cm3 of Y and add it to the cup
3. stir well and record the minimum/maximum temperature

Question 13

what is the equation of enthalpy change?

Answer 13

(heat change)/(amount of water formed, substance burnt or limiting reagent)

Question 14

what is the equation of heat change?

Answer 14

(mass) x (specific heat capacity) x (temperature change)

Question 15

what are some assumptions made when calculating enthalpy change?

Answer 15

• there is no heat loss to the surroundings
• the density is of water, 1.00 g/cm3
• the specific heat capacity is of water, 4.18 Jg-1K-1