3.1.4: Energetics

Question 1

What is enthalpy change (△H)?

Answer 1

the measurement of heat or temperature change at a constant pressure

• △H negative = exothermic (energy released)
• △H positive = endothermic (energy absorbed)

Question 2

What are pressure and temperature normally?

Answer 2

• pressure- 100kPa (approx. normal atmospheric Pressure)
• temperature- 298k (around normal room temp. 25°C)

Question 3

What is the standard state?

Answer 3

the state in which it exists at 298k and 100kPa
When enthalpy change is measured under a standard solution, it is written as △H°298

Question 4

What occurs during enthalpy change of formation (△H°f) ?

Answer 4

• when one mole or Substance is formed from its constituent elements under standard conditions, all reactants and products being in their standard states
  e.g. Na(s)+ 0.5Cl2(g) -> NaCl(s)

Question 5

What occurs during enthalpy change of combustion (△H°c) ?

Answer 5

• when a mole of substance is completely, burnt in oxygen under standard conditions, all reactats and products being in their standard states
• e.g. CH4(g) + 2O2(g)-> CO2(g)+ 2H2 (g)

Question 6

How can you measure enthalpy change?

Answer 6

• single colorimeter experiment
• styrofoam cup + thermometer

Question 7

What is the equation for enthalpy change?

Answer 7

heat energy change (J)= mass of heated substance (g) x specific heat capacity ( JgK-1) x temperature change (°C)

Q=mc△T

Question 8

What is Hess’ law?

Answer 8

the enthalpy change in a reaction is independent of the route taken and depends on initial and final status alone

Question 9

When is the enthalpy change 0 in a hess’ law reaction?

Answer 9

if an element is in its natural state

Question 10

How do you set out the Hess’ law equation?

Answer 10

FORMATION- arrows upwards (shared elements central) to form up
COMBUSTION- arrows downwards (CO2+H2O) to burn down

Question 11

What is bond association enthalpy?

Answer 11

The energy change required to break a covalent bond with all species in the gaseous state (this is the same as energy released when the same bond is formed)

Question 12

What is mean bond enthalpy?

Answer 12

The average value of bond dissociation enthalpy for a given type of Bond taken from a range of compounds
- as these are averages they give good approximate values and are quick and easy to use

Question 13

How can energy be lost in enthalpy change experiments?

Answer 13

• energy lost to surroundings
• incomplete combustion
• water/ alcohol evaporated
• not in standard conditions