Energetics

Question 1

Define: standard molar enthalpy of formation.

Answer 1

The enthalpy change when one mole of a substance is formed from its constituent elements with all reactants and products in their standard states; under standard conditions (100kPa, 298K).

Question 2

Define enthalpy change (ΔH).

Answer 2

Enthalpy change is the heat energy change of a reaction measured under under conditions of** constant/standard pressure**.
kJ mol^-1

Question 3

What is standard enthalpy change? (ΔH^o₂₉₈)

Answer 3

Enthalpy change measured under standard conditions; 100kPa, 298K. (25^oC)

Question 4

Define: standrad enthalpy of combustion.

Answer 4

Enthalpy change when 1 mole of substance is completely burned in oxygen with all reactants and products in their standrad states, under standard conditions. (100kPa, 298K)

Question 5

How do you calculate enthalpy change?

Answer 5

q = mcΔT

• q = heat loss/gain (J)
• m = mass of water/solution (g)
• c = specific heat capacity of water (4.18 J g^-1K^-1)
• ΔT = change in temperature of water/solution (K)

Question 6

State Hess’ s Law.

Answer 6

Hess’s Law states that the enthalpy change for a chemical reaction is independent of the route taken (it is the same overall whatever route the reaction takes).

Question 7

What occurs during a neutralisation reaction?

Answer 7

Acid + Alkali → Salt + Water
NaOH + HCl → NaCl + H2O

Question 8

What is mean bond enthalpy?

Answer 8

The average value of the bond dissociation enthalpy for a given type of bond, taken from a range of compounds.

E.g., O-H = +463 kJ mol^-1 ;different for H₂O value (+460 kJ mol^-1), but is averaged between other O-H enthalpies such as the O-H values in alcohols and carboxylic acids.

Average values from many different compounds.