Chem energetics

Question 1

standard enthalpy change of reaction

Answer 1

The standard enthalpy change of reaction is the energy change in a chemical reaction when the molar quantities of reactants stated in the chemical equation react under standard conditions

Question 2

standard enthalpy change of formation

Answer 2

The standard enthalpy change of formation of a substance is the energy change when 1 mole of the pure substance in a specified state is formed from its constituent elements in their standard states under standard conditions (i.e. 1 bar and 298 K)

Question 3

standard enthalpy change of combustion

Answer 3

The standard enthalpy change of combustion of a substance is the energy released when 1 mole of the substance is completely burnt in excess oxygen under standard conditions (i.e. 1 bar and 298 K).

Question 4

standard enthalpy change of neutralisation

Answer 4

The standard enthalpy change of neutralisation between an acid and a base is the energy change when the acid and the base react to form 1 mole of water under standard conditions (i.e. 1 bar and 298 K).

Question 5

bond dissociation energy

Answer 5

The bond dissociation energy of a X–Y bond is the energy required to break 1 mole of that particular X–Y bond in a particular compound in gaseous state.

Question 6

bond energy

Answer 6

The bond energy of a X–Y bond is the average energy absorbed when 1 mole of the X–Y bonds are broken in the gaseous state.

Question 7

lattice energy

Answer 7

The lattice energy (LE) of an ionic compound is the energy released when 1 mole of the solid ionic compound is formed from its constituent gaseous ions under standard conditions (i.e. 1 bar and 298 K).

Question 8

heat change

Answer 8

q = mcT / q = CT

Question 9

specific heat capacity

Answer 9

The specific heat capacity (c) of a substance is the quantity of heat
required to raise the temperature of 1 g of the substance by 1 C (or 1 K).

Question 10

heat capacity

Answer 10

The heat capacity (C) of a substance is the quantity of heat required to raise the temperature of the substance by 1 C (or 1 K).

Question 11

change in H formula

Answer 11

change in H (enthalpy change in reaction) = -q/n

enthalpy change in rxn: n= number of moles of limiting reagent
enthalpy change of combustion: n= no. of moles of substance burnt
enthalpy change of neutralisation: n= no of moles of water produced

Question 12

Hess’s Law

Answer 12

Hess’ Law states that the enthalpy change of a reaction is determined by the initial and final states of the system and is independent of the pathways taken

Question 13

Lattice energy

Answer 13

The lattice energy (LE) of an ionic compound is the energy released when 1 mole of the solid ionic compound is formed from its constituent gaseous ions under standard conditions (i.e. 1 bar and 298 K).

-The more negative (or greater the magnitude of) the lattice energy of an ionic compound, the stronger is the ionic bonding present in the compound.

Question 14

factors affecting LE

Answer 14

Effect of ionic charge
The bigger the cationic charge (q+) or anionic charge (q–), the
stronger is the attraction between the oppositely charged ions.
 the greater the magnitude of the lattice energy OR the more negative the lattice energy

Ionic Size
The smaller the cationic radius (r+) or the anionic radius (r–), the
shorter is the inter-ionic distance and hence the stronger is the
attraction between the oppositely charged ions.
 the greater the magnitude of the lattice energy OR the more negative the lattice energy

LE proportional to q+q-/r+ + r-