Chemical Energetics

Question 1

Define standard enthalpy change of reaction, ΔH_r

Answer 1

It is the energy change when molar quantities of reactants as stated in the thermochemical equation react together under standard conditions.

Question 2

What are the assumptions that are made when calculating the heat change for a reaction?

Answer 2

• there is no heat gained from or lost to the external environment including the calorimter
• the surrounding solution is so dilute such that the density and specific heat capacity of the solution is taken to be that of water
• the reaction takes place rapidly so that a maximum temperature is obtained before cooling sets in.

Question 3

Define the Standard Enthalpy Change of Neutralisation, ΔH_n

Answer 3

It is the energy evolved when one mole of water is formed during neutralisation of an acid an alkali under standard conditions.

Question 4

What is the sign of enthalpy change of neutralisation and why?

Answer 4

ΔH_n is -ve as energy is released when water is formed during neutralisation, and thus is is an exothermic reaction.

Question 5

Define the standard enthalpy change of combustion ΔH_c

Answer 5

It is the energy evolved when one mole of a substance is completely burnt in excess oxygen under standard conditions.

Question 6

What is the usual sign of enthaly change of combustion? In which case is it not so and why?

Answer 6

ΔH_c is usually -ve
It is only positive during the combustion of N₂.
- This is so as tge ebergy is required to break the strong triple bond.
- Excess oxygen is needed to ensure complete combustion

Question 7

Define the standard enthalpy change of atomisation
ΔH_atom of elements.

Answer 7

It is the energy absorbed when one mole of gaseous atoms formed from its element under standard conditions.

Question 8

Define the standard enthalpy change of atomisation ΔH_atom of compounds.

Answer 8

It is the energy absorbed when one mole of comounds is converted to its constituent gaseous atoms under standard conditions.

Question 9

Define the standard bond dissociation enthlapy ΔH_disso, or B.E

Answer 9

It is the energy needed to break one mole of covalent bonds between 2 atoms in the gaeous state under standard conditions.

Question 10

Define the first I.E

Answer 10

It is the energy absorbed when one mole of electrons is removed from one mole of gaseous atoms to form one mole of singly charged gaseous cations.

Question 11

Define the first electron affinity, 1^st E.A

Answer 11

It is the energy change when one mole of electron is added to one mole of gaseous atomsto form one mole of singly charged gaseous anions.

Question 12

Define the standard lattice energy, ΔH_latt or L.E.

Answer 12

It is the energy evolved when one mole of an ionic compound is formed from its constitutent gaseous ions under standard conditions.

Question 13

Define the standard enthalpy change of hydration, ΔH_hyd

Answer 13

It is the energy evolved when one mole of gaseous ions is surrounded by water molecules, forming a solution at infinite dilution under standard conditions.

Question 14

Define the standard enthalpy change of solution ΔH_sol

Answer 14

It is the energy change when one mole of substance is dissolved by solvent(usually water) such that further dilution produces no more energy change under standard conditions.

Question 15

Define the standard enthalpy change of formation, ΔH_f

Answer 15

It is the energy change when one mole of a compound is formed from its constituent elements in their standard states under standard conditions,

Question 16

What is the ΔH_f of an element in its standard state?

Answer 16

zero

Question 17

What is the equation of a reaction if values for ΔH_f of reactants and products are given?

Answer 17

ΔH_rxn= Σ aΔH_f of products -Σ bΔH_f of reactants.

Question 18

What is the equation of a reaction if values for ΔH_c of reactants and products are given?

ΔH_c

Answer 18

ΔH_rxn= Σ aΔH_c of reactants -Σ bΔH_c of procucts.

Question 19

What is the equation to determine enthalpy change of reaction using bond energy values?

Answer 19

ΔH_rxn= Σ energies of bonds broken -Σ energies of bonds formed.

Question 20

Define entropy.

Answer 20

Entropy is a measure of the randomness or disorder of a system.

Question 21

Explain how change in volume of gases affect entropy of system.

Answer 21

• In a larger volume of space, there are more ways to arrange the gas particles in the system.
• the gas expand from a less disordered system to a more disordered system.
  -Hence the entropy of the system increases. ΔS is positive.

Question 22

Explain how a change in state will affect entropy of system.

Answer 22

• During melting and boiling, there are more ways to arrange the particles in the system, from a less disordered solid phase to a more disordered liquid phase
• or from a less disordered liquid phase to a more disordered gaseous phase.
  -Hence, entropy of the system increases, and ∆S is positive.

Question 23

Explain how the change in number of particles (esp for gaseous systems) affect entropy of system.

Answer 23

• Entropy of the system decreases as there is a decrease in the total number of gas particles in the system (i.e. from 2 mol of gaseous reactant to 1 mol of gaseous product).
• There are less ways of arranging the particles in the system, resulting in a more disordered system to a less disordered system.
• ΔS is negative.

Question 24

Define Hess’ Law

Answer 24

Hess’ Law states that the enthalpy change for a chemical reaction is the same regardless of the route the reaction takes , provided the reactants and products, initial and final conditions are the same.

Question 25

What is the delta G for the dissolving of liquids of similar polarities?

Answer 25

Always negative (always spontaneous)

Question 26

What is the hydration energy of an ionic compound?

Answer 26

It is the sum of the hydration energy of all ions present.

Question 27

What is the standard conditions?

Answer 27

298K and 1bar