Energetics I

Question 1

Define standard enthalpy change of combustion.

Answer 1

Standard enthalpy change of combustion is the enthalpy change when 1 mole of the substance is completely burnt in excess oxygen under standard conditions of 298K and 1 bar.

Question 2

Define standard enthalpy change of neutralisation.

Answer 2

Standard enthalpy change of neutralisation is the enthalpy change when an acid is neutralised by a base to produce 1 mole of water under standard conditions of 298K and 1 bar

Question 3

Define standard enthalpy change of formation.

Answer 3

Standard enthalpy change of formation is the enthalpy change when 1 mole of the substance is formed from its elements in their standard states under standard conditions of 298K and 1 bar.

Question 4

Define bond energy.

Answer 4

Bond energy is the enthalpy change of when one mole of bonds between atoms of X and Y are broken in the gas phase. *ONLY FOR COVALENT BONDS

Question 5

Define the standard enthalpy change of atomisation.

Answer 5

Standard enthalpy change of atomisation is the enthalpy change when 1 mole of gaseous atoms is formed from the elements in its standard states under standard conditions of 298K and 1 bar.

Question 6

Define lattice energy.

Answer 6

Lattice energy is the enthalpy change when 1 mole of the IONIC SOLID is formed from its separate gaseous ions.

Question 7

Is exothermic energetically favourable? Why?

Answer 7

Enthalpy of product is lesser than reactant , making the product energetically more stable than reactants. This makes exothermic an energetically favourable reaction

Question 8

Is exothermic energetically favourable? Why?

Answer 8

Enthalpy of product is lesser than reactant , making the product energetically more stable than reactants. This makes exothermic an energetically favourable reaction

Question 9

Is endothermic energetically favourable? Why?

Answer 9

No. The enthalpy of the product is higher than the reaction , making the product energetically less stable than the reactant . The endothermic reaction is energetically not favourable.

Question 10

What are the standard conditions?

Answer 10

298K , 1 Bar , 1 mol and standard states

Question 11

Define standard enthalpy change of reaction.

Answer 11

It is the enthalpy change of when molar quantities of reactants , as specified by the balanced chemical equation , react completely to form products under standard conditions of 298K and 1 bar.

Question 12

What is the equation for enthalpy change of reaction?

Answer 12

(mc △T /amount of L.R) x stoichiometric coefficient of L.R

Question 13

What is the equation for the enthalpy change of combustion?

Answer 13

-(mc △T/amount of substance burnt) Jmol-1
mc △T= heat absorbed by water

Question 14

What are the 4 steps needed for % efficiency?

Answer 14

1. Heat absorbed by water = mc △T
2. Heat released by combustion = mc△T x 100/%
3. Enthalpy change of combustion = (-mc △T/ amount of substance burnt) x (100/%)
   4 % efficiency = (mc △T/mc △T x 100/%) x 100%

Question 15

What is the equation for Heat change of Calorimeter?

Answer 15

Heat change = mc △T + C△T , where C= heat capacity of Calorimeter.
Heat absorbed by water = mc △T
Heat absorbed by calorimeter = C △T
Heat change = Heat liberated by combustion of a substance = mc △T + C △T

Question 16

Which two standard enthalpy change is always negative? (exothermic)

Answer 16

Standard enthalpy change of combustion and neutralisation.

Question 17

How to calculate standard enthalpy change of neutralisation?

Answer 17

Find L.R , this equals to amount of H20
Amount of heat absorbed by water = mc △T
△H = -mc △T/mol of L.R

Question 18

Neutralisation reactions involving weak acid/base are less or more exothermic? Why?

Answer 18

Weak acids partially dissociates in water to produce H+ ions . Some of the energy released during neutralisation process is absorbed to complete the dissociation of the weak acid. Thus , the net heat evolved is lower making the enthalpy change of neutralisation is less exothermic.

Question 19

What is Hess’ Law?

Answer 19

Hess’ Law states that the enthalpy change for a reaction depends only on the initial and final states of the system , and is independent of the reaction pathway taken.

Question 20

What is the formula for enthalpy change of formation (universal)

Answer 20

∆Hr = ∑m∆Hf (products) – ∑n∆Hf (reactants) , where m and n represent stoichiometric coefficients

Question 21

What is the formula for enthalpy change of combustion? (universal)

Answer 21

∆Hr = ∑mHc(reactants) – ∑nHc(products)

Question 22

Enthalpy change of reaction using bond energy?

Answer 22

Enthalpy change of reaction

= ∑B.E. (bonds broken) − ∑ B.E. (bonds formed)

Question 23

Define standard enthalpy change of atomisation

Answer 23

Standard enthalpy change of atomisation is the enthalpy change when 1 mole of gaseous atoms is formed from the elements in its standard states under standard conditions of 298K and 1 bar

Question 24

How do you calculate lattice energy?

Answer 24

| (q+) x (q-) / (r+) + (r-) |

(q+) x (q-) / (r+) + (r-) |

Question 25

Define first electron affinity

Answer 25

First electron affinity is the enthalpy change of when 1 mole of gaseous atoms accepts one mole of electron to form 1 mole of gaseous singly charges anions.

Question 26

Why is the first electron affinity exothermic?

Answer 26

The first electron is attracted to the positive nucleus

Question 27

Why is the second electron affinity endothermic?

Answer 27

An electron is being added to anion , energy is required to overcome the electrostatic repulsion between the negatively charged anion and the negatively charged electron to be added.

Question 28

Define ionisation energy

Answer 28

Ionisation energy is the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form one mole of gaseous singly charged cations.

Question 29

Why is the ionisation energy endothermic?

Answer 29

Energy is required to overcome the electrostatic forces of attraction between the valence electrons and the nucleus

Question 30

Define standard enthalpy change of hydration.

Answer 30

Standard enthalpy change of hydration is the enthalpy change of when 1 mole of gaseous ions is dissolved in an infinite volume of water under standard conditions of 298K and 1 bar.

Question 31

Why is enthalpy change of hydration always exothermic?

Answer 31

Bond-forming takes place between gaseous ions and water molecules due to ion-dipole attractions. Since there is no bond-breaking , the enthalpy change of hydration of an ion is always exothermic.