Energetics, Gases, Kinetics

Question 1

What is Hess’s law?

Answer 1

enthalphy change for a chemical rxn is the same regardless of the pathway taken provided the initial states of the reactants and final states of the products are the same

Question 2

What is standard enthalpy change of formation? How to calculate ΔHrxn?

Answer 2

the enthalpy change when 1 mole of compound is formed from its constituent elements in their standard states at 298K and 1 bar
ΔHrxn = sum of ΔHproducts - sum of ΔHreactants

Question 3

What is standard enthalpy change of combustion? How to calculate ΔHrxn? Endothermic or exothermic?

Answer 3

the enthalpy change when 1 mole of a substance is completely burnt in excess oxygen at 298K and 1 bar
ΔHrxn = sum of ΔHreactants - sum of ΔHproducts

Question 4

What is bond energy? How to calculate ΔHrxn? Endothermic or exothermic?

Answer 4

enthalpy change to break 1 mole of a covalent bond between 2 atoms in the gaseous state
ΔHrxn = sum of BE(bb) - sum of BE(bf)
always endothermic

Question 5

What is 1st/2nd ionisation energy? Endothermic or exothermic?

Answer 5

1st IE: of an element is the enthalpy change to remove 1 mole of electrons from 1 mole of gaseous atoms to form one mole of single charged gaseous cations

2nd IE: of an element is the energy required to remove 1 mole of electrons from 1 mole of singly charged positive gaseous ions to form 1 mole of doubly charged gaseous cations

always endothermic

Question 6

What is 1st/2nd electron affinity? Endothermic or exothermic?

Answer 6

1st EA: of an element is the enthalpy change when one mole of gaseous atoms gains one mole of electrons to give one mole of singly charged gaseous anions
exothermic

2nd EA: of an element is the enthalpy change when one mole of singly charged negative gaseous ions gains one mole of electrons to give one mole of doubly charged gaseous anions
always endothermic

Question 7

What is lattice energy? Endothermic or exothermic? What is the equation?

Answer 7

enthalpy change when one mole of pure solid ionic compound is formed from its constituent gaseous ions
exothermic

Question 8

What is standard enthalpy change of hydration? Endothermic or exothermic?

Answer 8

enthalpy change when one mole of the gaseous ions is dissolved in a large amount of water to form one mole of aqueous ions at 298K and 1 bar
exothermic
ΔHrxn proportional to modulus q/r

Question 9

What is standard enthalpy change of solution Endothermic or exothermic?

Answer 9

enthalpy change when 1 mole of the substance is completely dissolved in a large excess of solvent to form an infinitely dilute solution at 298K and 1 bar

Question 10

What is formula for amount of heat absorbed/released?

Answer 10

q =mcΔT
m= mass of solution
ΔT of solution

Question 11

What is the formula for ΔHrxn for exothermic and endothermic rxns?

Answer 11

endothermic q/n

exothermic -q/n

Question 12

What is entropy change ΔS?

Answer 12

entropy is a quantity which measures the degree of disorder of a system

Question 13

What is gibbs free energy ΔG?

Answer 13

combines effects of enthalpy change and entropy change
ΔG = ΔH - TΔS
ΔG less than 0 feasible
ΔG = 0 eqm
ΔG more than 0 not feasible

Question 14

What is standard enthalpy change of rxn ΔHr?

Answer 14

the enthalpy change when molar quantities of reactants stated in the chemical equation react to form products at 298K and 1 bar

Question 15

What is standard enthalpy change of neutralisation?

Answer 15

the enthalpy change when an acid reacts with a base to form one mole of water in dilute aqueous solution at 298K and 1 bar

Question 16

What is standard enthalpy change of atomisation?

Answer 16

standard enthalpy change of atomisation for an element is the enthalpy change to form one mole of gaseous atom from the element in its standard state at 298K and 1 bar

standard enthalpy change of atomisation for a compound is the enthalpy change to form gaseous atoms from one mole of the compound at 298K and 1 bar