Chapter 17) ThemoDynamics

Question 1

Define Enthalpy change:

Answer 1

heat change at constant pressure

Question 2

what are the standard conditions of enthalpy change?

Answer 2

298Kelvin

100KPa

Question 3

Define the standard molar enthalpy of formation

Answer 3

is the enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions, all reactants and products in their standard states.

Question 4

Standard enthalpy of formation is always…

Answer 4

negative

Question 5

What is the standard enthalpy of formation of an elements?

Answer 5

0

Question 6

define the standard enthalpy of atomisation:

Answer 6

is the enthalpy change which accompanies the formation of one mole of gaseous atoms from the element in its standard state under standard conditions

Question 7

whats special about the products of atomisation?

Answer 7

they r always atoms , NOT molecules, even though ordinarily some substance wouldn’t be found in pairs in gas form like chlorine

Question 8

Define first ionisation energy:

Answer 8

The standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions each with a dingle POSITIVE charge

Question 9

Descibe 2nd ionisation energy

Answer 9

the loss of a mole of electrons from a mole of gasous singly positively charged ions.

Question 10

Define the first electron affinity:

Answer 10

the standard enthalpy change when a mole of gaseous atoms is converted to a mole of gaseous ions, with a single NEGATIVE CHARGE

Question 11

Define the 2nd electron affinity:

Answer 11

Enthalpy change when one mole of electrons is added to a mole of gaseous ions each with a single negative charge to form one mole of gaseous ions each with two negative charges.

Question 12

Define lattice enthalpy of formation:

Answer 12

the standard enthalpy change when one mole of solid ionic compound is formed from its gaseous ions.

Question 13

Describe (delta)L H of lattice enthalpy of formation ? explain why?

Answer 13

negative, as is exothermic reaction, as ionic bonds are made between oppositely charged ions in the lattice

Question 14

What is the lattice enthalpy of dissociation?

Answer 14

the standard enthalpy change when one mole of solid ionic compound dissociates into its gaseous ions

Question 15

Describe (delta) H of lattice enthalpy of dissociation ? explain why?

Answer 15

positive, as is endothermic reaction, as ionic bonds are broken between oppositely charged ions in the lattice

Question 16

whats the difference between first electron affinity and 2nd? explain:

Answer 16

1st=negative as energy is given out when an electron is attracted to the + nucleus of the atom.
2nd = positive as energy must be PUT IN to overcome the repulsion between an electron and a negatively charged ion.

Question 17

what does Hess’ law state:

Answer 17

the enthalpy change for a chemical reaction is always the same, whatever route is taken from the reactants to the products.

Question 18

Describe the (delta) H of ionisation enthalpies:

Answer 18

Always positive as energy has to be put in to pull an electron away from the attraction of the positively charged nucleus.

Question 19

Define enthalpy of hydration:

Answer 19

The standard enthalpy change when water molecules surround one mole of gaseous ions.

Question 20

Define enthalpy of solution:

Answer 20

the standard enthalpy change when one mole of solute dissolves completely in sufficient solvent to form a solution in which the molecules or ions are far enough apart not to interact with each other.

Question 21

Define mean bond enthalpy:

Answer 21

the enthalpy change when one mole of gaseous molecules each breaks a covalent bond to form 2 free radicals, averages over a range of compounds.

Question 22

How would you calculate the mean bond enthalpy of a C-H bond if you knew that the enthalpy of dissociation of methane = +1664 KJ/mol , also firstly why is this positive?

Answer 22

its positive as, you are breaking the covalent bonds so energy is going in.
1664/4=+416 KJ/mol

Question 23

Describe ionic bonding:

Answer 23

electrons are transferred from metal atoms to non-metal atoms
+ charged metal ions &- charged non-metal ions are formed that all have FULL STABLE outer shells of electrons
-ions arrange themselves into a lattice, so oppositely charged ions are touching each other.

Question 24

whats the difference between electron affinity and ionisation?

Answer 24

ionisation=electron loss

electron affinity= electron gain (like with heamoglobin and oxygen)