1.8- Thermodynamics

Question 1

Give the definition of enthalphy of atomisation

Answer 1

The enthalpy of atomisation of an element is the enthalpy change when 1 mole of gaseeous atom is formed from the element in its standard state

Question 2

What is the enthalpy of sublimation

Answer 2

The enthalpy change for a solid metal turning to gaseous atoms can also be called enthalpy of sublimation and will numerically be the same as the enthalpy of atomisation

Question 3

What is definition for bond dissociation enthalpy

Answer 3

The bond dissociation enthalpy is the standard molar enthalpy change when one mole of a covalent bond is broked into two gaseous atoms

Question 4

What is the bond dissociation enthalpy for diatomic molecules

Answer 4

For diatomic molecules the enthalpy of dissociation of the molecule is the same as 2x enthalpy of atomisation for an element

Question 5

What is the defintion of first ionisation enthalpy

Answer 5

The first ionisation enthalpy is the enthalpy change required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous ions with a 1+ charge

Question 6

What is the definition for the second ionisation enthalpy

Answer 6

The second ionisation is the enthalpy change to remove 1 mole of electrons from one mole of gaseous 1+ ions to produce one mole of gaseuous 2 + ions

Question 7

What is the definition for the first electron affinity

Answer 7

The first electron affinity is the enthalpy change that occurs when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of gaseous ions with a -1 charge, also this is exothermic for ions that normally form negative ions

Question 8

What is the definition for second electron affinity

Answer 8

The second electron affinity is the enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce 2- ions, this is endothermic because it takes energy to overcome the repulsive force between the negative ion and the electron

Question 9

What is the definition for lattice formation

Answer 9

The enthalpy of lattice formation is the standard enthalpy change when 1 mole of an ionic crystal lattice is formed from its constituent ions in gaseous form

Question 10

What is the enthalpy of lattice dissociation

Answer 10

The enthalpy of lattice dissociation is the standard enthalpy change when 1 mole of an ionic crystal lattice form is seperated into its constituent ions in gaseous form

Question 11

What is the enthalpy of hydration

Answer 11

Enthalpy change when one mole of gaseous ions become aqueous ions, this is always exothermic because bonds made between the ions and the water molecules

Question 12

What is the enthalpy solution

Answer 12

The enthalpy of solution is the standard enthalpy change when one mole of ionic solid dissolves in a large enough amount of water to ensure that the dissolved ions are well seperated and do not interact with one another

Question 13

What are the trends in lattice enthalpies

Answer 13

. Size of ions- The larger the ions, the less negative the enthalpies of lattice formation, as the ions become larger charges get further apart and have weaker attractive force

. The charges of an ion- The bigger the charge of the ion, the greater the attraction between the ions so the stronger the lattice enthalpy values

Question 14

What is the perfect ionic model

Answer 14

Theoretical lattice enthalpies assumes a perfect ionic model where the ions are 100% ionic and spherical and the attraction are purely electrostatic

Question 15

When is there a tendency towards covalent character

Answer 15

. When the positive ion is small
. The positive ion has multiple charges
. The negative ion is large
. The negative ion has multiple negative charges

Question 16

What happens when there is covalent character

Answer 16

The charge cloud gets distorted, the theoretical and experimental born haber values are different

Question 17

Why does calcium chloride have the formula CaCl2 and not CaCl and CaCl3

Answer 17

. The enthalpy of lattice formation for Ca+ and Cl- is not that endothermic as the charges are not that big

. The enthalpy of lattice formation for Ca2+ and Cl- is more endothermic as the charge of the calcium is greater, the lattice formed is more stable

. For the Ca3+ there is a very big increase in enthalpy to remove the 3rd electron from the Ca, this would not be compensated for by the stronger lattice enthalpy of formation, this would meen that the enthalpy of formation is then endothermic which is not feasible

Question 18

What is entropy

Answer 18

Entropy is a description of the number of ways atoms can share energy, if there is a large number of ways for arranging the energy then the system is disorded and the entropy is high

Question 19

When does a significant increase in the entropy occur

Answer 19

If there is a change of state from solid or liquid to gas
There is a significant increase in the number of molecules between the products and the reactants

Question 20

What is the formula for entropy

Answer 20

Entropy = Entropy of products - Entropy of reactants

Question 21

What is the formula for gibbs free energy

Answer 21

Gibbs = ^H - T^S

Make sure you convert the J into KJ

Question 22

What happens to gibbs free energy if there is an increase in entropy ]

Answer 22

If there is an increase in entropy then increasing temperature will make it more likely that G is negative and more likely that the reaction will occur

Question 23

What are the equations for enthalpy of solution

Answer 23

^H solution = ^H dissociation + ^H hydration (sum)
^H solution= -^H formation + ^H hydration (sum)

Question 24

Why are hydration enthalpies exothermic

Answer 24

Exothermic becuase negative ions are electrostatically attracted to the + H ions on the polar water molecules, positive ions are attracted to -O also

The higher the charge density the larger the hydration enthalpy, as the ions attract water molecules more strongly

Question 25

What does the enthalpy of solution tell us

Answer 25

Generally H solution is not very exo or endothermic so the hydration enthalpy is similar to the lattice enthalpym in general the substance is more soluble if the H sol is exothermic and if it insoluble it is likely that H sol is endothermic