Thermodynamics

Question 1

what is Hess’ law?

Answer 1

enthalpy change of reaction is independent of the route taken

Question 2

Define enthalpy of formation

Answer 2

enthalpy change when one mole of a substance is formed from its elements in standard conditions with all substances in their standard states

Question 3

what is Bond dissociation enthalpy?

Answer 3

endothermic
enthalpy change to break the bond in one mole of gaseous molecules to form gaseous atoms

X2(g) > 2X(g)

Question 4

what is the standard enthalpy of atomisation?

Answer 4

endothermic
The enthalpy change for the formation of one molecule of gaseous atom from the elements and its standard state

X(s) > X(g)

Question 5

what is first ionisation energy?

Answer 5

endothermic
enthalpy change when one mole of electrons is removed from one mole gaseous atoms to form one mole of gaseous 1+ ions

X(g) > X+(g) + e-

Question 6

What is second ionisation energy?

Answer 6

endothermic
enthalpy change when one mole of electrons is removed from one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions

X+(g) > X^2+(g) + 2e-

Question 7

why is second ionisation atom greater than the first?

Answer 7

more difficult to remove an electron from a +ive ion

Question 8

What is the first electron affinity?

Answer 8

exothermic
enthalpy change when one mole of electrons is added to one mole of gaseous atoms to form one mole of gaseous 1- ions

X(g) + e- > X-(g)

Question 9

Why is first electron affinity exothermic?

Answer 9

Due to attraction between the electron and nucleus

Question 10

What is the second electron affinity?

Answer 10

endothermic
The enthalpy change when one mole of electrons is added to one mole of gaseous 1- ions to form one mole of gaseous 2-

X-(g) + 2e- > X^2-(g)

Question 11

what is lattice formation enthalpy?

Answer 11

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

X+ + Y- > XY

Question 12

What is lattice dissociation enthalpy?

Answer 12

exothermic
The enthalpy change to separate one mole of solid ionic compound into gaseous ions

XY > Y- + X+

Question 13

What two factors affect lattice enthalpy?

Answer 13

charge and size of ions

Question 14

what is the theoretical value?

Answer 14

uses a perfect ionic model which assumes all ions are perfect spheres with a purely ionic attraction

Question 15

What does it mean if the experimental value is the same as the theoretical value?

Answer 15

There is no covalent character at the perfect ionic model

Question 16

what does it mean if the experimental value is different from the theoretical value?

Answer 16

It’s due to covalent character

Question 17

when are ions likely to be 100% ionic?

Answer 17

Where they are similar sizes that have the same charge

Question 18

when are ions likely to be ionic with covalent character?

Answer 18

When the cation is much smaller than the anion and is highly charged

Question 19

what is covalent character?

Answer 19

a Stronger ionic bond

Question 20

what does polarisation do to the experimental enthalpy of lattice formation?

Answer 20

it will be more exothermic than the theoretical value

Question 21

what does polarisation do to the experimental enthalpy of lattice dissociation ?

Answer 21

it will be more endothermic than the theoretical value

Question 22

What is enthalpy of solution?

Answer 22

Endo OR exothermic
Enthalpy change when one mole of a solid dissolves in water to form aqueous ions

NaCl(s) → Na+(aq) + Cl-(aq)

Question 23

What is enthalpy of hydration?

Answer 23

exothermic
Enthalpy change when one mole of gaseous ions become aqueous ions

Na+(g) → Na+(aq)

Question 24

Why is enthalpy of hydration exothermic?

Answer 24

due to attraction between the +ve ions and the delta -ve O in water and -ve ions and the delta +ve H

Question 25

What are the stages of solution formation

Answer 25

- lattice dissociation - hydration

Question 26

What is the equation for solution formation

Answer 26

Lattice diss + ∑Hydration

Question 27

How does charge effect lattice enthalpy?

Answer 27

higher charge = smaller radius **increasing the strength of attractions between ion and part of water** molecule with opposite charge

Question 28

what is entropy

Answer 28

a measure of **disorder** in a system

Question 29

When is a reaction feasible?

Answer 29

When there is an increase in disorder

Question 30

Which factors affect entropy

Answer 30

- how much disorder particles have - number of particles/moles

Question 31

How does number of particles/moles affect entropy

Answer 31

more moles = greater disorder

Question 32

What is the equation for entropy

Answer 32

∆S =∑prod - ∑reactants

Question 33

What is the equation for Gibbs free energy

Answer 33

∆G = ∆H - T∆S **(divide ∆S by 1000)**

Question 34

What are the units of ∆G

Answer 34

kJmol-1

Question 35

What are the units of ∆S

Answer 35

JK-1mol-1

Question 36

What is the value of ∆G in a feasible reaction

Answer 36

0 or below

Question 37

What is the equation for ∆S when the reaction is feasible

Answer 37

(∆G is 0) ∆H ÷ T

Question 38

Write the equation for ∆G to match y=mx+c

Answer 38

∆G = -∆S x T +∆H gradient = -∆S c (y intercept) = ∆H

Question 39

At what temperature will the reaction be feasible if ∆H -ve and ∆S +ve

Answer 39

any temperature

Question 40

At what temperature will the reaction be feasible if ∆H -ve and ∆S -ve

Answer 40

only at low temperatures

Question 41

At what temperature will the reaction be feasible if ∆H +ve and ∆S +ve

Answer 41

only at high temperatures

Question 42

At what temperature will the reaction be feasible if ∆H +ve and ∆S -ve

Answer 42

never

Question 43

How can ∆S explain why ∆G is negative

Answer 43

∆S is +ve moles increased meaning there was an **increase in disorder**

Question 44

How can ∆H explain why ∆G is negative

Answer 44

If Bonds broken = Bonds formed so ∆H is 0

Question 45

How can T∆S explain why ∆G is negative

Answer 45

if T∆S is bigger than ∆H then ∆G will be -ve