F325 - Energy

Question 0

What does Hess’s law state?

Answer 0

That if a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route.

Question 1

What is lattice enthalpy?

Answer 1

The enthalpy change that accompanies the formation of one mole of an ionic compound from it’s gaseous ions under standard conditions.

Question 2

What is the standard enthalpy change of formation?

Answer 2

The enthalpy change that takes place when one mole of a compound is formed from to constituent elements in their standard states under standard conditions.

Question 3

What is the enthalpy change of atomisation?

Answer 3

The enthalpy change that takes place when one mole of gaseous atoms forms from the elements in it’s standard state.

Question 4

What is the first ionisation energy?

Answer 4

The enthalpy change accompanying the removal of one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.

Question 5

What is the second ionisation energy?

Answer 5

The enthalpy change accompanying be removal of one electron from each ion in one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions.

Question 6

What is the first electron affinity?

Answer 6

The enthalpy change accompanying the addition of one electron to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions.

Question 7

What is the second electron affinity?

Answer 7

The enthalpy change accompanying the addition of one electron to each ion in one mole of gaseous 1- ions to form one mole of gaseous 2- ions.

Question 8

Why is the second electron affinity endothermic?

Answer 8

Have to overcome the repulsion between the negative ion and the electron, more energy is needed.

Question 9

Born Haber stuff

Answer 9

Page 168-172

Question 10

What is the standard enthalpy change of solution?

Answer 10

The enthalpy change that takes place when one mole of a compound is completely dissolved in water under standard conditions.

Question 11

What two stages are involved when a solid dissolves?

Answer 11

Breakdown of the ionic lattice -
Exact opposite of producing the lattice enthalpy.
-Delta H LE

Hydration -
Delta H hyd
Standard enthalpy change if hydration.

Positively charged ions attracted to negative oxygen atoms in water, and vice versa.

Question 12

What is the standard enthalpy change of hydration?

Answer 12

The enthalpy change that takes place when one mole of isolated gaseous ions is dissolved in water forming one mole of aqueous ions under standard conditions.

Question 13

What governs the size of lattice enthalpy and the size of hydration enthalpy?

Answer 13

Ionic size

Ionic charge

Question 14

How does size affect lattice enthalpy?

Answer 14

As ionic radius increases:
The attraction between the ions decrease.
The lattice enthalpy becomes less negative/less exothermic.

Question 15

How does ionic charge affect lattice enthalpy and hydration enthalpy?

Answer 15

Lattice enthalpy becomes more exothermic/more negative as the ionic charge increases.

Higher charged ions attract more strongly.
More energy to overcome.

Question 16

What is the result of a highly negative lattice enthalpy and hydration enthalpy?

Answer 16

Strong attraction between it’s ions, it will have a high melting point.

Question 17

What is entropy?

Answer 17

S

The Quantative measure of the degree of disorder in a system.

Question 18

What values can entropy be?

Answer 18

Always a positive number because all systems possess some degree of disorder because particles are always in constant motion.

Question 19

How does entropy increase?

Answer 19

With increasing temperature.
Molecules move faster.
Solid —> liquid —> gas

If a reaction produces more gaseous molecules entropy increases.

Question 20

What is the standard entropy change of reaction?

Answer 20

The entropy change that accompanies a reaction in the molar quantities expressed In a chemical equation under standard conditions, all reactants and products being in their standard states.

Question 21

What is free energy change?

Answer 21

Delta G
The balance between enthalpy, entropy and temperature for a process.

Delta G = delta H - T delta S.

A process can take place simultaneously when delta G < 0

Question 22

What makes a process spontaneous?

Answer 22

If a chemical system becomes more stable and it’s overall energy decreases.
The overall energy decrease is due to both entropy and enthalpy.

Question 23

What is T delta S?

Answer 23

Energy derived from entropy.
T is measured in kelvin.

As temperature increases, the energy derived from entropy becomes more significant.

Question 24

What factors affect whether or not a process is spontaneous?

Answer 24

Temperature, T, in kelvin. Entropy change, Delta S. Enthalpy change. Delta H, with the surroundings. Free energy change G = H - T/\S

Question 25

When is a reaction spontaneous?

Answer 25

When /\G is negative, as there must be a decrease in the overall energy served from both the enthalpy change and the entropy of a system.

Question 26

How do you calculate /\S?

Answer 26

Sum of S (products) - sum of S (reactants)

Question 27

What makes an endothermic reaction spontaneous?

Answer 27

/\S must be positive. | The temperature must be high enough so that T/\S > /\H

Question 28

How do you convert *C into K?

Answer 28

Add 273

Question 29

What is electrode potential?

Answer 29

Tendency to lose it gain electrons in the equilibrium.

Question 30

What is the job of the wire connection and salt bridge in a half cell?

Answer 30

The wire connects the two metals, allowing electrons to be transferred between the two half cells. Usually also has a voltmeter with a high resistance to minimise the current that flows. ``` The salt bridge connects the two solutions, allowing ions to be transferred between the Half cells. Usually KNO3 (aq) or NH4NO3 (aq) is used ```

Question 31

How do you make a salt bridge?

Answer 31

Using a strip of filter soaked in an aqueous solution of an ionic compound that does not react wig either of the Half cell solutions.

Question 32

Describe a metal/metal ion half cell.

Answer 32

Eg Zn and Cu. Zn releases electrons more readily. This is the negative electrode. It releases electrons into the wire, which flow to the Cu electrode. Zn loses electrons and it's equation moves to the left.

Question 33

What does the reading on a voltmeter show?

Answer 33

The potential difference of the cell, which is the different between the electrode potentials of the half cells.

Question 34

Describe a non metal/non metal ion half cell.

Answer 34

Eg hydrogen: 2H+ (aq) + 2e- ---> H2 (g) Used as a reference standard for the measurement of cell electromotive force. Non metal and it's aqueous ions. There is no electrode to connect the wire to, so a platinum electrode is placed in solution in contact with both H2 (g) and H+ (aq). The platinum is inert so does not react. It only transfers electrons in and out of the half cell via a connecting wire. Coated in platinum black. 1 mol/dm3 HCl for H+ ions. H2 (g) at 100kPa/1 atmosphere.

Question 35

Describe a metal ion/metal ion half cell.

Answer 35

Ions of the same element in different oxidation states. Eg Fe2+ + e- ---> Fe3+ A solution contains both Fe2+ and Fe3+ of equal concentrations - equimolar. An inert platinum electrode allows electrons to pass into or out if the half cell via a connecting wire.

Question 36

What is the use of a platinum electrode?

Answer 36

To allow electrons to pass into or out of the Half cell via a connecting wire. Inert so does not react.

Question 37

What is the standard electrode potential?

Answer 37

The electromotive force of a half cell compared with a standard hydrogen half cell, measured at 298K with solution concentrations of 1 mol/dm3 and a gas pressure of 100kPa/1 atmosphere.

Question 38

What Is e.m.f?

Answer 38

Electromotive force. | The voltage produced by a cell when no current flows.

Question 39

What is the standard electrode potential of a hydrogen half cell?

Answer 39

0V

Question 40

What I the cell reaction?

Answer 40

The sum of the reduction and oxidation half reactions taking place in each half cell.

Question 41

How do you calculate the standard cell potential?

Answer 41

Te difference between the standard electrode potentials of the half cells. Positive - negative.

Question 42

Why may a feasible reaction not take place?

Answer 42

The reaction rate is very slow due to a high activation energy. The conditions are not the standard conditions. Conditions may vary, pH temp etc. Standard electrode potentials apply to aqueous equilibria only.

Question 43

What are the three types of cells/batteries?

Answer 43

Non rechargeable cells. Rechargeable cells. Fuel cells

Question 44

What are the advantages of fuel cell vehicles?

Answer 44

Less pollution & CO2: Combustion of hydrocarbon fuels creates CO2, incomplete combustion creates CO. Hydrogen rich fuels only produce a small amount Of CO2. Greater efficiency: In petrol energy 80% of the chemical energy is wasted as heat. Hydrogen fuel cell vehicles are 40-60% efficient in converting the fuels energy. This drops fuel consumption.

Question 45

What methods of fuel cell vehicles have been developed?

Answer 45

Hydrogen rich fuel cells: CH3OH + H2O ---> 3H2 + CO2 The hydrogen is then fed into the fuel cells. Fuel cells using methanol have also been developed. Liquid is easier to store than hydrogen gas. Methanol can be generated from biomass.