Enthalpy And Entropy

Question 1

What is lattice enthalpy?

Answer 1

A measure of the strength of ionic bonding in a giant ionic lattice. The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions.

Question 2

What is always true of the enthalpy value for lattice enthalpy?

Answer 2

It is always an ENDOTHERMIC change and the value for the enthalpy change will always be negative

Question 3

What is a Born-Haber cycle?

Answer 3

The indirect determination of lattice enthalpy

Question 4

What are the three different processes in route 1 of a Born-Haber cycle?

Answer 4

Formation of gaseous atoms, formation of gaseous ions, lattice formation

Question 5

What is the standard enthalpy change of formation?

Answer 5

The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions with all reactants and products in their standard states

Question 6

What is the standard enthalpy change of atomisation?

Answer 6

The enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions

Question 7

What is the first ionisation energy?

Answer 7

The enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions

Question 8

Why are ionisation energies endothermic?

Answer 8

Because energy is required to overcome the attraction between a negative electron and the positive nucleus

Question 9

True or false? Electron affinity is the opposite of ionisation energy

Answer 9

True

Question 10

What is first electron affinity?

Answer 10

The enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions

Question 11

Why is the second electron affinity endothermic?

Answer 11

A second electron is being gained by a negative ion which repels the electron away. So, more energy must be put in to force the negatively charged electron onto the negative ion.

Question 12

True or false? Water molecules are able to break up the giant ionic lattice structure and overcome the strong electrostatic attractions between oppositely charged ions

Answer 12

True, this is what happens when a salt dissolves in water.

Question 13

What is the standard enthalpy of solution?

Answer 13

The enthalpy change that takes place when one mole of a solute dissolves in a solvent.

Question 14

Explain how the dipole on the water allows sodium chloride to dissolve in water.

Answer 14

The partial negative charge on the oxygen atom is attracted to the positive sodium ion. The partial positive charge on the hydrogen atoms is attracted to the negative chloride ion.

Question 15

What is the enthalpy change of hydration?

Answer 15

The enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions.

Question 16

Give three general properties of ionic compounds.

Answer 16

1) high melting and boiling points
2) soluble in polar solvents
3) conducts electricity when molten or in aqueous solution

Question 17

What effect does ionic size have on lattice enthalpy?

Answer 17

As the ionic radius increases, the attraction between ions decreases. This means that lattice enthalpy is less negative and so the melting point decreases

Question 18

What effect does ionic charge have on lattice enthalpy?

Answer 18

As the ionic charge increases the attraction between ions increases. Lattice enthalpy becomes more negative and so the melting point increases

Question 19

What uses do very stable metal oxides have?

Answer 19

Coating the inside of furnaces.

Question 20

What effect does ionic size have on hydration enthalpy?

Answer 20

Ionic radius increases so the attraction between the ion and the water molecules decreases. This makes the hydration energy less negative.

Question 21

What effect does ionic charge have on hydration enthalpy?

Answer 21

As the ionic charge increases the attraction to the water molecules also increases. This makes the hydration enthalpy more negative

Question 22

How would you predict the solubility of a compound knowing only the hydration and lattice enthalpies?

Answer 22

If the sum of the hydration enthalpies is larger than the size of the lattice enthalpy, the overall enthalpy change will be exothermic and the compound should dissolve.

Question 23

What is entropy (S)?

Answer 23

The dispersal of energy within the chemicals making up the chemical system

Question 24

What are the units for entropy?

Answer 24

JK^-1mol^-1 (joules/kelvin/mol)

Question 25

In general what is the order of states in terms of increasing entropy?

Answer 25

Solids, liquids, gases

Question 26

At 0K what would the entropy value of a substance be?

Answer 26

0

Question 27

If a system changes to become more random, energy is spread out more. Predict the direction of the enthalpy change

Answer 27

Positive enthalpy change

Question 28

If a system changes to become less random, energy becomes more concentrated. Predict the direction of the enthalpy change

Answer 28

The entropy change will be negative

Question 29

True or false? When any substance changes state from solid to liquid to gas its entropy increases

Answer 29

True!

Question 30

Which state is the most disordered?

Answer 30

Gas

Question 31

How can you predict the sign of the entropy change from an equation?

Answer 31

You look at the moles of gas on each side. If there are more moles of gas on the reactant side than the product side it is becoming more ordered so entropy change will be negative. The reverse is also true

Question 32

What is standard entropy?

Answer 32

The entropy of one mole of a substance under standard conditions

Question 33

What is the calculation for entropy change in a reaction?

Answer 33

Entropy change= sum of entropy change of products - sum of entropy change of reactants

Question 34

What is the meaning of the term feasibility?

Answer 34

Whether a reaction is able to happen and is energetically feasible (spontaneous)

Question 35

What is meant by free energy change?

Answer 35

The overall change in energy during a chemical reaction

Question 36

What two types of energy make up free energy change?

Answer 36

1) enthalpy change | 2) entropy change at the temperature of the reaction

Question 37

What is the Gibbs free energy equation?

Answer 37

Free energy change= enthalpy change with surroundings-temp(in K) multiplied by entropy change

Question 38

What must happen to the free energy for a reaction to be feasible?

Answer 38

It must decrease

Question 39

Using the Gibb's equation, what must you remember to do to the entropy change value?

Answer 39

Divide by 1000

Question 40

The free energy change is useful for predicting feasibility but many reactions have a negative free energy change and do not take place. Explain one reason why.

Answer 40

It may have a very large activation energy resulting in a very slow rate of reaction