Enthalpy

Question 1

Define activation energy

Answer 1

The minimum energy required to start a reaction by the breaking of bonds

Question 2

What are the standard conditions

Answer 2

Temperature= 298K (25°C)
Pressure= 100KPa (1atm)
Concentration of 1 moldm(-3)

Question 3

Define the standard enthalpy change of reaction

Answer 3

The enthalpy change that takes place when the number of moles of reactants specified in the balanced chemical equation react together under standard conditions

Question 4

Define standard enthalpy of formation

Answer 4

The enthalpy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions

Question 5

Define the standard enthalpy of combustion

Answer 5

The enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions

Question 6

Give the formula to calculate enthalpy changes from experimental values

Answer 6

Q=mc∆T

Q= heat change
m= mass of substance heated
c= specific heat capacity
∆T= change in temperature

Question 7

Define specific heat capacity

Answer 7

The quantity of energy required to heat 1g of a substance by 1°C

Question 8

Simple experiments to determine the enthalpy change of combustion often produce a result that’s less exothermic than the accepted value.
Suggest 2 reasons why there’s a difference

Answer 8

• Combustion may not have been complete
• Some heat energy may have been lost to the surroundings

Question 9

Give 3 reasons why it’s not always possible to measure an enthalpy change directly

Answer 9

• there’s a high activation energy
• there’s a slow rate of reaction
• multiple reactions are taking place

Question 10

Define Hess’s law

Answer 10

If a reaction can take place by more than 1 route and the initial and final conditions are the same, the total enthalpy change is also the same

Question 11

Define bond enthalpy

Answer 11

The enthalpy change that occurs when one mole of a given bond is broken in a molecule in the gas phase to form gaseous atoms

Question 12

Define average bond enthalpy

Answer 12

The average enthalpy change that takes place when 1 mole of a given bond is broken down in a molecule in the gas phase to form gaseous atoms

Question 13

What’s the formula for enthalpy of a reaction

Answer 13

∆H= bonds broken - bonds made

Question 14

Define lattice enthalpy

Answer 14

The enthalpy change that accompanies the formations of 1 mole of an ionic compound from its gaseous ions

Question 15

How do you know if lattice enthalpy is exothermic or endothermic

Answer 15

Lattice enthalpy is always an exothermic energy change

Question 16

What does it mean if there’s a large exothermic value for lattice enthalpy

Answer 16

There’s a large electrostatic attraction between the ions, showing that the bonds are strong

Question 17

Why don’t covalent compounds have a lattice enthalpy

Answer 17

Because they don’t have any ions in their structures

Question 18

Why can’t lattice enthalpy be measured directly by experiment

Answer 18

Because it’s impossible to form one mole of an ionic lattice from gaseous ions

Question 19

Define the standard enthalpy change of formation

Answer 19

The enthalpy change that occurs when one mole of a substance is formed from its elements in their standard states under standard conditions

Question 20

Define the standard enthalpy of atomisation

Answer 20

The enthalpy change accompanying the formation of one mole of gaseous atoms from its elements in its standard state

Question 21

How did you know if the standard enthalpy of atomisation is endothermic or exothermic

Answer 21

This is always endothermic as bonds are always broken

Question 22

Define the first ionisation energy

Answer 22

The energy required to remove 1 mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ion

Question 23

How do you know if the first ionisation energy is exothermic or endothermic

Answer 23

It’s always endothermic as energy is needed to overcome the attraction between the nucleus and outer electron

Question 25

Define the second ionisation energy

Answer 25

The energy required to remove one mole of electrons from one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions

Question 27

Define the first electron affinity

Answer 27

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

Question 28

How do you know if the first electron affinity is exothermic or endothermic

Answer 28

It's always exothermic as the electron is attracted into the outer shell of an atom by the nucleus

Question 29

Define the second electron affinity

Answer 29

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

Question 30

Give and explain all the factors that influence lattice enthalpy

Answer 30

1) Ionic size: - When the ions are smaller they can pack closer together and the attraction between the ions is stronger. Therefore, as the size of the ions increase, the lattice enthalpy becomes less negative. 2) Ionic charge: - As the charges on the ions increases, the electrostatic forces of attraction between them increases and the lattice enthalpy becomes more negative.

Question 31

Define the standard enthalpy of solution

Answer 31

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

Question 32

How do you know if the enthalpy change of solution is exothermic or endothermic

Answer 32

When the dissolving occurs, two processes occur: - breakdown of the ionic lattice into gaseous ions (endothermic) - hydration of the ions (exothermic). The overall enthalpy change depends on which of these energy changes is greater

Question 33

Define the enthalpy change of hydration

Answer 33

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 34

Give all the factors that affect enthalpy of hydration

Answer 34

1) ion size: - as the ions become smaller, the value of the enthalpy change of hydration becomes more negative. This is because hydration depends on the ability of an ion to attract and bond to a water molecule. Smaller ions exert more attraction on water molecules so more energy is released. 2) Charge: - an ion with a large charge exerts more attraction on water molecules so the enthalpy change is more exothermic.