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Z A Level Chemistry > Energy Entropy > Flashcards

Energy Entropy Flashcards

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1
Q

What is the first ionisation energy?

A

The enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form unipositve ions

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2
Q

What is the second ionisation energy?

A

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

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3
Q

What is the second electron affinity?

A

The enthalpy change when one mole of 1- gaseous ions gains one mole of electrons to from one mole of 2- ions

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4
Q

What is the first electron affinity?

A

The enthalpy change when one mole of gaseous atoms gain one mole of electrons to form uninegative ions

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5
Q

What is the enthalpy of formation? What type of change is this?

A
  • The energy change when one mole of a compound is formed from its constituent elements in their standers states under standard conditions of 298K and 1atm
  • Endothermic
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6
Q

What is lattice enthalpy?

A

The energy change when one mole of an ionic compound is formed from its gaseous ions under standard conditions of 298K and 1atm

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7
Q

What is enthalpy of atomisation?

A

The enthalpy change when one mole of gaseous atoms is formed from its element in its standard states

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8
Q

What is bond enthalpy?

A

The enthalpy change when one mole of gaseous covalent bonds is broken to form gaseous ions

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9
Q

What is the order of a born harber cycle?

A
  1. Enthalpy change of formation (DeltaHf)
  2. Enthalpy of atomisation
  3. First ionisation energy (for anion) (metal) (and more if needed)
  4. First electron affinity (for cation) (non-metal) (and more if needed)
    - When drawing this stage watch out and 1st EA may be endothermic but 2nd EA may be exothermic s check before drawing the cycle!
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10
Q

What is the standard enthalpy change of solution?

A
  • The enthalpy change that takes place when one mole of a compound is completely dissolved in water under standard conditions
  • NaCl(s) + aq –> Na+ (aq) + Cl- (aq)
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11
Q

What are the two things that take place in the enthalpy of solution? What type of changes are these?

A
  1. The breakdown of the ionic lattice into gaseous ions (endothermic): Breaking down the crystal lattice by overcoming the attractive forced between the ions. This process has the same magnitude as the lattice enthalpy but a different sign!
  2. Hydration of the ions (exothermic): the water molecules are attracted to, and surround the ions
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12
Q

What is the standard enthalpy change of hydration?

A

The enthalpy change that take space when one mole of aqueous ions are formed from their gaseous ions under standard conditions

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13
Q

How can lattice enthalpies be calculated?

A
  1. Lattice enthalpies can be calculated from enthalpies of solution and hydration using a born harber cycle and Hess’ law
  2. DeltaHle = DeltaHhyd - DeltaHsol
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14
Q

How does ionic size affect lattice enthalpy?

A
  1. Ionic radius increases
  2. Attraction between ions decreases
  3. Lattice energy is less negative
  4. Melting point decreases
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15
Q

How does ionic charge affect lattice enthalpy?

A
  1. Ionic charge increases
  2. Attraction between ions increases
  3. Lattice enthalpy becomes more negative
  4. Melting point increases
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16
Q

How does ionic size affect hydration?

A
  1. Ionic radius increases
  2. Attraction between ion and water molecules decreases
  3. Hydration energy less negative
17
Q

How does ionic charge affect hydration?

A
  1. Ionic charge increases
  2. Attraction with water molecules increases
  3. Hydration energy becomes more negative
18
Q

Why is magnesium oxide insoluble in water?

A
  1. Mg2+ and P2- both have a high charge density
  2. Therefore the DeltaHlattice is very exothermic
  3. Therefore it is insoluble because the hydration enthalpy does not compensate for the lattice enthalpy
19
Q

What determines if something dissolves?

A

If the enthalpy change of solution is exothermic it will dissolve if it is endothermic it will not dissolve

20
Q

Why is energy released when ions are hydrated?

A

Because bonds are formed between the gaseous ions and the water molecules which is an exothermic process

21
Q

Why is energy required to break up an ionic lattice?

A

As the electrostatic attraction between the ions needs to be overcome and this is endothermic

22
Q

Explain why the lattice enthalpy of NaBr is more exothermic than for KBr (2)

A
  1. Na+ has a smaller ionic radius than K+ and so has a higher charge density
  2. Therefore more energy is needed to break them apart
23
Q

Magnesium chloride has stronger ionic bonds than sodium chloride. Explain why (3)

A
  1. Mg2+ has a greater charge than Na+ and so has a greater charge density
  2. This means that there is a stronger electrostatic attraction to the chloride
  3. Therefore more energy is needed to break up
24
Q

Explain the differences between the enthalpy change of hydration values for Na+ and Mg2+ (3)

A
  1. Mg2+ has a more exothermic value because it has a larger charge density
  2. Therefore more energy is needed to?
25
Q

What is entropy? How do you calculate it?

A
  • It is measure of the dispersal of energy in a system which is greater, the more disorder a system
    1. We can think of entropy as a measure of randomness and chaos
    2. Every chemical reaction has an associated entropy change
    3. DeltaS = products - reactants
26
Q

How do you know the sign of DeltaS?

A
  1. If a system becomes more random, energy can be spread out more and deltaS is positive (more disorder) (liquid to gas)
  2. If a system changes to become less random, energy becomes more concentrated and deltaS is negative (less disorder) (liquid to solid)
  3. The greater the entropy, the greater the dispersal of energy and the greater the disorder
27
Q

Describe Gibbs Free Energy

A
  1. Reactions only happen when the products have a lower overall energy than the reactions
  2. Overall energy is represented by the Gibbs equations and has two contributions:
    - Enthalpy change
    - The entropy change at the temperature of the reaction
    - DeltaG = DeltaH -TDeltaS
  3. If DeltaG < 0 the reaction is said to b energetically feasibly and will occur at the given temperature
28
Q

What can Gibbs Free Energy tell us?

A
  1. Whether a reaction is feasible:
    -Feasible reaction are spontaneous at T when G<0
    -Feasible reactions have a lower overall energy in their products
  2. Why endothermic reactions can take place at room temperature
    -If DeltaH is positive, DeltaG is less than 0 when TDeltaS>DeltaH and Delta S is positive
  3. What the minima temperature for reaction feasibility might be
    T > DelatH / DeltaS
29
Q

What are some of the limitations of Gibbs Free Energy?

A
  • Some reactions have a negative deltaG bur do not seem to take place
    1. Diamond into graphite
    2. Decomposition of hydrogen peroxide into water and oxygen
30
Q

What does feasibility mean?

A

“Feasibility” just means that a reaction is theoretically possible at that temperature. It does not tell us anything about the speed of the reaction. Kinetics is just as important as thermodynamics when considering reaction. Some reactions are feasible, but infinitely slow due to a very large activation energy barrier

31
Q

What is entropy like in changes of state?

A
  1. Entropy increases during changes in state that give a more random arrangement of particles: solid->liquid->gas
  2. So when any substance changes state from a solid to liquid to gas, its entropy increases
    - Melting and boiling increase the randomness of particles
    - Energy is spread out more and DeltaS is positive
32
Q

What is entropy like in changes in the number of gaseous molecules?

A
  1. Reactions that produce gases result in an increase in entropy, for example calcium carbonate reaction with hydrochloric acid:
    - Production of a gas increases the disorder of particles
    - Energy is spread out more and deltaS is positive
  2. You can precept the sign of the entropy change for reaction where the reactant and products have different numbers of gas molecules, for example in the reaction of nitrogen and hydrogen gases to produce ammonia results in a decreases in the number of gas molecules
    - There is a decrease in the randomness of particles
    - The energy is spread out less and DeltaS is negative
33
Q

How could you obtain hydrogen gas for the Haber process?

A

Electrolysis of water

34
Q

Why may a reaction not be feasible at very high temperatures?

A

As temperature increase TdelatS becomes more negative and becomes too large in magnitude so that it outweighs deltaH and eventually deltaH-TdeltaS becomes positive

35
Q

Predict how the enthalpy changes of hydration of F- and Cl- would differ?

A
  1. Enthalpy of hydration for F- more negative/exothermic than for Cl- as F- has a smaller size than Cl-
  2. F- so smaller sized ion so greater attraction to H2O
36
Q

What is the reason for the difference in enthalpy change in solution for KF and RbF (more exothermic)?

A
  1. K+ ionic radius is smaller so greater charge density
  2. Lattice enthalpy of KF is more negative than RbF
  3. DelatHhydration of K+ is more negative than for Rb+ (K+ has greater attraction for H2O)
  4. Enthalpy change of solution affected more by lattice enthalpy than by hydration enthalpy
37
Q

Why does ammonium nitrate in the cold pack dissolve spontaneously in water even though this process is endothermic?

A
  1. During dissolving entropy/ disorder is increased

2. TdeltaS is greater than deltaH so deltaH-TDeltaS is negative

Z A Level Chemistry (13 decks)

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