Chapter 22 - enthalpy and entropy Flashcards by Rebecca Williams

Define lattice enthalpy

The enthalpy change that accompanies the formation of 1 mole of an ionic lattice from its gaseous ion`s under standard conditions of 298K and 100kPa

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Define standard enthalpy change of formation

The enthalpy change that takes place when 1 mole of a compound is formed from its elements under standard conditions of 100kPa and 298K with each element in their standard states

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Define standard enthalpy change of atomisation

The enthalpy change that takes place for the formation of 1 mole of an element in gaseous form from the element in its standard state under standard conditions of 100kPa and 298K

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Define first ionisation energy

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

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Define first electron affinity

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

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Define second ionisation energy

The enthalpy change required to remove 1 electron from each ion in 1 mole of 1+ ions to form 1 mole of 2+ ions

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Define second electron affinity

The enthalpy change that occurs when 1 electron is added to each ion in 1 mole of gaseous 1- ions to form 1 mole of gaseous 2- ions

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Is lattice enthalpy endothermic or exothermic? Why?

Exothermic because energy is given out when ionic bonds are formed from gaseous ions

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Is the standard enthalpy change of atomisation endothermic or exothermic? Why?

Endothermic because bonds are broken to form gaseous atoms which requires energy

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Is the standard enthalpy change of formation endothermic or exothermic?

It can be either but it is usually exothermic

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Is first ionisation energy endothermic or exothermic? Why?

Endothermic because the electron being lost has to overcome attraction from the nucleus in order to leave the atom which requires energy

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Is second ionisation energy endothermic or exothermic? Why?

Endothermic because the electron being lost has to overcome attraction from the nucleus in order to leave the atom which requires energy

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Is first electron affinity endothermic or exothermic? Why?

Exothermic because the electron is being attracted into the outer shell of an atom by the nucleus

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Is second electron affinity endothermic or exothermic? Why?

Endothermic because the electron is repelled by the 1- ions which has to be overcome which requires energy

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What does lattice enthalpy indicate?

The strength of an ionic lattice

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What does lattice enthalpy measure?

A measure of how strong an ionic bond is in a giant ionic lattice

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What is a Born - Haber cycle?

An energy diagram used to calculate lattice enthalpies by taking an indirect route

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List 2 factors that affect lattice enthalpy

size of ions

ionic charge

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Explain how ionic charge influences lattice enthalpy

Across a period, ionic charge increases, meaning attraction between ions increases, and lattice enthalpy becomes more exothermic, as ionic bonds are stronger between the oppositely charged ions, and thus lattice enthalpy is increased

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Explain how size of ions influences lattice enthalpy

Ionic size increases down a group
And the distance between ions in lattice increases as there are more shells
Therefore, the attraction between the ions decreases
And lattice enthalpy becomes less exothermic

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The more negative the lattice enthalpy the ____ the melting point

Higher

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Define enthalpy change of solution

The enthalpy change that takes place when 1 mole of a solute dissolves in a solvent under standard conditions of 298K and 100kPa

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Give an equation showing enthalpy change of a solution

NaCl (s) + aq –> Na+ (aq) + Cl- (aq)

Give the experimental procedure to calculate enthalpy change of solution

Weigh out sample using mass balance
Measure out known volume of water using a measuring cylinder
Measure the temperature of water using a thermometer
Stir the mixture using the thermometer until all has dissolved and temperature no longer changes. Record this value
Use q=mc. T to calculate energy in j
Use enthalpy change of solution =(q/1000 - needs to be in kJmol-1) /n to calculate enthalpy change, where n = moles and q= energy

What are the stages of dissolving a solute in water?

The ionic lattice is broken down into gaseous ions | The ions then become hydrated so the ions bond with water molecules

Define enthalpy change of hydration

The enthalpy change that takes place when gaseous ions are dissolved in water to form 1 mole of aqueous ions under standard conditions of 298K and 100kPa

List 2 factors affecting enthalpy of hydration

size of ions | ionic charge

Explain how size of ions affects enthalpy change of hydration

Distance between ions increases as ionic size increases Therefore, the attraction between the ions and water molecules decreases And hydration enthalpy becomes less exothermic

Explain how ionic charge of the enthalpy change of hydration

The attraction between ions and water molecules increases as ionic charge increases Hydration enthalpy therefore becomes more exothermic

What is entropy?

A measure of how dispersed energy is within a system

What does it mean when entropy is greater?

The dispersal of energy and the disorder of the molecules is greater

Does entropy increase or decrease over time?

increase, as energy will disperse over time

What are the units of entropy?

JK-1mol-1

Which has the greatest entropy? Solids, liquids or gases

gases, as they have the greatest dispersal of energy

At 0K how much entropy do perfect crystals have?

zero entropy

When a system changes, if the energy becomes ----- then the entropy will increase, and the entropy change is ---

more dispersed | positive

When a system changes, if the energy becomes -----then the entropy will decrease, and the entropy change is -----.

less dispersed/ more concentrated | negative

List 4 instances when entropy in a system increases

Melting Boiling Number of gaseous molecules increases Dissolving a substance

Define standard entropy

the entropy of 1 mole of a substance under standard conditions of 100kPa and 298K

Give the equation to calculate entropy change in a system

Entropy change = the sum of the entropy of the products – the sum of the entropy of the reactants

What does it mean for a reaction to be feasible?

It will occur spontaneously

When is a reaction feasible (in terms of energy change)

A chemical system becomes more stable and its overall energy decreases

What is overall energy of a system known as?

Free energy change

What does the Gibbs equation calculate?

The equation to calculate free energy change

What is the Gibbs equation (give formula)

Free energy change Jmol-1 = enthalpy change kJmol-1 – entropy change at a specific temperature

How do you calculate entropy change at a specific temperature?

(temperature K x entropy change JK-1mol-1)

How do you convert from celcius to kelvin?`

ADD 273

For a reaction to be feasible the free energy change has to be less than ___

zero

If the enthalpy change is negative and entropy change is positive will the reaction be feasible?

Yes, because the free energy change is less than zero. -10 - +10 = -20 Remember, you are subtracting entropy, so if it is positive, a negative sign and a positive sign make a negative

If the enthalpy change is positive and the entropy change is negative will the reaction be feasible? why?

No, because free energy change will always be more than 0 10 - - 10 = 20 Remember, you are subtracting entropy, and a negative sign and negative sign make a positive

What other factors need to be considered when considering if a reaction will occur?

activation energy | rate of reaction

If the enthalpy change is negative and the entropy is negative, what conditions are needed in order for the reaction to be feasible? Why?

Low temperatures are needed, so that T^S is smaller than enthalpy -10 --7 = -3

If the enthalpy change is positive and the entropy is positive what conditions are needed in order for the reaction to be feasible? Why?

High temperatures are needed, so that T^S is bigger than enthalpy, so that the overall reaction is negative 10 - +17 = -7

What is the enthalpy change of an endothermic reaction?

Positive

What is the enthalpy change of an exothermic reaction?

Negative

In order for an endothermic reaction to be feasible, what conditions are there?

T^S must be positive, and must be larger than ^H, so temperatures must be high so that the overall reaction is negative 10 - +17 = -7 - this makes sense with Le Chatelier so double check answers

In order for an exothermic reaction to be feasible, what conditions are there?

T^S must either be positive, or more less negative than ^H, so that the overall energy change is still negative. Either -10 - + 7 = -17 or -10 - - 3 = -7