Chapter 22 Flashcards
Why are solid ionic compounds very stable?
1) strength of ionic bonds
2) electrostatic attractions between oppositely-charged ions in ionic lattice
What does lattice enthalpy measure?
the strength of ionic bonding in a giant ionic lattice
What is lattice enthalpy?
the enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions
K⁺(g) + Cl⁻(g) –>
KCl(s)
Is lattice enthalpy endothermic or exothermic?
exothermic
Does lattice enthalpy break or make ionic bonds from separate gaseous ions?
make
Can lattice enthalpy be measured directly?
no
What is route 1 of a Born-Haber cycle?
1) formation of gaseous atoms
2) formation of gaseous ions
3) lattice formation
What is route 2 of a Born-Haber cycle?
converting the elements in their standard states directly to the ionic lattice - ΔfH
What is the standard enthalpy change of formation, ΔfH?
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
What is the standard enthalpy change of atomisation ΔatH?
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
Is ΔatH endothermic or exothermic?
endothermic
Why is ΔatH endothermic?
bonds are broken to form gaseous atoms
What is the first ionisation energy, ΔIEH?
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
Are ionisation energies endothermic or exothermic?
endothermic
Why are ionisation energies endothermic?
energy is required to overcome the attraction between a negative electron and the positive nucleus
What is the first electron affinity, ΔEAH?
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
Are first electron affinities endothermic or exothermic?
exothermic
Why are first electron affinities exothermic?
the electron being added is attracted towards the nucleus
Are second electron affinities endothermic or exothermic?
endothermic
Why are second electron affinities endothermic?
1) a second electron is being gained by a negative ion - repels the electron away
2) energy must be put in to force the negatively-charged electron onto the negative-ion
What is the standard enthalpy change of solution, ΔsolH?
the enthalpy change that takes place when one mole of a solute dissolves in a solvent
What happens with the standard enthalpy change of solution, ΔsolH is the solvent is water?
the ions from the ionic lattice finish up surrounded with water molecules as aqueous ions
Is the enthalpy change of solution endothermic or exothermic?
either depending on relative sizes of lattice enthalpy + enthalpy changes of hydration
What processes take place when a solid ionic compound dissolves in water?
1) ionic lattice breaks up
2) water molecules are attracted to, and surround, the ions
What energy changes take place when a solid ionic compound dissolves in water?
1) ionic lattice is broken forming separate gaseous ions
2) separate gaseous ions interact with polar water molecules to form hydrated aqueous ions - enthalpy change of hydration
What is the enthalpy change of hydration, ΔhydH?
the enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions
What are the properties of ionic compounds?
1) high melting + boiling points
2) soluble in polar solvents
3) conduct electricity when molten/in aqueous solution
What factors affect lattice enthalpy?
1) ionic size
2) ionic charge
How does ionic size affect lattice enthalpy down a group?
1) ionic radius increases
2) attraction between ions decreases
3) lattice energy less negative
4) melting point decreases
How does ionic charge affect lattice across down a period?
1) ionic charge increases
2) attraction between ions increases
3) lattice energy becomes more negative
4) melting point increases
What does the magnitude of lattice energy indicate?
1) melting point of an ionic compound
2) packing of ions in an ionic lattice
What factors affect hydration enthalpies?
1) ionic size
2) ionic charge
How does ionic size affect lattice enthalpy down a group?
1) ionic radius increases
2) attraction between ion and water molecules decreases
3) hydration energy less negative
How does ionic charge affect lattice enthalpy across a period?
1) ionic charge increases
2) attraction with water molecules increases
3) hydration energy becomes more negative
What has to happen to dissolve an ionic compound in water?
the attraction between the ions in the ionic lattice must be overcome
What does it mean when the sum of hydration enthalpies is larger than the magnitude of the lattice enthalpy?
1) overall energy change is exothermic
2) compound should dissolve
What does it mean when the sum of hydration enthalpies is smaller than the magnitude of the lattice enthalpy?
1) overall energy change is endothermic
2) compound should not dissolve
What do the reasons for solubility depend on?
1) temperature
2) entropy
What does entropy, S explain?
things that occur naturally
The greater the entropy, the greater the…
1) dispersal of energy
2) disorder
What is the term entropy used for?
the dispersal of energy within the chemicals making up the chemical system
What are the units of entropy?
J K⁻¹mol⁻¹
What are the general entropies with regard to different states?
1) solids = smallest entropies
2) liquids = greater entropies
3) gases = greatest entropies
Do systems that are more chaotic have a higher or lower entropy value?
higher
If a system becomes more random, is the entropy change positive or negative?
positive
If a system becomes less random, is the entropy change positive or negative?
negative
Does melting/boiling increase or decrease the randomness of particles?
increase
Do reactions that produce gases increase or decrease entropy?
increase
Why do reactions that produce gases increase entropy?
1) increases disorder of particles
2) energy is spread out more - change in entropy is positive
Do reactions that result in a decrease of gas molecules increase or decrease entropy?
decrease
What is the standard entropy S of a substance?
the entropy of one mole of a substance, under standard conditions
Are standard entropies positive or negative?
positive
entropy change of a reaction =
sum of standard entropies of products - sum of standard entropies of reactants
What is feasibility used to describe?
whether a reaction is able to happen + is energetically feasible (spontaneous)
What is the free energy change, ΔG?
overall energy change during a chemical reaction
What two types of energy is the free energy change, ΔG made up of?
1) enthalpy change ΔH - heat transfer between chemical system + surroundings
2) entropy change at the temperature of the reaction TΔS - dispersal of energy within the chemical system
ΔG =
ΔH - TΔS
