Enthalpy and Entropy (Chapter 22) Flashcards
What is the definition of lattice enthalpy?
The enthalpy change that accompanies the formation of one mole of an ionic compound (lattice) from its gaseous ions, under standard conditions
Give an example of a lattice enthalpy equation
Na+(g) + Cl-(g) = NaCl(s)
Is lattice enthalpy exothermic or endothermic?
Always exothermic, therefore enthalpy change is always negative
What is the definition of first electron affinity (E.A)?
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
What is the definition of second electron affinity (E.A)?
The enthalpy change when one electron is added to each 1- ion in one mole of gaseous 1- ions to form one mole of gaseous 2- ions
Why is 1st E.A exothermic?
Because the electron being added is attracted in towards the nucleus (doesn’t require energy, does it on its own) + mexo
Why is 2nd E.A endothermic?
- Because a second electron is being gained by a negative ion, which repels the electron away
- Therefore, energy must be put in to force the negatively-charged electron onto the negative ion
What is the definition of the enthalpy of atomisation?
The enthalpy change when one mole of gaseous atoms is formed from its element
Why is the enthalpy of atomisation endothermic?
Because bonds are broken to form gaseous atoms
What is the definition of the enthalpy change of solution?
The enthalpy change when one mole of a solute is completely dissolved in a solvent
What is the definition of the enthalpy change of hydration?
The enthalpy change that occurs when one mole of isolated gaseous ions is dissolved in water to form one mole of aqueous ions
What happens to the ions when an ionic compound is dissolved in water?
- The ions from the ionic lattice are surrounded with water molecules as aqueous ions
- The d- oxygen atom is attracted to the positive ion
- The d+ hydrogen atoms are attracted to the negative ion
What are the two processes that take place when a solid ionic compound dissolves in water?
- The ionic lattice breaks up
- Water molecules are attracted to, and surround, the ions
What are the two types of energy change involved when a solid ionic compound dissolves in water?
- The ionic lattice is broken up forming separate gaseous ions (opposite of lattice enthalpy)
- The separate gaseous ions interact with polar water molecules to form hydrated aqueous ions (enthalpy change of hydration)
Why can the enthalpy change of solution be exothermic or endothermic?
- It depends on the relative sizes of the lattice enthalpy and the enthalpy of hydration
What are the two factors that affect lattice enthalpy?
Ionic size and ionic charge (charge density)
How does ionic size affect lattice enthalpy?
- Ionic size increases
- Charge density decreases
- Attraction between ions decreases
- Lattice enthalpy becomes less negative (less exothermic)
- Melting point decreases
How does ionic charge affect lattice enthalpy?
- Ionic charge increases
- Charge density increases
- Attraction between ions increases
- Lattice enthalpy becomes more negative
- Melting point increases
Why does the ionic size of cations across a period decrease?
Because there are more protons attracting the same number of electrons
What is the effect of ionic charge and size for cations in period 3?
- Increasing charge gives more attraction
- Increasing size gives more attraction
What is the effect of ionic charge and size for anions in period 3?
- Increasing charge gives more attraction
- Increasing size gives less attraction
What is a use of stable metal oxides?
Protective coating for the inside of furnaces
How does ionic size affect hydration enthalpy?
- Ionic radius increases
- Attraction between ion and water molecules decreases
- Hydration enthalpy becomes less negative (bonds between ions and water molecules are weaker)
What must happen when an ionic compound is dissolved in water?
The attraction between the ions in the ionic lattice must be overcome
When should a compound dissolve?
When the sum of the hydration enthalpies is larger than the magnitude of the lattice enthalpy, the overall enthalpy change (of solution) will be exothermic
What is the symbol for enthalpy of solution?
deltaHsol
What is the symbol for enthalpy of hydration?
deltaHhyd
What is the symbol for entropy?
S
What is entropy?
- A measure of the dispersal of energy in a system which is greater, the more disordered a system
- A measure of the ‘disorder’ of a system
What happens in natural thermodynamic processes?
The sum of the entropies of the interacting thermodynamic systems increases i.e. the entropy of the universe must always increasee
What is the ‘universe’ in relation to entropy?
The system and the surroundings
What are the units for entropy?
J/mol/K
How can you calculate deltaS?
deltaS products - deltaS reactants
Why does entropy increase as substances go from solid to liquid to gas?
- Melting and boiling increase the randomness of particles
- Energy is spread out more (there is more disorder) and deltaS is positive
Why does the production of gas particles (or more moles of gas particles) increase entropy?
- Production of gas increases the disorder of particles as energy is spread out more
Are standard entropies positive or negative?
Always positive, because at 0K, entropy is 0
When does a reaction happen?
- When the products have an overall lower energy than the reactants
- When there is a decrease in free energy
- When deltaG < 0
What term is used to describe whether a reaction is able to happen?
Feasible or spontaneous
What is the free energy change (deltaG) ?
The overall change in energy during a chemical reaction
What is the relationship between deltaG, deltaH, deltaS and T?
deltaG = deltaH - TdeltaS
How do you calculate the minimum temperature that a reaction will take place?
Make deltaG equal to 0
Why don’t some reactions with a negative deltaG take place?
Because the reaction is too slow due to its very high activation energy
What is lattice enthalpy a measure of?
The strength of ionic bonding in a giant ionic lattice
How do you work out any missing values in a Born-Haber cycle?
Go from the start of the arrow to the end of the arrow the long way round
What do you need to remember when doing Born-Haber cycle calculations?
Negative values when going down the page, positive when going up
What is the relationship between enthalpy change of solution, hydration and lattice enthalpy?
Sol = -latt + hyd (hyd = sol + latt)
How does ionic charge affect hydration enthalpy?
- Ionic charge increases
- Attraction with water molecules increases
- Hydration enthalpy becomes more negative
How do you calculate entropy change?
The entropy of products - entropy of reactants
If a reaction occurs what must deltaG be?
Negative
If bubbles are produced in an experiment, what does this indicate about the entropy change?
That there is a positive entropy change
