Chapter 22 - Enthalpy and entropy Flashcards
What does lattice enthalpy measure?
The strength of the ionic bonds in a giant ionic lattice
What is the definition of lattice enthalpy?
The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions
Why will the value for lattice enthalpy always be negative?
It is an exothermic change
What is the name of the cycle used to indirectly determine the enthalpy change of a reaction?
The Born-Haber cycle
What are the three different processes covered in route 1?
Formation of gaseous atoms
Formation of gaseous ions
Lattice formation
What type of reaction of the formation of gaseous atoms?
Endothermic - bonds are broken
What type of reaction is the formation of gaseous ions?
Endothermic
What type of reaction is lattice enthalpy?
Exothermic
What processes are covered in route 2?
The enthalpy change of formation, which is exothermic
What is the standard enthalpy change of formation?
The enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in the standard state under standard conditions
What is the first ionisation energy?
The enthalpy change required to remove one electron from each atom in a mole of gaseous atoms
What type of reaction is ionisation energy?
Endothermic - energy is required to overcome the force of attraction between the nucleus and electron
What is the first electron affinity?
The enthalpy change that takes place when one electron is added to each atom in a mole of gaseous atoms
What type of reaction is electron affinity?
Exothermic - The electron being added is attracted to the nucleus
When are multiple ionisation energies required?
When the atom has more than one electron to lose
Why are second ionisation energies endothermic?
Energy must be used to ensure the negatively charged ion accepts the negatively charged electron
What is the standard enthalpy change of solution?
The enthalpy change that occurs when one mole of a solute dissolves in a solvent
Why does water dissolve sodium chloride?
Sodium chloride forms a giant ionic lattice with the Na+ ion and Cl- ion
Water consists of the H+ ion and the O2- ion
The oxygen is attracted to the sodium atom and the hydrogen is attracted to the chlorine atom
The ionic lattice becomes surrounded with water and breaks down
When using q=mcΔt, what is important to remember about mass?
It must be the mass of both the solution and the solute
What is the enthalpy change of hydration?
The enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions
Describe the dissolving process
The ionic lattice breaks up to form separate gaseous ions
The separate ions interact with the water molecules to form hydrate aqueous ions
What characteristics do ionic compounds tend to have?
High melting and boiling points
Soluble in polar solvents
Conduct electricity when molten or in solution
What are the two factors affecting lattice enthalpy?
Ionic charge and ionic size
What is the effect of ionic size on lattice enthalpy?
The ionic radius increases
The attraction between oppositely charged ions decreases
The lattice enthalpy is less negative
The melting point decreases
What is the effect of ionic charge on lattice enthalpy?
Ionic charge increases
Attraction between ions increases
Lattice enthalpy becomes more negative
Melting point increases
If a lattice enthalpy is very exothermic, what does this indicate about the melting point?
The melting point will be very high
What is the effect of ionic size on hydration enthalpy?
Ionic radius increases
The attraction between ion and water molecule decreases
The hydration enthalpy is less negative
What is the effect of ionic charge on hydration enthalpy?
Ionic charge increases
Attraction with the water molecules increase
The hydration enthalpy is more negative
When will a compound dissolve?
When the sum of the hydration enthalpies is larger than the lattice enthalpy and the reaction is exothermic
What does a high entropy value indicate?
High disorder and randomness
What is entropy?
The dispersal of energy within the chemicals making up a chemical system
What are the units of entropy?
J K-1 mol-1
Which state has the highest and lowest entropy values?
Highest = gases Lowest = solids
If a chemical system changes to become more random, what happens to the value of the entropy?
It increases
When there is a decrease in the number of gaseous molecules, hat happens to the entropy value and why?
It is negative - there is a decrease in the randomness of particles and the energy is less spread out
What is standard entropy?
The entropy of one mole of a substance under standard conditions
Will standard entropies always be positive or negative?
Positive
How do you calculate the entropy change?
(Entropy of products) - (entropy of reactants)
What does feasibility mean?
If a reaction is able to happen
What are the two types of energy making up free energy?
Enthalpy change and entropy change
What is the Gibbs’ equation?
ΔG = ΔH - TΔS
For a reaction to be feasible, what must the result of Gibbs’ equation be?
ΔG < 0
What is important to remember about ΔS when using Gibbs’ equation?
Must divide by 1,000 to be in kJ
How do you calculate the minimum temperature for feasibility?
Set Gibbs’ equation equal to 0
What is the main limitation of Gibbs’ free energy equation?
ΔG may show that a reaction is feasible but doesn’t take into account kinetics or the rate of reaction
