O - Enthalpies and dissolving Flashcards
What is lattice enthalpy a measure of?
A measure of ionic bond strength.
What can ionic compounds form?
Regular structures called giant ionic lattices.
What are the positive and negative ions held together by in giant ionic lattices?
Electrostatic attractions.
What happens when gaseous ions combine to make a solid lattice?
Energy is given out - this is called the lattice enthalpy.
What is the standard lattice enthalpy?
The enthalpy change when 1 mole of an ionic lattice is formed from its gaseous ions under standard conditions.
What are standard conditions (lattice enthalpy)?
298K (25^oC) and 100kPa.
Is lattice enthalpy always positive or negative?
Negative.
Why is lattice enthalpy always negative?
Because bonds are broken and energy is released.
The more negative the lattice enthalpy,…
…the stronger the bonding.
What is the value for lattice enthalpy like if the bonds are stronger?
More negative.
What does dissolving involve?
Enthalpy changes.
What happens when a solid ionic lattice dissolves in water?
1) Bonds between the ions break - this is endothermic and the enthalpy change is the opposite of the lattice enthalpy.
2) Bonds between the ions and the water are made (hydration) - this is the enthalpy change of hydration and it is always exothermic.
Is the enthalpy change of hydration always endothermic or exothermic?
Exothermic.
What is the enthalpy change of hydration of ions?
The enthalpy change when 1 mole of aqueous ions is formed from gaseous ions.
What is the enthalpy change of solution?
The overall effect on the enthalpy when something dissolves. It’s the net effect of the lattice enthalpy and the enthalpy change of hydration.
The enthalpy change when 1 mole of an ionic substance dissolves in enough solvent to form an infinitely dilute solution.
What is the enthalpy change of solution the net effect of?
The lattice enthalpy and the enthalpy change of hydration.
What is the net effect of the lattice enthalpy and the enthalpy change of hydration called?
The enthalpy change of solution.
What can you use an enthalpy cycle to find?
The enthalpy change of solution.
What does Hess’s law say?
That the total enthalpy change of a reaction is always the same, no matter which route is taken.
What is an infinitely dilute solution?
One which contains so much colvent that when you add more solvent, there is no change in concentration.
What can you use an enthalpy level diagram to find?
The enthalpy change of solution.
Briefly describe an experiment you can do to measure (and calculate) enthalpy change of solution
Put the reactants in a container, stick a thermometer in and measure the temperature change.
(It is best to use a polystyrene beaker with a lid (with a hole for the thermometer), so that you don’t lose of gain too much heat through the sides.
To calculate the enthalpy change you need to know the number of moles of the stuff that is reacting, the change in temperature, and how much stuff you have.
What affects the lattice enthalpy and enthalpy of hydration?
Charge density.
What does charge density affect?
The lattice enthalpy and enthalpy of hydration.
What is the charge density of an ion equal to?
Its charge divided by its radius.
What is the equation for charge density?
Charge density = charge/radius
What are ions with a higher charge density better at?
Attracting ions of the opposite charge.
Why are ions with a higher charge density better at attracting ions of the opposite charge?
Because the electrostatic attraction between ions with higher charge densities is much stronger than the attraction between ions with lower charge densities.
How does charge density affect lattice enthalpy?
Ions with a higher charge density are better at attracting each other in ionic lattices than those with lower charge densities. This means the ionic bonds are stronger, meaning more energy is released when the bonds are made, giving them a more exothermic lattice enthalpy.
How does charge density affect the enthalpy of hydration?
Ions with a higher charge density are better at attracting water molecules than those with lower charge densities. This means the intermolecular bonds are stronger, meaning more energy is released when the bonds are made giving them a more exothermic hydration enthalpy.
