O1 Flashcards
Describe the structure of ionic solids
- Held together by their opposite electrical charges between the ions
- Made up of anions and cations
- Giant ionic lattice - v. large no. of ions arranged in fixed positions
Describe the features of ionic solids (melting/boiling points?)
- They are hard and have high melting/boiling points
- This is because the lattice holds together very strongly
Give an example on an ionic solid and describe how the ions interact
NaCl
- Each Na+ is attracted to 6 Cl- around it but are repelled by other Na+ ions which are further away
- Overall, there are many attractions + repulsions within it but the attractions are stronger + so hold the lattice together
What happens when ionic substances dissolve readily in water?
- The ions become surrounded by water molecules
- The dissolved ions are no longer regularly arranged (randomly scattered)
- Once separated, the -ive and +ive ions behave independently
What is sea water made up of?
A mixture of -ive and +ive ions dissolved in water (ionic compounds are separated in sea what into +ive and -ive ions)
Is dissolving an ionic solid an endothermic or exothermic process?
Dissolving an ionic solid is an endothermic process
What is the interaction between water molecules and ions in solution called?
Ion-dipole interaction - tiny charges on the water molecule are attracted to the charges on the ions
Define lattice enthalpy
The enthalpy change when 1 mol of solid is formed by the coming together of separate ions
It measures the strength of ionic attractions in a lattice
In lattice enthalpy describe the changes of state
From gaseous state when ions are separate to solid state when they are together in lattice
Are lattice enthalpies negative or positive?
All lattice enthalpies are negative and so are exothermic (breaking down of a lattice)
What 2 factors affect lattice enthalpy?
Size and charge of ions
What makes lattice enthalpies more negative (exothermic)? What do the effect of these factors tell you about lattice enthalpy?
When:
- Ionic charges increase
- Ionic radii decrease (ion is smaller)
Therefore, lattice enthalpy becomes more -ive for ions with greater charge density
The greater the charge density of ions, the more -ive the lattice enthalpy is. Why is this?
- Ions with a higher charge (and ions that are closer together) attract one another more strongly
- Ions with a smaller radius can come closer together
- So stronger attractions means more negative lattice enthalpies
Describe water molecules (structure) and how they act in solution
The difference in electronegativity between the O and Hs means that the (covalent) bonds are polar - the water molecule acts as a tiny dipole
Describe ionic compounds (that are soluble) in solution
- Ions become separated from the lattice + surrounded by water
- Positive ions are surrounded by water molecules with the negative end of the dipole facing them (vice versa with -ive ions)
- These ions in solution are now hydrated (have water molecules bound to them)
How does charge density of the ion affect the number of water molecules it attracts?
The greater the charge density, the more water molecules it attracts so the bigger the hydrated ion is
What does the enthalpy change of hydration measure?
The strength of the attractions between ions + water molecules
Define enthalpy change of hydration
The enthalpy change for the formation of a solution of ions from 1 mole of gaseous ions
Are enthalpies of hydration negative or positive?
Enthalpies of hydration are always negative - exothermic
