22.2 Enthalpy Change In Solution And 22.3 Factors Affecting Lattice Enthalpy And Hydration Enthalpy Flashcards
How does water dissolve ionic latticed anyways
Water exists with dipoles because of electronegativity difference, and as it is an unsymmetrical shape! With partially positive on hydorgen and partially negative in oxygen
For ionic lattice to be dissolved, thr bonds made between water and the atoms energy released must be able to overcome the prior bonds (but we see how this changed)
How does the dissolved version of a lattice look like bond wise compared to before as solid
3d as solid, with ions attracted in giant lattice
After that thr partially positive hydrogens are attracted to negative ion, and partially negative oxygen to the positive ions, surrounding each ion like this above and below 3D wise
What is the standard enthlapy change of soltuion definition
The enthalpy change that takes place when one mole of a solute is dissolved in a solvent
(The overall energy change to dissolve one mole of a solute)
What is the general energy change of soltuon?
Both, sometimes negative and positive
How to calculate the enthalpy change of solution experimentally (same as yr 12 but extra step)
Weigh out mass of solute, now put volume of water and mass in, stir with thermometer until dissolved and measure until a constsnt temp
Now density assume to be 1g, so now extra step is to ADD THE MASS OF WATER AND THE MASS BECAUSE BOTH ARE CHANGING
Now do whole q = mcat, make it into KJ, find moles of salt used, divide by it , see TEMPERTAURE chnage and apply the sign
How do you represent enthalpy change of solution usign equations
We said it was when compound becomes dissolved
So NaCl(s) + aq —> Na+(aq) + Cl- (aq)
Hat happens when solute is dissolved and thus what are the two ENERGIES INVOLVED
when dissolved
- ionic lattice breaks up
- Agee molecules attract and surround the ions
Thus 1) the ionic lattice breaks by opposite energy ti lattice enthalpy , as now the solid becomes gaseous ions
2) water eats the ions and they become aqueous. This is called HYDRATION ENTHLAPY
What is hydration enthlapy
Enthnalpy change that is required to make one mole of gaseous ions into one mole of gaseous aqueous ions in water
So how can we form Hager cycles to link hydration energies and lattice energies and soltuon enthlapy Ed all together to find an UNKNOWN
- the starting point is the gaseous ions, and these want to end up becoming both AQUEOUS
1) route one is one by one hydration enthakpying them to become aqeuous
2) route two is first lattice enthlapy to become a lattice, then solution enthlapy , and that becomes solid to both aqueous
Manipulate to find u known
What’s the thing to lookout for basicslly for this one
Is that the nethakpy change or soltuion can sometimes be positive or negative so the graphs will change , as long as you remember they both start from the gaseous ions, whereas other from standard state, you’re goof
What’s the effect of increasing the size of the ion (with same charge) for:
- lattice enthlapy
- melting point
Why
Higher side = less negative lattice enthlapy (less Exothermic)
Why?
- bigger the size, bigger the radius, and for the same charge, this means less ionic attraction.
- scale of energy released when bonds made depend on the strength of bond. Thus less strength bond= means less energy released
- if less energy released, then value will be less negative, lattice enthlapy less negative
2) melting point
- lower value
(Easier to break bonds if less attraction )
What is the effect on ionic charge increases on
-lattice enthalpy
- melting point
Increase ionic charge for same size means attraction ionic increase
- this means stringer bond formed = more energy released
Thus MIRE NEGATIVE ENTHAKPY VALUE
- if more attraction, harder to break, melting point INCREASES
So what happens using this knowledge across a period to the right, to the left and going down a group
A) going to the left, we know the ionic charge increases, and as a result of the nuclear charge increasing, the radius also decreases
- thus both effects lead to INCREASED IONIC ATTRACTION = melting point inifrease + enthalpy more negative
B: going to the left, ionic charge can still increase but negatively, size increases as nuclear charge decreases, so
- these kinds counter each other out, hard to say
C) going down a group, the charge stays the same, but the radius increases, so the attraction decreases , and so enthalpy less nagetive and the melting point less too
What about ionic charge and size for HYDRATION ENTHALPY VALUE
First is hydration enthlapy Exothermic or endothermic always and why
1) always EXOTHERMIC , because it’s the enthalpy badcislly when a water bonds with ion, and this always releases energy
Same concept as before, the more stronger bond more energy release, more negative etc
So higher charge = higher attraction = more negative
Higher size = less attraction = less negative
What is the general idea to why a substance should be able to dissolve
Essentially what value of final enthalpy change is needed to dissolve
Why is this not always the case
We said for something to dissolve the energy released when water makes bonds must be greater than energy needed to break lattice.
Energy evolved when bonded with water is = the hydration enthlapy
Energy required to break lattice is same as to make it = lattice enthlapy
Thus if the hydration enthalpy is GREATER than lattice enthalpy, which means the enthakpy of solution, it SHOULD DISSOVE
2) however think about it, more hydration is negative so final answer should be negative and Exothermic
- but there are cases where ENDOTHERMIC values also dissolce
- this is to do with ENTRIPY
