chemistry topic 8

When dissolving ionic compounds in water, what bonds are broken and made

Ionic bonds in the solid and hydrogen bonds between water molecules are BROKEN

Ion dipole bonds between ions and water molecules are MADE

What is required for a substance to be soluble

The substance will be soluble if the bonds made are similar in strength to or stronger than the bonds broken

What is lattice enthalpy

The enthalpy change when 1 mole of a solid ionic compound is formed from separate gaseous ions

Eg Na+ (g) + Cl-(g) → NaCl(s)

Is lattice enthalpy exo or endo

Always exothermic as ionic bonds are being made (BMX)

What factor affect size of lattice enthalpy

The stronger the ionic bonding, the more negative the lattice enthalpy

• charge on the ions → ions with higher charge attract more strongly

• size of ions → the smaller the ionic radius the closer it can get to the opposite ion so stronger attractions

What is enthalpy of hydration

The enthalpy change when 1 mole of gaseous ions are added to water to give 1 mole of aqueous ions

Na+(g) + (aq) → Na+ (aq)

Is enthalpy oh hydration endo or exo

Exo as ion dipole bonds are made so ions in solution are hydrated

What factors affect size of hydration enthalpy

• higher charge

• Smaller ionic radius

As these ions have higher charge density

What is enthalpy change of solution

The enthalpy change when 1 mole of ionic solid dissolves in enough water to make an infinitely dilute solution

How do you draw an enthalpy diagram

• start near bottom of space with solid on horizontal line

• Draw a higher line for the gaseous ions (state symbols)

• Join 2 lines with a downward arrow for lattice enthalpy

• Draw a horizontal line on the right slightly above aqueous ions here

• Include arrow from gaseous to aqueous ions labelled with all the enthalpies of hydration

• Join solid to aqueous ions line with an arrow labelled triangle SolnH

What are the 3 possibilities for enthalpy of solution and solubility diagrams

1. Enthalpy of solution if exothermic overall so substances are almost all soluble and when they dissolve the solution warms up

2. Enthalpy of solution is endothermic and large so these solids are insoluble

3. Enthalpy of solution is endothermic but small so solids usually soluble but they have to take in heat to dissolve so the solution will feel cold

What’s the diagram for non polar solvents

Ionic substances do not dissolve so ions cannot form strong attractions with non polar solvent molecules so the down step is called enthalpy of solvated which is very s,all so enthalpy of solution is large and positive

What radiation does the sun give out

UV and visible

What radiation does the earth give out

Infrared

What do green house molecules do

Molecules in the atmosphere absorb some of the infrared radiation emitted from the earths surface and stop it from being re-radiated into space. Do not absorb uv or visible from the sun only ir which is vital for life or earth would be too cold to support life

How do green house molecules have an effect of warming the earth

• some of the IR is re emitted by molecules in all directions so some of the ir that was going out into space gets radiated back towards earth

• When molecules absorb certain frequencies of ir radiation their vibrational energy increases. When they collide with other molecules this extra energy is transferred increasing the kinetic energy of air molecules hence raising temperature

Which gases are green house gases

CO2 and H2O are very abundant, CH4, N2O, CFCs, O3 less abundant

What is the IR window

The IR spectrum which water doesn’t absorb as it’s very abundant but only absorbs certain frequencies. Those not absorb are the window which escape and gases put into troposphere like CO2 and CH4 can absorb

What’s used to predict earths climate

Complex computer modelling

What’s an alkali

A soluble base

What’s an acid

A proton donor

What’s a base

A proton acceptor

What’s a conjugate base/acid

When an acid has donated protons, what’s left is its conjugate base

When a base has accepted protons, what’s left is its conjugate acid

What’s special about water

It can act as both an acid and a base → amphoteric

Why is water neutral in pH

Because a tiny % of water molecules dissociate into ions, so equal quantities of [H+] and [OH-] ie a neutral solution

What’s a strong acid

fully dissociates in water into ions so very good proton donor eg HCl → H+ and Cl- (fully arrow used)

What’s a weak acid

Partially dissociates in water into ions. Once ions form they start joining back together so the reaction is an equilibrium ( therefore use double headed arrow)

How do indicators work

Indicators themselves are weak acids, but ones where HA is a different colour to A-. HA <=> H+ + A-

If acid is added, H+ conc increases so equilibrium shifts to the left to use up some of the extra H+. Most of the indicator is present as HA so we see that colour

If alkali is added, it reacts with hence removes H+ so the equilibrium shifts right to replace H+. Most of the indicator is now in the form A- so we see that colour

what is the ionic product of water

Kw=[H+][OH-] which is constant unless temperatyre changes

if water can still conduct a small current even when pure, what does this prove?

that water partially split into ions. conductivity of pure water is very low, showing equilibrium position is far on the left

H2O <=> H+ + OH-

whats the ionic product of pure water/any aqueous solution

1×10-14 mold2dm-6

[H+]=[OH-]=1×10-7

whats pH

the measure of concentration of H+

pH=-log[H+]

what type of reaction is Kw and how does temp effect this

H2O <=> H+ + OH-

must be endothermic as bonds being broken, so if temperature increases, equilibrium will shift to the right so Kw will increase

what is Ka

the acid dissociation constant

its a constant for a given acid which only changes if the temperature changes

how is Ka and pKa affected

the stronger the acid, the bigger the Ka value, lower the pKa value

what is pKa

pKa = -logKa or Ka = 10^-pKa

used as Ka are usually awkward numbers

how do pH and pKa differ

pH varies for a given acid with its concentration

pKa is a constant for given acid whatever its concentration

why would 2 pKa values be given for this acid   

it has 2 carboxylic groups

how do you use Ka to calculate pH of a weak acid solution

make 2 approximations

1. that [H+] = [A-], so the top line of Ka is written as [H⁺]²

2. conc of [HA-] at equilib = conc of HA put in to the solution (ie [HA] = conc of the solution) → assumes no HA ionises

so Ka = [H+]² / conc of solution then can do -log to find pH of acid

how do oceans remove CO2

partly by marine life photosynthesising

partly by CO2 dissolving

as concentration of CO2 in the atmosphere rises, what will happen to the concentration of H+ in the sea

1. as the concentration of CO2 in the air rises, the position of equilibrium (a) shifts right

2. so [CO2(aq)] rises and hence equilibrium (b) shifts right minimising the change

3. so [HCO3-] rises and hence equilibrium c shifts right minimising the change

4. so concentration of H+ in the ocean rises

what is a buffer solution

• a buffer solution is a solution which minimises change in pH

• when acid or alkali is added to it

• in small quantities (or when diluted)

what are buffers made of?

theyre solutions containing

• a weak acid and one of its salts (eg CH3COOH and CH3COONa)

• a weak base and one of its salts (eg NH3 and NH4Cl)

how can you make eg CH3COOH/CH3COONa buffer?

• adding both CH3COOH and CH3COONa to a solution or

• adding NaOH to a solution of CH3COOH, making sure the acid is in excess to produce the left, so as long as some of the acid is left you will have a buffer

how does a buffer solution work? eg CH3COOH/CH3COONa

1. start by writing the weak acid equilibrium equation for the weak acid in the buffer, then keep referring to which way it shifts

2. when asked how it works, always start writing the equation of the weak acid equilibrium.

• if acid is added, the equilibrium shifts to the left to minimise the change by using up most of the extra H+ (plenty CH3COO- to do this , but wouldn’t be plenty if we just had a weak acid) so pH hardly changes as all the extra H+ is removed

• if alkali is added, the added OH- reacts with hence removes H+ making water, but equilibrium shift to the right to minimise the change so more CH3COOH dissociates replacing almost all he H+ hence pH hardly changes (plenty CH3COOH to do this)

how do you do calculations involving buffer solutions

[HA] = conc of the weak acid in the buffer mixture

[A-] = conc of the salt in the buffer mixture

so Ka of weak acid in it = [H+][salt]/[acid]

so [H+] = Ka x [acid]/[salt] then do -log to find pH

what does the Ka expression of a buffer show?

[H+] = Ka x [acid]/[salt]

1. why pH is unaffected as the fraction [acid]/[salt] stays unaltered as both concs decrease

2. how to prepare buffer solutions of different pHs by picking a weak acid with pKa close to the pH you want the bugger to be, then fine tune the pH by altering acid and salt

3. for a buffer which the conc of the acid and the salt are equal, they cancel out in the expression so [H+]=ka so pH=pKa

What happens when ionic substances dissolve in water

They completely separate from each other, becoming surrounded by water molecules as water molecules are polar and form ion-dipole bonds with ions → hydrated ions

What is solubility

The maximum amount of solid which will dissolve in a certain solvent

What’s the units for solubility

Often gdm-3 and changes with temp, to convert to moldm-3 just divide by molar mass.

What does sparingly soluble mean

It only dissolves a little bit (some ionic solids are insoluble, but do in fact dissolve a tiny bit)

What happens when a sparingly soluble solid is dissolved in water to give a saturated solution

An equilibrium is established between the undissolved solid and dissolved ions

Eg AgCl(s) <=> Ag+ (aq) + Cl- (aq)

What is the solubility product

Ksp = [Ag+(aq)] [Cl-(aq)]

Which always has the same value for a given solution at a given temp

State symbols are required for the mark

What does Ksp give

The maximum concentration of ions that be in solution before a ppt forms

How do you work out with Ksp if a ppt will form

If the value of [A][B] in the mixture is more than the Ksp value given, a ppt will be given.

If it is smaller, no ppt will form

How do you calculate Ksp when mixing solutions

Work out the new concentration of each ion by working out the moles then diving by the new volume

How do you calculate solubility product from solubility

Write the equilibrium then Ksp equation, then plug in the concentration

What if there are 2 moles of an ion

[2x]²

What is common ion effect

Ksp is a constant even if there is another source of one of the other ions. If there is then the solubility of the solid will be lowered

What are practical considerations to make as the idea is to make a saturated solution of the solid and carry a suitable titration to find the conc of one of the ions

• ensure the solution is saturated→ excess solid is added to water and stirred thoroughly

• Should be filtered to remove any remaining solid

• Accurately measure a volume of the solution using a pipette and put in conical flask

• Conc of standard solution used in titration should be adjusted to ensure a big enough titre is given to avoid large % errors

• Repeat to get concocrsnt results, go drop wise near end point

How do you calculate entropy change of the system

Units are JK-1mol-1 (J K moling)

what is entropy

The measure of the number of ways that particles can be arranged (or measure of disorder)

What effects entropy

1. Physical states - solids < liquids < gases as solids are in a more ordered arrangement in fixed positions, where as gases are more disordered as the particles are more spread out

2. the amount of energy a substance has - the more energy quanta a substance has, the more ways they can be arranged → heating a substance increases the number of energy quanta so increases entropy or bigger/heavier molecules have higher entropies as their energy levels are closer together so more energy quanta

3. number of particles - the more particles you got, the more ways they and their energy can be arranged so increase in number of moles increases entropy

what decides if a process will or wont happen?

for a process to happen (be spontaneous/feasible), the overall entropy change, Δ_totalS must be positive

how do you calculate Δ_totalS?

Δ_totalS = Δ_systemS + Δ_surroundingsS units are JK-1mol-1

where Δ_surroundingsS= -ΔH/T

have to convert ΔH from kJmol-1 to Jmol-1

how can you use entropy calculations to find the melting point

the temperature when Δ_totalS is zero, so when rearranging calculation

T= ΔH/Δ_sysS

whats entropy at equilibrium

Δ_totalS = 0 with neither direction favoured, so Kc=1

Give examples of processes which have increasing disorder and are exothermic, giving the temperatures they will occur at

Ice melting, NaOH dissolving.

All temperatures because Δ_totalS is positive at all temperatures

Give examples of processes which have decreasing disorder and are endothermic, giving the temperatures they will occur at

No temperature because Δ_totalS is negative at all temperatures

Give examples of processes which have increasing disorder and are endothermic, giving the temperatures they will occur at

NaCl dissolving, thermal decomposition

Eg melting, boiling, dissolving ionic solutions occurs when Δ_totalS is positive which is when ΔH/T<Δ_sysS which is more likely at higher temperatures ie will work better at a certain higher temp

Give examples of processes which have decreasing disorder and are exothermic, giving the temperatures they will occur at

Water freezing, 2Cl* - Cl2

eg freezing, dissolving gases

works when the magnitude of ΔH/T>Δ_sysS so more likely at lower temperature ie will work below a certain temperature and work better

Many salts with a positive enthalpy change of solution are readily soluble in water. Explain this in terms of Δ_totS, Δ_sysS and Δ_surrS for the dissolving process

• Δ_sysS for the dissolving must be positive

• Δ_sysS = Δ_sysS + Δ_sysS

• Δ_sysS is -ve here as Δ_sysS is the +ve and temperature is always +ve

• so for the Δ_sysS to be +ve, Δ_sysS must be +ve ( and > Δ_sysS)

• often the case for dissolving as the ions more disordered when in solution : more ways of arranging

do feasible reactions always happen?

no because Δ_totS tells us nothing about Ea. If Ea is very high the reaction may not happen spontaneously. It may need igniting in some way eg using a use to overcome the high Ea