Oceans

When dissolving ionic compounds in water, what bonds are broken

• ionic bonds in the solid

• Hydrogen bonds between water molecules

When dissolving ionic compounds, what bonds are made

Ion- dipole bonds between the ions and water molecules

How do you know if a substance is soluble

If the bonds made are similar in strength or stronger than the bonds broken

What are some insoluble ionic compounds

AgCl, Agbr

BaSO4

Cu(OH)2

Fe(OH)2

CaCO3

Definition of lattice enthalpy and symbol

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

Example equation of lattice enthalpy for sodium chloride

In lattice enthalpy endothermic or exothermic and why

ALWAYS EXOTHERMIC because ionic bonds are being made

Is it endothermic or exothermic to dissolve an ionic compounds in water and how do you do it

We need to break up the lattice to get separate ions, so we need to do the reverse of lattice enthalpy.

This will always be endothermic as bonds are being broken

How does a lattice enthalpy become more negative

The stronger ionic bonding, the more negative the lattice enthalpy

Which factors affect the size of lattice enthalpy for different compounds

1. The charge on the ions- Ions with higher charges attract together more strongly. This means more energy is needed to break the lattice so lattice enthalpy is more exothermic

2. Size of ions- ions with small ionic radius can get closer to the opposite ions in the lattice, giving stronger attractions so more exothermic lattice energy (not as big of a factor as charge)

Enthalpy of hydration definition

The enthalpy change when 1 mole of gaseous ions are added to water to give 1 mole of aqueous ions to give an infinitely dilute so.lution

Example equation of enthalpy of hydration

Why is enthalpy of hydration always negative

Always negative for positive and negative ions as ion dipole bonds are being made between the polar water molecules and the fully charged ions. The ions are hydrated- they have water molecules bound to them

Draw a diagram of a hydrated sodium ion with charges and partial charges

What factors affect the size of thee hydration enthalpy

Hydration enthalpy is more exothermic for ions which have a higher charge and a smaller ionic radius so have a higher charge density. Smaller ions get closer to the water molecules so there’s stronger ion dipole bonds and more water molecule bind to them

For example mg2+ and Na+ Mg2+ has higher charge density as higher charge and smaller size

Enthalpy change of solution

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

Is enthalpy change of solution endothermic or exothermic

Depends on relative size of lattice enthalpy breaking step (endothermic) and hydration of ions step (exothermic)

How to draw enthalpy level diagram

How to calculate enthalpy of solution

What are the 3 possibilities for enthalpy of solution and solubility

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

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

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

Non-polar solvents

Ionic substances do not dissolve. The ions cannot form strong attractions with non-polar solvent molecules so the down step (enthalpy of solvation) is very small making enthalpy of hydration large and positive

Radiation in and out relating to the greenhouse effect

Hot objects send out electromagnetic radiation. The hotter the object, the higher the energy of this radiation. The sun (surface temperature of 6000K) gives out mainly UV and visible radiation. When this radiation reaches earth, part is absorbed by the earths surface and atmosphere, and part is reflected back into space. The part that gets absorbed warms up the earth and the earth send electromagnetic radiation back into space, but the earth is a lot cooler than the sun so it gives out lower energy radiation (infra red)

A steady state is reached where the earth is radiating energy into space as fast as it is absorbing it, so the earths average temperature stays constant

What is the role of greenhouse gasses

• greenhouse gas molecules in the atmosphere absorb some of the infrared red radiation emitted from the earths surface and stop it being re-radiated into space.

This warms the earth because:

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

• When molecules absorb certain frequencies of IR radiation, their vibrational energy increases: the bonds vibrate more vigorously. When they collide with other molecules in the atmosphere this extra energy is transferred increasing the kinetic energy of air molecules, raising the temperature of the atmosphere

• They do not absorb UV or visible from the sun but let it in but do absorb IR and stop IR radiation from earth getting out- without this life would be too cold to support life but human actions is changing this balance by adding additional greenhouse gasses in atmosphere

What are the very abundant greenhouse gasses

CO2

H2O

What are the less abundant greenhouse gasses

CH4

N2O

CFC

O3

What is the Infra red window

Water vapour is the most abundant greenhouse gas in the atmosphere but water only absorbs certain frequencies of IR and the rest of the IR spectrum is the IR window where infra red can escape without being absorbed. If human activity puts gasses into the troposphere which absorb IR in this window there will have big greenhouse impact with CO2 and CH4 being important examples. The higher conc of greenhouse gasses in troposphere the more IR absorbed increasing greenhouse effect

Is CO2 a problem

Increase in CO2 in atmosphere is due to more use of fossil fuels worldwide

A lot of extra CO2 is removed by oceans by dissolving and marine plants like phytoplankton

What is an acid

Proton donor

What is an alkali

A proton acceptor

When an acid has donated H+ what is given

When a base has accepted H+ what is given

Acid leaves a conjugate base

Base leaves a conjugate acid

Why can water act as an acid and a base

Acid because it can donate H+ leaving OH- and can accept H+ giving H3O+

Why is water neutral in PH

As the tiny percentage of water molecules that dissosciate into ions give equal quantities of H+ and OH- so [H+] = [OH-]

What is the name of something that acts as both an acid or a base

Amphoteric

What is a strong acid and how do you write the equation

An acid which when dissolved in water is fully dissociated into ions so a strong acid is a very good H+ donor.

Use a full arrow in equation to show complete reaction

Show equation for HCL as a strong acid

How do H+ ions usually exist and what is the dot and cross diagram and the equation forming it from HCl

As oxoniuum ion H3O+

What is a weak acid and how do you write the equation for one

An acid that when dissolved in water only very slightly dissociates into ions and once ions form they start joining back together so reaction is in equilibrium. Use equilibrium arrow

Write equation for weak acid of HF

How do indicators work

Indicators are weak acids but where HA is a different colour to A-.

If acid is added, conc H+ increases so equilibrium shifts left to use up some extra H+ and most of indicator is present as HA so we see that colour. If alkali is added, it reacts with and removes H+ so equilibrium shifts right to replace H+. Most of indicator is now in the A- form so we see that colour

What is evidence that water is partially split into ions

Even when completely pure water will still conduct a current so water must be partially split into ions as ions carry current and this is autoionisation of water.

Why is conductivity of pure water low

As almost all water is unionised as position of equilibrium is far on the left

Write the Kc expression of water and state why [H2O] can be taken as a constant

As [H2O] is so much larger than [H+] or [OH-] it is taken as constant

Rewrite the water Kc expression with H2O as a constant and What is the new constant called

What is the molar concentrations of [H2O] [H+] and [OH-] at 25 degrees

Kw= 1X10-14 mol2dm-6

[H+] = 1X10-7 mol dm-3

[OH-] = 1x10-7 mol dm-3

Work out the concentration of H+, OH- ions in a o.1 mol dm-3 solution of HCl

What is pH

Measure of concentration of H+

What is the equation to work out pH

How many decimal points do we write ph to

2 decimal points e.g. 7.00

How to work out PH of strong acids vs strong alkali

Acid PH scale

Alkali PH scale

How to calculate PH at various points in an acid-base titration

Effect of temperature on PH values

Kw is 1X10-14 only at 25 degrees so Ph 7 is only at 25 degrees

Water dissociating into its ions is endothermic as bonds are being broken so at a higher temperature equilibrium will shift right so Kw will increase

If the solution is neutral for example water what is the main rule

[H+] = [OH-] so do square root of Kw to find out there conc

How to calculate [H+] from PH

'Answer this question

Why is it harder to work out PH of weak acids then strong acids

As strong acids are fully ionised but weak acids are not fully dissociated so we need a measure of how dissociated they are

Write the Kc equation for weak acids

Rearrange Kc for weak acids to get Ka equation and units of Ka

What does Ka stand for

Acid dissociation constant

What makes Ka value bigger

The stronger the acid is the bigger the Ka value

What factor changes the value of Ka

Ka only changes if temperature changes

What is pKa

Another way of showing Ka

How do you work out pKa

PKa = -logKa

What is the trend in Ka vs pKa if an acid is getting stronger

The stronger the acid the larger the Ka value but smaller the pKa value

How to work out Ka from pKa

What are the 2 approximations for Ka

How to find Ka of a weak acid from its PH and concentration

How do oceans remove CO2

Marine life photosynthesise

And CO2 dissolves

What are the 3 equilibrium equations for CO2 being removed by the oceans from the atmosphere

As the concentration of CO2 in the atmosphere rises what will happen to concentration of H+ (aq) in the sea

Even with concentration of H+ increasing in ocean why are the oceans not becoming significantly more acidic

When the conc of a weak acid increases, PH does not change as much as it would for a strong acid which means the ocean can act as a buffer

What are the main concerns for the ocean when the concentration of H+ is increasing

Worried about the effects of ocean acidification and warming of sea water on coral reefs ecosystems

Definition of a buffer

a buffer solution is a solution which minimises changes in PH when acid or alkali is added to it in small quantities or when diluted so they maintain an almost constant PH

What are buffers made of

Make the rest of buffers flashcards

How do ionic substances dissolve in water

Many ionic substances dissolve in water, the ions completely separate from each other, each becoming surrounded by water molecules as water molecules are polar and form ion dipole bonds with the ions.

They are said to be hydrated ions and ions can react separately from each other

How do ionic solids dissolve

They dissolve only to a tiny extent as in a saturated solution a dynamic equilibrium exists between dissolved ions and the undissolved solid

Write an equation between saturated solution of silver chloride in contact with un dissolved Silver chloride

What is solubility product

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

Which always has the same value for a given solution at a given temperature and constant unless temperature changes

Need state symbols for a mark

What does Ksp give

Maximum conc of ions that be in a solution before a ppt forms

How to predict with Ksp if a pot will form

How to work out Ksp when mixing solutions

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

Calculating solubility product from solubility and vice versa

Common ion effect

Determining solubility products by experiment aim

To make a saturated solution of solid and do a titration to find the concentration of on of the ions it cotains. By knowing its formula the concentration of the other ion can be determined and using both concentrations Ksp can be calculated

Determining solubility products by experiment practical

• to ensure solution is saturated excess solid is added to water and the mixture stirred thoroughly so no more solid will dissolve

• Before titrating, the solution should be filtered to remove remaining solid- if any solid was remaining during titration when one of the ions reacts more solid would dissolve affecting the results

• Accurately measure a volume of solution using a pipettes and put into a conical flask

• The conc of standard solution in the titration in the burette should be adjusted so a big enough titre is given to avoid large % errors. It will need diluting using a pipettes and volumetric flask

• Usual titration procedure

Why do oil and water not mix

Here ae strong attractive forces of hydrogen on ding between after molecules, holding them together and preventing molecules that cannot form hydrogen bods from mixing

Symbol for entropy

S

Entropy definition

A measure of disorder or of the number of ways of arranging the particles and their associated energy quanta

What has a higher entropy a mixture or separate susatnces

A mixture because there are more ways of arranging particles in a mixture

Which states have highest and lowest entropy and why

Solids have lower because in a solid particles are in a more ordered arrangement , in fixed positions. In a gas the particles are spread out and moving in random directions at different speeds so is much more disordered. So when a liquid boils change in entropy is positive as entropy increased.

Also when molecules get bigger their entropy increases

Do the molecules in a substance have equal entropy

No the molecules in a substance don’t all have equal energy which gives more entropy. Molecules hav electronic, vibrational, rotational and translational energy and these energies ae quantised

What effect does heating a substance have on entropy

Heating increases number of nervy quanta available and increases entropy. Substances with bigger molecules / heavier atoms in their molecules have higher entropies as energy levels are closer together so there are more energy quanta at a given temperature