The Oceans Flashcards
Carbon dioxide dissolves in water giving the overall equation
CO2(aq) + H2O(l) 2H+(aq) + CO3^2- (aq)
Product is carbonic acid
Methods to reduce CO2 in atmosphere
More economical use of fossil fuels
Alternatives to fossil fuels
Capture and storage of CO2
Increased levels of photosynthesis
Entropy is a measure of
Order
Entropy change for a chemical system is
The difference between the entropies of the reactants and products. P-R
Entropy change of the surroundings
Depends on the transfer of heat to and from the surroundings I.e enthalpy change.
Delta Ssurr = delta H (enthalpy change) / T
T is in kelvin. 0 –> +273
Total entropy change is…
Sum of delta system add delta surroundings.
Total entropy predictions:
Delta total is positive then a reaction will occur spontaneously
Delta total = 0 then reaction is in equilibrium
Ionic compounds in water
Dissolve in water, ionic lattice is broken up and the ions seperate and become hydrated. Energy is used to break up lattice but energy is given out when ions are hydrated, ion- dipole bonds are formed.
Lattice (formation) enthalpy definition :
Enthalpy change when one mole of a solid is formed from its separate gaseous ions.
Magnitude of delta LE is an indication for bond strength in the lattice
ALWAYS NEGATIVE - bonds forming
Bigger charges and or smaller ions…._____ ionic bonds_______ lattice enthalpy
Stronger
Larger (or more negative )
Ionic radius of an element depends on
Nuclear charge (atomic number) - the bigger the nuclear charge the smaller the ion ( for atoms with the same number of filled energy levels) The number of full energy levels - the more levels, the bigger the ion
Enthalpy change of hydration definition
Enthalpy change when an aqueous solution is formed from one mole of gaseous ions.
Ion-dipole bonds form - ALWAYS NEGATIVE.
The greater the enthalpy change of hydration the…
Stronger the ion-dipole attractions and the greater the number of water molecules surrounding the ion.
Strength of ion - dipole attraction depends on the charge and size of the ions:
Bigger the charge&/or small the size of ions the stronger the ion -dipole bonds - larger the enthalpy of hydration.
Enthalpy change of solution definition
Is the enthalpy change when one mole of a solute dissolves to form a dilute solution. Can be thought of as a two step process - breaking down the lattice and then hydrating the gaseous ions produced (water is solvent)
Enthalpy change of solution =
Delta Hhyd (cation) + delta Hhyd(anion) - deltaHle
Enthalpy change of solvation
Enthalpy change when a solution is formed from 1 mole of gaseous ions using a solvent OTHER than water.
If delta H solution is negative or slightly positive
Then solid will dissolve as entropy change will be favorable
If delta H solution is large and positive then
Solid will not dissolve even tho enthalpy change is favorable because too much energy is required.
Ionic solutes in non polar solvents
-Delta H solvation is tiny because there is little attraction between the ions and solvent.
Strong acids in water
Almost donate all their protons - said to have undergone complete dissociation
Eg HCL, sulfuric acid and nitric acid (HNO3)
Weak acids in water ..
Only a small proportion of the acid molecules donate their protons. Said to have undergone incomplete dissociation
Eg Carboxylic acids and carbonic acid
Ka
Acidity constant or acid dissociation constant. Greater the value of Ka the Stronger the acid.
Equation for Ka
= (H+) (acid-) / (acid)
All aqueous
For very weak acids we can use something instead of Ka for ease
pKa = -log (Ka)
Calculation to work out pH
pH= -log(H+ (aq))
Monoprotic acids
Have one acidic hydrogen - they release one proton per molecule
Strong acid assumptions
Because they fully dissociate we can assume that concentration of acid is the same as concentration of hydrogen ions in that solution if acid is Monoprotic
Weak acids assumptions
Because it hardly dissociates we say that acid concentration remains same.
Also for every one mole of H+ there’s one mole of acid-. conc =
What’s needed to calculate weak acid
Concentration of the acid and its dissociation constant
Strong bases in water
Gives (OH-) is fully dissociated
Ionic product of water:
Used to calc pH of strong bases
Kw = (H+)(OH-) = 1X10-14 mol2dm-6 at 298K
Calculating pH of strong base
Fully dissociates so conc of base = conc of OH-
Kw =(H+)(OH-)
So (H+) = Kw/(OH-)
Take -log(H+)
Buffer solution definition
Solutions that have almost constant pH, despite dilution of small additions of acid or alkali
Buffer solutions contain
Either a weak acid and on of its salts - e.g ethanoic acid and sodium ethanoate
Or a weak base and one of its salts - ammonia and ammonium chloride
All buffers contain large amounts of…
Proton donor - weak acid or conjugate acid
And large amounts of proton acceptor - weak base or conjugate base.
Any additions react with these large amounts and maintain the stable pH
Two assumptions made in buffer calculations
All the anions (A-) Have come from the salt, so contribution from the acid is negligible
Concentration of the acid in solution (HA(aq)) is the same as conc of acid put into solution.
Calculations with buffers
Ka= (H+)(A-) / (HA)
Using assumptions can be shown that Ka=(H+) X ((salt)/(acid))
Finding pH of a buffer solution
If Ka of the weak acid is known and conc of both salt and weak acid then (H+) can be calculated and hence the pH can be also.
Whenever solutions are mixed, the individual concentrations are reduced in proportion to their volumes in the mixture. Equal amounts = halved
Making buffer solution of a specific pH
Use Ka= (H+) X (salt)/acid
Choose right weak acid with correct Ka or pKa and then calculate ratio of acid : salt
Buffers in action
CO2 dissolved in oceans and limestone rocks (Calcium carbonate)
Carbonic acid produced acts as weak acid
Limestone acts as as anion sink (CO3^2-)
Means as CO2 atmospheric levels increase and dissolve to form carbonic acid the pH can be maintained
Unique properties of water
High bp for a compound with low relative molecular mass
Has a high specific heat capacity ( takes a lot of energy to raise temperature) large masses of water can carry a lot of energy in the oceans and distribute it across the worlds oceans.
Has a high enthalpy change of vaporization
Density of ice is lower than liquid water, result of bond angles - ice has a very open structure with large spaces.
ALL DUE TO H BONDING, WATER CAN FORM 2 HBONDS PER MOLECULE.
