The Oceans Flashcards
What happens when ionic compounds dissolve in water?
The ionic lattice breaks up
New bonds are formed between water molecules and the seperate ions
What is the enthalpy change of solution?
What is its symbol?
ΔsolutionH
The enthalpy change when 1mol solute dissolves to form a solution
e.g. CaCl2(s) + (aq) → Ca2+(aq) + 2Cl-(aq)
What is lattice enthalpy?
What is its symbol?
ΔLEH
Enthalpy change when 1mol ionic solid formed from gaseous ions
e.g. Ca2+(g) + 2Cl-(g) → CaCl2(s)
What is enthalpy change of hydration?
What is its symbol?
ΔhydH
Enthalpy change when 1mol gaseous ions is hydrated by forming bonds to water molecules
e.g. Ca2+(g) + (aq) → Ca2+(aq)
What is a hydrated ion?
Ion bonded to/surrounded by water molecules
Bonded by ion-dipole bonds
What factors will cause lattice enthalpy to be more negative?
Ions in lattice more highly charged
Ions in lattice smaller
i.e. more negative if ions have greater charge density
What factors will cause the enthalpy change of hydration (of ions) to be more negative?
Ionic charge is greater
Ionic radius is smaller
i.e. more negative if ions have higher charge density
Why will the enthalpy change of hydration be more negative if the ion has a greater charge density?
Because this means the ion attracts more water molecules/forms stronger ion-dipole bonds
Hence the energy released by bond forming is greater
If a different solvent is used to water, what is enthalpy change of hydration called and what is its symbol?
Enthalpy change of solvation
ΔsolvH
What formula can be used to measure ΔsolH experimentally?
ΔsolH = (ΔhydH(cation) + ΔhydH(anion)) - ΔLEH
Can be used to construct a Hess’ Cycle
Draw an enthalpy level diagram for the dissolution of a water-soluble salt
Draw an enthalpy level diagram for an insoluble salt
Draw an enthalpy level diagram for a salt that may be soluble
Many ionic substances are soluble in water.
Explain why
Ionic bonds are broken when an ionic substance dissolves. Some hydrogen bonds also break between water molecules
Ion-dipole bonds form between water molecules and the free ions
The strength of the bonds formed is similar to the strength of the bonds broken
So the energy released by bond formation is sufficient to compensate for the energy required to break the bonds between ions
Sodium chloride doesn’t dissolve in cyclohexane.
Explain why
In order to dissolve, ionic bonds would need to break between the ions in the sodium chloride lattice
Instantaneous dipole-induced dipole bonds would also need to break between the cyclohexane molecules
Because cyclohexane molecules don’t have permanent dipole bonds, only weak ion-dipole bonds can form between cyclohexane molecules are the free ions
The strength of the bonds that could form is much weaker than the bonds that would need to break
So the energy released by bond formation is not sufficient to compensate for the energy required to break the bonds between ions
What is entropy?
A measure of the number of ways in which molecules + energy quanta can be arranged
Shown by the symbol S
What is a feasible reaction/process?
One which can occur without any energy input
However may occur very slowly (or have a high EA)
A reaction is feasible if ΔtotS is positive
List the states of matter in order of increasing entropy
(from lowest to highest)
Solids
Liquids
Solutions
Gases
Why do gases have a greater entropy than solids?
Particles in a solid are rigidly fixed in place, whereas particles in a gas are free to move around + take up many different positions
Hence there are more ways of arranging the particles + energy quanta, so entropy is greater
What is ΔtotS?
The total change in entropy of a process
Is the sum of 2 entropy changes:
The entropy change of the system - ΔsysS
The entropy change of the surroundings - ΔsurrS
What is the formula for calculating the total entropy change of a process/reaction?
ΔtotS = ΔsysS + ΔsurrS
What is the formula for calculating ΔsysS?
ΔsysS = ΣS (products) - ΣS (reactants)
What is the formula for calculating ΔsurrS?
ΔsurrS = - ΔH / T
Where T is temp given in K
ΔH given in Jmol-1
If the forwards reaction of a reversible reaction is feasible (at a given temp), what signs will ΔtotS for the forwards and backwards reactions have?
For the forwards (feasible) reaction: +
For the backwards (unfeasable) reaction: -
What will happen if ΔtotS = 0 for a reversible process?
The process will reach equilibrium where neither forwards/backwards favoured
How can you predict from a balanced equation whether or not a reaction is feasible at room temperature?
If the states of the products have a higher entropy than those of the reactants (e.g. are gaseous not solid) it will be feasible
If there are more molecules of product than reaction it will be feasible as this means there are more ways of arranging the molecules + energy quanta
Why might a reaction still not occur even if the entropy change is feasible?
Because the Ea might be very high - not taken into account when calculating entropy changes
Describe the greenhouse effect
Solar energy reaches the Earth mainly as visible + UV radiation
The Earth absorbs some of this radiation + radiates some in the form of IR
Water vapour in the atmosphere absorb some frequences of IR, helping keep the temp. of the Earth constant
However, greenhouse gases in the troposphere absorbs the remaining freqs. of IR (the ‘IR window’)
This causes the vibrational energy of their bonds to increase + energy is transfered to other molecules by collisions, increasing their kinetic energy and thus raising temp.
Greenhouse gagses also re-emit some of the absorbed IR in all directions, contributing to the heating of the Earth
Which part of the atmosphere are greenhouse gases found in?
The troposphere
Name 2 greenhouse gases
Methan
CO2
What can enhance the greenhouse effect/make it worse?
Increased concentrations of greenhouse gases
Why can the oceans help decrease the effect of global warming?
The can dissolve/absorb some CO2 preventing it building up in the atmosphere and acting as a greenhouse gas
What is the drawback to the oceans absorbing CO2?
It leads to ocean acidification
CO2 is soluble in water
It acts as a base and absorbs H+ ions to form hydrogencarbonate ions
Some hydrogencarbonate ions dissociate to form carbonate ions
Write out equations for these reactions
CO2(aq) + H2O(l) ⇌ HCO3-(aq) + H+(aq)
HCO3-(aq) ⇌ CO3-(aq) + H+(aq)
What is an acid?
A substance which donates H+ ions when in an aqueous solution
It is a proton donor
What is a base?
A substance that accepts H+ ions
Is a proton acceptor
What is the Brønsted-Lowry theory?
Used to categorise acids + bases
Based on ability to transfer/accept H+ ions
What is a conjugate acid-base pair?
The substance that is formed from the dissociation of an acid but can act as a base to reverse the reaction/dissociation
HA(aq) ⇌ H+(aq) + A-(aq)
HA and A- are the conjugate acid-base pair
What is a conjugate acid?
A species that donates protons in the reverse reaction of a dissociation of an acid
What is a strong acid?
An acid which fully dissociates into H+ and A- ions
What is a weak acid?
An acid which doesn’t dissociate fully
Undergoes an incomplete ionisation to H+ and A- so reaches equilibrium
For a strong acid, how does the concentration of [HA] relate to the concentration of [H+]?
They are equal if the acid is monoprotic
What are the units for pH?
It doesn’t have any
What is the formula to calculate pH?
pH = -log10 [H+]
What is Ka?
What equation is used to calculate it?
The acidity constant
Ka = [A-][H+] / [HA]
How does Ka change if the strength of an acid increases?
The stronger the acid, the greater Ka
What is pKa?
Used when the value for Ka very small due to acid being very weak
pKa = -log Ka
What does a larger pKa value signify?
That the acid is very weak
The higher pKa, the weaker the acid
What assumptions are made when doing calculations to do with weak acids?
In equilibrium concentration [HA] for a weak acid is the same as the initial concentration of the acid
The equilibrium concentration of [A-] is equal to the equilibrium concentration of [H+].
A few protons will be provided by water but these are insignificant compared to those donated by the acid
What does monoprotic mean?
That an acid will donate 1 H+ (proton) when it dissociates as it contains 1 (acidic) H
Calculate the pH of 0.01moldm-3 CH3COOH (Ka = 1.7x10-5moldm-3)
CH3COOH(aq) ⇌ CH3COO-(aq) + H+(aq)
Write an expression for Ka:
Ka = [CH3COO-][H+] / [CH3COOH]
Assumption 1: Few molecules dissociate, so [CH3COOH] = 0.01moldm-3
Assumption 2: For every 1mol H+ there’s 1mol CH3COO- so [H+] = [CH3COO-]
So Ka = [H+]2 / 0.01 = 1.7x10-5
[H+] = √1.7x10-7
[H+] = 4.12x10-5
pH = log10 (4.12x10-5) = 3.4
What assumptions can be made when doing calculations with strong bases?
The base fully dissociates so [OH-] is equal to the initial concentration of the base
That any dissociation from water molecules is insignificant so can be ignored
Calculate the pH of a 0.01moldm-3 solution of NaOH
NaOH → Na+ + OH-
Strong bases fully dissociate so [OH-] = 0.01moldm-3
Kw = [H+] [OH-]
[H+] = Kw / [OH-]
[H+] = 1x10-14mol2dm-6 / 0.01moldm-3 = 1x10-12moldm-3
pH = -log (1x10120 = 12.0
What is Kw?
The ionic product of water
Formed due to the slight dissociation of water
H2O(l) ⇌ H+(aq) + OH-(aq)
Kw = [H+] [OH-] = 1x10-14mol2dm-6 at 298K
Used instead of Ka in calculations involving strong bases
What is a buffer solution?
A solution which has little/no change in pH when a small amount of acid/alkali added
Made from a weak acid/base and one of its salts
What do all buffer solutions contain?
Why?
Large amounts of a proton donor - either weak acid or conjugate acid
Large amounts of proton acceptor - either weak base of conjugate base
Means that any additions of acid/alkali react with these + keeps pH constant
A buffer solution is made of ethanoic acid and sodium ethanoate.
A small amount of acid is added. Explain how equilibrium changes.
CH3COOH ⇌ CH3COO- + H+
Equilibrium shifts to the left to oppose the change in conc. of H+
This is done by the ethanoate ions reacting with the excess H+ to produce more ethanoic acid
Hence the pH remains constant
Weak acids partially dissociate + form an equilbrium that also included a conjugate acid.
Explain why, despite the fact a conjugate acid is formed anyway, a salt of the acid used to make a buffer must also be added
Because the acid alone doesn’t dissociate to produce enough conjugate acid ions to oppose a change in pH if more H+ ions were added
Give some example of buffers in action
Shampoos
In food + drink as ‘acidity regulators’
In the blood
Explain why buffers are needed in the blood
To protect the body from changes in pH due to the formation of CO2 + H+ in metabolic processes
Otherwise these pH changes could affect the action of enzymes which is bad for our health
Calculate the pH of a buffer solution made by mixing equal volumes of 0.20moldm-3 ethanoic acid + 0.10moldm-3 sodium ethanoate solutions
(For ethanoic acid, Ka = 1.7x10-5moldm-3 at 298K)
By mixing equal vols, each original conc halved, so:
[CH3COOH] = 0.10moldm-3 [CH3COO-] = 0.05moldm-3
Ka = [H+] [CH3COO-] / [CH3COOH]
[H+] = Ka x [CH3COOH] / [CH3COO-]
[H+] = 1.7x10-5 x 0.1 / 0.05 = 3.5x10-5
pH = -log (3.4x10-5) = 4.5
What values are required in order to calculate the pH of a buffer solution?
Ka of the weak acid
Conc. of the salt + weak acid
Needed to calculate [H+] and therefore pH
What is Ksp?
The solubility product
An equilibrium constant for the dissolution of a sparingly soluble salt
Write the equation for the dissolution of a sparingly soluble ionic solid
AB(s) ⇌ A+(aq) + B-(aq)
What is the formula for the solubilty product?
Ksp = [A+] [B-]
Why doesn’t the equilibrium constant for the dissolution of sparingly soluble ionic solids (the solubilty product) include the concentration of the original ionic salt?
Because the concentration of a solid cannot be measured
What is the equation for Ksp of CaI2?
CaI2 ⇌ Ca2+ + 2I-
x ⇌ x + 2x
Ksp = [Ca2+] [2I-]2
How can Ksp be used to predict whether a precipitate will form?
If the product of the conc. of the ions is smaller/equal to value for Ksp the ions will stay in solution and a precipitate will not form
How can the solubility product be determined experimentally?
In order to do so, the concentrations of the ions in solution needs to be measured
e.g. with Ca(OH)2 this can be done via titration to determine [OH-]
Undissolved salt should be filtered before titrating
The conc. [Ca2+] can then be found once [OH-] known using the molar ratio ([OH-] will be x2 [Ca2+])
Will a precipitate from a solution at 298K in which [Ag+] and [Cl-] = 5.0x10-6moldm-3?
Ksp [AgCl] = 2.0x10-10mol2dm-6 at 298K
[Ag+] and [Cl-] = (5x10-6) x (5x10-6) = 2.5x10-12mol2dm-6
Answer smaller than Ksp so no precipitate will form
