[Year 2] O Flashcards
Describe the greenhouse effect.
- Most UV radiation from the sun is absorbed by gasses in the earth’s atmosphere.
- Some is reflected back into space.
- Some reaches the surface of Earth which is absorbed.
- This heats up the surface of the earth and radiates IR back towards space.
- Some IR escapes through the ‘IR window’, as they are not absorbed by the gases in the atmosphere.
- Some IR is absorbed by gases in the troposphere and reemitted in all directions included back towards earth.
Give examples of greenhouse gasses?
CO₂
H₂O
CH₄
What happens when greenhouse gasses absorb radiation?
- Their bonds in the molecules vibrate more.
- The vibrational energy is transferred to neighbouring particles during collisions.
- These particles now have more kinetic energy and their temperature starts to rise.
How can polar molecules be dissolved?
Using polar solvents.
What is the difference between hydration and solvation?
- Hydration is dissolving with water as a solvent.
- Whereas solvation is dissolving with a non-water solvent.
What happens when a salt dissolves in water?
- Water is polar.
- Its 𝛿+ Hs are attracted to the negative ions
- Its 𝛿- O is attracted to the positive ions.
- The water molecules surround the ions during hydration.
- This ion-dipole interaction must be stronger/as strong as the bond broken for it to dissolve.
Why might molecules like Al₂O₃ not dissolve in water?
- The bond between Al and O ions are stronger than the ion-dipole interaction that would form.
How do some non-polar molecules dissolve in water?
Through hydrogen bonds.
e.g. alcohols disolving in water
Why don’t polar molecules like haloalkanes dissolve in water?
- they have weak dipoles.
- so water will form a stronger intermolecular bond than with the haloalkane.
How can you dissolve a haloalkane?
Use a solvent that can interact via pertinent dipole-dipole interactions.
How do non-polar substances dissolve in non-polar solvents?
- by forming instantaneous dipole-induced dipole forces.
What is the standard lattice enthalpy?
The energy change when 𝟭 𝗺𝗼𝗹𝗲 of an ionic compound is formed from its 𝗴𝗮𝘀𝗲𝗼𝘂𝘀 ions under 𝘀𝘁𝗮𝗻𝗱𝗮𝗿𝗱 𝗰𝗼𝗻𝗱𝗶𝘁𝗶𝗼𝗻𝘀.
(298K/100kPa)
What does the sign and magnitude of the standard lattice enthalpy tell you?
- Always negative, as energy is released as bonds form.
- more negative = stronger bonding.
What is the enthalpy change of solution?
The energy change when 𝟭 𝗺𝗼𝗹𝗲 of an ionic compound is dissolved in the 𝗺𝗶𝗻𝗶𝗺𝘂𝗺 𝗮𝗺𝗼𝘂𝗻𝘁 𝗼𝗳 𝘀𝗼𝗹𝘃𝗲𝗻𝘁 to ensure 𝗻𝗼 𝗳𝘂𝗿𝘁𝗵𝗲𝗿 enthalpy change upon further dilution.
What is the enthalpy change of hydration?
The energy change when 𝟭 𝗺𝗼𝗹𝗲 of aqueous ions are formed from its 𝗴𝗮𝘀𝗲𝗼𝘂𝘀 ions under 𝘀𝘁𝗮𝗻𝗱𝗮𝗿𝗱 𝗰𝗼𝗻𝗱𝗶𝘁𝗶𝗼𝗻𝘀.
How is the lattice enthalpy, enthalpy of solution and enthalpy of hydration linked?
enthalpy of solution = enthalpy of hydration - lattice enthalpy.
What happens if the enthalpy change of hydration is larger (in magnitude) than the lattice enthalpy?
- The enthalpy change of solution will be negative.
- So the enthalpy change of solution will release energy.
- So the process is exothermic.
How can you find the enthalpy change of solution practically?
Calorimetry.
Describe calorimetry to find the enthalpy change of solution practically
- In a polystyrene cup add the solvent and measure the initial temperature of the solvent,
- measure out your solid solute in a weighing boat using scales.
- add the solute to the solvent and stir. Put a lid over the polystyrene cup (leave a hole in the lid to keep the thermometer in.
- Note down the maximum temperature reached after the solute was added.
- use the equation E = mcΔT to work out the enthalpy.
(mass in grams, T can be in any unit). - Work out the moles of solute added by doing moles = mass/Mr
- Divide this by the energy to find the enthalpy change of solution.
Why might the enthalpy change of the solution from a practical be different than the actual value?
- heat lost to surroundigns.
What affects the lattice enthalpy?
- Size of ions (ionic radii)
- Size of change (bigger difference in charge = stronger bond).
How do the ionic radii affect the lattice enthalpy?
- The smaller the ionic radii, the stronger the electrostatic attraction between ions.
- as smaller ions can pack together more closely.
- and so have a larger charge density.
- Thus more energy is required to overcome the stronger forces.
How does the size of charge affect the lattice enthalpy?
- Bigger the charge on an ion, the stronger the electrostatic attraction between ions.
- Thus more energy is required to overcome these forces.
What is entropy?
The measure of disorder in a system.
- It is the number of ways energy can be shared out in a system.
- Given the letter S.
Why do gases have a larger entropy than solids?
- Gases are more disordered.
- So there are more ways to arrange the atoms/energy than in a solid.
- And so it has a higher entropy.
How does the number of particles affect the entropy of a reaction?
- If there are more moles of a particle then there are more ways energy can be distributed.
- and so the entropy has increased.
How is the entropy change calculated?
ΔₛᵧₛS = S(products) - S(reactants)
Unit: JK⁻¹mol⁻¹
How can the total entropy change be calculated?
ΔₜₒₜS = ΔₛᵧₛS + ΔₛᵤᵣᵣS
ΔₛᵤᵣᵣS = -ΔH/T (convert ΔH to Jmol⁻¹)
What must be true for a reaction to be feasible?
The total entropy change must be positive or zero.
What is solubility?
The maximum amount of solid that will dissolve in a solvent.
Units: gdm⁻³
What is the ‘point of saturation’?
The point where water (or a solvent) can no longer hold any more solute.
How do you get moldm⁻³ from gdm⁻³
divide by Mr of solid
as mole = mass/Mr
What is the solubility product (Kₛₚ)?
- The equilibrium constant for a sparingly soluble product in a saturated solution.
What is the formula for Kₛₚ?
Use the general equation AₓBᵧ₍ₛ₎ ⇌ xAʸ⁺₍ₐᵩ₎ + yBˣ⁻₍ₐᵩ₎
Same as K𝒸 but only the aqueous products…
Kₛₚ = [Aʸ⁺₍ₐᵩ₎]ˣ [Bˣ⁻₍ₐᵩ₎]ʸ
(always include state symbols)
How would you work through the following question…
Calculate the solubility product of Li₂CO₃ when the solubility of the is 13.3gdm⁻³ at 20ᵒC.
- write Kₛₚ expression using the reaction.
Li₂CO₃₍ₛ₎ ⇌ 2Li⁺₍ₐᵩ₎ + CO₃²⁻₍ₐᵩ₎
Kₛₚ = [Li⁺₍ₐᵩ₎]² [CO₃²⁻₍ₐᵩ₎₎]
- work out the solubility of Li₂CO₃ in moldm⁻³
- 3/Mr(Li₂CO₃) = 13.3/74 = 0.18moldm⁻³
- We know 1 mol of Li₂CO₃ dissocaites to form 2 moles of Li⁺ and 1 mole of CO₃²⁻ therfore…
[Li⁺₍ₐᵩ₎] = 0.18 x 2 =0.36
[CO₃²⁻₍ₐᵩ₎] =0.18
- Put into equation
Kₛₚ = 0.36² x 0.18 = 0.023
- Work out units.
Kₛₚ = (moldm⁻³)² x (moldm⁻³) = (moldm⁻³)³ = (mol³dm⁻⁹)
Answer = 0.023mol³dm⁻⁹
What is a feature of Brønsted-Lowry acid?
- proton donors.
How do H⁺ ions actually exist in water?
As hydroxonium ions (H₃O⁺)
What is a feature of Brønsted-Lowry base?
- proton acceptors
Give a general equation for a Brønsted-Lowry acid in water?
HA₍ₐᵩ₎ + H₂O₍ₗ₎ ⇌ H₃O⁺₍ₐᵩ₎ + A⁻₍ₐᵩ₎
Give a general equation for a Brønsted-Lowry base in water?
B₍ₐᵩ₎ + H₂O₍ₗ₎ ⇌ BH⁺₍ₐᵩ₎ + OH⁻₍ₐᵩ₎
Where is the position of equilibrium in weak acids, and what does this mean?
- More to the left.
- So backwards reaction is favoured.
- So dissociate poorly
- Therefore not too many H⁺ produced.
Where is the position of equilibrium in strong acids, and what does this mean?
- More to the right.
- So forward reaction is favoured strongly.
- So dissociate almost completely.
- Therefore many H⁺ produced.
Where is the position of equilibrium in strong bases, and what does this mean?
- More to the right.
- So forward reaction is favoured strongly.
- So dissociate almost completely.
- Therefore many OH⁻ produced.
Where is the position of equilibrium in weak bases, and what does this mean?
- More to the left.
- So backwards reaction is favoured.
- So dissociate poorly.
- Therefore not too many OH⁻ produced.
What happens to protons in acid-base reactions?
They are exchanged.
What is the role of water in the following reaction…
HA₍ₐᵩ₎ + H₂O₍ₗ₎ ⇌ H₃O⁺₍ₐᵩ₎ + A⁻₍ₐᵩ₎
- acts as a base.
- as it accepts protons.
What is a conjugate acid?
A species that has gained a proton.
What is a conjugate base?
A species that has lost a proton.
What are the conjugate pairs in the following reaction…
HA + B ⇌ BH⁺ A⁻
- HA and A⁻ are pairs with HA as the acid and A⁻ as the base.
- B and BH⁺ are pairs with B as the base and BH⁺ as the acid.
What assumptions can you make about the concentrations in…
- A neutral solution?
- A strong acid?
- Neutral solution: [H⁺] = [OH⁻]
- Strong Acids: [H⁺] = [Acid]
What is the pH equation?
pH = -log₁₀[H⁺]
[H⁺] = 10⁻ᵖᴴ
What must you factor in when finding the pH of acids like H₂SO₄
- Two protons are produced for every 1 acid.
- Therfore [H⁺] = 2[Acid]
How does pure water exist in reality?
In equilibrium with its ions…
2H₂O ⇌ H₃O⁺ + OH⁻
which simplifies to…
H₂O ⇌ H⁺ + OH⁻
Use the equilibrium of pure water to derive Kw.
H₂O ⇌ H⁺ + OH⁻
Kc = [H⁺] [OH⁻] / [H₂O]
Water dissociated very weakly, therefore, we can assume the concentration of water is a constant.
So we can multiply out [H₂O] on both sides to get…
Kw = [H⁺] [OH⁻]
What assumptions can we make when using Kw for pure water and Kw at standard conditions?
Kw = 1x10⁻¹⁴ (this is at standard conditions and will change if the temperature changes.)
In pure water… [H⁺] = [OH⁻]
therefore…
Kw = [H⁺]²
How do you work out the pH of strong bases?
- strong bases dissociate to produce OH⁻.
- Kw = [H⁺] [OH⁻]
- therefore…
[H⁺] = Kw / [OH⁻] - then put into pH equation
What constant is used to help work out the pH of weak acids?
Ka
Use the equilibrium of weak acids to derive Ka.
- HA ⇌ H⁺ + A⁻
- Kc = [H⁺] [A⁻]/[Ha]
- Only a small amount of weak acids dissociates so… [HA₍ₐᵩ₎]equilibrium ≈ [HA₍ₐᵩ₎]start
- Ka = [H⁺] [A⁻]/[Ha]ₛₜₐᵣₜ
- The dissociation of acid is much greater than of water so we can assume all the H⁺ comes from the acid so…
[H⁺₍ₐᵩ₎] ≈ [A⁻₍ₐᵩ₎] - Ka = [H⁺]²/[Ha]ₛₜₐᵣₜ
How do you work out the pH of a weak acid?
- Ka = [H⁺]²/[Ha]ₛₜₐᵣₜ
- [H⁺]² = Ka x [Ha]ₛₜₐᵣₜ
- [H⁺] = √(Ka x [Ha]ₛₜₐᵣₜ)
- then put into pH equation
What is pKa?
Another way of measuring the strength of an acid
equations:
pKa = -log₁₀Ka
Ka = 10⁻ᵖᴷᵃ
What is a buffer?
A chemical that resists a change in pH when small amounts of acid or base are added.
What are acidic buffers?
- something that resist the change of pH in order to keep a solution below pH7
- it is made from a weak acid and it’s salt.
Ethanoic acid (CH₃COOH) can be used as a buffer with sodium ethanoate (CH₃COO⁻Na⁺).
Describe what happened when an acid is added?
- Ethanoic acid is a weak acid in equilibrium…
CH₃COOH₍ₐᵩ₎ ⇌ CH₃COOH₍ₐᵩ₎ + H⁺ - Because it is a weak acid its equilibrium lies far to the left so the concentrations are…
CH₃COOH₍ₐᵩ₎ ⇌ CH₃COOH₍ₐᵩ₎ + H⁺
[High] [Low] [Low] - Sodium ethanoate dissociates fully in equilibrium…
CH₃COO⁻Na⁺ ⇌ CH₃COO⁻ + Na⁺ - So its equilibrium lies far to the right and ist concentration are…
CH₃COO⁻Na⁺ ⇌ CH₃COO⁻ + Na⁺
[Low] [High] [High] - When an acid is added its H⁺ react with a negative ion.
- This is CH₃COO⁻ from the sodium ethanoate equation, as there is a high concentration of these.
- This produces more CH₃COOH using the additional H⁺ returning the pH to normal and shifting the equilibrium to the left.
Ethanoic acid (CH₃COOH) can be used as a buffer with sodium ethanoate (CH₃COO⁻Na⁺).
Describe what happened when a base is added?
- Ethanoic acid is a weak acid in equilibrium…
CH₃COOH₍ₐᵩ₎ ⇌ CH₃COOH₍ₐᵩ₎ + H⁺ - Because it is a weak acid its equilibrium lies far to the left so the concentrations are…
CH₃COOH₍ₐᵩ₎ ⇌ CH₃COOH₍ₐᵩ₎ + H⁺
[High] [Low] [Low] - Sodium ethanoate dissociates fully in equilibrium…
CH₃COO⁻Na⁺ ⇌ CH₃COO⁻ + Na⁺ - So its equilibrium lies far to the right and its concentration are…
CH₃COO⁻Na⁺ ⇌ CH₃COO⁻ + Na⁺
[Low] [High] [High] - When a base is added its OH⁻ ions react with a positive charge.
- This is not the Na⁺ however as this will create a base and will dissociate back to OH⁻.
- So this reacts with the H⁺.
- Because there is a low contraction of H⁺ as this is used up, it shifts the equilibrium of the ethanoic acid equation to the right.
- This drives the forward reaction and so produces more H⁺ to return the pH to normal
How would you work through the following question…
Calculate the pH of a buffer that contains 2.35x10⁻² mol of methanoic acid and 1.84x10⁻² mol of sodium methanoate in a 1dm³ solutions. The value of Ka at 25°C for methanoic acid is 1.78x10⁻⁴
- Write Ka expression from the equilibrium equation.
HCOOH₍ₐᵩ₎ ⇌ H⁺₍ₐᵩ₎ + HCOO⁻₍ₐᵩ₎
Ka = [H⁺][HCOO⁻] / [HCOOH] - we assume that salts dissociate fully and weak acids dissociate poorly so…
[salt] = [A⁻] AND [HA₍ₐᵩ₎]equilibrium ≈ [HA₍ₐᵩ₎]start - [H⁺] = Ka x [HCOOH] / [HCOO⁻]
- [H⁺] = 1.78x10⁻⁴ x 2.35x10⁻²) / 1.84x10⁻²
- [H⁺] = 2.27x10⁻⁴ moldm⁻³
- plug into pH equitation
- pH = 3.64
