Acids, Bases and pH Flashcards
What is the Brønsted-Lowry theory?
A theory that describes acid-base interactions in terms of proton transfer between chemical species
What are Brønsted-Lowry acids?
Proton donors
Why are Brønsted-Lowry acids called proton donors?
They release hydrogen ions when they’re mixed with water
Write the general equation for a general Brønsted-Lowry acid (HA) mixed with water
HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq)
What are Brønsted-Lowry bases?
Proton acceptors
Why are Brønsted-Lowry bases called proton acceptors?
When they’re in a solution, they grab hydrogen ions from water molecules
Write the general equation for a general Brønsted-Lowry base (B) in a solution
B(aq) + H2O(l) ⇌ BH+(aq) + OH-(aq)
What do acids and bases do in water?
Break up into positive and negative ions (called dissociation)
What do strong acids do in water?
They dissociate almost completely in water - nearly all H+ ions are released
What do strong bases do in water?
They dissociate almost completely in water
What do weak acids do in water?
They dissociate only partially - so only a small amount of H+ ions are formed
What do weak bases do in water?
They only slightly dissociate
Describe the equilibrium position in the reversible reaction when a strong acid mixes with a water
The equilibrium lies far to the right, which is why nearly all the H+ ions are released
Describe the equilibrium position in the reversible reaction when a weak acid mixes with a water
The equilibrium lies far to the left, which is why only a small amount of the H+ ions are released
What are monobasic acids?
An acid that releases one H+ ion per molecule
What are monobasic acids also called?
Monoprotic acids (because an H+ ion is just a proton)
Give 2 examples of monobasic acids
HCl
HNO3
What are dibasic acids?
An acid that releases two H+ ion per molecule
Give an example of dibasic acids
H2SO4
What are tribasic acids?
An acid that releases three H+ ions per molecule
Give an example of a tribasic acid
H3PO4
What are tribasic and dibasic acids examples of?
Polyprotic acids
What is a species?
A type of chemical - could be a molecule, atom or an ion
What are conjugate pairs?
Species that are linked by the transfer of a proton (H+ ion)
What is the general equation for the equilibrium reaction between an acid and a base? Show the conjugate pairs
HA + B ⇌ BH+ + A-
HA ⇌ A- is conjugate pair 1 (acid1 —> base1)
B ⇌ BH+ is conjugate pair 2 (base2 —> acid2)
In a balanced chemical equation, where will you find conjugate pairs?
On either side of the chemical equation
Describe a conjugate pair and the proton transfer between them
When a conjugate acid loses a proton, it forms a conjugate base. But when this conjugate base gains a proton, it forms a conjugate acid
What is the conjugate acid when water reacts with acids?
H3O+
What is the conjugate base when water reacts with bases?
OH-
What is the general equation for when an acid dissolves in water? State the conjugate pairs
HA + H2O ⇌ H3O+ + A-
HA (acid1) ⇌ A- (base1)
H2O (base2) ⇌ H3O+ (acid2)
What is the general equation for when a base dissolves in water? State the conjugate pairs
B + H2O ⇌ BH+ + OH-
B (base1) ⇌ BH+ (acid1)
H20 (acid2) ⇌ OH- (base2)
How do you identify conjugate pairs?
By looking at the chemical equation for 2 species that are linked by proton transfer
Two species can’t be a conjugate pair if…?
… they are on the same side of the chemical equation
What are the products of the reaction between a reactive metal and acid?
Salt and Hydrogen gas
In the reaction between a metal and an acid, what is oxidised and what is reduced?
The metal atoms donate electrons to the H+ ions in the acid solution. The metal atoms are oxidised and the H+ ions are reduced
What are the products of the reaction between an acid and a carbonate?
Carbon dioxide, water and a salt
What is the product when an acid reacts with a base or an alkali?
Water
What ions are produced in the reaction between an acid and an alkali?
Acids produce H+ ions and the alkalis produce OH- ions when dissolved in water
What is an alkali?
A base that releases OH- ions when they dissolve in water
What type of reaction is Acid + Base?
Neutralisation
What is the ionic equation for the reaction between an acid and an alkali?
H+(aq) + OH-(aq) —> H2O(l)
What is the ionic equation for the reaction between an acid and an insoluble base?
H+(aq) + O2-(s) —> H2O(l)
What is pH?
A measure of the hydrogen ion concentration in a solution
What is the equation for pH?
-log[H+]
What is the range of the pH scale?
0 (very acidic) to 14 (very alkaline)
What is the pH of water?
pH7 (neutral)
For a monobasic acid, how can you work out the concentration of the H+ ion if you know the concertation of the acid?
Each mole of acid produces one mole of hydrogen ions, so the H+ concentration is the same as the acid concentration
What are the products when water dissociates?
Hydroxonium ions and Hydroxide ions
What equilibrium exists in water, then simplify it?
H2O(l) + H2O(l) ⇌ H3O+(aq) + OH-(aq)
remove H2O from both sides:
H2O ⇌ OH-(aq) + H+(aq)
Why does the equilibrium for the dissociation of water lie far to the left?
Because water only dissociates a tiny bit, so most is just H2O
What is the ionic product of water?
An equilibrium constant specific to the dissociation of water at a constant temperature
What is the equation for the ionic product of water?
Kw = [H+][OH-]
What are the units of Kw?
mol^2dm^-6 (always because you’re multiplying 2 concentrations together)
What type of process is the dissociation of water?
Endothermic (+ΔH)
What happens to the value of Kw at a fixed temperature?
It remains constant
What happens to the position of the equilibrium (and value of Kw) when the temperature of the solution is increased?
The dissociation of water is endothermic, therefore the equilibrium shifts to the right to (dissociates more) so the value of Kw increases
What happens to the position of the equilibrium (and value of Kw) when the temperature of the solution is decreased?
The dissociation of water is endothermic, therefore the equilibrium shifts to the left to (dissociates less) so the value of Kw decreases
What can be said about the pH of water?
It is not always 7, it changes depending on the temperature
How do you find the ionic product of pure water?
Kw = [H+]^2
Why is the equation for the ionic product of pure water, Kw = [H+]^2?
In pure water, there is always one H+ ion for each OH- ion, so the concentrations are the same so equal to [H+]^2
How do you find the pH of a strong base?
- Find the values of Kw and [OH-]
- Substitute into equation for ionic product of water and solve for [H+]
- Substitute this value into the equation for pH
For a strong base, how do you find [OH-] if you know the concentration of the base?
Strong bases fully ionise in water - donating one mole of OH- ions per mole of base. Therefore concentration of base = [OH-]
How do you find the concentration of a strong base if you know the pH?
-Use equation [H+] = 10^-pH to find the value of [H+]
-Substitute value of [H+] and Kw into equation for ionic product of water to work out [OH-]
= [OH-] = concentration of base
What is the acid dissociation constant, Ka?
An equilibrium constant specific to weak acids that relates the acid concentration to the [H+] concentration
What are the units of the acid dissociation constant?
mol dm^-3
What is the equation for the acid dissociation constant?
Ka = [A-][H+] / [HA]start = [H+]^2 / [HA]
What are the 2 assumptions made when finding Ka?
- Only works for weak acids
- Because strong acids dissociate more, there is a significant difference between [HA] start and [HA] equilibrium
How does temperature affect Ka?
The Ka is fixed at a given temeprature
If dissociation is endothermic, what effect on Ka does increasing the temperature have?
Shifts the equilibrium to the right (dissociates more), so Ka increases
If dissociation is endothermic, what effect on Ka does decreasing the temperature have?
Shifts the equilibrium to the left (dissociates less), so Ka decreases
If dissociation is exothermic, what effect on Ka does increasing the temperature have?
Shifts the equilibrium to the left (dissociates less), so Ka decreases
If dissociation is exothermic, what effect on Ka does decreasing the temperature have?
Shifts the equilibrium to the right (dissociates more), so Ka increases
What are the 4 steps to find the pH using Ka?
1: Write an expression for Ka for the weak acid
2: Rearrange to solve for [H+]^2
3: Take the square root of the number to find [H+]
4: Substitute [H+] into the pH equation
What are the 4 steps to find the concentration of a weak acid using pH and Ka?
1: Substitute pH into the inverse pH equation to get [H+]
2: Write an expression for Ka
3: Rearrange equation to give concentration of acid
4: Substitute values of Ka and [H+] to solve for [HA]
What are the 3 steps to find Ka of weak acid from concentration and pH?
1: Use the pH to find [H+]
2: Write an expression for Ka
Substitute values of [H+] and [HA] to find Ka
What is pKa?
Logarithmic constant
Why do scientists often use pKa instead of Ka?
Because Ka varies so much between different acids, it makes it easier to handle numbers
What is the equation for pKa?
pKa = -log(Ka)
What is the equation for Ka if given the value of pKa?
Ka = 10^-pKa (inverse log)
The larger the value of pKa, the…?
weaker the acid
What are the usual pH values for a weak acid?
pH2-pH5
What is a buffer?
A solution that minimises changes in pH when small amounts of acid or base are added
What is the job of a buffer?
To not stop the pH from changing, but to make the change very slight when an acid or base are added
In what situation do buffers only work in?
When small amounts of acid or base are added
Why do buffers only work for small amounts of acid or bases?
Because if add too much acid or base, the buffer won’t be able to cope
What are the 2 types of buffers?
Acidic buffers
Basic buffers
What is the pH range of an acidic buffer?
What are the 2 ways of making an acidic buffer?
- From a weak acid and the salt of its conjugate base
- From an excess of weak acid and some strong alkali
How do you make an acidic buffer from a weak acid and the salt of its conjugate base?
Mix a weak acid and the salt of its conjugate base. A small amount of the acid will dissociate, but most will be undissociated. The salt will fully dissociate to form the conjugate base of the acid in the solution. The solution will contain a mixture of weak acid and its conjugate base, so would act as an acidic buffer
How do you make an acidic buffer from an excess of weak acid and some strong alkali?
Mix the excess of weak acid and some strong alkali. Some of the acid neutralises to make a salt, but some is left un-neutralised. The reaction mixture would then contain a weak acid and its conjugate base, so would act as an acidic buffer
When making an acidic buffer from an excess of weak acid and some strong alkali, why do you need to have an excess amount of acid rather than an excess amount of alkali?
Because there was an excess amount of alkali, all of the acid would react, so there’d be no acid left
What does an acidic buffer solution contain?
Lots of dissociated acid and the acids conjugate base
When you prepare an acidic buffer, what equilibrium is set up between the weak acid and the conjugate base?
HA ⇌ H+ + A-
How does an acidic buffer work?
When you alter the concentration for H+ or OH- ions in the buffer solution, the equilibrium position shifts to counteract the change. The conjugate base can react to ‘mop up’ excess H+ ions and the conjugate acid can dissociate to release H+ ions if there’s too much base
How does an acidic buffer resist an acid?
If you add a small amount of acid, the H+ concentration increases. Most of the extra H+ ions join the A- ions to form HA. This causes the equilibrium to shift to the left, meaning the H+ concentration reduces to its original level, so the pH stays the same
How does an acidic buffer resist a base?
If a small amount of base is added, the OH- concentration increases. Most of the extra OH- ions react with the H+ ions to form water, decreasing the H+ concentration. This causes the equilibrium to shift to the right, so that more of the acid dissociates and releases H+ ions, increasing the H+ concentration to its original value, so the pH stays the same
What range of pH does blood need to be kept at?
Between pH 7.35 and pH 7.45
What is the name of the buffer that controls the pH in our blood?
Carbonic acid-hydrogencarbonate buffer
What is the equilibrium set up in the blood by the buffer?
H2CO3 ⇌ H+ + HCO3-
How does the buffer system in the blood keep the pH the same if a small amount of acid is added?
The HCO3- ions from the carbonic acid-hydrogencarbonate buffer will react with the excess H+ ions, causing the equilibrium to shift left, reducing the H+ concentration
How does the buffer system in the blood keep the pH the same if the concentration of H+ ions drops?
More H2CO3 molecules from the carbonic acid-hydrogencarbonate buffer will dissociate, so the equilibrium will shift to the right meaning that the concentration of H+ ions increases
What process controls the levels of H2CO3 (carbonic acid) in the blood?
Respiration
How does respiration control the levels of H2CO3 (carbonic acid) in the blood, and state the equilibrium set up?
By breathing out CO2, the level of H2CO3 is reduced because it moves the equilibrium to the right
H2CO3 ⇌ H2O + CO2
How are the levels of HCO3- controlled in the body?
By the kidneys, if there is excess HCO3-, it is excreted out the body as urine
What are the 2 assumptions you need to make when calculating the pH of an acidic buffer?
- The salt of the conjugate base is fully dissociated, so the equilibrium concentration of A- is the same as the initial concentration of the salt
- HA is only slightly dissociated, so its equilibrium concentration is the same as its initial concentration
What are the 4 steps to calculate the pH of a buffer?
1: Write out the expression for Ka of a weak acid, and rearrange to solve for [H+]
2: Substitute the value of Ka and equilibrium concentrations into the expression
3: Solve the equation for [H+]
4: Substitute the value of [H+] into pH equation
What are the 5 steps to calculate the equilibrium concentrations if, to make the buffer, you mixed an acid with a base?
1: Write out the equation for the neutralisation reaction
2: Calculate number of moles of acid and base at the start using volumes and concentration given in question
3: Use the molar ratios in the equation to work out the moles of acid and salt left at the end of the reaction
4: Calculate concentration of acid and salt in the buffer solution by dividing by the volume of the solution
5: Use these to calculate pH by using the Ka equation and pH equation
What is a pH meter?
An electronic device that measures the pH of a solution
How do you calibrate a pH meter?
- Place the bulb of the pH meter into distilled water and allow the reading to settle, then adjust the reading to pH7
- Do the same with 2 standard solutions of pH4 and pH10, making sure to rinse the probe off with distilled water each time
What does a pH meter look like?
They have a probe that you place in the solution, and at the bottom of the probe there is a delicate bulb
How do you use a pH meter after it has been calibrated?
Place the probe in the liquid and let the reading settle down
What should you do to the pH meter after having taken a reading?
Rinse the probe with distilled water
How do you plot a pH curve for a titration?
By plotting the pH of the titration mixture against the amount of base added as the titration goes on
Compare the starting point for the pH graphs of strong acids and weak acids
A strong acid titration will start at a much lower pH than a weak acid titration
Why does the graph on a pH curve increase by a large amount straight away?
To start with, adding small amounts of base has little effect on the pH of the solution
For all of the pH graphs for the titration of an acid, except from the weak acid/weak base curve, what does the midpoint of the vertical part of the graph mean?
That is the end point of the titration, when all the acid is neutralised
What can be said about the amount of weak base that needs to be added to a strong acid, compared to a strong base, for the pH to start increasing to pH7?
You need to add more weak base than strong base to a strong acid to cause a change in pH
What can be said about the amount of weak base that needs to be added to a weak acid, compared to a strong base, for the pH to start increasing to pH7?
You need to add less strong base to a weak acid to see a large change in pH
For a pH graph, what does the final pH depend on?
The strength of the base- the stronger the base the higher the pH is at the end
What is an indicator?
A solution that changes colour to show you when the sample has been neutralised
What are indicators and how doe they work?
They’re weak acids that have different coloured conjugate pairs. As the pH of the solution changes during a titration, the equilibrium concentrations of the conjugate pairs will also change
What 2 indicators are used in acid-base titrations?
Methyl orange
Phenolphthalein
How does phenolphthalein work as an indicator?
The protonated form is colourless, and the conjugate base is pink. At low pH, the concentration of H+ ions is high so most of the phenolphthalein is protonated, so the solution is colourless. As the pH rises, the concentration of H+ ions decreases, shifting the equilibrium to the right, so more of the phenolphthalein deprotonates, causing the solution to turn pink
What is the equilibrium set up for the indicator phenolphthalein?
phenolphthalein-H ⇌ phenolphthalein- + H+
How does methyl orange work as an indicator?
The protonated form is red and the conjugate base is yellow. At low pH, the concentration of H+ ions is high, so most pf the methyl orange is protonated, so the solution is red. As the pH increases, the concentration of H+ decreases, so the equilibrium shifts to the right, meaning more of the methyl orange deprotonates, causing the solution to start turning yellow
What factors affect which indicator you should use for a titration?
You want an indicator that change colour over a narrow pH range, and one that changes colour at the end point of the titration
For a strong acid/strong alkali titration, should you use methyl orange (colour change between pH3.1 and pH4.4), phenolphthalein (colour change between pH8.3 and pH10) or either?
Either, because there’s a rapid pH change over the range of both indicators
For a strong acid/weak alkali titration, should you use methyl orange (colour change between pH3.1 and pH4.4), phenolphthalein (colour change between pH8.3 and pH10) or either?
On methyl orange, because the pH changes rapidly in the range of methyl orange, but not phenolphthalein
For a weak acid/strong alkali titration, should you use methyl orange (colour change between pH3.1 and pH4.4), phenolphthalein (colour change between pH8.3 and pH10) or either?
Phenolphthalein, because the rapid pH change occurs over the phenolphthalein range, but not methyl orange
For a weak acid/weak alkali titration, should you use methyl orange (colour change between pH3.1 and pH4.4), phenolphthalein (colour change between pH8.3 and pH10) or neither?
Neither, because there is no sharp change in pH, so neither indicator will work
What do you have to use to find the end point of a weak acid/weak alkali titration?
A pH meter
What is the pH range of an indicator?
The range in which the colour changes
