12. Acid Base Equilibrium Flashcards
What is a bronsted Lowry acid
A proton donor that releases hydrogen ions when they’re mixed with water
What is a bronsted Lowry base
A proton acceptor. When in solution they grab hydrogen ions from the water molecules
What is a strong acid
An acid that dissociates almost completely in water. - nearly all of the H+ ions will be released
What is a weak acid
A weak acid only slightly dissociates in water so small amounts of H+ ions are formed
What is a conjugate pair
Species that are linked by the transfer of a proton and are always on opposite sides of the reaction.
What is a conjugate base
The species that has lost a proton and the species that has gained the proton is the conjugate acid
What happens when acids and bases react together
A salt and water is produced
What does the concentration of H+ ion have to be equal to, to form a neutral solution
OH- ions
What is the standard enthalpy change of neutralisation
The enthalpy change when solutions of an acid and a base react together under standard conditions to produce 1 mole of water
What is the pH scale
A measure of the hydrogen ion concentration
Equation for pH
pH = -log [H+]
What can strong acids be called and what does it mean
Monoprotic - it means that one mole of acid produces one mole of hydrogen ions as they can only release 1 protons into solution
What is a polyprotic acid
An acid that can release more than one proton into solution so one mole of acid released more than one mole of [H+] ions
How to find the pH of weak acids
Ka=[H+][A+]/[HA]
Ka=[H+]^2/[HA]
Why can water be an acid and a base
It can act as an acid by donating a proton but can also act as a base by accepting a proton. This means that hydroxonium and hydroxide ions are present in water at the same time, therefore water is in equilibrium.
Why is the concentration of water considered to be a constant value
This is due to water only dissociating a small amount and the amount of water compared to the amount of H+ and OH- meaning its concentration is deemed constant
Equation for Kw
Kw = [H+][OH-]
Kw for pure water is….
Always the same value at a given temperature.
pKw =
-log Kw
Kw from pKw =
10^-pKw
How to measure the pH of a solution using a pH meter
Make sure the pH meter is correctly calibrated by placing into deionised water and setting the reading to 7. Doing the same for solutions pH 4 and pH 10. Make sure to rinse with deionised water between readings.
Place the probe in the solution and leave to settle before recording a result.
Ensure you rinse the probe with deionised water between each measurement
What do pH titrations tell you
Exactly how much base is needed to neutralise a quantity of acid.
How to plot a pH curve
Plot the pH of the titrated mixture against the amount of base that has been added.
How to choose an indicator for a pH titration
Choosing the correct indicator is important as some indicators only work in between a certain pH range.
Methyl orange red 3.1 ——-4.4 yellow
Phenolphthalein colourless 8.3 ——-10 pink
What is a Buffer
A solution that has minimal changes in pH when small amounts of acid or bases are added.
What is an acidic buffer
They have a pH less than 7 and are made up of an equilibrium between a weak acid and its conjugate base.
Buffer action on a titration curve
When the curve levels off this is due to a buffer solution formed to resist further dramatic change to pH
What pH does blood need to be kept at
pH 7.4
How is the pH of the blood controlled
Using carbonic acid-hydrogen carbonate buffer system. Carbonic acid dissociates into H+ ions and HCO3- ions which react with excess H+ ions if the concentrations of these in the blood rises.
If the concentration of H+ ions falls then H2CO3 from the buffer system dissociates and equilibrium shifts so that the original value of H+ ions returns
What are levels of H2CO3 controlled by
Respiration by breathing out CO2
What are the levels of HCO3 controlled by
The kidneys with excess being excreted in the urine
What is the Henderson-Hasselbalch equation
pH = pKa + log ([A-]/[HA])
First equivalence point is…
When enough base has been added to neutralise the first acidic proton
The second equivalence point is…..
When enough base has been added to neutralise the second acidic proton in polyprotic acids
