Buffers (21) Flashcards
What is a buffer solution?
A solution that minimises changes in pH when small amounts of acids or bases are added.
What components do buffer solutions contain?
- They contain a weak acid to remove the alkali
- They contain a conjugate base to remove the added -acid
What happens if you add acids and alkalis to a buffer?
If you add both, the components in the buffer will both react with them to neutralise it, but they will eventually be used up which means it will no longer be a buffer
What do you add to make a buffer based on a weak acid?
A weak acid and its conjugate base
How do you prepare buffer from a weak acid and its salt?
You can prepare a buffer solution by adding ethanoic acid ( a weak acid) and its salt - sodium ethanoate.
Ethanoic acid will partially dissociate. This acts as you conjugate acid
Ethanoate will split into ions when dissolved in water and this will act as your conjugate base component
How do you prepare a buffer by partial neutralisation?
You add an aqueous alkali to an excess of weak acid which will partially neutralise it to form the conjugate base. Some weak acid is left over and this results in the salt of the weak acid and any unreacted weak acid
What reservoirs are used to remove an added acid and alkali?
You can use ethanoic acid and CH3COO- ions. The equilibrium lies to the left in ethanoic acid, then when you add CH3COO-, it shifts even further left which reduces the amount of H+ even more, leaving you with just the 2 components
Explain the action of a buffer solution
Adding a conjugate acid-base pair (buffer) will control the pH using equilibrium. The equilibrium will shift, depending on what concentration increases.
What happens if an acid is added to your buffer solution?
The conjugate base removes the added acid:
- The conc of H+ will increase
- The H+ ions will react with the conjugate base in the buffer
- The equilibrium will shift left, because of the increased H+ concentration and this will remove most H+ ions
What happens if an acid is added to your buffer solution?
The weak acid removes the added alkali
- The OH- conc will increase
- The small conc of H+ reacts with the OH- to make H20
- Because the H+ conc is decreasing, the equilibrium will shift right to restore the H+ ions
When is a buffer most effective?
When the conc of weak acid and conjugate base are the same.
- The pH of the buffer will be the same value as the pKa of HA.
How to calculate the pH of a buffer solution?
Ka x conc of weak acid / conc of the conjugate base
How would you work out the pH of a buffer if they gave you a strong base?
You would add the strong base and weak acid together for a partial neutralisation. You would then find the conc of the conjugate base = conc of strong base
Then you find the conc of the weak acid by doing the conc of the weak acid before - conc of strong base. the apply the buffer solution formula.
Why is it important to maintain the pH of the blood?
Very important for maintaining the optimum conditions for enzymes to take part in.
What is the buffer solution of the blood?
Carbonic acid hydrogencarbonate controls the blood plasma
What is the pH of the blood plasma?
Between 7.35 and 7.45
What happens if the blood pH falls below 7.35?
Acidosis occurs which is a condition that causes shortness of breath and fatigue
What happens if the blood pH rises above 7.45?
Alkalosis occurs which can cause muscle spasms, light-headedness
Explain what happens in the carbonic acid hydrogencarbonate system if an acid is added
The H+ conc increases and will react with the conjugate base (HCO3-) more. This means equilibrium will shift left to remove most H+ ions, more H2CO3 will be made
Explain what happens in the carbonic acid hydrogencarbonate system if an alkali is added
The OH- conc increases which then reacts with the small amount of H+ in the buffer to form water. This decreases the amount of H+ so the equilibrium shifts right. More H2CO3 will dissosciate
How do you use a pH meter?
- Add your acid to a conical flask making sure its a specific volume
- Place the electrode of the pH meter in the flask and recording the pH
- Add alkali into the acid and swirl before recording the result
- When the pH starts changing rapidly, add the alkali dropwise and record results.
- Repeat this until you get concordant results which are within 0.1cm of each other
Explain a titration curve
When the base is first added, there is a large excess of acid, so the pH will increase slowly, small gradient. Then the pH will start increasing more rapidly as you add more base which creates a vertical section on the graph as the acid is being used up.
There will now be an excess of base so the pH will only start rising slowly again
What is the equivalence point?
The equivalence point on the titration curve is the centre of the vertical section. This is when a solution reacts exactly with the volume of another solution.
What is the titration curve when you add an acid to a base?
It is the reverse, it will start from the top, decrease slowly, have the vertical section and then decrease again.
What is the endpoint and how can you use an indicator to find this?
The endpoint is when there are equal quantities of the weak acid and conjugate base. This is detected using an indicator like methyl orange. When it is a weak acid, it is red when it is a base it is yellow. At the end point, it will be orange
Is an indicator acid or alkali and what does this mean?
It is a weak acid which means that the equilibrium will be shifted towards the weak acid in acidic conditions and towards the conjugate base in basic conditions. This changes the colour
What happens when you add methyl orange to your acid during a titration?
The colour will be red because the concentrations of H+ ions move the position to the left, making the pH smaller
What happens to the indicator when a base is added to your titration?
OH- ions will increase and will react with the H+ ions in the indicator. This will decrease the conc of protons and shift the equilibrium to the right, causing a colour change to orange at the end point and then to yellow
What happens to your indicator if an acid is added to a base in your titration?
The H+ ions in the acid will react with the conjugate base in the indicator, which means your H+ conc is decreasing, so the equilibrium shifts left and the colour will go to orange at the endpoint and then red.
How can you choose an indicator?
You must pick one which has a colour change that coincides with the vertical section of the pH titration curve. The endpoint and equivalence point would be the same.
What indicator do you choose for a weak acid weak base titration?
You don’t choose one as there is no vertical section.
What is the difference between equivalence point and endpoint?
The equivalence point is the centre of the vertical section on a titration curve where the volume of one solution reacts exactly with the volume of another solution
The endpoint is when the indicator is the centre of 2 colour extremes and where the conc of HA is the conc of A-
