Buffers | A-level Chemistry | OCR, AQA, Edexcel
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Buffers in a Snap! Unlock
Buffers in a Snap!
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The key points covered in this video include:
1. What is a buffer solution?
2. Acidic Buffers
3. How buffers work
4. Basic Buffers
5. Naturally occuring buffers
6. Calculating the pH of buffers
What is a Buffer Solution?
Controlling the pH of a solution is often important, especially biological systems. Buffers provide a way to do this by taking advantage of equilibria to control the amount H+ ions and OH- ions present in a solution. A buffer is a solution which minimises changes to pH upon the addition of a small amount of an acid or a base. It’s important to realise that buffers cannot stop a pH change from happening entirely, they can just minimise it.
Acid Buffers
The purpose of a buffer is to try and counteract any small changes in the pH and so in the concentration of H+ ions. Acidic buffers are a mix of a weak acid, HA and it’s conjugate base, A-. The weak acid’s role: What happens to the salt of the weak acid: The excess of Ch3COO-(aq) pushes the dissociation equilibrium towards the left. Acidic Buffer = weak acid + conjugate base. We could have set up this system differently. If instead of mixing the acid and the salt we mixed excess acid with a strong base then the base and acid react: From here everything is the same as above.
How do Acidic Buffers work?
What if we add a small amount of acid? This will increase [H+]. Because the acid and the conjugate alkali are in excess, the added H+ will be almost entirely used up to re-establish the equilibrium and keep Ka constant. Ka is constant. What does this new equilibrium look like? The concentration of H+ ions has only changed very slightly compared to before we added the acid so the pH only changes very slightly. What if we add a small amount of alkali? This will decrease [H+]. Because the acid and the conjugate alkali are in excess, the decrease in H+ will be almost entirely replaced to re-establish the equilibrium and keep Ka constant. the concentration of H+ ions has only changed very slightly compared to before we added the alkali so the pH only changes very slightly.
Basic Buffers
These are made from a weak bases and their conjugate acid and maintain pH values above 7.00. When we add a small amount of acid: The excess H+ is all used up and the concentrations of NH3(aq) and NH4+(aq) remain roughly constant as they are in excess. When we add a small amount of alkali: The excess OH- is all used up and the concentrations of NH3(aq) and NH4+(aq) remain roughly constant as they are in excess. Basic Buffer = Base + Conjugate Acid.
Naturally Occurring Buffers
Buffers appear a lot in biological systems because in a lt of organisms, pH needs to be carefully controlled. In humans our blood plasma needs a pH value of 7.35 - 7.45, it’s bad news if it drops below 7.35 or raises above 7.45. There are multiple buffers systems to keep our blood in this narrow pH range, the most important is the carbonic acid-hydrocarbonate ion buffer: The buffer action works to stabilise the pH. Most materials in the blood are acidic so the reaction that happens most is: An enzyme then converts the carbonic acid which accumulates into aqueous CO2 which can be converted into carbon dioxide gas in the lungs and exhaled.Rapid breathing will remove more CO2 so this is one of the symptoms of acidosis as your body tries to remove the excess acid.
Calculating the pH of a Buffer
If we want to find the pH of any solution we need to calculate the concentration of the H+ ions. We can look up the Ka for the acid and work from there: Just as we did when calculating pH for weak acids we assume: Because the salt is ionic, it dissociates fully: We can now calculate the concentration of H+ ions and then compute the pH.
Summary
A buffer solution minimises changes to the pH when we add small amounts of acid or base
Acidic buffers are made from a weak acid and its conjugate base
Buffers use an equilibrium to react with any extra H+ or OH- ions
Carbonic acid and hydrocarbonate ions form a buffer to maintain the pH of human blood
We can use the expression for Ka of the acid in a buffer to find the pH
Unlock the full A-level Chemistry course at http://bit.ly/2lOThgX created by Barney Fidler, Chemistry expert at SnapRevise.
SnapRevise is the UK’s leading A-level and GCSE revision & exam preparation resource offering comprehensive video courses created by A* tutors. Our courses are designed around the OCR, AQA, SNAB, Edexcel B, WJEC, CIE and IAL exam boards, concisely covering all the important concepts required by each specification. In addition to all the content videos, our courses include hundreds of exam question videos, where we show you how to tackle questions and walk you through step by step how to score full marks.
Sign up today and together, let’s make A-level Chemistry a walk in the park!
The key points covered in this video include:
1. What is a buffer solution?
2. Acidic Buffers
3. How buffers work
4. Basic Buffers
5. Naturally occuring buffers
6. Calculating the pH of buffers
What is a Buffer Solution?
Controlling the pH of a solution is often important, especially biological systems. Buffers provide a way to do this by taking advantage of equilibria to control the amount H+ ions and OH- ions present in a solution. A buffer is a solution which minimises changes to pH upon the addition of a small amount of an acid or a base. It’s important to realise that buffers cannot stop a pH change from happening entirely, they can just minimise it.
Acid Buffers
The purpose of a buffer is to try and counteract any small changes in the pH and so in the concentration of H+ ions. Acidic buffers are a mix of a weak acid, HA and it’s conjugate base, A-. The weak acid’s role: What happens to the salt of the weak acid: The excess of Ch3COO-(aq) pushes the dissociation equilibrium towards the left. Acidic Buffer = weak acid + conjugate base. We could have set up this system differently. If instead of mixing the acid and the salt we mixed excess acid with a strong base then the base and acid react: From here everything is the same as above.
How do Acidic Buffers work?
What if we add a small amount of acid? This will increase [H+]. Because the acid and the conjugate alkali are in excess, the added H+ will be almost entirely used up to re-establish the equilibrium and keep Ka constant. Ka is constant. What does this new equilibrium look like? The concentration of H+ ions has only changed very slightly compared to before we added the acid so the pH only changes very slightly. What if we add a small amount of alkali? This will decrease [H+]. Because the acid and the conjugate alkali are in excess, the decrease in H+ will be almost entirely replaced to re-establish the equilibrium and keep Ka constant. the concentration of H+ ions has only changed very slightly compared to before we added the alkali so the pH only changes very slightly.
Basic Buffers
These are made from a weak bases and their conjugate acid and maintain pH values above 7.00. When we add a small amount of acid: The excess H+ is all used up and the concentrations of NH3(aq) and NH4+(aq) remain roughly constant as they are in excess. When we add a small amount of alkali: The excess OH- is all used up and the concentrations of NH3(aq) and NH4+(aq) remain roughly constant as they are in excess. Basic Buffer = Base + Conjugate Acid.
Naturally Occurring Buffers
Buffers appear a lot in biological systems because in a lt of organisms, pH needs to be carefully controlled. In humans our blood plasma needs a pH value of 7.35 - 7.45, it’s bad news if it drops below 7.35 or raises above 7.45. There are multiple buffers systems to keep our blood in this narrow pH range, the most important is the carbonic acid-hydrocarbonate ion buffer: The buffer action works to stabilise the pH. Most materials in the blood are acidic so the reaction that happens most is: An enzyme then converts the carbonic acid which accumulates into aqueous CO2 which can be converted into carbon dioxide gas in the lungs and exhaled.Rapid breathing will remove more CO2 so this is one of the symptoms of acidosis as your body tries to remove the excess acid.
Calculating the pH of a Buffer
If we want to find the pH of any solution we need to calculate the concentration of the H+ ions. We can look up the Ka for the acid and work from there: Just as we did when calculating pH for weak acids we assume: Because the salt is ionic, it dissociates fully: We can now calculate the concentration of H+ ions and then compute the pH.
Summary
A buffer solution minimises changes to the pH when we add small amounts of acid or base
Acidic buffers are made from a weak acid and its conjugate base
Buffers use an equilibrium to react with any extra H+ or OH- ions
Carbonic acid and hydrocarbonate ions form a buffer to maintain the pH of human blood
We can use the expression for Ka of the acid in a buffer to find the pH
5 سال پیش
در تاریخ 1398/06/21 منتشر شده
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