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Chemistry 5.1.3 - Acids, bases and buffers > Buffers and Neutralisation > Flashcards

Buffers and Neutralisation Flashcards

1
Q

What is a buffer?

A

A system that minimises PH changes when small amounts of acid or bases are added

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2
Q

What are the two ways of preparing bases?

A
  1. A weak acid and its salt
  2. Partial neutralisation of a weak acid
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3
Q

How do you form a buffer from a weak acid and its salt?

A

A buffer solution can be made by mixing a weak acid and its salt.
The weak acid partially dissociates into conjugate base minimally making it the source of the acid
The salt of the weak acid then dissolves completely into a conjugate base, providing component 2

Component 1
CH3COOH = H+ + CH3COO-

Component 2
CH3COO- Na+ = CH3COO- + Na+

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4
Q

How do you form a buffer by partial neutralisation?

A

A small amount of aqueous alkali is added to a weak acid
The weak acid is partially neutralised to form its conjugate base
Some of the unreacted weak acid is left over acting as a source of the acid

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5
Q

What are the steps of calculating partial neutralisation?

A

Find the moles of the weak acid used initially and the moles of the alkali
To calculate the moles of acid left, subtract the moles of the acid - the alkali
Put into the rearranged Ka equation to find the concentration of H+
Use H+ to find the PH of the buffer solution

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6
Q

What is the action of a buffer solution when an acid is added?

A

On addition of H+
HA = H+ + A-
The concentration of H+ increases
H+ reacts with the conjugate base A-
This causes equilibrium to shift to the left, removing the excess H+

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7
Q

What is the action of a buffer solution when an alkali is added?

A

On addition of OH-
HA = H+ + A-
The concentration of OH- increases
OH- then reacts with H+, forming water
This causes equillibrium to shift to the right to increase the concentration of H+ ions

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8
Q

When does acidosis and alkalosis occur?

A

Acidosis = when PH falls below 7.35
Alkalosis = when PH rises above 7.45

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9
Q

What is the equillibrium occuring in blood?

A

H2CO3 = H+ + HCO3-

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10
Q

What occurs upon the addition of H+ in blood?

A

H+ increases
H+ reacts with HCO3-
Equilibrium then shifts to the left using up the excess H+
H2CO3 = H+ + HCO3-

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11
Q

What occurs upon the addition of OH- in blood?

A

OH- increases
OH- reacts with H+, forming water
Equilibrium then shifts to the right to restore the H+ ions
H2CO3 = H+ + HCO3-

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12
Q

Draw the shape of a strong acid/strong base curve

A
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13
Q

Draw the shape of a weak acid/strong base curve

A
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14
Q

Draw the shape of a strong acid/weak base curve

A
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15
Q

Draw the shape of a weak acid/weak base curve

A
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16
Q

Draw the inverse of each curve

A
17
Q

How do you use a PH meter?

A
  • Using a pipette, add a measured volume of acid to a conical flask
  • Place the electrode of a PH meter in the flask
  • Add the base, 1cm3 at a time
  • After each addition, swirl the contents whilst keeping the PH meter in the flask
  • Record the PH and the total base added
  • Repeat steps until the PH changes
18
Q

What is the equivalence point?

A

The point where the volume of one solution reacts exactly with the volume of the other solution

19
Q

What is the end point?

A

the point at which an indicator changes colour at a certain pH

20
Q

How do you choose an indicator to use?

A

An indicator that has an end point within the vertical section

21
Q

What is the end point of an indicator?

A

The PKA of an indicator

22
Q

What are indicators technically?

A

Indicators are like weak acids and work as they have different conjugate pairs
The colour is dependent on protonation or deprotonation

23
Q

How do indicators work on addition of H+?

A

The H+ reacts with A-
The equilibrium shifts to the left, restoring HA
The colour change changes to orange at endpoint and then red when HA is restored
HA = A-
red orange

24
Q

How do indicators work on addition of OH-?

A

OH- reacts with H+, forming H20
Equilibrium shifts to the right, restoring H+
This causes a colour change to orange at end point and then yellow when H+ is restored
HA = A-
red orange

Chemistry 5.1.3 - Acids, bases and buffers (4 decks)

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