QCE Senior Chemistry U3: Acid-Base (QLD Teachers)

What happens when the pH of a solution is equal to the pKa of an indicator?

The indicator will change colour

What type of substance does this equation represent? BOH → B+(aq) + OH-(aq)

A strong base

What is the definition of a weak base in terms of its Kb?

Low Kb - Weak bases

What is the relationship between the pH range of an acid-base indicator and its pKa value?

A higher pKa correlates to a higher end point of the indicator

What is the definition of a weak acid in terms of its Ka ?

Ka < 1 Weak acids

What is a conjugate acid?

An acid formed when a base accepts a proton (H+) B: + H2O → B:H+ + OH- (B:H+ is the conjugate acid)

What is an amphiprotic substance?

A substance that can act as either an acid or a base

What is an example of a common amphiprotic substance?

Examples could include - water, amino acids and hydrogen carbonate ions (HCO3- )

What is a conjugate pair?

Two species that differ by a proton (H+)

Identify the conjugate acid produced in this reaction: NH4+ + OH- ⇌ NH3 + H2O

H2O

What is a conjugate base?

A base formed by removing a proton (H+) from an acid

Identify the conjugate base produced in this reaction: NH4+ + OH- ⇌ NH3 + H2O

NH3

How does the Brønsted-Lowry model define an acid and a base?

Acids as any species that can donate protons (H+). Bases as any species that accept protons (H+)

What is the conjugate base of CH3COOH?

CH3COO-

What is the conjugate acid of the following base: OH- ?

H2O

Why is water a weak electrolyte?

It can self-ionise, but equilibrium lies to the left H2O ⇌ H+(aq) + OH-(aq)

What is Kw?

Kw is the ionic product constant of water. It is the equilibrium constant for the self-ionisation of water. Kw can be used to calculate the concentration of hydrogen ions from the concentration of hydroxide ions in a solution

How is pH calculated?

The pH of a solution can be calculated from the concentration of hydrogen ions using the relationship pH = -log10 [H+ ]

How is pOH calculated?

pH = -log10 [OH-]

Under what circumstances could the numerical value of Kw differ?

Different temperatures. Kw is 10-14 at just below 25oC

What is self-ionisation of water?

Self-ionisation of water refers to the reaction in which a water molecule loses a hydrogen ion (leaving OH-) and the hydrogen ion immediately reacts with another water molecule to form a hydronium ion (H3O+ )

What is pH?

pH is a figure expressing the acidity or alkalinity of a solution on a logarithmic scale

How is [H+ ] calculated?

[H+ ]= 10^(-pH)

How is [OH- ] calculated?

[OH-] can be obtained using the pH or pOH value: pH + pOH = 14 pOH = 14 - pH pOH= --log10 [OH- ] [OH-] = 10^(-pOH)

pH + pOH = ?

14 or Kw (at just below 25oC)

How do we calculate Kw?

Kw = Ka x Kb

How do we calculate concentration?

C = n / V

What are acid-base indicators?

Weak acids or bases, which dissociate slightly in water and form ions

How are strong acids defined?

Strong acids are defined fully ionising / dissociating (Ka > 1 )

How are weak bases defined?

Weak bases are defined as having a low Kb

How do you calculate Ka?

Ka = [H3O+ ] x [A-] / [HA]

How do you calculate Kb?

Kb = [BH+ ] x [OH-] / [B]

How are weak acids defined?

Weak acids are defined as having a Ka < 1

How are strong bases defined?

Strong bases are defined as having a high Kb

What type of substance does this indicator represent? HIn ⇌ H+(aq) + In-(aq)

A weak acid

What are examples of Strong Acids?

Hydrochloric Acid (HCl) Nitric Acid (HNO3) Sulfuric Acid (H2SO4)

What are examples of Strong Bases?

All group 1 hydroxides (LiOH, NaOH, KOH, RbOH, CsOH) Barium hydroxide (Ba(OH)2)

What is the dissociation expression for a base?

Kb = [products] / [reactants]

What is the definition of a strong acid in terms of its Ka?

Ka > 1 is a strong acid

Which Acid and Bases (strong or weak) completely ionise in water?

Strong acids and bases

What are examples of Weak Acids?

Carboxylic acid (R-C(=O)-OH), carbonic acid (H2CO3), aqueous carbon dioxide (CO2 (aq))

What are examples of Weak Bases?

Ammonia (NH3), amines

What substance states are included in a dissociation expression?

Aqueous (aq)

What is the definition of a strong base in terms of its Kb?

High Kb - Strong bases

The strength of an acid is explained by the degree of ...

dissociation of the acid at equilibrium in an aqueous solution

What is the Bronsted-Lowry definition of an acid?

A proton donor

What is a diprotic acid?

An acid that can donate two protons per molecule

What is a polyprotic acid?

An acid that can donate more than one proton per molecule

Name an example of a triprotic acid

H3PO4 - Phosphoric acid

What is a monoprotic acid?

An acid that can donate one proton per molecule

Name an example of a monoprotic acid

HCl - hydrochloric acid HBr - hydrogen bromide HNO3 - nitric acid CH3COOH - ethanoic acid

Name an example of a diprotic acid

H2SO4 - sulphuric acid H2CO3 - carbonic acid

What is meant by the "strength" of an acid or a base

The level of dissociation that occurs in the acid or base.

If an acid or a base completely ionises in water what is it called?

Strong acid or base

What is hydronium?

H3O+

What does it mean if an acid is dilute?

It means it has a small number of moles of the acid in a litre of water

Why are acids and bases good electrolytes?

They can dissociate into positive and negative ions which means they can conduct an electrical charge

What type of substance has a pH of 1?

A strong acid

If an acid or a base only partially ionises in water what is it called?

A weak acid or base

What does it mean if an acid is concentrated?

It means it has a large number of moles of the acid in a litre of water

What can be tested experimentally to determine the strength of an acid or base?

The electrical conductivity of the acid or base

Which have better electrical conductivity, strong or weak acids (or bases)?

Strong acids (or bases)

What type of substance has a pH of 7?

A neutral substance

What type of substance has a pH of 13?

A strong base

What is a conjugate acid?

An acid formed when a base accepts an H+

What is the conjugate base of ammonium (NH4+)?

NH3

What does amphiprotic mean?

A molecule or ion that can donate of accept a proton. Therefore it acts as both an acid and a base.

What substances make up a buffer?

A weak acid and its conjugate base or a weak base and its conjugate acid

What is the Bronsted-Lowy definition of a base?

A species that accepts protons

What is a conjugate base?

A base formed by removing an H+ from an acid

What is the conjugate acid of hydroxide (OH- )?

H2O

What is a buffer solution?

A solution that resists changes in pH when small quantities of acids or bases are added to it

What does Ka represent?

The equilibrium constant for the dissociation of an acid in an aqueous solution

Is the conjugate base of a weak acid, stronger or weaker?

Stronger

Is Kw temperature dependent?

Yes

What is pKb a measure of?

The strength of a base

Is a base strong or weak if its pKb is large?

Weak

What strength is an acid or base that is used as an indicator?

They are weak acids and bases

What is pKa a measure of?

The strength of an acid

Is an acid strong or weak if it has a low pKa?

Strong

What is an indicator?

A chemical substance that changes colour at particular pH values

How are the acid and its conjugate base different in indicators

They are different colours

At what point does an indicator change colour?

When the pH of the solution changes past the pKa of the indicator

What is the equivalence point?

The point in a titration when neutralisation is reached (i.e. when moles H+ = moles OH-)

What is the analyte?

The solution in a titration whose concentration is unknown

When choosing an indicator for a titration what are you looking for?

An indicator whose end point is close to the equivalence point.

What is the buffer point on a weak acid or weak base titration curve?

It is the point where pH = pKa. It is at half the equivalence volume.

What is a titration?

A technique used to determine the unknown concentration of a solution

What is the "end point" ?

The point in a titration when the indicator changes colour

What is the titrand?

The solution in a titration whose concentration is known

What is meant by a standard solution in a titration?

A solution whose concentration is accurately known

What does the buffer region look like on a weak acid or base titration curve?

It is the (almost) horizontal area that occurs before the equivalence point.

What impact does having water in the burette have in a titration?

It will decrease the concentration of the titrant and therefore a greater volume will be needed to neutralise the analyte

What is an aliquot?

A fixed volume of liquid measured by a pipette

How often should a titration be performed?

Until you have obtained three titres that differ by less than or equal to 0.1mL

What impact does having water in the flask holding the analyte have in a titration?

No impact as the number of moles of the substance remains the same

What is parallax error?

When the volume of a liquid is read when the eye level is above or below the meniscus. It leads to inaccurate volumes being determined

What is the dissociation expression for an acid?

Ka = [H+] [A-] / [HA]