Unit 1: Section 9 - Acids, Bases and pH Flashcards by Lowri Jones

Define a Bronsted-Lowry acid

Proton donor

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Define a Bronsted Lowry base

Proton acceptor

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What ion causes a solution to be acidic? ( answers) Name and formula

H+, or more accurately H3O+ (oxonium ion), as protons react with H2O to form it

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What ion causes a solution to be alkaline?

-OH (hydroxide ion)

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Write an equation for the ionisation of water (2)

2H2O(l) ⇌ H3O+ (aq) + -OH (aq)
OR
H2O (l) ⇌ H+(aq) + -OH(aq)

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Derive Kw using the equation for the ionisation of water

Keq = [H+] [OH-] / [H2O]
[H2O]Keq = [H+] [OH-]
[H2O] is so large compared to [H+] and [Oh-] that [H2O]Keq can be considered to be constant.
[H2O]Keq = Kw
∴ Kw = [H+] [OH-]

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What is the value of Kw at 298K?

1.0 x 10^-14

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What physical factors affect the value of Kw? How do they affect it?

Temperature only - if temperature is increased, the equilibrium moves to the right Kw increases and the pH of pure water decreases

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Why is pure water still neutral, even if pH does not equal 7?

[H+] = [OH-]

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Give an expression for pH in terms of H+

pH = -log[H+]

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What is the relationship between pH and concentration of H+?

Lower pH = higher concentration of H+

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If 2 solutions have a pH difference of 1, what is the difference in [H+]?

A factor of 10

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How do you find [H+] from pH?

[H+] = 10^-pH

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How do you find [OH-] from pH?

Find [H+], use Kw to calculate [OH-]

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What is the difference when finding [H+] from the concentrations of diprotic and tripotic acids?

Need to multiply the concentration of the acid by the number of protons to find [H+]

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How do you calculate the pH of a strong alkaline solution?

Use Kw to calculate [H+] from [OH-]
Use pH = -log[H+]

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Define the term strong acid

One which fully dissociates in water ( HX -> H+ + X-)

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Define the term strong base

One which fully dissociates in water (XOH -> X+ + OH-)

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What is the difference between concentrated and strong?

Concentrated means many mol per dm^3, strong refers to amount of dissociation

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What is a weak acid and a weak base?

They only partially dissociate into their ions

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Give some examples of strong acids

HCl, H2SO4 and H3PO4

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Give some examples of strong bases

NaOH, CaCO3 and Na2CO3

Give some examples of weak acids

CH3COOH (ethanoic), any organic acid

Give some examples of weak bases

NH3

What is Ka? (expression)

For acid HA, HA ⇌ H+ + A- Ka = [H+] [A-] / [HA]

How would you work out the pH of a weak acid?

* Use the equation for Ka, subbing in values for [A-] and [HA] * Use pH = -log[H+] equation to find pH

What is a titration?

* The addition of an acid/base of known titration to a base/acid of unknown titration to determine the concentration * An indicator is used to show neutralization has occurred, as is a pH meter

Draw the titration curve for a strong acid with a strong base

Draw the titration curve for a strong acid with a weak base added

Draw the titration curve for a weak acid with a weak base added

Draw the titration curve for a weak acid with a strong base added

Define the term equivalence point

The point at which the exact volume of base has been added to just neutralise the acid, or vice-versa

What generally happens to the pH of the solution around the equivalence point?

There is a large and rapid change in pH, except in the weak-weak titration

How would you calculate the concentration of a reactant if you know the volume and the conc of the other reactant and the volume of that reactant added?

1. Calculate moles of one reactant 2. Used balanced equation to work out moles of other Use conc = mol/vol to calculate concentration

What is the end point?

* The volume of acid or alkali added when the indicator just changes colour * If the right indicator is chosen, equivalence point = end point

What are the properties of a good indicator for a reaction? (3)

* Sharp colour change (not gradual - no more than one drop of acid/alkali needed for colour change * End point must be the same as the equivalence point, or titration gives wrong answer * Distinct colour change so it is obvious when the end point has been reached

What indicator would you use for a strong acid-strong base titration?

Phenolphthalein or methyl orange, but phenolphthalein is usually used as a clearer colour change

What indicator would you use for a strong acid-weak base titration?

Methyl orange

What indicator would you use for a strong-base-weak acid titration?

Phenolphthalein

What indicator would you use for a weak acid-weak base titration?

Neither methyl orange or phenolphthalein is suitable, as neither give a sharp change at the end point

What colour is methyl orange in acid? In alkali? At what pH does it change?

Red in acid; yellow in alkali. Changes at about pH 4-5 Approx same pKa value

What colour is phenolphthalein in acid? In alkali? At what pH does it change?

Colourless in acid; pink in alkali Changes at about pH 9-10 Approx same pKa values

What is half-neutralisation point?

When volume = half the volume that has been added at the equivalence point

Define a buffer solution

A solution that resist changes in pH when a small amount of acid/alkali are added

What do acidic buffer solutions contain in general terms?

A weak acid and a soluble salt of that acid that fully dissociates

Write a reaction for an acidic buffer with added acid

A- + H+ -> HA, opposes addition of H+

Write a reaction for an acidic buffer with added alkali

HA + OH- -> H2O + A-

How else can you achieve an acidic buffer solution other than just mixing the constituents?

Neutralise half of a weak acid (meaning the acid must be in excess) with an alkali - this forms a weak acid/soluble salt mixture

What do basic buffer solutions contain in general terms?

Weak base and soluble salt of that weak base

How can you calculate the pH of buffer solutions?

Use the Ka of the weak acid, sub in [A-] and [HA], calculate [H+] -> pH

How can you calculate the new pH of a buffer solution when acid or base is added?

Calculate number of moles of H+ and A- and HA before acid or base is added Use equations to work out new moles of A- and HA -> find [H+] ->pH

Which buffer system maintains blood at 7.4? What happens when acid/alkali is added?

H+ + HCO3- ⇌ CO2 + H2O Add OH- -> reacts with H+ to form H2O, then shifts equilibrium left to restore H+ lost Add H+ -> equilibrium shifts to the right, removing excess H+

What products are buffers found in?

Shampoos, detergents -> important to keep pH right to avoid damage to skin, hair, fabrics