Chapter 20 Flashcards by Lozza Thompson

What is an acid?

It is when a aqueous solution dissociates releasing H+ ions

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What is an alkali?

A soluble base which releases OH- ions when it dissociates

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What is a neutralisation reaction?

acid + base –> salt (H+ is replaced with a metal ion or NH4+)

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What is a Bronsted Lowry acid?

A proton donor

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What is a Bronsted Lowry base?

A proton acceptor

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What does an acid base reaction involve?

A proton transfer

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What is a strong acid?

When it fully dissociate in water releasing H+ ions eg HCl

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What are the conjugate acid base pairs in HCl.

HCl and Cl- are the conjugate base pairs

In the forward direction Cl- is the conjugate base as HCl releases a proton
In the reverse direction HCl is the conjugate acid as the Cl- accepts a proton

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What is a weak acid?

An incomplete dissociation of H+ ions this is shown by an equilibrium. e.g CH3COOH

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What are the conjugate base pairs in ethanoic acid?

CH3COO- is the conjugate base of CH3COOH

- CH3COOH is the conjugate acid of CH3COO-

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What happens when an acid releases a proton?

It forms a conjugate base

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What happens when a base accepts a proton?

It forms a conjugate acid

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What is a conjugate acid base pair?

It’s when two species can be interconverted by a proton transfer.

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What are monobasic, dibasic, tribasic acids?

Acids which refer to the number of H+ in the acid which can be replaced per mole in an acid base reaction. The number of hydrogen is in the formula give a clue to the type of acid.

e.g HCl - monobasic CH3COOH - monobasic
H3BO3 - tribasic

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What is the simplified formula for neutralisation?

H+ + OH- –> H2O

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Describe the dissociation using a hydronium ion?

In aqueous solution dissociation requires a proton transfer from an acid to a base which won’t happen unless water is present. Water acts as a base as accept a proton. The hydronium ion is the conjugate acid as H2O excepted a H plus ions (it’s an active acid ingredient in any aqueous acid).

H3O+ + OH- –> 2H2O

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What are Spectator ions?

Ions don’t change in the reaction these can be cancelled out when writing an ionic equation.

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Show the equation for neutralisation of acid with carbonates?

acid + carbonate –> salt + carbon dioxide + water

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Show the equation for neutralisation of acid with metal oxides/hydroxide?

acid + metal oxide/hydroxide –> salt + water

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Show the equation for neutralisation of acids with alkalis ?

acid + alkali –> salt + water

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Show the equation for neutralisation of acids with metals?

acid + metal –> salt + hydrogen

Undergo a redox reaction

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How can an acid be written in a equation

2H+

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What does pH> 7 show?

Increasing alkalinity

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What does pH< 7 show?

Increasing acidity

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What does pH = 7 show?

Neutral

What does a low [H+] match with?

A high pH

What does a high [H+] match with?

A low pH

What is the equation for pH and [H+]?

``` pH = -log[H+] [H+] = 10^ -pH ```

How many times concentrated is pH 1 to pH 2

x 10

If you had to dilute a solution from pH1 to pH4 what would it require?

A dilution of 1000 times

How many times more does pH1 have of H+ compared to pH 14?

10^13

What is the general formula for a strong acid?

HA --> H+ + A-

Describe the relationship between [H+] and [HA] in a strong acid?

[H+] = [HA]

What is the acid dissociation constant Ka?

The extent of dissociation is different for different weak acids, so the position of equilibrium differs. The further to the right the stronger the weak acid.

What is the equation for Ka?

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

What is Ka standardised at?

25 degrees and is temperature dependent

How do you work out Ka?

10^ -pKa

How do you work out pKa?

pKa = -logKa (should be given to 2dp)

What happens to the value of Ka and pKa when the acid is stronger?

LARGER the Ka value | SMALLER the pKa value

What happens to the value of Ka and pKa when the acid is weaker?

SMALLER the Ka value | LARGER the pKa value

Describe the relationship between [H+] and [HA] in a weak acid?

[H+] not equal [HA]

What does [H+] depend on?

Conc of acid [HA] | Ka

What happens when HA molecules dissolve?

[H+] and [A-] form equal quanities

What is approximation 1 ?

[HA] dissociates to produce [H+] = [A-] There will be a small conc of H+ from water dissociation but it can be neglected. [H+] = [A-]

What is approximation 2?

[HA] eqm = [HA]start - [H+]eqm As the dissociation of weak acids is small you can assume [HA] > [H+] and you can neglect any decrease in HA concentration from the dissociation. [HA] eqm = [HA] inital

From approximation 2 how to work out Ka?

Ka = [H+]^2 / [HA]

From approximation 2 how to work out [H+]?

square root Ka x [HA]

Accuracy on approximation 1?

[H+] = [HA]. At 25 degrees water is pH 7, so [H+]= 1 x 10^-7. If weak acid> ph6 i.e [H+] of less than 1 x 10^-6 the contribution of [H+] from water dissociation will be significant, so breaks down very weak or dilute acids

Accuracy on approximation 2?

[HA] eqm = [H+] inital | The approximation breaks down for stronger weaker acids (Ka > 1 x 10 ^-2 )

How can Ka be determined?

Preparing a standard solution of the week acid of known concentration. Measuring pH with a pH meter of a standard solution.

As water ionises very slightly what does it act as?

Both an acid and a base setting up the acid base equilibrium. H2O + H2O --> H3O- + OH-

As water dissociation is very small what can the concentrations be regarded as?

As the concentration is basically remain and changed they can be regarded as a constant. Ka (constant ) x [H2O] (constant ) = [H+][OH-]

What is Kw?

The ionic product of water. It is essentially an equilibrium constant which controls [H+] and [OH-] concentrations. This value varies with temperature at 298K (25 degrees) = 1.00 x 10^-14 mol2dm-6 Kw = [H+][OH-] It sets up the neutral point of the pH scale

When is the solution neutral?

[H+] = [OH-]

When is a solution acidic?

[H+] > [OH-]

When is a solution alkaline?

[H+] < [OH-]

What will acidic solution still contain?

It will still contain OH- ions but just a small proportion compared to H+ ions.

What is a strong base?

An alkali which completely dissociate in solution releasing OH- ions e.g group 1 and group 2 hydroxides.

How is the pH of a strong base calculated?

The concentration of the base and the ionic product of water Kw

Four whole number PH is what must the [H+] | [OH-]concentrations add up to?

-14

What are weak bases?

They are also alkali. Ammonia gas partially dissolves in water releasing OH ions. An equilibrium is set up with a position to the left. One mole of NH3 releases far less than 1 mole of OH- ions.