Acids and bases Flashcards by Angel Ssemanda

What is a Brønsted Lowry Acid

Proton donor

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Why is H2SO4 a stronger acid compared to HA?

it can donate more protons

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What is a Brønsted Lowry base

proton acceptor

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What is a strong acid

completely dissociates into ions in aq solution

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What is a monoprotic acid

donates 1 H+ per molecule

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What is a Diprotic acid?

Donates 2 H+ ions per molecule

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What is a Strong base?

*completely dissociates** into ions in aq solution

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What is a weak acid?

partially dissociates into ions in aq solution

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What is a weak base?

slightly dissociates to give ions in aq solution

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What is the equation for pH?

-log10[H+]

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As the concentration of H+ ions increase what happens to the pH?

decrease

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How can you calculate H+ from pH

H+ = 10^-pH

(divide by no. H)

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What is the equation for water

H20 ⇌ H+ + OH-

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Why is the concentration of water very high?

water is weakly dissociated so equilibrium lies far to the left

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What is the equation for Kw?

Kw = [H+] [OH-]

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What is the equation for Kw in pure water

Kw = [H+]^2

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What is a monoprotic base?

base that can accept one proton

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What is a diprotic base?

Base that can accent two protons

(has 2 OH’s e.g Ba(OH)2)

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How can you calculate the pH of a Strong base?

use concentration of the base to find [OH-]

use Kw to find [H+]

convert [H+] into pH

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how can you calculate the concentration og [OH-] from pH

use pH to find [H+]

use Kw to find [OH-]

use [OH-] to find conc of base

divide by no. of OH

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How can you calculate the pH of a strong acid after it has been diluted with water?

calculate moles of [H+]

work out new volume

convert to concentration by dividing by volume

calculate pH

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How can you calculate the pH of a strong base after dilution?

calculate moles of [OH-]

calculate the new volume

convert to concentration

[H+] = Kw / [OH-]

calculate pH

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What is the value of Kw at room temperature?

1 x 10^-14

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How can you calculate the pH of a mixture of acid and alkali?

calculate moles of H+
Calculate moles of OH-
Work out which one is in excess (largest number) and calculate the remaining moles
convert two concentration by by the total volume

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Why are strong acids not in equilibrium?

They fully ionise in water

why is the Kc of weak acids/bases small?

weak acids/bases partially ionise in water and the equilibrium lies to the left

What is the equilibrium of weak acids?

HA ⇌ H+ + A-

what is the equation for the acid dissociation constant Ka?

Ka = [H+] [A-] ————- [HA]

what are the units of Ka?

mol dm-3

what is the equation for dissociation of an acid?

eg ethanoic acid CH3COOH ⇌ H+ + CH3COO-

which acids have a Ka less than one?

Weak acids

Which assets have a KA more than one?

Strong acids

what is the formula for pKa

pKa = -logKa

how can you determine Ka from a pKa value

Ka = 10^-pKa

what are the two assumptions for an aqueous solution a weak acids?

- The **dissociation of HA is very small** so the [HA] remains effectively constant - [HA] at equilibrium = [HA] initial (1:1)

What is the simplest equation for dissociation of weak acids?

Ka = [H+]^2 -——— [HA]

what happens in an acid base titration?

Base is added from a burette to a measured amount of an acid with a known concentration until an indicator shows that it has been **neutralised**

what is a PH meter?

Digital device that can determine the pH of a solution by a place saying the pro in it It can track pH during a titration Must be calibrated before use

Why must PH metres be calibrated?

Because BH means to do not give accurate readings overtime

how do you calibrate a pH meter

Place probe and buffer solution of known pH and plot calibration curve - Rinse probe with deionised water and dry - Place in pH 4 buffer - Take out rinse and place in pH 7 and 10 - Plot a graph pH buffer against measured pH

how do you do an acid based titration with a ph probe

- Measure the pH of unknown volume of acid - Add base from the burettr in small portions - Stir - Record pH after each addition - Add base drop by drop at a near the end point - after the end point add base slowly until it is in excess

how does the pH change in an acid base titration?

As the basis added the pH gradually increases

When does the pH highly increase in a titration?

when one drop of the base changes the mixture from being acid in access to being base in excess there is a rising pH until it levels off

what is the equivalence point?

When the moles of alkali added equals a mole of acid present (1:1) rapid change in pH

What is the endpoint of a titration?

when the indicator changes colour The end point should concede with the equivalence point

What is the pH range for an indicator for a titration with a strong acid and strong base

36960

What is the pH range for an indicator in a titration with a weak acid and strong base

37113

What is the pH range for an indicator for a titration with a strong acid and weak base

36956

What is the pH range for an indicator in a titration with a weak acid and a weak base?

There is no suitable indicator because there is no sharp end point

how do you use a pH code to find the concentration of an acid?

- Work out the moles of base - 1:1 ratios of acid are the same - Workout concentration (moles/olume)

what is the half equivalence point?

put halfway along the axis between the zero and equivalence point pH = pKa

how do you calculate Ka from a titration pH curve?

- divide the volume equivalence by 2 to find the volume at half equivalence - read the pH at half equivalence this will be pKa -10^-pKa to find Ka

What is an indicator?

A weak acid whose acid form (HIn-) and base form (In/)have different colours

what colour is phenophthalein?

acid - colourless base - pink

What colour is methyl orange?

acid - red base - yellow

what properties must a suitable indicator for a titration need?

- colour change must be sharp rather than gradual at the end point - end point given by the indicator must be the same as the equivalence point otherwise titration will give us the wrong answer - Indicator should give a distinct colour change

What is a buffer?

solution that maintains a constant pH despite the addition of acid/base/water

What is the pH of an acidic buffer?

below 7

What is the pH of a basic buffer?

above 7

What is an acidic buffer?

a weak acid and its salt

How can an acidic buffer be made by adding acid to salt?

adding a solution of the salt to the weak acid (vice versa) the salt **dissociates completely** and the acid **slightly**

How can you make an acidic buffer by partial neutralisation?

by adding a strong base to the weak acid - the salt is made as a product

Where does equilibrium moves if H+ are added

H+ reacts with A- eqm moves to **left** to decrease moles of H+

Where does equilibrium moves if OH- ions are added?

alkali is added they react with H+ forming H2O and eqm moves to **right** and more acid is dissociated to replace H+

What is the equation for making a buffer using weak acid and its salt?

Ka = [H+] [A-] ————- [HA] A- = moles of salt HA= moles of acid

What effect does adding a strong acid have on a buffer

Increases moles of HA Decreases moles of A-

What effect does adding a strong base have on a buffer?

Decreases moles of HA Increases moles of A-