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Chemistry > Acids and Bases > Flashcards

Acids and Bases Flashcards

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1
Q

What is a Brønsted acid and base in terms of protons and hydrogen ions

A

Acid = proton donor, release H+
Base = proton acceptor, attract H+

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2
Q

What does Brønsted acid-base equilibria involve

A

Transfer of protons

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3
Q

Equation that related pH and H+

A

pH = -log10[H+]
[H+] = 10^-pH

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4
Q

What is Ka

A

An equilibrium constant for the dissociation of an acid

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5
Q

What is the equation for Kw in pure water

A

Kw = [H+]^2

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6
Q

Equation for pKa

A

pKa = -log10Ka

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7
Q

What does it mean when acids and bases dissociate in water

A

Break up into positively and negatively charged ions

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8
Q

What dissociates the most and why

A

Strong acids = releasing all H+ ions
Strong bases = ionise completely
Weaker bonds dissociate more

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9
Q

What happens to the equilibrium if you add more acid or base in acid + base = salt + water

A

Position of equilibrium shifts right
(opposite happens if you add more salt or water)

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10
Q

What does the dissociation of water equation look like

A

H2O <–> H+ + OH-

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11
Q

What is a conjugate acid-base pair

A

A conjugate acid-base pair is two species that are different from each other by an H+ ion

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12
Q

what is Kc

A

the equilibrium constant, measured using products and reactants in either the gaseous or aqueous state

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13
Q

What is Kw

A

Kw i= autoprolysis constant of water
(at 25 degrees Celsius)
Is always = 1.0 x 10^-14.

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14
Q

What is the Kc and Kw equation for the dissociation of water

A

Kc = [H+][OH-] / [H2O]

Kw = [H+][OH-]

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15
Q

Why is [H2O] not shown in the Kw expression

A

[H2O] is almost constant

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16
Q

What is a pH scale a measure of

A

Hydrogen ion concentration in a solution
measured in mol dm-3

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17
Q

What is the equation for strong acid ionisation

A

HA (aq) -> H+ (aq) + A- (aq)

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18
Q

What is the equivalence point

A

where the amount of acid neutralises the alkali or vice versa

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19
Q

Kc equations

A

Kc x H2O = [H2O][OH-]

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20
Q

Strong base ionisation equation

A

BOH (aq) → B+ (aq) + OH- (aq)

21
Q

Strong base equations:

A

[H+] = Kw/[OH-}

[H+] = 1 x 10^-14 / [OH-]

pH = -log[H+] = -log (Kw (1 x 10^=14) / [OH-])

22
Q

Calculate the pH of 0.15 mol dm-3 sodium hydroxide, NaOH

A

[H+] = Kw ÷ [OH-]

[H+] = (1 x 10-14) / 0.15 = 6.66 x 10-14

pH = -log[H+]

  = -log 6.66 x 10-14  = 13.17
23
Q

Calculate the hydroxide concentration of a solution of sodium hydroxide when the pH is 10.50

A

Step 1: Calculate H concentration
[H+]= 10-pH

[H+]= 10-10.50

[H+]= 3.16 x 10-11 mol dm-3

Step 2: Rearrange the ionic product of water

Kw = [H+] [OH-]

[OH-]= Kw ÷ [H+]

Step 3: Find the concentration of hydroxide ions

Kw is 1 x 10-14 mol2 dm-6,

[OH-]= (1 x 10-14) ÷ (3.16 x 10-11)

[OH-]= 3.16 x 10-4 mol dm-3

24
Q

What is Ka

A

Dissociation constant for a weak acid

25
Q

Equation for Pka

A

pKa = -log10Ka

26
Q

Ka equation

A

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

27
Q

What are you looking for when you are calculating:

pH
Ionic product of water
Weak acid dissociation constant
Weak acid pH

A

pH = -log10[H+]
Ionic product of water = Kw = [H+][OH-]
Equation for pKa = -log10 Ka
Weak acid pH = Ka = [H+][A-] / [HA]

28
Q

What does the higher or lower the value of Ka mean

A

higher the value of Ka the more dissociated the acid and the stronger it is
The lower the value of Ka the weaker the acid

29
Q

How to find the pH of weak acids step by step

A

1) Write Ka for weak acid
( Ka = [H+]^2 / [HA] )

2) Rearrange equation and substitute to find [H+]^2
3) Square root the number to find [H+]
4) Substitute [H+] into pH
( pH = -log10 [H+] )

30
Q

How to find concentration of weak acids step by step

A

1) Substitute pH into pH equation to calculate [H+]
( [H+]=10^-pH )
2) Write an expression for Ka
3) Rerrange to give concentration
( [H+]^2 / Ka )
4) Substitute Ka and [H+] into equation

31
Q

Find Ka of weak acids step by step

A

1) Use pH of acid to find [H+]
2) Write an expression for Ka
3) Substitute values for [H+] and [A-]

32
Q

Step by step calculating Ka

A

1) Concert pKa to Ka
( Ka = 10^-pKa )
2) Write Ka expression
3) Rerrange to give [H+]^2
4) Square root it
5) Substitute [H+] into pH equation and solve

33
Q

How to calibrate pH meter

A

1) Rinse probe with distilled water, shake off excess, place in standard buffer solution, record pH reading in table
2) Repeat solutions of other concentrations, rinsing the probe between each step
3) Plot graph of recorded pH against pH of buffer

34
Q

What is Kw When going from [OH-] to [H+]

A

Kw = [H+][OH-]

35
Q

As temperature increases, what happens to pH?

A

pH drops because water dissociated more so there are more H+

36
Q

How to find the equivalence point in a titration graph?

A

In the middle of the graph where the line is completely straight going up

37
Q

How to find the half equivalence point?

A

Half neutralised
So distance between straight line going up, half of it

38
Q

What is the half equivalence point

A

PH = pKa

39
Q

Pure weak acid equation by itself

A

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

40
Q

describe how the student would carry out a titration to obtain a pH curve

A

1) Use pH probe to measure the initial pH of ammonia solution
2) Add Hcl acid 2cm^3 at a time
3) Stir mixture
4) Record pH after each addition
5) Reduce size portions when close to the end point
6) Repeat until acid in excess
7) Plot a graph of the results

41
Q

Explain why [H2O] is not shown in the Kw expression

A

[H2O] is very large compared to [H+
] and [OH-]

42
Q

Explain why the value of Kw increases as the temperature increases

A

Equilibrium is endothermic
so shifts to the right to oppose temperature increase

43
Q

Explain why water is neutral at 50 °C

A

[H+] = [OH–]

44
Q

Suggest why the pH probe is washed with distilled water between each of
the calibration measurements.

A

To wash off any residual solution/substance (which could
interfere with the reading)

45
Q

Explain why the volume of sodium hydroxide solution added between each
pH measurement is smaller as the end point of the titration is approached.

A

To wash off any residual solution/substance (which could
interfere with the reading)

46
Q

How do you know if the indicators are suitable in the titration

A

All will have a colour change/pH range within the steep/vertical part
of the titration curve

47
Q

State the meaning of the term weak acid.

A

partially dissociates (in water to form H+ ions)

48
Q
A

Chemistry (12 decks)

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