Chapter 20 Acids

Question 1

What does the Arrhenius model of acids and bases state?

Answer 1

Acids disassociate and release H+ ions when dissolved in solution
Alkalis disassociate and release OH- ions when dissolved in solution

Question 2

What is a Brønsted-Lowry acid?

Answer 2

A proton donor

Question 3

What is a Brønsted-Lowry base?

Answer 3

A proton acceptor

Question 4

What happens when H+ ions dissolve in water really?

Answer 4

Proton transferred from the acid to a water molecule
This forms the hydroxonium ion, which is what we really mean by protons
The charge density is too high to allow lone protons

Question 5

How do you name acid-base pairs?

Answer 5

Use the same molecule for the numbers
E.g Acid 1 becomes B1
Base 2 becomes Acid 2

The previous acid, then becomes a base

Question 6

What do mono, di, and tribasic mean?

Answer 6

They refer to the number of hydrogen ions that can be replaced when making a salt
1, 2 or 3
e.g HCl, H2SO4, H3BO4

Monobasic acid- release 1 mol of protons when 1 mol of acid is dissolved in water, each molecule with release 1 proton when the acid is dissolved

Question 7

How do you calculate pH?

Answer 7

pH= log10([H+])

[H+]= 10^-pH

Question 8

How do you calculate pH for strong acids? What pH are acids/alkalis?

Answer 8

As fully disassociate, -log10(concentration) x number of protons that disassociate

Acids<7 Alkalis>7 neutral=7

Question 9

What is Ka, what is special about it?

Answer 9

The acid-disassociation constant
Special equilibrium for weak acids
As an equilibrium constant, changes with temperature

Question 10

What is pKa? How does acid strength vary with pKa?

Answer 10

Easier to compare Ka
pKa= -log10(Ka)
The lower the pKa, the stronger the acid (like pH)

Question 11

What are the assumptions used for calculating the pH of weak acids?

Answer 11

The concentration of H+ = A- at equilibrium
The concentration of HA at the start is the same at equilibrium

Question 12

When do the pH assumptions break down?

Answer 12

For H+ = A-, ignores disassociation of water as normally small so negligible. But if the acid is weak enough, the H+ from water can become important
So when pH>6

For HA start = equilibrium
If a stronger weak acid, significant amount disassociate to reduce concentration,
when Ka> 10^ -2

Question 13

What is the equation for calculating pH of weak acids using assumptions?

Answer 13

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

Question 14

How do you right acid base equilibrium between acids?

Answer 14

Normally an equilibrium sign
Proton from the stronger acid donated to the weaker acid
Charges

Question 15

What is the ionisation of water?

Answer 15

Proton transferred from one water molecule to another
Simplified, splits into H+ and OH-

Question 16

What is Kw?

Answer 16

Ionic product of water
Equilibrium constant for disassociation of water
A constant= concentration of water x Ka

Question 17

What is the concentration of water? What is the numerical value of Kw?

Answer 17

55.6 mol/dm3, think about moles in 1dm3 density
1 x 10^-14

Question 18

Why is Kw important with pH?

Answer 18

The concentration of H+ multiplied by OH- is always Kw
When they are equal, gives the pH for neutral, 7 at 25 degrees celcius

Question 19

At higher temperatures, is pH 7 still neutral?

Answer 19

No, as the forward reaction is endothermic, Kw increases
More disassociation
So the pH is higher, as the pH scale runs longer
But water is always neutral, as there is the same concentration of protons as hydroxide ions

Question 20

How can you calculate the pH of strong bases using pOH?

Answer 20

pOH of strong base = -log10(concentration strong base )
pH= 14- p(OH)

14 only works at 25 degrees celcius, as pH scale runs different

Question 21

How does equivalence point relate to Ka?

Answer 21

Look at equivalence point volume (half way of vertical)
Half of this volume, look the pH
pH=Pka
10^-pH= Ka