Acid - Base equilibria

Question 1

How to calculate H+ concentration and hence pH for a WEAK acid

Answer 1

• use the Ka equation (conc. products/ conc. reactants)

- then once you have the H+ from this, use -log[H+] = pH to calculate pH

Question 2

Equation to calculate pH

Answer 2

-log[H+] = pH

Question 3

Equation to calculate H+ concentration

Answer 3

10^-pH= H+

Question 4

Definition of pH

Answer 4

-log[H+] = pH (this is the definition, don’t just say the concentration of H+ ions)

Question 5

What is Brønsted–Lowry acid?

Answer 5

a proton donor

Question 6

What is a Brønsted–Lowry base?

Answer 6

a proton acceptor

Question 7

What do acid-base reactions involve the transfer of?

Answer 7

protons

Question 8

Hydrogen ion concentration units

Answer 8

mol dm^3

Question 9

How to identify Brønsted–Lowry conjugate acid-base pairs? Explain using this example.

H2O (l) + CO3 2- (aq) ⇄ OH- + HCO3 -

Answer 9

1) Identify the acid on the reactant side.

HELP: an acid is a proton donor so it will be the reactant that starts off with a proton (hydrogen). But then you need to look at the products to find that exact same species but without the hydrogen which the original reactant had now donated.

EXPLANATION: the acid is H2O because it is on the LHS and it contains a hydrogen which it donates meaning that on the RHS it then becomes just an OH- (because it has donated an electron)

2) identify the thing that pairs with the acid you just identified. This thing is called the conjugate base. (NOT conjugate acid. acid + base go together)

HELP: a base is a proton acceptor so you must find the species on the RHS * that, when you look at the backwards reaction, it now accepts a proton from the acid you just identified. So look for the species on the LHS which when you read from right to left, has a hydrogen added to it and becomes something else.

*(always the RHS because this is where you will find things that have formed after either losing/gaining a proton)

EXPLANATION: so we know the acid is H2O so we need to find its conjugate base on the RHS which, in the backwards reaction, accepts a proton from the H2O acid we identified earlier. So if you look, OH- , reading from left to right (backwards reaction) turns into H2O and is accepting a prton so OH- is the proton acceptor and hence the conjugate base

EASIER WAY TO DO STEP 2 BUT IT WOULD MEAN NOT UNDERSTANDING THE CONCEPT:

to identify the thing that pairs with the acid you just identified. (This thing is called the conjugate base) then just search for the exact same species (as the acid) on the RHS but with a hydrogen knocked off. So in this case, H2O, our acid, with a hydrogen knocked off is just OH- and thus this is the conjugate base.

3) Because the acid and base are always on the reactant side, the other species on that side is the opposite to the species next to it. So in this case, CO3 2- is the base.

The actual reason it is a base is because it is a proton acceptor; it turns to HCO3 - on the RHS having accepted a proton.

4) Then the conjugate acid is what the base on the reactant side turn into after a proton has been added. So in this case it turns into HCO3 - and therefore it is the conjugate acid.

Question 10

what does a strong acid in terms of dissociation mean?

Answer 10

it has lots of H+ ions and fully dissociates into ions

Question 11

what does a weak acid in terms of dissociation mean?

Answer 11

it has less H+ ions and only partially dissociates into its ions

Question 12

-log is not the same as

Answer 12

reverse log

Question 13

What does log(100) equal?

Answer 13

2

Question 14

Why does only a weak acid, not a strong acid have an equilibrium arrow?

Answer 14

because it only PARTIALLY dissociates into ions

some ions will rejoin and ASSOCIATE again (hence a backwards reaction would be occurring)

Question 15

What does what is the log of x (number quoted) mean?

Answer 15

It means what power do I need to raise ten to in order to get to the number I’m quoting (x)

Question 16

Once you work out the H+ concentration of a weak acid, why is the value lower than the general concentration in the main acid in the reactants?

Answer 16

because weak acids partially dissociate (basically less H+ ions detach or whatever and so less floating around freely hence a lower H+ concentration)

Question 17

What does Ka mean?

Answer 17

Ka means the acid dissociation constant, it’s a measure of how much an acid splits up into H+ In solution.

Question 18

When you have a strong monoprotic acid why do you know that whatever the concentration is, the concentration of H+ ions after dissociation willl be the same?

Answer 18

Because strong acids fully dissociate (so all the H+ ions are floating around freely

Question 19

When ONLY do we use Ka?

Answer 19

When finding the H+ concentration of a weak acid

Question 20

What type of things are strong and weak acids usually?

Answer 20

strong - inorganic acids

weak - organic acids

Question 21

in the bronsten thing, which side of the equation are conjugates and the other thing?

Answer 21

the other thing - acid and base - RHS

conjugates - conjugate acid and conjugate base - LHS

Question 22

Ka general equation

Answer 22

reactant x reactant

Question 23

Ka units

check?/?

Answer 23

mol dm^3

Question 24

a shift to the left causes Ka to

Answer 24

decrease

Question 25

a shift to the right causes Ka to

Answer 25

increase

Question 26

What happens to Ka when a forwards reaction speeds up?

Answer 26

forwards reaction speeds up
shift to the right
products increase
Ka goes up (because it is products/reactants so if products are increasing and reactants are decreasing then you divide a larger number by a smaller number which overall gives you a larger number)
so Ka value increases

(a shift to the right causes Ka to increase)

Question 27

log(x) means ….

Answer 27

10 to the power of what to get x