Acids and bases

What is a Bronsted-Lowry acid and base?

Acid: a proton donor

Base: a proton acceptor

What is the H3O+ ion called?

Oxonium or hydronium ion

Does water act as an acid or base?

Both

Why can water act as both an acid and a base?

One water molecule acting as base can accept a hydrogen ion from a second molecule that acts as an acid, which forms a hydronium and hydroxide ion.

How is an equilibrium set up in water?

The hydronium ion is a strong acid and the hydroxide ion is a strong base, so they react to produce water again as soon as they form so an equilibrium is set up.

What is the equation for the equilibrium in water?

2H2O ⇄ H3O+ + OH-

or: H2O ⇄ H+ + OH-

When is the equilibrium of water established?

In water and all aqueous solutions

What is the Kw expression?

Kw = [H+] [OH-]

What is the value and units of Kw?

1.00 × 10⁻¹⁴ mol²dm^-6

What does neutral mean?

That the concentration of H+ and OH- ions is equal

Does the value of Kw vary with temperature?

Yes

What does pH measure?

Hydrogen ion concentration in a solution

What is the equation for pH?

-log10 [H+]

How is the pH scale logarithmic?

Each successive whole number pH has a difference of a factor of 10.

What number of decimal places should pH be quoted to?

2

What is a strong acid?

An acid which fully dissociates in aqueous solution

What are some examples of strong acids?

HNO3, H2SO4, HCl

What are the steps to calculating the pH of a strong acid?

Firstly use the concentration of the acid to find [H+], then convert this into pH

Equation to find [H+] if pH is known?

[H+] = 10 ^-pH

What is a diprotic acid?

An acid with two acidic hydrogens (that can be donated to a base)

Weak acid definition?

An acid that is only partially dissociated in aqueous solution

What is a strong base?

Proton acceptor that is fully dissociated in aqueous solution

What is the main example of a strong soluble base?

Metal hydroxide

What are the steps to calculating the pH of a strong base?

1. Use concentration of base to find [OH-]

2. Use Kw to find [H+]

3. Convert H+ into pH

What are the steps to finding the overall pH of a mixture of acid and base?

1. Find moles of hydrogen ions and hydroxide ions

2. Find the moles in excess by taking large away from small

3. Convert this into a concentration by dividing by total volume

4. If H+ was in excess go straight to calculating pH

5. If OH- is in excess calculate [H+] using Kw and rearrange, then calculate pH

What are the steps to calculating change in pH?

1. Calculate initial pH of the solution

2. Calculate moles in excess when the new solution is added

3. Use this to calculate concentration of H+ if its in excess or OH- if that is in excess.

4. For OH- use Kw to find H+ concentration then calculate pH.

What is a weak acid?

When molecules only partially dissociate in water.

What are some examples of weakness acids?

Citric acid, carboxylic acids

What can be used to represent a weak acid if its full formula is too long?

HA or HX, and A- or X- for the negative ion

What does a larger Ka mean?

That equilibrium lies further to the right meaning the acid is more dissociated, therefore a stronger acid.

What is Ka?

The dissociation constant for a weak acid.

What is the general equation for Ka?

Ka = [H+] [A-]/

[HA]

What are the usual units for Ka?

Moldm-3

What is pKa?

-log10 Ka

What is pKa used for?

In calculations to give a measure of how strong a weak acid is as a smaller value indicates a stronger acid

What are the steps for calculating pH using acid concentration and pKa?

1. Convert pKa to Ka using 10 -pKa

2. Multiply by the acid concentration then square root to find [H+] using Ka constant equation

3. Find pH using -log10

What are the steps to finding pKa using pH and concentration of weak acid?

1. Find [H+] using 10 -pH

2. Square this then divide by concentration to find Ka

3. Convert to pKa using -log10

What is the usual degree of dissociation of a weak acid?

4/1000

What is the trend in electrical conductivity as concentration increases?

As concentration of acid goes up conductivity also increases because there are more dissociated ions to carry charge.