Topic 18 - Acids and Bases Flashcards
Defintion of Bronsted Lowry acid
Proton donor
(Donates H+ ions)
Defintion of Bronsted Lowry base
Proton acceptor
Equation when strong acid is added to water
HA(aq) + H2O(l) –> H3O+ (aq) + A-
Equation for when base is added to water
B(aq) + H2O <—> BH+(aq) + OH-(aq)
Defintion of monoprotic acid
Acid that releases one H+ ion per molecule
Definintion of diprotic acid
Acid that realeases two H+ ions per molecule
Why is the logarithmic scale used for PH values?
The concentration of hydrogen ions can vary widely so alog scale is used. A minus sign is used to get rid of negative values.
The smaller the pH, the greater the concentration of H+
Equation for calculating H+ from pH
[H+] = 10^-pH
Equation for calculating the pH of strong acids
pH = -log [H+]
Value for Kw of an aqueous solution at a given temp
1.00x10^14 mol2dm-6
Defintion of neutral
[H+] = [OH-]
Things you need to know for calculating the pH of strong bases
- Strong bases fully ionise in water.
- They donate one mole OH- per mole of base
- The concentration of OH- ions is the same as the concentration of the base
Steps for calculating the pH of an alkaline solution
- Find the calues of Kw and [OH-]
- Rearrange the equation, substitute the values for Kw and [OH-] into the equation and solve it to find [H+]
- Put the [H+] into the pH equation
pH = log10 [H+]
Things you need to know calcualting the pH of weak acids - Ka
Only a small number of the molecules break aprt to form ions
HX <—> H+ + X-
For a weak aqueous solution acid, HA:
HA(aq) <–> H+(aq) + A-(aq)
Kc= [H+][A-] / [HA]
For a weak acid the symbol Ka is used and is called the acid dissaoction constant
Ka= [H+][A-] / [HA]
When dealing with weak acids you can assume that all the [H+] = [A-]
So the simplified version is:
Ka = [H+]2 / [HA]
Solution of a weak acid in water with nothing else added
[H+] = [A-]
[HA] ~ [HA] initial
The concentration of HA at equilibrium is virtually the same as it was before any o fit dissociated as so little dissociates
Steps for finding the pH of weak acids
- Write the expression for Ka for the weak acid
- Rearrange the equation and substitute in the values you know to find [H+]^2
- Take the square root to find [H+]
- Substitute [H+] into the pH equation to find the pH
Steps for finding the concentration of weak acid
- Substitute the pH into the inverse pH equation to calculate [H+]
- Write an expression for Ka
- Rearrange equation and substitue the values for ka and [H+] into the equation and solve
Use and formula of pKa
The value of Ka massively varies from one acid to the next. This can make numbers difficult to convert/ manage so pKa can be used.
pKa= -log10(Ka)
Steps of calculating the pH of mixtures
- Write a balanced equation
- Calculate moles of H+
- Calculate moles of OH-
- Calculate moles in xs either H+ or OH-
- Calculate pH
Things need to know for calculating mixtures of weak acids and strong bases
When a weak acid reacts with a strong base, for every mole of OH- added, one moles of HA is used up and one mole of A- is formed.
Calculating mixtures of weak acids and strong bases
- If XS HA
- Calculate moles of HA
- Calculate the moles of OH-
- Calculate moles XS HA or OH-
- Calculate moles HA left and A- formed
- Calculate [HA] leftover and [A-] formed
- Use Ka to find [H+]
- Find pH
Calculating mixtures of weak acids and strong bases
- If XS OH-
- Calculate moles of HA
- Calculate the moles of OH-
- Calculate moles XS HA or OH-
- Calculate [OH-]
- Use Kw to find [H+]
- Find pH
Calculating mixtures of weak acids and strong bases
If mole HA = OH-
- Calculate moles of HA
- Calculate the moles of OH-
- Calculate moles XS HA or OH-
- pH = pKa of weak acid
Vertical point and indicator of Strong acid and Strong base
pH 3-10
Phenolphthalein
