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
Vertical point and indicator of Strong acid and weak base
pH 3-8
Phenolphthalein
Suitable indicatior weak acid and strong base
Methyl orange
Vertical point and indicator of weak acid and weak base
No vertical point so no indicator used
Uses of buffers in everyday life
Shampoo
Biological washing powder
Indicator colours acid to base and PH range
Methyl orange- Red to yellow 3.2 to 4.4
Phenolphthalein- Colourless to pink 8.2 to 10.0
Equations to show action of buffer
Addition of acid: CH3COO- + H+ —–> CH3COOH
Addition of base: CH3COOH + OH- —–> CH3COO- + H2O
Meaning of the term buffer solution
A solution which resists/ minimises change in PH when small amounts of acid/ alkali added
Use equations to show how blood resists changes in PH
