Acids And Bases Flashcards
What is a Bronsted-Lowry acid
A proton donor
What is a bronsted-lowery base
A proton acceptor
What does bronsted Lowry refer to
Exchange of protons
What is pH
-log10 /H+/
What is a strong acid
Strong acids fully dissociate in water
What assumption can be made when calculating pH of a strong acid
The concentration of hydrogen ions in a MONOPROTIC acid is the SAME as the concentration of the acid
How many decimal places is pH measured to
2 d.p.
How do you find /H+/ from pH
/H+/ = 1 x 10^-pH
What is equilibrium for water
H2O <=> H+ (aq) + OH- (aq)
What is the equilibrium expression for water
Kc = /H+/ /OH-/
/H2O/
What can the equilibrium expressions for water be simplified to
Kw = /H+/ /OH-/
Why can the equilibrium expression for water be simplied
Pure water doesn’t dissociate much so we assume the concentrations stays constant, so isn’t included in Kw expression
What is the kw for water
At 25 degrees c, Kw = 1 x 10^-14 mol2dm-6
What can we assume for pure water or neutral solutions
/H+/ = /OH-/
What can the equilibrium expression be simplified to when solution is neutral
Kw = /H+/^2
When does Kw change
At different temperatures
Why does Kw change with temperature
Position of equilibrium changes, so /H+/ changes, so pH changes
What is the concentration of a strong base equal to
/OH-/
What are the steps to calculating the pH of a strong base
1) Find /H+/ using the Kw expression
2) pH = -log10 /H+/
Describe a weak acid
An acid that doesn’t fully dissociate
What is the equilibrium equation for weak acids
HA + H2O <=> H3O + A-
What does the equilibrium equation for weak acids simplify to
HA <=> H+ + A-
What is the weak acids dissociation expression
Ka = /H+/ /A- /
/HA/
What does a larger Ka value mean
Stronger acid
What can the weak acid equilibrium expression be simplified to
Ka = /H+/^2
/HA/inital
What are the assumptions made to simplify the weak acids equilibrium expression
1) /H+/ = /A-/ as they dissociate in a 1:1 ration
2) amount of dissociation is small, so assume that initial concentration of undissociated acid remains constant
How do you work out pH of a weak acid
PKa = -log10 (Ka)
How do you work out Ka from pKa
Ka = 1 x10^ -pKa
What are the steps to work out a strong acid + strong base neutralisation
1) work out the moles of acid and base
2) work out which (acid or base) is in excess
What is the next step of strong acid + strong base neutralisation if ACID IS IN EXCESS
3) /H+/ = moles excess H+ / total volume
4) pH = -log10 /H+/
What is the next step of strong acid + strong base neutralisation IF BASE IS IN EXCESS
3) /OH-/ = moles excess OH- / total volume
4) /H+/ = Kw / /OH-/
5) pH = -log10 /H+/
What do you do in a neutralisation calculation if a diprotic acid is used
Multiply moles x 2
What are the steps for a weak acid, strong base neutralisation calculation
1) work out moles of acid and base
2) If excess acid, work out new concentration of HA
3) work out concentration of salt /A-/ formed
4) rearrange Ka expression for /H+/
5) pH = -log10 /H+/
How to work out new concentration of HA if excess acid
(Weak acid, strong base neutralisation)
/HA/ = inital moles HA - moles OH- / total volume
How to work out concentration of salt /A-/ formed
(Weak acid, strong base neutralisation)
/A-/ = moles OH- added / total volume
When can Ka expression simplify further
Weak acid reacts with exactly half the neutralisation volume of alkali
What is assumed when Ka is simplified at half neutralisation
/HA/ = /A-/
What does the Ka expression simplify to at half neutralisation
Ka = /H+/
What does pH equal at half neutralisation
PH = pKa
In questions, what is the volume of alkali equal to
Half acid volume
How to calculate pH of a diluted strong acid
/H+/ = /H+/old x old volume
New volume
How to calculate pH of a diluted base
/OH-/ = /OH-/old x Old volume
New volume
/H+/ = Kw
/OH-/
On a titration curve, where is the buffer region
When the graph horizontally plateaus
Define a buffer solution
One where the pH does not change significantly when small amounts of acid o alkali are added to it
How to make a basic buffer
Weak base and a salt of that weak base
How do you make the salt of a weak base
React weak base with strong acid
How do you make an acidic buffer
Weak acid and a salt of that weak acid
How do you make the salt of a weak acid
React weak acid with strong base
Give an example of components of an acidic buffer
Ethanoic acid and sodium ethanoate
Give an example of components of a basic buffer
Ammonia and ammonium chloride
What does a buffer contain a reservoir of
Reservoir of HA and A- ions
What is the equilibrium equation for an acidic buffer
Acid <=> conjugate base + /H+/
What happens to equilibrium if small amounts of acid are added to an acidic buffer
Equilibrium shifts to the left, removing nearly all H+ ions added
Why does pH stay constant when acid is added to acidic buffer
There’s a large concentration of the salt ion in the buffer, so ratio of acid to conjugate base remain constant
What forms when small amounts of alkali are added to an acidic buffer
The OH- ions react with H+ ions to form water
What happens to the position of equilibrium when alkali is added to acidic buffer
Equilibrium shifts right to produce more H+ ions
Suggest a way salt content can be added to a buffer
Solid salt or salt solution could be added
What values can you use instead of concentration when making a buffer by adding solid salt
The mole values of HA and A- as they have the same new final volume
How does adding a small amount of alkali to buffer affect moles
Increase moles of alkali by number added
Decrease moles of acid by number added
Find new concentrations using these new values for moles
What happens to moles when small amount of acid is added to buffer
Increase moles of acid by number added
Decrease moles of alkali by number added
Find new concentrations using these new values for moles
How will diluting a buffer solution affect pH
Diluting with water wont change the ph
Why does diluting buffer with water not change its pH
The ratio of HA to A- stays constant as both diluted by same proportion
