Acids and Bases Flashcards
Define a Brønsted-Lowry Acid?
A proton Donor
Define Brønsted-Lowry Base
Proton Acceptor
What ion causes a solution to be acidic
The oxonium ion
H3O+
Protons react with H2O to form it
What ion causes a solution to be alkaline
-OH
Hydroxide ion
Write the equation for the ionisation of water
Derive the Kw Equation using the ionisation of water
Ka = [H3O+][OH-] / [H2O]
[H2O] Kw = [H3O+][OH-]
[H2O] is so large in comparison to [H3O+] and [OH-] That it is considered to remain constant
Kw = [H3O+][OH-]
State the value of kw at 298k
10^-14
What physical factors affect the value of Kw and explain why
Temperature only
If temperature is increased the eqm moves in the endothermic direction (right)
Kw therefore increases and the Ph of pure water decreases
Why is pure water still neutral even if the ph doesn’t equal 7?
[H3O+] = [OH-]
What is the equation for Ph?
What bace is the logarithm to
Ph = -log(H3O+)
Base 10
A lower ph has a _____________________________
Higher concentration of [H+] present
A Ph difference of 1 is a ______________ difference in [H+]
A factor of 10
How do you find [OH-] From ph
1) PH —-> [H3O+]
2) Kw = [H3O+][OH-]
3) Kw at 298k = 10^-14
Explain the difference between monoprotic, diperotic and triprotic acids?
Produce different amounts of Protons per mol of acid
Monoprotic —> 1 Mol of [H+] from 1 mol of acid
DiProtic ——> 2 Mol of [H+] from 1 mol of acid
Triprotic ——> 3 Mol of [H+] from 1 mol of acid
How do you calculate the PH of a strong alkaline solution?
1) Kw = [H+][OH-]
2) ph = -log([H+])
Define the term strong acid?
An acid which dissociates fully into their ions within aqueous conditions
Define the term strong bace
A bace which fully dissociates into their ions in water
What is the difference between a strong acid and a concentrated one?
Concentrated acids have many moles per unit volume
Strong acids dissociate fully in water
What is a weak acid and bace
Acids and braces which only dissociate partially into their ions in aqueous conditions
Name some strong acid examples
H2SO4 - Diprotic
HCl - Monoprotic
H3PO4 - Triprotic
Give some examples of strong bases?
NaOH
CaCO3
Na2CO3
Give an example of a weak acid
Ethanoic acid - CH3COOH
Any organic acid
Give an example of a weak Base?
NH3 Ammonia
Write the expression for Ka
Ka = [H+][A-] / [HA]
Where
HA (reversible) [H+][A-]
How would you work out the PH of a weak acid?
1) Ka = [H+][A-] / [HA]
2) Rearrange to calculate [H+]
3) Ph = -log[H+]
What is a titration?
The addition of a base/acid of know concentration
Titration by adding it to a acid/bace of unknown concentration to determine the unknown concentration
A Indicator (Ph probe) is used to show that neutralisation has occurred
Draw a labled diagram for the set up for a titration
Draw the pH Curve for strong acid into a
1) Strong Base
2) Weak Bace
Draw the pH Curve for weak acid into a
1) Strong Base
2) Weak Bace
Define the term equivalence point
The point at which the exact volume of base/acid has been added to neutralise the acid/bace
What happens to the ph of the solution around the equivalence point
Large and rapid change of Ph (unless weak-weak)
When mol of H+ = mol of OH-
What are the properties of a well chosen indicator (3)
. Sharp colour change changes colour over a small range of ph values
. Colour change must be at the steep/vertical section of the curve
. Distinct colour change so it is obvious when the endo point has been reached
What indicator would you use for a strong acid- strong bace titration
Phenolphthalein or methyl orange
Explain the properties of methyl orange indicator
Red in acidic solutions
yellow in basic solutions
Colour changes between:
3.1-4.4
Explain the properties of Phenolphthalein
Colourless in acidic solutions and pink in basic solutions
Changes colour between
8.3-10.0 ph
What is the half-neutralisation point
What is it the same as?
Point where the volume of bace/acid added has reacted with half of the molecules of the original solution
Pka value = Ph
What indicator would you use for a strong acid - weak bace titration
Methyl Orange
What indicator would you use for a strong base- weak acid titration
Phenolphthalein
What indicator would you use for a weak acid - weak base titration
Neither methyl orange or phenolphthalein is suitable
Neither give a sharp change at the end point
Why does the ph equal the pka at the half neutralisation point?
Define a buffer solution
A solution that resists changes in ph when small amounts of acid/alkali are added
What do acidic buffer solutions contain
A weak acids and a soluble salt of that acid that fully dissociates
What happens if (H+) are added to an acidic buffer?
H+ reacts with the organic acids ion
The organic acids ion is in high concentration due to the soluble salt of that acid
This means that eqm shifts to the left to compensate for the increase in H+
so CH3COOH is produced more
What happens when we add a base (OH-) to an acidic buffer
The OH- ions react with the h+ ions in the solution producing water
There is a low concentration of these ions within the solution, so to compensate the eqm shifts to the right to utilise more of the organic acid.
What does a basic buffer contain?
Weak base and a soluble salt of that weak base that fully dissociates
What happens if you add a base (Oh-) to a basic buffer
OH- reacts with the basic ion
The basic ion is in high concentration due to the soluble salt of that base
This means eqm of weak base has shifted to the left to compensate for the change, producing more of the weak bace and water
What happens when you had H+ to a basic buffer
The H+ ions react with the OH- ions in the solution producing water
There is a low concentration of these ions within the solution, so to compensate the eqm shifts to the right to utilise more of the Base and water .
Write the equibrium reactions for an acidic buffer.
Use Ethanoic acid and Sodium ethanoate
Write the equilibrium equations for a basic buffer
use Ammonia and Ammonium Chloride
Write the reaction for an acidic buffer with added acid
A- + H+ —-> HA
Write a reaction for an acidic buffer with added alkali
H+ + Oh- —> H2O
How to you form an acidic buffer other then mixing the constituents
Neutralise half of a weak acid with an alkali, this forms a weak acid / soluble salt mixture
When calculation the ph of a buffer what assumptions do we make?
Salts are dissociated fully
and weak acids dissociate very poorly, (conc of acid at start of calc = conc of acid )
[Salt] = [A-] (conc of salt is this initial concentration of A- ions)
[HA start] = [HA Eqm] (conc of acid is taken as it’s initial conc)
Where are buffer solutions found in commercial products?
Shampoo - stops hair becoming alkali and therefore dry
Washing powder - contains enzymes that require specific ph’s
Blood
Why does KW increase as temperature increases
Equilibrium is endothermic
Equilibrium shifts to the right to minimise temperature change
H20 (reversible) H+ + OH-
which increases the kw equation
Both Acids are in the form of RCOOH
ethanoic acid R = CH3
ethanediotic acid R =COOH
COOH on ethanediotic acid contains two very electronegative oxygen atoms
has a negative inductive effect compared with the CH3 group’s positive inductive effect
as a result O-H bond in ethanediotic acid is more polarised resulting into more H+dissociation
ethanediotic acid is stronger
As it hasn’t reacted with anything [H+] = [A-]
C
Propanoic acid has more of the negative inductive effect then Methanol
So there’s the addition of more (OH-) in solution then (H3O+)
So higher PH
Smaller PH
Magnesium Hydroxide is less double then Calcium Hydroxide
So it will Dissociate until it’s ions less readily in water
In Pure water [H+] = [OH-]
So Ratio of A- : HA remains constant as Equilibrium shifts equally in both directions
As H+ = Ka[HA] / [A-]
H+ stays constant, so Ph stays constant
The Ph of barium hydroxide is lower at 50 Degrees then at 10 degrees
At 50 degrees a 25cm^3 sample was neutralised by 22.45Cm^3 of HCL
Would the volume of hydrochloric acid added to 25Cm^3 of Barium hydroxide at 10 degrees be lower/the same/ higher then at 50 degrees
Explain your answer
The same —> Same number of [OH] ions
Explanation (No marks) :
Neutralisation is when all of the [H+] ions have reacted with [OH-] ions
Barium hydroxide at different a higher PH means that more OH- ions have been dissociated from the positive barium ion, but the amount of [OH] ions in solution are exactly the same.
So The same amount of Acid is needed
State how a buffer solution can be made from solutions of Potassium Hydroxide and Ethanoic acid
How does this solution resist a change in PH if a small acid is added
Add sufficient KOH so that the acid contains a mixture of Ethanoic acid and Ethanoate ions.
Soluble salt/weak acid mixture
Ethanoate ion reacts with the H+ to form CH3COO
This shifts the equilibrium to the left as in a weak acid changes in conc of conjugate bace and h+ will has a greater effect then changes in conc of the acid. , utilising more of the H+ ions in solution to form Ethanoic acid. as H+ has a lower conc then A- increase in H+ have the most effect
Name something you always forget about this topic
Ka expression you need to divide by the total volume!!!!!!!!!!!!!!!!!!!
Which structures are held together by disulphide bridges
Tertiary and Quaternary
It takes me X Cm^3 of NaOH to neutralise a monoprotic acid
Explain a method to calculate the original concentration of HX
In the reaction OH- ions react with the Acid to produce a salt + water
Therefore mol of OH- ions = mol of acid in the original solution
