Acids And Bases Flashcards
What did arrhenius describe an acid as?
A substance that contains hydrogen and releases hydrogen ions when it dissolves in water
What is a strong acid?
An acid that dissociates (ionises) fully in water. There would be no molecules of e.g. Hydeogen chloride present
What is a weak acid?
An acid that does not dissociate fully in water. The solution is an equilibrium mixture that contains e.g. ethanoic acid molecules, hydrogen ions and ethanoate ions
What does arrhenius’ theory state that a base is?
A substance that reacts with H+ to form water
How do you write the reaction between magnesium oxide and hydrogen chloride?
MgO(s) + 2HCl (aq) -> Mgcl2 (aq) + H2O (l)
Or its ionic form
MgO(s) + 2H+ (aq) -> Mg2+(aq) + H2O(l)
How does a hydrogen ion exist in water?
It forms a bond with a water molecule to form a hydronium ion (H3O+)
How would you write the reaction of HCl with water taking into account hydronium ions?
HCl (aq) + H2O (l) -> H3O (aq) + Cl-
How is the general reaction of an acid with an alkali written?
H3O+(aq) + OH- (aq) -> 2H2O (l)
What did Johannes Bronsted and Thomas Lowry define an acid as?
A substance that donates protons in a reaction. It is a proton donor
What dis Bronsted and Lowry define a base as?
A substance that accepts protons in a reaction
It is a proton acceptor
What is a neutralisation reaction?
When an acid and a base react to form a salt and water only. The base accepts one or more hydrogen ions from the acid
What does an ionic equation have?
Everything is written as it’s ions and the ions that appear on both sides of the equation are removed
What are spectator ions?
Ions that do not take part in the reaction. They appear on both sides of the equation
What is the conjugate base of an acid?
The negative ion of the acid that acts as a base as it recombines with the hydrogen ion to reform the acid molecule in a dynamic equilibrium
What is the conjugate acid?
When a base e.g. NH3 reacts with H+ to form a positive ion such as NH4+ the positive ion formed is called the conjugate acid
What do the terms strong and weak refer to?
The extent of dissociation
What do the terms concentrated and dilute refer to?
The amount of acid in solution
If the equilibrium lies very far to the right is the acid a strong or weak acid?
Strong
What is the pH scale and what is it used for?
It is a log scale making the numbers easier to handle. It is used to measure acidity
How do you work out pH?
pH = -log10 [H+]
Where H+ is the molar hydrogen ion concentration of the solution
For strong acids how do we calculate the pH?
We do -log10[H+] where [H+] is equal to the concentration of the acid
pH indicators and pH meters are used to obtain values of pH for the solution
How does the concentrarion vary when you move up or down the pH scale by one unit?
The hydrogen ion concentration changes by a factor of 10
How is the strength of a weak acid determined?
By the position of equilibrium. It can be described using equilibrium constants. These are called acid dissociation constants Ka
How are dissociation constants calculated?
In a similar way to equilibrium constants for reversible reactions, with the products divided by the reactants
What is the general expression for the dissociation constant Ka of any weak acid of general formula HA?
HA H+ + A-
Ka = [H+][A-]/ [HA]
Where all the concentrations are those at equilibrouk and the units are mol dm -3
How do you use Ka to find the value of pKa and how do you use pKa to find the value of Ka?
pKa = -log10Ka
Ka = 10^-pka
What happens in regard to equilibria with diprotic acids?
Two equilibria are established in solution and each one has an equilibrium constant
What is a diprotic acid?
An acid that can release more than one H+ ion
Why can we assume that H2O is a constant?
Because water only dissociates to a small extent and undissociated water molecules are present in much greater numbers than the ions
What is the expression for the ionic product of water Kw and whats it’s value and 298 K?
Kw = [H+][OH-]
= 10 z14 mol2 dm-6
Why is a solution of water always neutral?
Because the concentration of hydrogen ions equals the concentration of hydroxide ions
How do you calculate the pH of pure water?
You square root the ionic product of water Kw (divide the power by two)
Is the ionisation of water endothermic or exothermic?
Endothermic
What happens when a strong base is dissolved in water?
It produces hydroxide ions
What do we need to do to calculate the pH of an alkaline solution?
1) calculate the concentration of hydroxide ions OH-
2) use the ionic product of water Kw to calculate the concentration of hydrogen ions
3) calculate the pH from the hydrogen ion concentration
What js a pH curve?
A plot on a graph that shows measured pH against quantity of solution added (in a titration)
How would you carry out a pH titration of a strong base with a weak acid?
1) calibrate the pH meter using solutions of known pH
2) use a pipette to measure the required volume of the weak acid into a beaker
3) set up a magnetic or mechanical stirrer to mix the contents of the beaker when base is added. If such a stirrer is not available the solution can be stirred with a stirring rod after each addition before the pH is measured
4) fill the burette with the strong base that will be added to the acid
5) measure the pH of the weak acid
6) add 1.0 cm3 quantities of the base to the acid, stirring between additions and measuring the pH before adding the next quantity of base. Continue adding until there is no further change of pH
7) plot a graph of ph against volume of base added
What does the pH curve look like for a strong acid with a strong base and explain it in regard to a strong base being added
Check snap camera roll
- initially when a small volume of OH- is added to the acid the proportion of H+ removed is quite small compared with the total amount of H+ present. The increase in pH is very small
- as more OH- is added the proportion of H+ being removed each time relative to the total amount increases so there is a greater increase in pH
- the change becomes greater when the end point is close
- the graph then shows a very steep rise and at the end point the pH is 7 ( pH of pure water)
What is the pH curve for a weak acid with a strong base and describe it
Check snap camera roll
- The pH curve starts at a higher pH value, since the weak acid is only slightly dissociated producing a low [H+] at equilibrium
- CH3COO- acts as a weak conjugate base and recombines with H+ to form CH3COOH.
- the end point pH is greater than 7
What does the pH graph look like for a strong acid with a weak base and explain it
Check snap camera roll
- it baisically follows the same pattern for a strong acid but rhe end point is at a pH of less than 7
- an equilibrium is set up between the weak base and its conjugate acid
- this increases the concentration of H+ and lowers the pH compared with that for a solution of a strong base
What does the pH curve look like for the titration of a weak bade with a weak acid and explain it
Check snap camera roll
When a weak base is titrated with a weak acid the variation in pH with the volume of titre is more gradual and it is not possible to drtect a accurate end point so a pH meter should be used
What are acid base indicators?
They are solutions of compounds that have different colours below and above a specific concentration of H+. They are weak acids (usually written Hln) wnere the dissociated form ln- and the undissociated form Hln have different colours
What is the equilibrium for a reaction with undissociated and dissociated indicators?
Hln H+ + ln-
When should the indicator change colour?
When there is a large change of pH e.g. at the end point of reaction. This is because if an acid is added or is neutralised by a base during a titration then the concentration of [H+] changes.
What is the end point of the titration?
When the concentrations of the acid and base forms of the indicator are equal
Why does the colour of one form of the indicator need to be intense?
So that only two or three drops can be used to give a clearly visible change and so this doesn’t affect the concentration of [H+]
When choosing an indicator what should you consider?
A suitable indicator must change colour in the pH ranhe that corresponds to the steep part of the pH curve, where there is a marked change in pH and the indicator will change colour completely.
What is the best way to choose an indicator for a titration?
Look at the pH curve for the particular titration and choose an indicator where the pH range of activity for that indicator matches the end point of the titration
What is a solution that can resist a change in pH when small amounts of acid or base are added?
A buffer
What do buffers generally contain?
Either:
• a weak acid and a large amount of a strong base salt of the acid (an acid buffer) e.g. ethanoic acid with sodium ethanoate
• a weak base and a large amount of a strong- acid salt of the base (an alkaline buffer) e.g. ammonia with ammonium chloride
What do buffers generally contain?
Either:
• a weak acid and a large amount of a strong base salt of the acid (an acid buffer) e.g. ethanoic acid with sodium ethanoate
• a weak base and a large amount of a strong- acid salt of the base (an alkaline buffer) e.g. ammonia with ammonium chloride
What happens if
H
How does a mixture of ethanouc acid, CH3COOH and ethanoate, CH3COO- work as a buffer?
Ethanoic acid partially dissociates in water. Sodium ethanoate CH3COONa dissolves and dissociates completely in water:
CH3COOH CH3COO- + H+
CH3COONa -> CH3 COO - + Na+
- if an acid is added to the buffer the equilibrium adjusts as H+ combines with some of the large amount of CH3COO- to give more undissociated acid and the position of equilibrium shifts to the left
- if alkali is added to the buffer the equilibrium adjusts as OH- reacts with H+ neutralising it while some of the large amount of CH3COOH present dissociates to give more H+. The position of Equilibrium shifts to the right
What does a buffer do?
Maintains the equillibrium reaction and the pH remains steady except in the presence of large quantities of acid or alkali. Ka remains constant throughout
