Unit 1.7 - Simple Equilibra and acid-base reactions Flashcards
What are the characteristics of chemical equilibrium?
- The reaction is reversible
- The equilibrium is dynamic (i.e - the reaction is still proceeding)
- The rate of the forward reaction is equal to the rate of the backward reaction
- At equilibrium, properties such as concentration and pressure are constant
- Equilibrium can only be attained in a closed system
Symbol for reversible reaction
—>
<—
—>
<— meaning
reversible reaction
What does it mean that the equilibrium is dynamic during chemical equilibrium?
The reaction is still proceeding
Dynamic equilibrium definition
A reversible reaction where the rate of the forward reaction is equal to the rate of the backward reaction
What are the observations we can make due to the fact that the rate of the forward reaction is equal to the rate of the backward reaction in chemical equilibrium?
No observable change in the system
Which properties are constant in chemical equilibrium?
Pressure and temperature, for example
In what type of system only can equilibrium be attained?
Closed
What’s a fizzy drink can an example of?
A closed system within which equilibrium can be attained
Example of a closed system
Fizzy drink can
Why would there be no observable change in a system in chemical equilibrium?
The rate of the forward reaction is equal to the rate of the backward reaction
What is “the rate of the forward reaction is equal to the rate of the backward reaction” the definition for?
Dynamic equilibrium
Compare a system in dynamic equilibrium in different conditions
Fixed conditions - stable
Sensitive to alterations in conditions
When is a system in dynamic equilibrium stable?
Under fixed conditions
What are the reactants and products constantly doing in a reversible reaction?
Reactants - constantly forming products
Products - constantly forming reactants
Describe the amount of products and reactants in a system in dynamic equilibium
Constant, not equal
Are the amount of reactants and products in a system in dynamic equilibrium equal?
No, but they are constant
What is a system in dynamic equilibrium sensitive to?
Alteration in conditions
Homogenous system
All the substances are in the same physical state (e.g - g)
What type of system is one where all of the substances are on the same physical state?
Homogenous
The Haber process equation + what is it an example of?
N2 (g) + 3H2 (g) —> 2NH3 (g)
<—
A homogenous system (all g)
Give two examples of homogenous systems
- Formation of ammonia in the haber process
- the esterification between an organic acid and an alcohol
Heterogeneous systems
The substances are in two or more physical states
What type of system is one where the substances are in two or more physical states?
Heterogeneous system
Le Chatelier’s Principle
If a system at equilibrium is subjected to a change, the position of equilibrium will move to counteract the change imposed
“If a system at equilibrium is subjected to a change, the position of equilibrium will move to counteract he change imposed” - What is this?
Le Chatelier’s Principle
Position of equilibrium
The equilibrium state under a particular set of conditions
What are the factors that affect the position of equilibrium?
- Catalysts (but not really)
- Concentration
- Pressure
- Temperature
What do the following factors all affect - catalyst, concentration, pressure and temperature?
The position of equilibrium
How do catalysts affect the position of equilibrium?
They increase the rates of the forward and backward reactions to the same extent, therefore they have no effect on the position of equilibrium
How does concentration effect the position of equilibrium?
It moves towards what we want more of (the thing that HASN’T increased in concentration)
If we have more reactants than products, how does the position of equilibrium move?
Starts at the left, so moves to the right to form more products
Why does the position of equilibrium move when the individual concentrations of elements are changed in a reaction?
To remove the change
(Reduce this concentration again)
In what type of reactions would the total pressure affect the position of equilibrium?
Gaseous reactions
How do we move the position of equilibrium if the pressure within a system is increased?
Moves it towards the side with the fewest GASEOUS molecules
(Compare the sum of the molecules - big numbers - on both sides)
What do we need to take notice of in order to move the position of equilibrium when the total pressure has changed?
The number of molecules on both sides of the equation (big numbers)
What does a greater number of gas molecules lead to?
A greater pressure
What leads to a greater pressure?
A greater number of gas molecules
Why do we move the position of equilibrium to the side containing the least molecules if the total pressure of a reaction has increased?
As it reduces the amount of molecules in a given volume (i.e - the pressure)
What is increasing the volume the same as doing?
Reducing the pressure
What is reducing the pressure the same as doing?
Increasing the volume
If the volume of a system is increased, what happens to the position of equilibrium and why?
It moves to the side containing the most molecules, as this will increase the pressure
(Increasing the volume is the same as reducing the pressure, so the system needs to try to increase it like this)
Enthalpy change symbol
ΔH
ΔH meaning
enthalpy change
What does the Enthalpy change show?
The value of energy of the forward reaction
Which reaction does the Enthalpy change always show the value of energy for?
Forward reaction
What does a negative Enthalpy change value represent?
An exothermic reaction
What does a positive Enthalpy change value represent?
An endothermic reaction
Exothermic reaction shown as the enthalpy change
Negative value
Endothermic reaction represented by the enthalpy change
Positive value
What do endothermic reactions do and what are they favoured by?
Absorb heat
favoured by increasing the temperature
What do exothermic reaction do and what are they favoured by?
Release heat
Favoured by decreasing the temperature
If the temperature increases in a reaction, what does the position of equilibrium do and why?
Moves it in the direction of the endothermic reaction to reduce it
Why does moving the position of equilibrium towards the endothermic reaction reduce the affect of the change in temperature when it’s increased?
During an endothermic reaction energy is transferred from the surroundings
How do you calculate the concentration of an acid if given its pH?
1- Concentration of H+ ions (protons)
2- Molar ratio
What type of acid is ethanoic acid?
Organic
Which acid is an organic acid?
ethanoic
What type of acids are sulphuric, hydrochloric and nitric acid?
Mineral/inorganic
Which acids are mineral/inorganic?
Sulphuric, hydrochloric and nitric acid
Electrolyte
A substance which will conduct electricity in an aqueous solution (or molten)
What does an electrolyte conduct electricity in?
Aqueous solution or molten
What can’t ions do in a solid?
Move
Strong electrolyte
Is largely dissociated into its ions in aqueous solution
What are largely dissociated into its ions in aqueous solution?
Strong electrolytes
Strong electrolytes examples
Soluble ionic compounds…
1. All salts which are soluble in water
2. Mineral/inorganic acids - sulphuric, hydrochloric and nitric acid
3. Alkalis such as sodium hydroxide
(Aq) meaning
Water molecules surrounding it
Dissociated
Split into its ions in solution
What’s the word for something being split into its ions in solution?
Dissociated
State symbol for water molecules completely surrounding something
(aq)
What is the % of electrolytes split into ions if they’re strong electrolytes? Why?
100%
Completely dissociated into its ions
Monophonic/monobasic
Releases 1 proton
Word for releases 1 proton
Monophonic/monobasic
Diprotic
Releases 2 protons
Word for releases 2 protons
Diprotic
Proton symbol
H+ (aq)
H+ (aq) meaning
Proton
What does every acid, strong or weak, give out?
Protons
What does every kind of itself give out protons?
Acids
Weak electrolyte
Is largely undissociated into its ions in aqueous solution - the degree of dissociation is often less than 1%
What’s the degree of dissociation for weak electrolytes?
Often less than 1%
What’s largely undissociated into its ions in aqueous solution?
Weak electrolytes
Examples of weak electrolytes
- Organic acids (e.g. - ethanoic acids)
- Ammonia (weak base)
- Water
What is ammonia?
A weak base
What does “weak” and “strong” refer to when discussing acids?
The amount of dissociation
Which phrases are used to describe the amount of dissociation in acids?
Weak or strong
Non-electrolytes
Do not conduct electricity and are not dissociated to any appreciable extent into ions in aqueous solution
What do not conduct electricity and aren’t dissociated to any appreciable extent into ion in aqueous solution?
Non-electrolytes
Examples of non-electrolytes
- Sugar
- Ethanol
Lowry-Bronsted theory
An acid is a substance which donates protons in aqueous solution
A base is a substance which accepts protons in aqueous solution
What is an acid? What is this based on?
A substance which donates protons in aqueous solution
Lowry-Bronsted theory
What is a base? What is this based on?
A substance which accepts protons in aqueous solution
Lowry-Bronsted theory
In what type of solution are protons accepted and donated with acids and bases?
Aqueous
What does an acid give out?
A base and a proton
Which substance donates protons in aqueous solution?
Acids
Which substance accepts protons in aqueous solution?
Bases
Can we permanently decide whether a chemical is an acid or a base?
No, only for that particular reaction in which is takes part
Describe strong acids and bases
Almost completely dissociated into their ions
Describe weak acids and bases
Largely undissociated into their ions in aqueous solution
What does the concentration of an acid refer to?
The amount of moles of acid present in a given volume
What is the amount of moles in a given volume if acid?
Its concentration
Concentrated acid
Large amount of moles of the acid present
Dilute acid
Small amount of moles of the acid present
What type of acid has a large amount of moles of the acid in it?
Concentrated
What type of acid has a small amount of moles of the acid present?
Dilute
What could be technically used as an indicator of acidic strength?
Ka - the acid dissociation constant
What can the acid dissociation constant be used as?
An indicator of acidic strength
What’s wrong with using the acid dissociation constant?
The numbers are difficult to handle
What’s easier to follow than using the acid dissociation constant to indicate acidic strength?
Hydrogen ion concentration expressed as pH
What is pH a measure of?
The [H+ (aq)] concentration
pH definition
-log10 [H+ (aq)]
(small 10)
What’s good about the pH scale?
A simple and widely applicable method to measuring the acidity, alkalinity or basicity of a substance in aqueous solution
What’s the pH scale used for?
Measuring the acidity of a substance in aqueous solution
Measuring the alkalinity or basicity of a substance in aqueous solution
What happens as the pH increases by 1 on the pH scale?
Concentration of H+ ions decreases by a factor of 10
(Lower pH —> higher concentration of H+ ions —> more acidic)
Relationship between the amount of H+ ions in an acid and its acidity
Low pH = higher concentration of H+ ions = more acidic
What’s the concentration of H+ ions in low pH levels and what does this mean?
High = more acidic
What can pH be measured using?
pH meter
Universal indicator
Positives of using a pH meter to measure pH
Accuracy - measures to within +-0.01 of a pH
Negatives of using a pH meter to measure pH
Often needs to be calibrated against a solution of known pH before use
Positives of using a universal indicator to measure pH
Paper or solution
Quick and cheap
Negatives of using a universal indicator to measure pH
Not especially accurate
What is a universal indicator ideal for measuring?
Soil pH
What is pH dependant on?
Concentration and temperature of a substance
What are pH values usually quoted on in terms of temperature?
25 degrees Celsius
What can be done in terms of pH with strong acids and why?
Why can’t this be done with weak acids?
Can calculate the pH directly if the concentrations of the solutions are known - they’re assumed to be totally dissociated into their ions
Cannot be done so directly with weak acids - only a small fraction of the molecules are dissociated into ions
Are negative pH’s possible?
Yes
What’s the first thing to do when answering pH based questions?
Write down how it dissociates into its ions
How do you calculate the pH of a substance from its concentration?
- Write down how it dissociates into its ions
- Molar ratio comparison between the starting reactant and the H+ ions
- Use the equation -log10[H+(aq)]
How do you calculate the hydrogen ion concentration from pH values?
[H+] = 10^-pH
Equation for calculating the hydrogen ion concentration of a substance
[H+] = 10^-pH
Concentration shorthand
[ ]
What does [ ] mean?
Concentration shorthand
How do you calculate the concentration of an acid from its pH?
- Calculate the hydrogen ion concentration with [H+] = 10^-pH
- Molar ratio comparison between the acid and H+ ions
How can equilibrium position be described in precise terms?
By combining the equilibrium concentrations to give the value of an equilibrium constant
Equilibrium constant symbol
Kc
What’s Kc?
The equilibrium constant
What’s c in Kc (the equilibrium constant)?
Ratio of concentrations
Equilibrium constant equation
Kc = [products]
——————
[reactants]
(Remember square brackets means concentration!)
what are concentrations raised to the power of when calculating Kc?
The molar ratios
What do we do with the molar ratios (big numbers) when calculating Kc?
Make them powers
What do we do if there’s no unit for Kc?
Physically write “No unit”
How do we work out the unit of Kc?
Remember the indices rule - dividing = minus
So, count the amount of moldm^-3 you have at the top and bottom and minus them
The answer goes to the mol section, and the dm’s original ^-3 is multiplied by this
What do we do if we get the small number on top when calculating Kc’s unit?
Do the reciprocal…
Not moldm^-3 but
dm^-3mol-1
If the value of Kc is larger than 1, what does this tell us?
More products than reactants in the equilibrium mixture - position of equilibrium lies to the right
If the value of Kc is smaller than 1, what does this tell us?
More reactants than products in the equilibrium mixture - position of equilibrium lies to the left
If the position of equilibrium lies to the left, what type of value would Kc be?
Smaller than 1
If the position of equilibrium lies to the right, what type of value is Kc?
Larger than 1
What can change the value of Kc? Why?
ONLY changes in temperature
The value of Kc is constant for a particular equilibrium reaction at constant temperature
What is the value of Kc constant for?
A particular equilibrium reaction at constant temperature
What doesn’t change Kc’s value? What do these do instead?
Concentration and pressure
Can only shift the position of equililbrium
What can and can’t change the value of Kc?
Can - temperature
Can’t - concentration and pressure
What does Kc give us information about?
Concentrations achieved at equilibrium
What does Kc tell us nothing about?
How quickly equilibrium is reached
When do we do a reciprocal with the units of Kc?
When the number on top is smaller than the bottom
When we have an answer for the indices for the unit of Kc that’s bigger than 1 or zero, what do we do?
The answer goes to the mol
Multiply the answer with the original dm^-3
If the values of Kc are all very similar in an experiment, can it be considered constant?
Yes, as there’s a margin for error with experimental data
What’s it important to state if it’s included in the question in terms of equililbrium reactions?
The colour change
When is a base called an alkali?
When dissolved in water
What’s called an alkali if dissolved in water?
A base
What do you call compounds or molecules that can act as both an acid and a base?
Amphoteric
Amphoteric
A compound or a molecule that can act as both an acid and a base
Where is ethanoic acid found?
In vinegar
What type of acid does vinegar contain?
Ethanoic acid
What forms with weak electrolytes and why?
An equilibrium as it only partly dissociates
Which type of electrolyte forms equililbrium and why?
Weak, as if only partly dissociates
What is Ka?
The acid dissociation constant
How do you calculate the pH of an alkali?
14-(-log10 [OH-])
What’s 14-(-log10[OH-]) for?
Calculating the pH of an alkali
Are there protons involved in an base reaction?
A small amount, as the base dissolves in water
Diprotic acid example
H2SO4 (sulphuric acid)
What do we need to remember to do with Diprotic acids? Why?
Multiply their concentration by 2 in a pH calculation - they release 2 H+ ions per molecule
Why do we need to multiply the concentration given by 2 when calculating pH of Diprotic acids?
They release 2 H+ ions per molecule
What do Diprotic acids do?
Release 2 H+ ions per molecule
What release 2 H+ ions per molecule?
Diprotic acids
What actually are all of the examples of strong electrolytes?
Soluble ionic compounds
What type of analysis is an acid base titration?
Volumetric
What’s measured during an a acid-base titration?
The volume of one solution (e.g - acid) that reacts exactly with a known volume of another solution (e.g - a base)
How do we measure the precise point of neutralisation using an acid-base titration?
Using an indicator
What does an indicator help us do during an acid base titration?
Measure the precise point of neutralisation
What are acid base titrations used for?
-The preparation of salts
-Measuring exact concentrations of acid or base solution
What must we have in order to measure the exact concentration of an acid or base solution?
One of the solutions must have been standardized
Why must one of the solutions be standardized during an acid base titration?
To measure EXACT concentrations of acid or base solution
What’s a standard solution used for?
Finding out about the substances dissolved in the other solution
What’s used to find out about a substance dissolved in the other solution?
A standard solution
How is a standard solution made?
By dissolving an accurate mass of reactant into a known volume of water
What does dissolving an accurate mass of reactant into a known volume of water give us?
A standard solution
What does a standard solution give us?
An accurate concentration that can be used to work out the concentration of another solution that it reacts with
What gives us an accurate concentration that can be used to work out the concentration of another solution that it reacts with during a titration?
A standard solution
Features of a primary standard
- High purity
-Stability
-Low hygroscopicity
-High molar mass
What are high purity, stability, low hydrgoscopicity and high molar mass all features of?
A primary standard
Hygroscopicity
The tendency of a solid substance to absorb moisture from the surrounding atmosphere (e.g - CO2 and H2O)
What’s the tendency of a solid substance to absorb moisture from the surrounding atmosphere?
Hydroscipicity
What’s an example of a substance that cannot be used as a primary standard and why?
Sodium hydroxide
Reacts with atmospheric CO2
What can’t sodium hydroxide be used as and why?
A primary standard - reacts with atmospheric CO2
Give 2 examples of primary standards commonly used in Chemistry
- Potassium hydrogen phthalate (KHP)
- Sodium carbonate
What are Potassium Hydrogen Phthalate (KHP) and Sodium Carbonate examples of?
Primary standards commonly used in Chemistry
What’s potassium hydrogen phthalate (KHP) used for? Why?
The standardisation of aqueous base solutions
(It’s a stable acid, and its concentration doesn’t change over time)
KHP
Potassium hydrogen phthalate
Which primary standard is often used for the standardisation of aqueous base solutions and why?
Potassium hydrogen phthalate (KHP)
stable acid, concentration doesn’t change over time
What’s sodium carbonate used for?
The standardisation of acid solutions
Which primary standard is often used for the standardisation of acid solutions?
Sodium carbonate
What’s the apparatus required for preparing a standard solution?
Volumetric flask
Funnel
Digital scales
Weighing bottle
Beaker
Glass rod
How much standard solution does our method help us make?
250cm^3
What’s the first step of preparing a standard solution?
Accurately weigh a known amount of the solid sample in a weighing bottle to 3d.p
How many decimal places do we accurately weigh the known amount of solid sample for a standard solution?
3 d.p
What do we do after we’ve accurately weighed our known amount of solid sample in a weighing bottle when preparing a standard solution?
Transfer all the solid into a 250cm^3 beaker then add no more than 150cm^3 distilled water and stir with a glass rod until the solid has completely dissolved
Size of the beaker the solids are transferred into during an acid base titration
250cm^3
How much distilled water do we add to the accurately weighed solid for a standard solution?
No more than 150cm^3
What do we need to remember to do when transferring the contents of the beaker with the solids to the volumetric flask when preparing a standard solution? Why?
Don’t remove the glass rod when pouring - will lose ions
Why shouldn’t we remove the glass rod when transferring contents from the beaker to the volumetric flask when preparing a standard solution?
We will lose ions
How could we lose ions carelessly when preparing a standard solution?
Removing the glass rod when transferring form the beaker to the volumetric flask
What do we do after dissolving the solid in distilled water when preparing a standard solution?
Use a funnel to transfer the contents of the beaker to a 250cm^3 volumetric flask
How do we transfer the contents of the beaker to the volumetric flask when preparing a standard solution?
Using a funnel
What’s the volume of the volumetric flask used when preparing a standard solution?
250cm^3
What do we do after transferring the contents of the beaker to the volumetric flask when preparing a standard solution?
Rinse the beaker and glass rod thoroughly, at least 3 times, with distilled water and carefully pour the rinsings into the volumetric flask
What do we need to rinse when preparing a standard solution?
The beaker and the glass rod
What do we do with the rinsings of our beaker and glass rod when preparing a standard solution?
Pour them into the volumetric flask each time
How many time do we rinse the beaker and glass rod when preparing a standard solution?
At least 3 times
What do we do after rinsing the beaker and glass rod when preparing a standard solution?
Carefully add distilled water to the volumetric flask until the bottom of the meniscus is level with the horizontal line on the neck of the flask - the flask and graduation mark must be at eye level when the final drops are added using a Pasteur pipette
How do we know how much distilled water to add to the volumetric flask when preparing a standard solution?
Add until the bottom of the meniscus is level with the horizontal line on the neck of the flask
What must be at eye level when adding the final drops of distilled water to the volumetric flask when preparing a standard solution?
The flask and graduation mark
How is the distilled water added to the volumetric flask when preparing a standard solution?
Using a Pasteur pipette
What must the flask and graduation mark be at when adding the final drops of distilled water when preparing a standard solution?
At eye level
What’s the final step of preparing a standard solution?
Holding the stopper into the neck of the flask, carefully turn the flask upside down several times to thoroughly mix the solution
How do we thoroughly mix the solution when preparing a primary standard?
Hold the stopper into the neck of the flask, carefully turn the flask upside down several times
What’s the apparatus required for performing a titration?
-Burette (+stand and clamp)
-Pipette
-Funnel
-Conical flask
-White tile
What’s the volume of the pipette used during a titration?
25cm^3
What’s the volume of the burette used during a titration?
50cm^3
What’s the purpose of performing a titration?
To work out the concentration of an acid by titrating it against a standard solution of an alkali
What’s the first step of performing a titration?
Using a funnel, pour a small volume of the acid into the burette to rinse it out
What do we use to rinse out the burette?
Use a funnel to pour a small volume of acid in
What do we do after rinsing out the burette when performing a titration?
Fill the burette with the acid using a funnel (make sure the jet is filled) and remove the funnel before recording the initial burette reading
What do we use to fill up the burette with acid and what must we remember about it?
A funnel
Remember to remove it before recording the initial burette reading
What do we do before recording the initial burette reading?
Remove the funnel
What 2 things must we make sure before taking the initial burette reading when performing a titration?
-the funnel has been removed
-the jet is filled
What do we do after recording the initial burette reading when performing a titration?
Rinse the volumetric pipette with a small volume of alkali solution
What type of pipette is used during a titration?
Volumetric
What’s the volumetric pipette rinsed with during a titration?
Alkali solution
What’s the burette rinsed with during a titration?
Acid solution
What do we do when performing a titration after rinsing the volumetric pipette with a small volume of alkali solution?
Accurately pipette 25cm^3 of alkali and place it into a conical flask
How much alkali do we pipette with the volumetric pipette during a titration?
25cm^3
Where do we place the alkali that we pipette with a volumetric pipette during a titration?
Into a conical flask
What do we do after adding the alkali to the conical flask when performing a titration?
Add 2/3 drops of a suitable indicator
How much indicator do we add when performing a titration?
2/3 drops
Give 2 examples of suitable indicators for a titration
Phenolphthalein or methyl orange
What are both phenolphthalein and methyl orange?
Indicators
What do we do after adding the indicator to the alkali solution in a conical flask during a titration?
Open the tap on the burette to start adding the acid to the conical flask. Swirl the mixture continuously the mix it.
How do we ensure that the acid from the burette and the alkali from the conical flask are mixing?
Swirl the mixture continuously
What do we open to release the acid into the alkali in the conical flask during a titration?
The tap on the burette
What does opening the tap on a burette do?
Release the acid to the alkali in the conical flask
What do we do when the acid is being added to the alkali in the conical flask during a titration?
Whilst titrating the standard solution with the acid, the colour of the indicator will start to change. When this happens, slow down the flow of acid from the burette so that it is dripping into the conical flask one drop at a time.
What do we do when the colour of the indicator starts to change during a titration?
Slow down the flow of acid from the burette so that it’s dripping into the conical flask 1 drop at a time
What is the end point of a titration?
When 1 drop of acid indicator permanently changes the colour of the indicator
What’s the point when 1 drop of acid will permanently change the colour of the indicator?
The end point
What helps us see the end point of a titration? What is the end point?
A white tile, as it’s when 1 drop of acid will change the indicator colour permanently
What do we do after the end point of a titration?
Record the volume of acid needed to neutralise the alkali
Subtracting the volume at the beginning from this gives us our titre
How do we get our titre?
By subtracting the volume at the beginning from the volume at the end
What does subtracting the volume of acid in the burette at the beginning from the volume needed at the end give us?
Our titre
What’s the final step of performing a titration?
Rinse out the conical flask, top up the acid in the burette and repeat the steps to get 2 concordant results (within 0.2cm^3 of each other)
How many times do we repeat the titration?
Until we get 2 concordant results (within 0.2cm^3 of each other)
Concordant results (titration)
Within 0.2cm^3 of each other
What do we call results that are within 0.2cm^3 of each other during a titration?
Concordant results
Why can’t we use standard titration methods sometimes? What do we do instead?
- Reaction between determined substance and titration can be too slow
- Problems with the end point determination
- Base could be an insoluble salt \
Use a back titration instead
What would factors like the reaction between determined substance and titration being too slow, problems with the end point determination and a base being an insoluble salt lead to?
Not being able to use standard titration methods, and therefor have to use a back titration
What happens during a back titration?
Known excess of one reagent A reacts with an unknown amount of reagent B, and at the end of the reaction, the amount of reagent A that remains is found by titration
A simple calculation gives the amount of reagent A that’s been used and the amount of reagent B that’s reacted
Why is a back titration called a back titration?
Have to track back with calculations
What are the steps of a back titration calculation?
- Write out acid/base titration reaction
- Calculate the moles of the thing you have concentration and volume of (usually the alkali) - from the titration!
- Use this to calculate the moles of the acid
- If only 25cm^3 used, but 250cm^3 batch made, must x10
- Calculate the original moles of acid (in excess from 1st reaction)
- Original moles (ans 5) minus moles left over from reaction (ans. 3 or 4) - gives the moles of acid that were used in the reaction
- Write out the original reaction (not the titration reaction)
- Use the moles that reacted (ans 6) and the ratio in the original reaction to calculate moles of whatever the acid is reacting with
- Use this to calculate the mass (m = n x M)
- Calculate the % (if required) —> mass calculated
———————— x100
Original mass of object (e.g - tablet)
How do you work out the moles of acid that were used in the reaction? (back titration)
Original amount of moles - moles left over from reaction (from titration)
Why would you use a large conical flask for the back titration practical?
To prevent acid from escaping
What do we do when calculating the mean titre?
-Don’t necessarily go for the last 3 titres
-Don’t include irregular values
What are some possible reasons for incorrect titres? (5)
Errors while reading the burette
Difficult to see the indicator’s colour change
Jet not filled
Burette not rinsed beforehand
Funnel kept in burette
What does including a rough titration affect for out results?
Their validity
What affects the validity of titration results?
Including a rough titration
How many times is the beaker and stirring rod rinsed when preparing a standard solution?
3 times
To how many decimal places must we write ALL of the figures in a table for titrations?
2 d.p.
Why do we add an excess of acid during a double titration?
Allows 100% of the solute to dissolve
When does Kc’s value get larger?
When the reaction shifts right
When does Kc’s value get smaller?
When the reaction shifts left
What happens to Kc’s value when the reaction shifts right?
Gets larger
What happens to Kc’s value when the reaction shifts left?
Gets smaller
Is there a greater % error in measuring small or large volumes?
Greater % error in measuring small volumes
When do we get the total error of a piece of equipment to multiply more than once? (Percentage error)
If used more than 1 piece of equipment, or the same one more than once
Why might incorrect titres be obtained when completing a titration? (5)
-funnel kept in burette
-difficulty seeing the colour change of an indicator
-jet of burette not filled
-air bubble in burette
-equipment not rinsed with acid beforehand
Why is equipment rinsed before use?
To remove SOLUBLE impurities
Definition of a reversible reaction
Products react to form reactants and vice versa
Products react to form reactants and vice versa
Reversible reaction
How should a product be treated to obtain results for quantitative analysis?
Filtered
Washed to remove soluble impurities
Dried to a constant mass to ensure all water is removed
Purpose of fluoride ions in drinking water
Strengthens teeth enamel
Decreases tooth decay
How would we treat an ore to get ions in solution?
Add acid
How would we obtain 0.050moldm-3 HCl from 2moldm-3 HCl?
Dilute by a factor of 40 (2/0.050)
Use pipette and volumetric flask
What’s the purpose of filtering vs the purpose of washing a product of a reaction?
Filtering —> remove insoluble impurities
Wash —> remove soluble impurities
What’s the only thing that can change the value of Kc?
Temperature
What volume of solution do we always use in a pipette for a titration?
25cm^3
When a Kc question says that the moles of 2 different things are the same, what do we do?
Use x squared
To how many decimal places do we record titration values?
2
What does preparing a standard solution give us?
An accurate concentration
What happens if we don’t invert the flask after preparing a standard solution?
Fluctuations in the titration values
What will be wrong with our titre is the jet of a burette isn’t full when performing a titration?
End point overshoot
What effect will leaving the funnel in the burette have on the value of our titres?
Higher titres than expected
Why do we need to rinse a burette with whatever were using in our titration before using it?
To remove traces of water that could dilute the titrant
Why does not having the jet filled in a titration lead to an end point overshoot?
Acid used to fill the jet
