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Acid alkali and titrations revision - Coggle Diagram
Acid alkali and titrations revision
What is an acid and a base
Acid is a substance which donates H+ in solution and a base donates OH-
Acids generally have a
pH < 7
Bases generally have a
pH > 7
What colour does a weak acid turn in universal indicator?
yellow/orange
summarising indicators:
Simple Indicators:
have more than one colour form depending on pH.
Litmus
- red in acidic; blue in alkaline; purple in neutral (pH 7)
Methyl Orange
- red in acidic; yellow in alkaline (changes at pH 4)
Phenolphthalein
- colourless in acidic; bright pink in alkaline (changes pH 9)
Universal Indicator:
a mixture of dyes, which change colour in a gradual way over a range of pH.
Titrations
Materials: (bulb) safety filler, pipette, conical flask, (clamp) stand, burette, beaker
Titrations can be used to follow the movement of an acid-alkali neutralisation reaction.
It can be used to calculate the amount of acid or alkali that reacts with a given volume of acid or alkali.
In a neutralisation titration, a solution of unknown concentration of acid or base is titrated against a solution of known concentration of acid or base. The unknown solution's concentration can then be determined.
Calculations:
eg:
25.0 cm3 of a solution of sodium hydroxide solution required 21.5 cm3 of 0.100 mol/dm3 sulfuric acid for neutralisation.
H2SO4(aq) + 2 NaOH(aq) o Na2SO4(aq) + 2 H2O(l)
Find the concentration of the sodium hydroxide solution in mol/dm3, give your answer to 3 sig f
Step 1: identify the balanced equation and convert the volume of sulfuric acid from cm3 to dm3 by dividing by 1000 = 0.0215dm3
Step 2: find the amount of moles in sulfuric acid given the concentration (concentration= mass x volume) = 0.00215 mol
Step 3: identify the ratio and use the moles of sulfuric acid and times them by 2 to get the moles of sodium hydroxide: 0.00430 mol
Use the moles and volume (don't forget to convert to dm3) of sodium hydroxide to find the concentration of sodium hydroxide which is: 0.172mol/dm3
Things to remember while carrying out a titration: example of titration used to f
ind the volume of hydrochloric acid needed to neutralise a particular volume and concentration of sodium hydroxide
Rinse the burette with distilled water and sodium carbonate solution
This is because distilled water removes impurities, and then the sodium carbonate rinsing ensures the solution isn't diluted
Burette readings need to be recorded to 2 decimal places
Rinse the pipette with Hydrochloric acid
dropwise add the solution from burette into the conical flask (add a drop, swirl, add a drop swirl) so we don't accidentally add too much/ overshoot the endpoint
When you have 2 concordant results (within 0.2 cm3 of each other) you can stop carrying out the titration
Method
Using a small funnel, pour a few cubic centimetres of hydrochloric acid into the burette, with the tap open and a beaker under the open tap. Once the tip of the burette is full of solution, close the tap and add more solution up to the zero mark. (Do not re-use the acid in the beaker – this should be rinsed down the sink.)
Use a pipette with pipette filler to transfer 25 cm3 of 0.4 mol sodium hydroxide solution to the conical flask, and add two drops of methyl orange indicator. Swirl gently to mix. Place the flask on a white tile or piece of clean white paper under the burette tap.
Add the hydrochloric acid to the sodium hydroxide solution in small volumes, swirling gently after each addition. Continue until the solution just turns from yellow-orange to red and record the reading on the burette at this point. This coloured solution should now be rinsed down the sink.
Repeat the titration, adding the hydrochloric acid more quickly until within 5 cm3 of the endpoint you saw last time. Then add the acid drop by drop, swirling each time until the colour changes from yellow orange to red. Record your results.
Repeat the titration if there is time.
The endpoint of a titration is the point at which the indicator changes colour.
