Acids, alkalis and titrations (+ indicators)

Range of pH scale + when it is considered a mild/strong alkali or acid

What is an indicator?

Substance that changes colour depending on pH

Universal indicator to figure out pH

Universal indicator changes colour gradually over a range of pH values

It isn’t very accurate

The colour of paper or solution is checked against a chart to find the pH

Red in acid, blue in alkaline and green in neutral

Litmus paper (when do we use it? Details?)

We use it when we already know more or less what we are sampling

Red litmus to check for an alkaline/base → if alkaline, turns red litmus to blue

Blue litmus to check for an acid → if acidic, turns blue litmus to red

We can’t test for a neutral pH (eso nos dijo Mr d’Acunzo) pero el textbook dice que litmus paper is purple in neutral

Phenolphthalein

Colourless in acid

Pink in alkali

Very very pale pink if neutral

Methyl orange

Turns red in acidic solution, yellow in alkaline and orange in neutral

pH meter

Most accurate (Gives you value to 1 or 2 d.p)

H+ = OH- H+ > OH- = pH higher than 7 H+ < OH- = pH lower than 7

When we are measuring pH, what are we actually measuring?

The concentration of H+ ions in the solution

What are acids? What happens when acids are in water? What happens when acids react in terms of its hydrogen?

Substances that when dissolved, produces/donates hydrogen ions (H+), act as a source of hydrogen ions (H+) in solution

• They all have an H that is given out when dissolved in water (in solution)

They dissociate (break apart) to form hydrogen ions, for example:

• HCl(aq) → H+(aq) + Cl-(aq)

• H2SO4(aq) → 2H+(aq) + SO4²-(aq)

The hydrogen is replaced by something else; all acids have replaceable H (some have 2 H and one of them might be replaceable but the other not). For example, when HCl reacts with sodium hydroxide we obtain:

• HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

Having an H in front of its formula is an indicator that it may be an acid but it is NOT PROOF

How do we know if an acid is strong?

Common acids we need to learn, give their formula: hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid

The more easily it releases the H+ ion, the stronger the acid

Formulas

• HCl

• H2SO4

• HNO3

• H3PO4

What are alkalis? Tell me examples of alkalis

Alkalis are substances that when dissolved, they dissociate to give hydroxide ions (OH-)

• NaOH(aq) → Na+(aq) + OH-(aq)

Ammonia (NH3) is an alkali that reacts with the water to form ammonium ions and hydroxide ions:

• NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq)

• ⇌ This means it is a reversible reaction so reaction can proceed in both directions: reactants can form products and products can also revert back to reactants. This means that the reaction doesn’t go to completion.

Group 1 hydroxides

There are not many soluble carbonates, but sodium carbonate and potassium carbonate are both alkalis.

• This is due to the OH- ions in solution

• Only some of the carbonate ions react with water, so these solutions are only weakly alkaline

What are bases + the common ones

Bases can neutralise acids by combining with the hydrogen ions in them, that includes alkalis but also chemicals that are insoluble

• Metal oxides (e.g CaO)

• Metal carbonates (Na2CO3)

• Metal hydroxides

• Ammonia (NH3)

Important: in Edexcel exam board when talking about bases they usually mean insoluble ones.

Acids react with bases/alkalis in a neutralisation reaction

They neutralise

Can you tell me the ionic equation of neutralisation in terms of H+ ions and OH- ions?

H+(aq) + OH-(aq) → H2O(l)

An acid is a proton ___ and a base is a proton ___

Donor, acceptor

Remember that when we dissolve an acid and a base in water, acid ionises/dissociates so for example HCl → Cl- + H+ ions. If we used a base like CuO it first reacts with water to form Cu(OH)2 (copper hydroxide) y de ahí se saca Cu2+ + OH- ions

• We say proton transfer because an H+ ion is just a proton (atomic no is 1 so only on electron and one proton so if we quitar el electrons there is just a proton left).

• OH- receives the proton (H+) to form water - H2O

• Cl- and Cu react to form a salt, CuCl2

START OF TRIPLE CONTENT: Titrations

What are titrations used for? What is the indicator we used in titration practical?

Titration is used to follow the course of a neutralisation reaction between an acid and alkali.

Titration can be used to find out how much of the acid/alkali reacts with a certain volume of the alkali/acid.

Phenolphthalein

Describe a precise method a student can use to find out how much sodium hydrogen sulfate solution is required to neutralise 25.0cm² of sodium hydroxide solution

• occasional in ms: rinse the burette with distilled water

• Fill the burette with sodium hydrogensulfate (if you aren’t told the chemical DONT PUT THE NAME)

• Add sodium hydrogensulfate to sodium hydroxide (in conical flask) until indicator changes colour

• Swirl the conical flask as the sodium hydrogensulfate is added

• Close to the endpoint add the sodium hydrogensulfate dropwise

• Repeat until two concordant results are obtained

What apparatus is used to measure the volume of 25cm³ of sodium hydroxide? Why don’t we use a measuring cylinder?

Why do we need to rinse the burette with distilled water?

What does it mean by “two concordant results”?

Pipette. A pipette gives a much more precise (use precise, not accurate in this case) measurement.

To avoid contamination/remove any chemical residue from previous experiments.

2 results with a difference of 0.2 cm³ or less

Why is a white tile placed underneath the conical flask?

Why do we use a conical flask instead of a beaker?

To allow any colour changes in the indicator to be more visible

So that when swirling there is no risk of spillage