Acids, Alkalis and Solubility (PAPER ONE) Flashcards
Where do acids and alkalis lie on the pH scale?
Neutral - 7
Acids - Less than 7
Alkalis - More than 7
What is universal indicator?
An indicator made from a mixture of different indicators and produces a range of colours based on the pH
What colour does methyl orange indicator turn in acid/alkaline solutions?
Acid - red
Alkaline - yellow
What colour does litmus indicator turn in acid/alkaline solutions?
Acid - red
Alkaline - blue
What colour does phenolphthalein indicator turn in acid/alkaline solutions?
Acid - colourless
Alkaline - pink
What’s an ion?
An atom that has become charged by losing/gaining electrons (CHARGED PARTICLE)
What would happen when an acid is dissolved in water? - how is hydrochloric acid formed?
When acids are dissolved in water, they produce an excess of hydrogen ions (H+)
e.g. hydrochloric acid is formed when hydrogen chloride dissolves in water and splits into H+ and Cl- ions
What happens when alkalis are put in water?
Alkalis produce an excess of OH- ions when in water
e.g. sodium hydroxide splits into Na+ and OH- ions when it dissolves
What happens to the acidity/alkalinity and pH of a solution,
as more hydrogen/hydroxide ions are added?
As more hydrogen ions (H+) are added to the same volume, the concentration increases. The higher the concentration of H+ ions, the more acidic the solution becomes, which makes the pH lower
The higher the concentration of hydroxide ions (OH-) in a solution, the more alkaline it becomes, which increases the pH
How is the concentration of OH- ions and H+ ions in water?
Water is a pure substance with a pH of 7
So, it contains low and equal concentrations of OH- and H+ ions
How do you work out the concentration of a solution?
Concentration (g dmcubed) = amount dissolved/volume of solution
How do you convert cmcubed to dmcubed?
cmcubed —-> dmcubed = divide cmcubed by 1000
What’s a concentrated solution?
A concentrated solution is a solution that contains a lot of dissolved solute per unit volume
How does concentration of hydrogen ions in an acid relate to the pH scale?
If the concentration of hydrogen ions is increased by
a factor of 10, the pH decreases by 1.
If the concentration decreases by a factor of 10, the pH increases by 1
e.g. a solution with a pH of 5 would have a concentration of H+ ions 10 x 10 = 100 times greater than a solution of pH 7
An acid with a pH of 3 is diluted by a factor of 10. What will the
new pH be?
pH 4
What’s the difference between strong and weak acids?
Strong acids - Their molecules completely dissociate (break up) into ions when they dissolve in water, they produce a high concentration of H+ ions. They tend to have low pHs (0-2)
weak acids - Their molecules do not dissociate completely into ions in solution produce a low concentration of H+ ions, ionisation of weak acids are reversible reactions, pHs 2-6
What’s a base
Any substance that reacts with an acid to form a salt and water only - all metal oxides are bases
What happens during acid-alkali neutralisation reactions? (detail)
During neutralisations, the hydrogen ions in the acid combine with oxide ions to form water. This removes the hydrogen ions, therefore increasing the pH which makes the solution become more neutral and less acidic
Complete the word equation: magnesium oxide + sulfuric acid —->
magnesium oxide + sulfuric acid —> magnesium sulfate + water
Core practical: Preparing copper sulfate
1) Measure 20cmcubed of dilute sulfuric acid using measuring cylinder and pour it into a conical flask
2) Warm the acid in a 50’c water bath
3) Add copper oxide powder to acid and stir
4) If all the copper oxide reacts, so disappears, add some more and stop when the copper oxide is in excess and no longer reacts
5) Filter the mixture and transfer the filtrate into evaporating basin
6) Heat evaporating basin by placing it over a beaker of water heated by a Bunsen burner and stop heating when crystals start to form
7) Pour solution into a watch glass and leave for a few days to allow all the water to evaporate
Why is an excess of the base always added to the reaction?
To ensure that all the acid is used up
What’s a base that can dissolve in water called?
Bases that can be dissolved in water are called alkalis
Why are all alkalis bases, but not all bases
are alkalis?
Not all bases are soluble, only soluble bases are alkalis
Which bases are alkalis?
Group 1 hydroxides - e.g. sodium hydroxide
Group 2 hydroxides - e.g. calcium hydroxide
What is the format for the chemical formula of group 2 hydroxides?
chemical symbol(OH)2
What happens during a neutralisation reaction? (for the formation of salt)
The hydrogen ions from the acid react with the hydroxide ions from the alkali. So, water is formed in the reaction.
The other ions from the acid and alkali stay in the solution as ions of the dissolved salt. These ions combine to form a salt when the water evaporates
What type of solution is it important to have before evaporating the water for crystallisation, when obtaining soluble salts from alkalis? - why?
It is important to have a neutral solution before evaporating the water, because otherwise you will contaminate the
salt with an excess of one reactant
What are the products when acids react with metal carbonates?
Salt, water, carbon dioxide
What would you see when an acid reacts with metal carbonates?
Bubbles of carbon dioxide
solid metal carbonate disappears if there is enough acid to react with it
What happens in terms of ions during a reaction between an acid and metal carbonate?
The hydrogen ions from the acids react with the
carbonate ions to form water and carbon dioxide molecules
What are the products when a metal reacts with an acid?
Salt + hydrogen
What’s a precipitate reaction?
A reaction in which 2 soluble substances in solutions cause an insoluble precipitate to form
What’s a precipitate?
The insoluble salt formed when two soluble salts react
Test for hydrogen
Use a burning splint held at the open end of a test tube of the gas
▪ Creates a ‘squeaky pop’ sound
Test for carbon dioxide
Bubble the gas through the limewater (calcium hydroxide solution) and it will turn cloudy
What’s a neutralisation reaction?
Reaction between an acid and a base
What’s the ionic equation for an acid and alkali neutralisation reaction?
OH- + H+ —–> H2O
If the concentration of H+ ions increases by a factor of 100, what happens to the pH?
Decreases by 2
In terms of ions, why is pH 7 neutral?
Has an equal concentration of OH- and H+ ions
What are the formulas for…
Hydrochloric acid
Nitric acid
Sulfuric acid
—–
Sodium hydroxide
Potassium hydroxide
Calcium hydroxide
HCl
HNO3
H2SO4
—-
NaOH
KOH
Ca(OH)2
Which ionic compounds are soluble/insoluble in water?
SOLUBLE:
- All common sodium, potassium and ammonium salts
- All nitrates
- Most chlorides (Silver and Lead chloride are INSOLUBLE)
- Most sulfates (Lead, barium and calcium sulfate are INSOLUBLE)
INSOLUBLE:
- Most carbonates (Sodium, potassium and ammonium carbonate are SOLUBLE)
- Most hydroxides (sodium, potassium, ammonium hydroxide are SOLUBLE)
What do titrations do?
They allow us to accurately measure the volumes of an acid and an alkali required for neutralisation
Explain how to carry out a titration.
1) Use a volumetric pipette to transfer 25cmcubed alkali into a conical flask (use a pipette filler)
2) Add 5 drops of e.g. phenolphthalein indicator to the alkali in the conical flask
3) Place on white tile to see the colour change more clearly
4) Fill the burette with an acid.
5) Add acid to the alkali until the solution is neutral. Once the solution begins to go colourless/a colour change is seen, add the acid drop by drop, swirling the conical flask
6) Once a full colour change is completed, record the volume of acid added from the burette - bottom of the meniscus at eye level to prevent parallax error
What does the solubility table tell us?
If the product of a precipitate reaction is insoluble, it will form as a precipitate
Lead Nitrate + Sodium Chloride –>
Lead Chloride (precipitate as it’s insoluble) + Sodium Nitrate
What’s an example of a base?
Metal oxides (insoluble in water)
