Chemistry: Chemical Changes TOPIC 3
- Ions that make aqueous solutions acidic are hydrogen ions. Ions that make aqueous
solutions alkaline are hydroxide ions.
- The pH scale ranges from 0-14 and measures the acidity/alkalinity of a solution. Acid-less
than 7, Alkali more than 7 and neutral solution is pH 7. (strongest acid is pH 1 and strongest
alkali pH 14).
- To measure pH you can use a universal indicator and a pH probe:
- Phenolphthalein is colourless in acid and pink in alkali
- Methyl Orange is red in acid and yellow in alkali
- Blue litmus paper in acid turns red and in alkali stays blue
- The problems with universal indicator, is that the colour of the solution is matched to a pH
colour chart, but is quite subjective as people may disagree with colour choices (not an exact
pH value)
- If an acid has a pH value of 1, then there will be a high concentration of hydrogen ions in the
acid, if for an alkaline solution has a low pH of 8 then there will be a low concentration of
hydroxide ions
- Concentrated Acids have more moles of acid per unit volume of water than dilute acids.
Concentration of an acid is not the same as strength. Strength refers to whether the acid has
completely dissolved in water or not ( a weak acid partially dissociates in water)
- All alkalis are soluble
Neutralisation reactions:
- A neutralisation reaction is a reaction between an acid and a base. A base is any substance
that reacts with an acid to form a salt and water only: acid + base → salt + water
- In an acid-alkali neutralisation reaction, hydrogen ions from the acid react with the
hydroxide ions from the alkali to form water.
Acid formulas: (products)
- Acid + metal -> salt and hydrogen
- Acid + metal carbonate -> salt and water and carbon dioxide
- Acid + metal oxide-> salt and water
- Acid + metal hydroxide-> salt and water
● Metal oxides are normally bases (because insoluble)
● Metal hydroxides are bases/alkalis if insoluble/soluble
● To name salts: ○ first part is simply the name of the metal in the oxide/hydroxide/ carbonate ○
second part comes from the acid:
■ hydrochloric acid (HCl)- chloride
■ nitric acid (HNO3 )- nitrate
■ sulfuric acid (H2SO4 )- sulfate
Note: if both (acid and soluble reactant) are soluble then use titration to produce a salt
, HINT: Any precipitate will be formed if an insoluble salt is the product.
Core practical: Investigate the preparation of pure, dry hydrated copper sulfate crystals starting
from copper oxide including the use of a water bath
● Method: add an excess of copper oxide (insoluble) to your acid (sulphuric acid H2SO4 - as you
are making copper SULFATE)
○ Use a filter and filter paper to filter off any copper oxide that hasn’t reacted (your solution
should be blue as copper sulfate solution has been formed) Use excess copper oxide to make
sure it’s reacted)
○ evaporate off the water by placing your final solution in a water bath (crystallisation)
Electrolysis:
- Electrolyte- an ionic compound in its molten state that carries charged ions
- Electrolysis- a process with uses electrical energy to decompose/breakdown electrolytes
- Cathode-negative electrode (metals always move here as they have positive ions-cations)
- Anode-positive electrode (negative ions-anions- always move here) opposites attract
- Cathode-reduction (gains electrons)
- Anode- oxidation (loses electrons)
When you have a ionic solution (NOT a molten ionic compound), your solution will contain: the
ions that make up the ionic compound, and the ions in water (OH- and H +)
● the cathode (-):
○ hydrogen (from H + in water) is produced UNLESS the + ions in the ionic compound are from a
metal less reactive than hydrogen ○ if the metal is less reactive, it will be produced instead
● at the anode (+): ○ oxygen (from OH in water) will be produced UNLESS the ionic compound
contains halide ions (Cl- , Br- , I - )
○ if there are halide ions, the halogen will be produced instead (e.g. Cl2 )
- Ions that make aqueous solutions acidic are hydrogen ions. Ions that make aqueous
solutions alkaline are hydroxide ions.
- The pH scale ranges from 0-14 and measures the acidity/alkalinity of a solution. Acid-less
than 7, Alkali more than 7 and neutral solution is pH 7. (strongest acid is pH 1 and strongest
alkali pH 14).
- To measure pH you can use a universal indicator and a pH probe:
- Phenolphthalein is colourless in acid and pink in alkali
- Methyl Orange is red in acid and yellow in alkali
- Blue litmus paper in acid turns red and in alkali stays blue
- The problems with universal indicator, is that the colour of the solution is matched to a pH
colour chart, but is quite subjective as people may disagree with colour choices (not an exact
pH value)
- If an acid has a pH value of 1, then there will be a high concentration of hydrogen ions in the
acid, if for an alkaline solution has a low pH of 8 then there will be a low concentration of
hydroxide ions
- Concentrated Acids have more moles of acid per unit volume of water than dilute acids.
Concentration of an acid is not the same as strength. Strength refers to whether the acid has
completely dissolved in water or not ( a weak acid partially dissociates in water)
- All alkalis are soluble
Neutralisation reactions:
- A neutralisation reaction is a reaction between an acid and a base. A base is any substance
that reacts with an acid to form a salt and water only: acid + base → salt + water
- In an acid-alkali neutralisation reaction, hydrogen ions from the acid react with the
hydroxide ions from the alkali to form water.
Acid formulas: (products)
- Acid + metal -> salt and hydrogen
- Acid + metal carbonate -> salt and water and carbon dioxide
- Acid + metal oxide-> salt and water
- Acid + metal hydroxide-> salt and water
● Metal oxides are normally bases (because insoluble)
● Metal hydroxides are bases/alkalis if insoluble/soluble
● To name salts: ○ first part is simply the name of the metal in the oxide/hydroxide/ carbonate ○
second part comes from the acid:
■ hydrochloric acid (HCl)- chloride
■ nitric acid (HNO3 )- nitrate
■ sulfuric acid (H2SO4 )- sulfate
Note: if both (acid and soluble reactant) are soluble then use titration to produce a salt
, HINT: Any precipitate will be formed if an insoluble salt is the product.
Core practical: Investigate the preparation of pure, dry hydrated copper sulfate crystals starting
from copper oxide including the use of a water bath
● Method: add an excess of copper oxide (insoluble) to your acid (sulphuric acid H2SO4 - as you
are making copper SULFATE)
○ Use a filter and filter paper to filter off any copper oxide that hasn’t reacted (your solution
should be blue as copper sulfate solution has been formed) Use excess copper oxide to make
sure it’s reacted)
○ evaporate off the water by placing your final solution in a water bath (crystallisation)
Electrolysis:
- Electrolyte- an ionic compound in its molten state that carries charged ions
- Electrolysis- a process with uses electrical energy to decompose/breakdown electrolytes
- Cathode-negative electrode (metals always move here as they have positive ions-cations)
- Anode-positive electrode (negative ions-anions- always move here) opposites attract
- Cathode-reduction (gains electrons)
- Anode- oxidation (loses electrons)
When you have a ionic solution (NOT a molten ionic compound), your solution will contain: the
ions that make up the ionic compound, and the ions in water (OH- and H +)
● the cathode (-):
○ hydrogen (from H + in water) is produced UNLESS the + ions in the ionic compound are from a
metal less reactive than hydrogen ○ if the metal is less reactive, it will be produced instead
● at the anode (+): ○ oxygen (from OH in water) will be produced UNLESS the ionic compound
contains halide ions (Cl- , Br- , I - )
○ if there are halide ions, the halogen will be produced instead (e.g. Cl2 )
