6.4- Acid reactions depend on strength and concentration

Question 1

Acids

Answer 1

• All acids contain hydrogen
• Strong acids are dangerous because they are corrosive and can cause severe burns
• Weak acids are much less reactive and found in food and drinks
• Most acid are molecular compounds (covalent bonds)- when they dissolve in water, they form ions

o Ionisation reaction

o All acids donate a hydrogen ion to the base

o A hydrogen ion is a hydrogen atom that has lost its electron (just a proton)

Question 2

Bases

Answer 2

• A substance that gains a hydrogen ion

o Causes litmus paper to turn blue

• Although all bases have a pH greater than 7, some are more reactive than others
• E.g. caustic soda (NaOH) can burn the skin

Question 3

Neutralisation reaction

Answer 3

• When an acid and base are mixed, hydrogen ions from the acid combine with hydroxide ions commonly found in bases to form water
• The remaining ions form a metal salt
• Water is considered neutral (pH 7)
• Acid + base (metal hydroxide) -> water + metal salt
• E.g. HCI + NaOH -> H2O + NaCl

Question 4

Acid and metal reactions

Answer 4

• Reaction between acids and metals e.g. acid rain

o carbon dioxide or sulfur dioxide in the air cause the formation of acid rain

• acid + metal -> salt + hydrogen
• e.g. sulfuric acid + zinc -> zinc sulfate + hydrogen

o H2SO4 (aq) + Zn(s) -> ZnSO4 (aq) + H2 (g)

• Only the more reactive metals will form a salt and hydrogen when reacting with acids

Question 5

Acid and metal oxides

Answer 5

• an acid and a metal oxide react to form a metal salt and water
• acid + metal oxide -> metal salt +water
• example:
  - nitric acid + magnesium oxide -> magnesium nitrate + water
  - 2HNO, (aq) + MgO(s) -> Mg(NO^ 3)^2(aq) + H^20(1)

Question 6

Acids and metal carbonates

Answer 6

• An acid and a metal carbonate react to form a metal salt, and carbon dioxide
• acid + metal carbonate -> metal salt + water + carbon dioxide
• For example, acid rain can affect the calcium carbonate that makes up marble
• nitric acid + calcium carbonate -> calcium nitrate + water + carbon dioxide
• 2HNO, (aq) + CaCO, (s) -> Ca(NO^ 3)^2(aq) + H^20(1) + CO^2(g)

Question 7

Concentration or strong?

Answer 7

• Concentration of an acid or base is a measure of how many molecules of an acid or base are present in each litre of solution
• A concentrated acid/base has little water – molecules of acid/base
• Strength of an acid is a measure of how readily it will give away hydrogen ions to a base
• Acid strength is compared at the same concentration- usually very low, a very dilute (watered down) solution