Acids react with reactive metals to form salt + hydrogen gas (refer to Reactivity Series) General equation:
- metal + acid → salt + hydrogen
Example:
- magnesium + dilute sulfuric acid → magnesium sulfate + hydrogen
- Mg(s) +
- H2SO4(aq) → MgSO4(aq) + H2(g)
Salts are called:
- sulfates when formed from sulfuric acid
- Nitrates when formed with nitric acid
- Chlorides when form with hydrochloric acid
- Sulfites are formed with sulfurous acid
- Phosphates are formed from phosphoric acid
- Ethanoate is formed from ethanoic acid.
Test for hydrogen gas:
- placing burning splint at the mouth of the test tube
- Hydrogen gas extinguishes the burning splint with a ‘pop’ sound refer to Test for Gasses
Acids and unreactive metals ⇒ not all metals react with acids to give salt and hydrogen gas
- when unreactive metals - eg: copper and silver → added to dilute acids → there is no reaction Reactivity Series Main Note
- Lead appears not to react with hydrochloric acid and sulfuric acid. Initial reaction between lead and acid produces a layer of lead(II)chloride or lead(II)sulfate. Salts Main Note
- This layer is insoluble in water and quickly forms a coating around the metal.
- Coating protects the metal from further reaction with the acid.
Decreasing order of reactivity: Potassium, sodium, calcium, magnesium, aluminium, zinc, iron, tin, lead, (hydrogen), copper, silver, gold, platinum. Only metals before hydrogen gas will react with acids to produce hydrogen gas.
Some reactive metals react with some acids to form salts which are insoluble in water. Thus, these metals also appeared not to react with acids. This is because the reaction ends shortly after it begins. (refer to Reactivity Series Main Note
Calcium: calcium sulfate is formed and it is insoluble and coats around the calcium, so it prevents calcium from reacting with surrounding sulfuric acid.
Insoluble salts: lead(II)sulfate, calcium sulfate, barium sulfate, silver chloride, lead(II)chloride, lead(II)iodide. (refer to Salts Main Note)
Special Case: Aluminium + Acids ⇒ aluminium reacts with oxygen in the air to form a layer of insoluble aluminium oxide (base) that coats around the aluminium ⇒ when aluminium is placed in HCL → there was no visible reaction → after a while bubbles started to form and continued to be produced rapidly
there was a delay due to the fact that the aluminium oxide reacted with the acid first*