Acids give hydrogen ions and alkalis give hydroxide ions
Definition
Acid
A substance that forms hydrogen ions when it dissolves in water.
Definition
Alkali
A base that dissolves in water, giving a solution that contains hydroxide ions.
- Hydrogen chloride releases hydrogen ions, H+\text{H}^+H+, and chloride ions, Cl−\text{Cl}^-Cl−, when it dissolves in water, forming hydrochloric acid.
- The three common laboratory acids are hydrochloric acid, HCl\text{HCl}HCl, sulfuric acid, H2SO4\text{H}_2\text{SO}_4H2SO4, and nitric acid, HNO3\text{HNO}_3HNO3.
- Each of them releases H+\text{H}^+H+ ions in water, and these ions give acids their shared reactions.
- Sodium hydroxide, NaOH\text{NaOH}NaOH, dissolves in water to give a solution containing hydroxide ions, OH−\text{OH}^-OH−.
- Potassium hydroxide and calcium hydroxide are other alkalis, and each supplies OH−\text{OH}^-OH− ions in solution.
- Dry hydrogen chloride gas does not turn dry indicator paper red, because hydrogen ions form only when it dissolves in water.
Note
The negative ion left by each acid names its salts: chloride from hydrochloric acid, sulfate from sulfuric acid and nitrate from nitric acid.
Neutralisation forms a salt and water
Definition
Base
A substance that reacts with an acid to form a salt and water, such as a metal oxide or metal hydroxide.
Definition
Salt
An ionic compound formed when the hydrogen in an acid is replaced by a metal or another positive ion.
- Metal oxides and metal hydroxides, such as copper(II) oxide and sodium hydroxide, act as bases.
- Sodium hydroxide dissolves in water, so it is also an alkali, while copper(II) oxide is insoluble and is not.
- Every neutralisation follows the same word equation: acid + base →\rightarrow→ salt + water
- The salt takes its positive ion from the base and its negative ion from the acid.
- Sodium hydroxide and hydrochloric acid give sodium chloride: NaOH(aq)+HCl(aq)→NaCl(aq)+H2O(l)\text{NaOH(aq)} + \text{HCl(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)}NaOH(aq)+HCl(aq)→NaCl(aq)+H2O(l)
- Copper(II) oxide and sulfuric acid give copper(II) sulfate: CuO(s)+H2SO4(aq)→CuSO4(aq)+H2O(l)\text{CuO(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{CuSO}_4\text{(aq)} + \text{H}_2\text{O(l)}CuO(s)+H2SO4(aq)→CuSO4(aq)+H2O(l)
Practical
Making copper(II) sulfate from copper(II) oxide
- Aim: make pure, dry crystals of copper(II) sulfate from an insoluble base.
- Apparatus: a 100 cm3100\,\text{cm}^3100cm3 beaker, a 25 cm325\,\text{cm}^325cm3 measuring cylinder, a Bunsen burner, a tripod and gauze, a heatproof mat, a spatula, a glass rod, a filter funnel and filter paper, a conical flask, an evaporating basin, dilute sulfuric acid, copper(II) oxide and a digital balance.
Method
- Measure 25 cm325\,\text{cm}^325cm3 of dilute sulfuric acid into the beaker and warm it gently without boiling, because the reaction is faster when the acid is warm.
- Add copper(II) oxide a spatula at a time, stirring, until some black solid stays undissolved.
- Filter the warm mixture into a conical flask to remove the excess copper(II) oxide, collecting the blue copper(II) sulfate solution as the filtrate.
- Pour the filtrate into an evaporating basin and heat it gently until crystals start to form on a cooled glass rod.
- Leave the basin in a cool place so that the solution crystallises.
- Pat the crystals dry with filter paper and weigh them, so that the yield can be compared with the expected mass.
Accuracy and safety
- Using the base in excess makes sure that all the acid reacts, so no acid is left in the final salt.
- Do not boil the filtrate dry, because the crystals would lose their water of crystallisation.
- Wear eye protection, because sulfuric acid is an irritant and can spit when warm.
- Copper compounds are harmful, so avoid skin contact and wash your hands afterwards.
Hydrogen ions and hydroxide ions react to form water
Definition
Neutralisation
A reaction between an acid and a base or alkali that forms a salt, and usually water.
- Hydrochloric acid contains hydrogen ions and chloride ions, and sodium hydroxide solution contains sodium ions and hydroxide ions.
- When the solutions mix, each H+\text{H}^+H+ ion joins an OH−\text{OH}^-OH− ion to make a water molecule: H+(aq)+OH−(aq)→H2O(l)\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}H+(aq)+OH−(aq)→H2O(l)
- The sodium and chloride ions do not react, so they are left dissolved in the water as sodium chloride.
- Every acid gives H+\text{H}^+H+ ions and every alkali gives OH−\text{OH}^-OH− ions, so the same equation describes every aqueous neutralisation.
- Nitric acid with potassium hydroxide, or sulfuric acid with sodium hydroxide, produces a different salt, but the reaction that forms the water is the same.
- When all the H+\text{H}^+H+ ions have reacted with an equal number of OH−\text{OH}^-OH− ions, the solution is neutral.
Key Idea
The water in a neutralisation comes from hydrogen ions joining hydroxide ions, while the salt comes from the ions that do not react.
Carbonates and some metals also react with acids
Definition
Carbonate
A compound containing the carbonate ion, which reacts with acids to give a salt, water and carbon dioxide.
- An acid and a metal carbonate react to form a salt, water and carbon dioxide.
- Calcium carbonate reacts with hydrochloric acid like this: CaCO3(s)+2HCl(aq)→CaCl2(aq)+H2O(l)+CO2(g)\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}CaCO3(s)+2HCl(aq)→CaCl2(aq)+H2O(l)+CO2(g)
- The carbon dioxide makes the mixture fizz, and it turns limewater milky.
- Metals above hydrogen in the reactivity series, such as magnesium, zinc and iron, react with acids to form a salt and hydrogen.
- Magnesium reacts with hydrochloric acid like this: Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g)\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g)
- Copper, silver and gold are below hydrogen, so they do not react with dilute acids.
Common Mistake
Do not write water as a product when a metal reacts with an acid, because the products are a salt and hydrogen only.
Writing balanced equations for acid reactions
- First decide the type of reaction, because it fixes the products: a base gives a salt and water, a carbonate adds carbon dioxide and a metal gives hydrogen in place of water.
- Name the salt from the metal and the acid, so zinc and sulfuric acid give zinc sulfate.
- Write the salt formula from the ion charges, so Mg2+\text{Mg}^{2+}Mg2+ with Cl−\text{Cl}^-Cl− gives MgCl2\text{MgCl}_2MgCl2.
- Balance the equation by changing the large numbers in front of formulae, never the small numbers inside them.
- Check that every element, including oxygen and hydrogen, has the same number of atoms on both sides.
Example
- Write a balanced equation for sodium carbonate reacting with sulfuric acid.
- The reactants are a carbonate and an acid, so the products are a salt, water and carbon dioxide.
- The salt contains Na+\text{Na}^+Na+ and SO42−\text{SO}_4^{2-}SO42− ions, so its formula is Na2SO4\text{Na}_2\text{SO}_4Na2SO4.
- Write the equation: Na2CO3+H2SO4→Na2SO4+H2O+CO2\text{Na}_2\text{CO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + \text{H}_2\text{O} + \text{CO}_2Na2CO3+H2SO4→Na2SO4+H2O+CO2
- Count the atoms: there are 222 Na, 111 C, 111 S, 777 O and 222 H on each side, so the equation is balanced.
Self review
- Which ion do all acids form when they dissolve in water?
- What are the products when an acid reacts with a base?
- What forms when H+\text{H}^+H+ ions react with OH−\text{OH}^-OH− ions in a neutralisation?
- What three products form when zinc carbonate reacts with hydrochloric acid?
- Write a balanced equation for zinc reacting with sulfuric acid.
