GCSE OCR Gateway C3:3c-g Neutralisation and Redox Equations of acids

New OCR Gateway specification from September 2016 Higher tier: grades 9 to 4: 

In this and subsequent posts I’m simply going to explain and illustrate each learning objective as they come up in the topics in the new GCSE specification.
 
I’m giving you my notes from each lesson.

You can really get ahead of your class if you follow this blog and all the posts that will appear here about the new GCSEs over the coming months. 

This rejigging of the specification is just that: there is nothing really new here it has all been with us for the past half century at least.
 
That written in italics is for the higher tier paper only.

Neutralisation equations

C3.3c recall that acids form hydrogen ions when they dissolve in water and solutions of alkalis contain hydroxide ions.

[C3.1c use the names and symbols of common elements from a supplied periodic table to write formulae and balanced chemical equations where appropriate for the first 20 elements, Groups 1, 7, and 0 and other common elements included within the specification.]

Acids:

An acid can be defined as a water-soluble compound that releases hydrogen ions (protons) into aqueous solution.

Briefly an acid is a proton donor. 

e.g. hydrochloric acid

Hydrochloric acid forms when the gas hydrogen chloride (HCl) dissolves in water and the hydrogen chloride molecules split into chloride ions (Cl^—) and hydrogen ions (H+) in the water.

Here’s the equation for this:   HCl(g)  +  H₂O(l)  ➞  H+ (aq)  +   Cl^— (aq)

Alkalis:

Alkalis are soluble bases usually metal oxides that dissolve in water to form a metal hydroxide.

A good example of a metal oxide base is sodium oxide (Na₂O) which reacts with water to form an alkaline solution of sodium hydroxide (NaOH). 

Sodium hydroxide solution contains both sodium ions (Na+) and hydroxide ions (OH^—).

Here is the equation:

NaOH(s)  +  H₂O(l)   →   Na+(aq)   +    OH^—(aq)

C3.3d describe neutralization as acid reacting with alkali or a base to form a salt plus water.

[C3.1b use the names and symbols of common elements and compounds and the principle of conservation of mass to write formulae and balanced chemical equations and half equations]

When an acid reacts with a base (or a soluble base an alkali) the reaction is called neutralization and a salt and water are produced.

Let’s see how we can write some balanced chemical equations for neutralization of common acids with common alkalis and bases.

Example 1:  Hydrochloric acid + Sodium hydroxide 

Write the word equation following the template:

Acid    +    alkali     →     salt   +   water

Hydrochloric acid   +   sodium hydroxide → sodium chloride  +  water

Add symbols for the acid and the alkali:

HCl     +     NaOH      →    

Identify the elements of the water formed

HCl     +     NaOH      →    

Add one mole of water to the right hand side and the elements remaining form the salt formula.

 HCl     +     NaOH      →     NaCl     +     H₂O

Example 2: sulphuric acid and potassium hydroxide

Write the word equation following the template:

Acid    +    alkali     →     salt   +   water

sulphuric acid    +   potassium hydroxide → potassium sulfate  +  water

Add symbols for the acid and the alkali:

H₂SO₄     +     KOH      →    

Identify the elements of the water formed

H₂SO₄     +     KOH      →    

There is enough hydrogen to form two moles of water so add two mole of water to the right hand side and double up the KOH. 

The elements remaining form the salt formula.

H₂SO₄     +     2KOH      →     K₂SO₄     +    2H2O

Here is a Lab you can follow to make a salt from an acid and a base.

(WS1.4a Production of a pure dry sample of a salt.)

C3.3e recognise that aqueous neutralisation reactions can be generalised to hydrogen ions reacting with hydroxide ions to form water.

Here is an example we have already shown in a different post

Neutralization reaction.

Think of the reaction between sodium hydroxide and nitric acid.

NaOH   +    HNO₃    =    NaNO₃     +    H₂O

Let’s add the state symbols next

NaOH(aq)   +    HNO₃(aq)    =    NaNO₃(aq)     +    H₂O(l)

Now we have to find out which ions do not change from the left hand side of the equation to the right hand side.

Again in this case as with all neutralizations of an acid by a base the metal ion and the acid anion never change.

These are easy to spot because they form the salt on the right hand side.

In this case they are Na+  and NO₃^—.

These are the spectator ions. 

what remains is this ionic equation:

       OH^—(aq) +      H+(aq)  =       H₂O(l)

which means that in the reaction between sodium hydroxide and nitric acid it is the hydroxide ion from the base and the hydrogen ions from the acid that react, they neutralize each other, to form water. 

Every hydroxide base reacting with an acid behaves in the same way so the ionic equation will be of the same type.

C3.3f recall that carbonates and some metals react with acids and write balanced equations predicting products from given reactants

Carbonates neutralize acids but metals reduce acids.

The two reactions are fundamentally different.

The first is neutralization, the second redox.

Carbonates and acids:

When an acid reacts with a base, like a carbonate, the reaction is called neutralization and a salt, water and carbon dioxide are produced.

This is the template:

Acid   +   carbonate    →   salt    +   water   +    carbon dioxide

Let’s see how we can write some balanced chemical equations for neutralization of common acids with common carbonates.

Example 1:  Hydrochloric acid + Sodium carbonate 

Write the word equation following the template:

Acid    +    carbonate     →     salt   +   water   +   carbon dioxide

Hydrochloric acid + sodium carbonate → sodium chloride + water + carbon dioxide

Add symbols for the acid and the base:

HCl     +     Na₂CO₃      →    

Identify the elements that will form the water and the carbon dioxide.

HCl     +     Na₂CO₃      →    

Add one mole of water to the right hand side and a mole of carbon dioxide, the elements remaining form the salt formula.

 HCl     +     Na₂CO₃      →   salt          +     H₂O   +    CO₂

But there are not sufficient hydrogens on the left so balance them up adding a 2 to the left hand side

2HCl     +     Na₂CO₃      →           +     H₂O   +    CO₂

Note how the hydrogens combine with one oxygen atom from the carbonate to form water and leave CO₂ over.

The remaining elements form 2 moles of the salt sodium chloride

2HCl     +     Na₂CO₃      →     2NaCl      +     H₂O   +    CO₂

So the balanced equation is:

2HCl     +     Na₂CO₃      →     2NaCl      +     H₂O   +    CO₂

Example 2: sulphuric acid and potassium carbonate

Write the word equation following the template:

Acid    +    carbonate     →     salt   +   water  + carbon dioxide

sulphuric acid + potassium carbonate → potassium sulfate  +  water + carbon dioxide

Add symbols for the acid and the carbonate:

H₂SO₄     +     K₂CO₃      →    

Identify the elements that will form water and carbon dioxide

H₂SO₄     +     K₂CO₃      →    

There is enough hydrogen to form one mole of water and one mole of carbon dioxide so add them to the right hand side.

H₂SO₄     +     K₂CO₃     →            +    H₂O      +     CO₂

The elements remaining form the salt formula.

H₂SO₄     +     K₂CO₃     →      K₂SO₄      +    H₂O      +     CO₂

So the balanced equation is:

H₂SO₄     +     K₂CO₃     →      K₂SO₄      +    H₂O      +     CO₂

Can you balance these equations"
1. Nitric acid and copper carbonate
2. Sulphuric acid and copper carbonate
3. Hydrochloric acid and copper carbonate
4. Nitric acid and potassium carbonate
5. Sulphuric acid and sodium carbonate
6. Hydrochloric acid and lithium carbonate

Here is a Lab you can follow to make a salt from an acid and a carbonate

[This reaction was discussed here:

Think of the reaction between sodium carbonate and hydrochloric acid.

Na₂CO₃(s)  +   2HCl(aq)   =   2NaCl(aq)   +   H₂O(l)  +    CO₂(g)

Now we have to find out which ions do not change from the left hand side of the equation to the right hand side.

In this case as with all neutralizations of an acid by a base the metal ion and the acid anion never change.

These are easy to spot because they form the salt on the right hand side.

In this case they are Na+  and Cl—.

These are the spectator ions.

what remains is this:

CO₃ ^2— (s)     +     2H+(aq)       =       H₂O(l)     +       CO₂ (g)

which means that in the reaction between sodium carbonate and hydrochloric acid it is the carbonate ion and the hydrogen ions from the acid that react, they neutralize each other, to form water and carbon dioxide. 

Every carbonate reacting with an acid behaves in the same way so the ionic equation will be the same type.]

Metals and Acids

When an acid reacts with a metal, it has to be a reactive metal the stands above hydrogen in the reactivity series.

Reactivity series:

                           Potassium K

                           Sodium Na

                           Lithium Li

                           Calcium Ca

                           Magnesium Mg

                           Aluminium Al

                           Zinc Zn

                           Iron Fe

                           Tin Sn

                           Lead Pb

                           [Hydrogen H₂]

                           Copper Cu

                           Silver Ag

The best metals are this highlighted in bold italic since they are reactive enough but not too reactive as to react dangerously with the acid or the water that is in a diluted acidic solution.

The reaction is called redox and a salt and hydrogen are produced.

This is the template:

Acid   +  reactive metal    →   salt    +   hydrogen

Let’s see how we can write some balanced chemical equations for the reduction of common acids using reactive metals like magnesium.

Example 1:  Hydrochloric acid + magnesium 

Write the word equation following the template:

Acid    +    metal     →     salt   +   hydrogen

Hydrochloric acid + magnesium → magnesium chloride + hydrogen

Add symbols for the acid and the metal:

HCl     +     Mg     →    

Recognise that the hydrogen formed is a diatomic molecule H₂

Identify the elements that will form the hydrogen.

HCl     +     Mg      →     salt     +     H₂

Add one extra mole of hydrochloric acid to the left hand side to give enough hydrogen to form the hydrogen gas on the right hand side.

2HCl     +     Mg      →     salt     +     H₂

The remaining elements form the salt formula.

2HCl     +     Mg      →    MgCl₂    +     H₂

So the balanced equation is:

2HCl     +     Mg      →    MgCl₂    +     H₂

Example 2: sulphuric acid and zinc

Write the word equation following the template:

Acid    +    metal     →     salt   +   hydrogen

Sulfuric acid  +   zinc   →   zinc sulfate   +    hydrogen

Add symbols for the acid and the base:

H₂SO₄     +     Zn     →    

Recognise that the hydrogen formed is a diatomic molecule H₂

Identify the elements that will form the hydrogen.

H₂SO₄     +     Zn     →     salt    +      H₂

As both sides contain the same number of hydrogen atoms there is no need to add any further molecules to either side. 

H₂SO₄     +     Zn     →     salt    +      H₂

The remaining elements form the salt formula.

H₂SO₄     +     Zn     →     ZnSO₄   +      H₂

So the balanced equation is:

H₂SO₄     +     Zn     →     ZnSO₄   +      H₂

Here is a Lab you can follow to make a salt from an acid and a metal.