GCSE OCR Gateway Chemistry C3.1c, e and f Chemical and Ionic Equations.

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.

C3 Chemical Reactions

C3.1 Introducing chemical reactions

a) Equations and state symbols

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.

Magnesium (Mg) reacts well with hydrochloric acid (HCl)

Here’s the chemical equation:

Mg     +    2HCl     =      MgCl₂      +     H₂

Let’s add the state symbols:

Mg(s)    +    2HCl(aq)      =      MgCl₂(aq)    +     H₂(g)

Which is what C3.1f wants us to do.

C3.1f describe the physical states of products and reactants using state symbols (s, l, g and aq)

Here is another equation:

Sodium carbonate reacts with hydrochloric acid

Na₂CO₃   +   2HCl      =    2NaCl   +     H₂O     +    CO₂

And now add the state symbols:

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

And that equation contains all four symbols

S for solid at room temperature

L for liquid at room temperature

G for gas at room temperature

Aq for an aqueous solution i.e. a solution of stuff dissolved in water.

Remember the stuff in italics is for the higher tier grades only.

b) Ionic equations

C3.1e construct balanced ionic equations

Ionic equations fit precipitation, redox and neutralization reactions.

To construct balanced ionic equations you need to identify the spectator ions in a reaction.

Spectator ions are any ion that just do not get involved in the reaction.

You construct a balanced ionic equation from the overall equation for a chemical reaction.

Example 1:

Think of the reaction between lead nitrate solution and sodium iodide solution.

Here is the equation with state symbols

Pb(NO₃)₂    +  2NaI    =   PbI₂     +   2NaNO₃

Let’s add the state symbols

Pb(NO₃)₂(aq)    +  2NaI(aq)   =   PbI₂(s)     +   2NaNO₃(aq)

Now look at what happens to the sodium and the nitrate ions.

Both stay in solution(aq) but the lead and iodide ions form an insoluble solid (s) precipitate.

So the sodium and nitrate ions are spectator ions because they remain in exactly the same state after the reaction as before.

So let’s ignore these two ions and see what’s left over in the equation: 

This leaves the ionic equation below:

Pb²⁺ (aq)     +    2I^—(aq)     =     PbI₂(s)

And this is the ionic equation for the reaction.

Example 2:

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.

Example 3:

Another type of 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.

Example 4:

Magnesium reacting with hydrochloric acid.

Mg(s)    +    2HCl(aq)      =      MgCl₂(aq)    +     H₂(g)

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

In this case as with all redox reactions of an acid by a metal the acid anion never changes.

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

In this case it is only Cl^—.

This is the spectator ion.

what remains is this:

Mg (s)     +     2H+(aq)       =       Mg ²⁺(aq)     +       H₂ (g)

which means that in the reaction between magnesium and hydrochloric acid it is the magnesium atom and the hydrogen ions from the acid that react, they exchange electrons.

We can write this reaction as two half equations:

Mg (s)     =   Mg ²⁺(aq)     +    2e^—

And

2H+(aq)       +     2e^—      =        H₂ (g)

The first half equation shows the loss of two electrons or the oxidation of the magnesium atom.

The second half equation shows the gain of the same two electrons by the two hydrogen ions to form hydrogen gas: the reduction of the hydrogen ions.

This is a redox reaction because it involves the loss and gain of electrons.

Every metal reacting with an acid behaves in the same way and the ionic equation will be of the same type.