1.2.5

# Balanced Equations

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## Balanced Equations

Balanced equations are absolutely crucial to any kind of quantitative chemistry.

### What are they?

• There are a few key features of balanced equations:
• They must have an equal number of atoms on either side.
• You add more atoms by adding reactants or products to either side.
• There's no method to do this apart from trial and error - just find what works.

### Why are they useful?

• Balanced equations can be used to calculate masses of reactants and volumes of gases.
• We'll demonstrate these via some worked examples in this session.

### Titration

• Titration is an experimental technique for finding the concentrations of solutions.
• Titrations use balanced equations to calculate the concentration of an unknown solution from a volume of a known solution.
• The experimental technique involves the use of a burette, and often an indicator.
• A small volume of solution is dropped from the burette into the reaction flask. This is repeated until the indicator shows no further reaction happening.

## Ionic Equations

Ionic equations are used to describe reactions in solution.

### Ionic solids

• Ionic solids will dissolve in solution and will split into their constituent ions.
• E.g. NaCl(aq) ⇌ Na+(aq) + Cl(aq)

### Reacting in solution

• Some of the ions dissolved in the solution will react.
• But, some of the ions dissolved will not be involved in any reaction.
• These ions are called spectator ions.

### Ionic equations

• Ionic equations are equations that have excluded all the non-reacting spectator ions.
• The charge on either side of the ionic equation should be equal.

## State Symbols

State symbols are letters that tell us what state of matter a substance is in. There are four different state symbols:

### (s)

• (s) is used for solids.
• E.g. Mg(s)
• E.g. Na(s)

### (l)

• (l) is used for pure liquids.
• E.g. H2O(l)
• E.g. Hg(l) (mercury is liquid at room temperature).

### (aq)

• (aq) is used for aqueous solutions (substances dissolved in water), and therefore it is used more often than (l).
• E.g. NaOH(aq)
• E.g. H2SO4(aq)

### (g)

• (g) is used for gases.
• E.g. H2(g)
• E.g. CO2(g)

### Why are they useful?

• State symbols are useful to chemists because the state a reactant is produced in can affect experimental techniques.
• For example, you wouldn't want to carry out a reaction that generates 30 moles of CO2(g) in a 10cm3 sealed vessel - it would explode.
• If you left your state symbols out, you might try this.

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