How to WriteNET IONIC EQUATIONS

Aqueous Reactions

• Many chemical reactions occur in water, i.e. in solution

• Two things we must consider– Solubility of reactants and products– Electrolytic properties of reactants and

products

Solubility

• Amount of substance that can be dissolved in a given quantity of solvent– Usually g/100mL

• All substances are soluble to some degree, but if their solubility is very low, we can consider them essentially insoluble

• Solubility Rules– You MUST MEMORIZE these

Electrolytic Properties

• An electrolyte (or strong electrolyte) is a substance whose solution contains ions and therefore conducts electricity– Example:

NaCl dissolves in water and the resulting solution conducts electricity (which makes it an electrolyte)Explanation: When NaCl dissolves, it actually dissociates or ionizes (breaks apart into the ions that make it up)NaCl (s) → Na+(aq) + Cl-(aq)

ALL SOLUBLE STRONG ELECTROLYTES DO THIS

• Note that because we use the word “solution”, electrolytes must be soluble (however the reverse statement is not true)

Strong Electrolytes

• All soluble ionic compounds (salts)• All strong acids (memorize list)

– HCl, HBr, HI, HClO3, HClO4, HNO3, H2SO4

– Note that all other acids are weak electrolytes (pay particular attention to HF and CH3COOH- they are the two most common weak acids and you do NOT write them in dissociated form in a reaction)

• All strong bases– Soluble metal hydroxides (alkali metals and Ca, Sr,

Ba)

• A nonelectrolyte is a substance whose solution does not contain ions and therefore does not conduct electricity– A substance can be soluble, but a nonelectrolyte

• Example: sucrose (sugar)- it dissolves in water, but the resulting solution does not conduct electricity (which makes it a nonelectrolyte)

Explanation: When sucrose dissolves, it does not dissociate into ions (its not made of ions to start with)

C12H22O11(s) → C12H22O11(aq)

ALL SOLUBLE NONELECTROLYTES DO THIS

• A weak electrolyte is a substance whose solution contains a small amount of ions, and therefore conducts electricity to some extent– Example: Ethanoic (acetic) acid solution weakly conducts

electricityExplanation: some of the molecules dissociate into ions (~3%), but most do not.

CH3COOH(aq) ↔ CH3COO-(aq) + H+(aq)

*note the double arrow- this indicates that this reaction is a dynamic equilibrium, that is both the forward and reverse reactions are occurring at the same rate, but the total amount of the undissociated form is much more than the amount of the dissociated form (therefore the difference in font size). Also, a solution of a weak electrolyte (or any equilibrium reaction) consists of a mixture of reactants and products.

Writing “ionic” equations

• From now on- we will write chemical equations in “ionic” form; i.e. we will show soluble strong electrolytes that are in solution as they really are- as ions– Example:

NaCl(aq) + AgNO3(aq) → AgCl(s) + NaNO3(aq)

NOW BECOMESNa+(aq) + Cl-(aq) + Ag+(aq) + NO3

-(aq) → AgCl(s) + Na+(aq) + NO3-(aq)

NaCl(aq) + AgNO3(aq) → AgCl(s) + NaNO3(aq)NOW BECOMES

Na+(aq) + Cl-(aq) + Ag+(aq) + NO3-(aq) → AgCl(s) + Na+(aq) + NO3

-(aq)

• Questions:1. Why did NaCl(aq) become Na+(aq) + Cl-(aq) ?

– Because NaCl(aq) is soluble, in solution, and a strong electrolyte, so it actually exists as ions in the reaction

2. Why did AgNO3(aq) become Ag+(aq) + NO3-(aq) ?

– Because AgNO3(aq) is soluble, in solution, and a strong electrolyte, so it actually exists as ions in the reaction

3. Why did AgCl(s) remain AgCl(s) in the ionic equation?

– Because AgCl(s) is NOT SOLUBLE, therefore it is a solid precipitate- if its not in solution, it can’t exist as ions

4. Why did NaNO3(aq) become Na+(aq) + NO3-(aq) ?

– Because NaNO3(aq) is soluble, in solution, and a strong electrolyte, so it actually exists as ions in the reaction

So how do we know when to write substances in dissociated form?

• ONLY when they are1. Soluble (how do you know? Solubility rules)

AND

2. In solution (how do you know? Given in the question) AND

3. A strong electrolyte (how do you know? Strong electrolytes consist of soluble salts, strong acids, and strong bases)

Steps to writing an ionic equation

• NaCl(aq) + AgNO3(aq) → AgCl(s) + NaNO3(aq)

• Soluble? YES YES NO YES• In Solution? YES YES NA YES• Str. Electrolyte? YES YES NA YES• Write as ions? YES YES NO YES• Na+(aq) + Cl-(aq) + Ag+(aq) + NO3

-(aq) → AgCl(s) + Na+(aq) + NO3-(aq)

• Write in dissociated form ONLY when the answer to all three questions is YES

Na+(aq) + Cl-(aq) + Ag+(aq) + NO3-(aq) → AgCl(s) + Na+(aq) + NO3

-(aq)

• What do you notice about the reactant side of this equation compared to the product side? – Na+(aq) appears on both sides

• Is it a reactant and a product?

– NO3-(aq) appears on both sides

• Is it a reactant and a product?

Na+(aq) + Cl-(aq) + Ag+(aq) + NO3-(aq) → AgCl(s) + Na+(aq) + NO3

-(aq)BECOMES

Ag+(aq) + Cl-(aq) → AgCl(s)

• Not exactly- they are called SPECTATOR IONS- although they are indeed present in the solution, they do not actually participate in the reaction- they are just “watching” from the “sidelines”

• In a NET IONIC EQUATION, the spectator ions are omitted (not shown) because they are of no significance to the reaction that is taking place– Which in this case is the reaction of silver ions and

chloride ions to give solid silver chloride

Steps to writing a NET Ionic Equation

• Molecular Eqn NaCl(aq) + AgNO3(aq) → AgCl(s) + NaNO3(aq)

• Soluble? YES YES NO YES• In Solution? YES YES NA YES• Str. Electrolyte? YES YES NA YES• Write as ions? YES YES NO YES• Ionic equation Na+(aq) + Cl-(aq) + Ag+(aq) + NO3

-(aq) → AgCl(s) + Na+(aq) + NO3-(aq)

• Spec. ions? Na+(aq) + Cl-(aq) + Ag+(aq) + NO3-(aq) → AgCl(s) + Na+(aq) + NO3

-(aq)

• Omit spectators• Net ionic equation Ag+(aq) + Cl-(aq) → AgCl(s)