Predicting Products – Double Replacement Reactions
Remember that double replacement reactions involve the swapping of the cations between the two reactants to produce two new product compounds.
Use the valences of the elements to create the formulae of the substances. The valence that an element or polyatomic ion starts with as a reactant will continue to be its valence even when combined into the new product (for instance, iron uses its +3 valence as a reactant and as a product).
iron (III) nitrate + sodium hydroxide → iron (III) hydroxide + sodium nitrate
Fe(NO3)3 + 3NaOH → Fe(OH)3 + 3NaNO3
Notice that a precipitate forms, but what combination of ions caused this?
Using Solubility Rules
So, how does one determine if an ionic compound is soluble in water, you might ask. By, using the solubility rules, of course.
Keep in mind:
When the rule tells you that the compound is "soluble" you will indicate this with the state symbol (aq).
When the rule tells you that the compound has "low solubility" that means it is not soluble to any appreciable amount and would precipitate out of solution. So, you indicate this with the state symbol (s).
Let's try some examples:
So, let's go back to the example from the beginning of the lesson. Using the rules, we find that iron (III) hydroxide is insoluble and sodium nitrate is soluble. We use (s) to represent a precipitate or solid and (aq) to represent the compound which remains in solution, which is said to be aqueous.
Fe(NO3)3(aq) + 3NaOH(aq) → Fe(OH)3(s) + 3NaNO3(aq)
Total & Net Ionic Equations
A total ionic equation is a chemical reaction equation that shows the separation of soluble ionic compounds into their ions.
A net ionic equation is a chemical reaction equation that shows only reacting particles.
Spectator ions are those ions which appear on both sides of the equation and are crossed out.
TryIt! Finish the word equation below and create a balanced equation. Using the solubility rules, indicate whether the products are soluble or not. Also, write out the total ionic and net ionic equations.
iron
(III) nitrate + sodium hydroxide →
The answer, and an accompanying video, can be found below
TryIt! Part 2 Finish the word equation below and create a balanced equation.
Using the solubility rules, indicate whether the products are soluble
or not. Also, write out the total ionic and net ionic equations.
silver nitrate + magnesium chloride →
The answer can be found below.
Special Double Replacement Reactions
Reactions that Produce Gas
Some reactions are not straight forward. For instance the mixing of acid and washing soda results in a two step reaction, since the carbonic acid produced (through a double replacement reaction) in the first step decomposes in the second step:
2HCl(aq) + Na2CO3(aq) →
2NaCl(aq) + H2CO3(aq)
H2CO3(aq) →
H2O(l) + CO2(g)
___________________________________________________
2HCl(aq) + Na2CO3(aq) → 2NaCl(aq) + H2O(l) + CO2(g)
Notice that the equations in the two steps, when added together, allow for the crossing out of carbonic acid. The equation below the line shows the overall reaction.
Neutralization Reaction
Mixing an acid and a base results in the production of a salt and water.
Acid + Base → a Salt + Water
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
TryIt! Answer:
iron (III) nitrate + sodium hydroxide → iron (III) hydroxide + sodium nitrate
Fe(NO3)3(aq) + 3NaOH(aq) → Fe(OH)3(s) + 3NaNO3(aq)
Fe3+(aq) + 3NO3-(aq) + 3Na+(aq) + 3OH-(aq) → Fe(OH)3(s) + 3Na+(aq) + 3NO3-(aq)
Fe3+(aq) + 3OH-(aq) → Fe(OH)3(s)
Check out this video, if you want to see me arrive at the answer above. When I recorded the video, I had my phone turned sideways, so try not to get a neck crick - sorry.
TryIt! Part 2 Answer:
Homework:
Answers:
