Balanced net ionic equation for redox reaction

[LizziePearl]

Just need some idea of where to start on this one...am in my first Chemistry class and it is an intensive so we're not spending a lot of time on things. This one just has me stumped from the beginning so just some guidance on where to begin would be a huge help...

Problem: Gold metal will dissolve only in aqua regia, a mixture of concentraed hydrochloric acid and concentrated nitric acid in 3:1 volume ratio. The products of the reaction between gold and the concentrated acids are AuCL4-(aq), NO(g), and H2O.
a. Write a balanced net ionic equation for the redox reaction, treating HCL and HNO3 as strong acids.
b. What stoichiometric ratio of hydrochloric acid to nitric acid should be used?
c. What volumes of 12 M HCl and 16 M HNO3 are required to furnish the Cl- and NO3- ions to react with 25.0 g of gold?

I started with trying to write the equation given but wasn't sure it was correct because of the statement "3:1 ratio". This is what I started with:
Au+Cl+HNO3 -> AuCl4-+NO+H2O

Then I started trying to do the balanced redox equation by breaking it down but so I started by...
Au -> AuCl4-
HCl -> H2O
HNO3 -> NO

Is this the correct initial approach?

 

[vsage]

I don't really like the idea of you treating strong acids as not dissociated on the reactant side of the equation. Remember that $$HNO_3$$ exists as $$H^+$$ and $$NO_3^-$$ in solution, for example. That being said, your half reactions need to be tweaked a little.

 

[thepatientmental]

Yes, your initial approach is correct. To write the balanced net ionic equation for the redox reaction, you will need to first identify the oxidation states of each element involved. In this case, gold is being oxidized from a neutral state to a +4 state, while nitrogen is being reduced from a +5 state to a 0 state. The balanced net ionic equation will involve the transfer of electrons from one species to another.

Based on the given information, the balanced net ionic equation for the redox reaction can be written as:

Au(s) + 4H+(aq) + NO3-(aq) -> AuCl4-(aq) + NO(g) + 2H2O(l) + 3e-

To balance the equation, you will need to add H+ ions and water molecules on the left side to balance the hydrogen and oxygen atoms. You will also need to add electrons on the left side to balance the charges. The final balanced equation will be:

Au(s) + 4H+(aq) + NO3-(aq) -> AuCl4-(aq) + NO(g) + 2H2O(l) + 3e-

For part b, the stoichiometric ratio of hydrochloric acid to nitric acid can be calculated by using the coefficients in the balanced equation. In this case, the ratio is 4:1, meaning that 4 moles of HCl are needed for every 1 mole of HNO3.

For part c, you will need to use the molar mass of gold to convert the given mass into moles. Then, using the stoichiometric ratio calculated in part b, you can determine the moles of HCl and HNO3 needed. Finally, using the molarities of the acids, you can calculate the volume of each acid needed.