Does Polyprotic Strong Acid Completely Dissociate in a Net Ionic Equation?

• apchemstudent
In summary, the net ionic equations for reactions involving polyprotic strong acids assume complete dissociation of the acid and show only the ions that are involved in the reaction. In the reactions given, aluminum chloride and sulfuric acid produce 3 hydrogen sulfate ions and calcium and water produce 2 hydroxide ions. In the reaction between calcium hydroxide and ammonium phosphate, 3 calcium ions, 6 hydroxide ions, 6 ammonium ions, and 2 phosphate ions are involved. In the reaction between sulfuric acid and magnesium hydroxide, 2 hydrogen ions, 2 hydroxide ions, 1 sulfate ion, and 1 water molecule are involved. And in the reaction between phosphoric acid
apchemstudent
I have to write net ionic equations for reactions. I have a big question to ask about polyprotic strong acids. Do we assume that the acid completely dissociates in a net ionic equation, such as H3PO4 and H2SO4?

aluminum chloride + sulfuric acid -> aluminum sulfate + hydrogen chloride

I got 3HSO4(-) -> 3SO4(2-) + 3H(+)
If i assumed H2SO4 completely dissociates then there wouldn't be a net ionic equation. Everything cancels out.

Calciuim + water -> calcium hydroxide + H2

I got Ca + 2H2O -> Ca(2+) + 2OH(-) + H2

Is this correct? I'm using a chart that says calcium hydroxide is soluble so that's why they are in ionic form.

Calcium hydroxide + ammonium phosphate -> calcium phosphate + ammonia + water

3 Ca(+) + 6 OH(-) + 6NH4(+) + 2PO4(3-) -> Ca3(PO4)2 + 6NH3 + 6H2O
I think this is correct. In a book i have, they kept the hydroxide ion on the reactant side, instead of removing a hyroxide ion from both sides.

sulfuric acid + Magnesium hydroxide -> magnesium sulfate + water
This is another one i have problems on.
Again I am using a book that says that MgOH is soluble(my teacher's book)

H+ + HSO4(-) + 2OH(-) -> SO4(2-) + 2H2O

Is this it? I mean, a hydroxide ion and a hydrogen ion can combine to form water in the reactant, so that

HSO4(-) + OH(-) -> SO4(2-) + H2O

So I'm not sure which one is correct.
Again, the complete dissociation question... the net ionic can also be 2 H(+) + 2 OH(-) ->2 H2O

Which one is the correct one?

As well, phosphoric acid + magnesium hydroxide -> magnesium phosphate + water

6 H(+) + 2 PO4(3-) + 3 Mg(2+) + 6 OH(-) -> Mg3(PO4)2 + 6H2O

Last edited:
I'm guessing that e.g.

$$H_3PO_4_{(g)} + H20_{(l)} \xrightarrow~3H_3O^{+}_{(aq)} + PO_4^{-3}_{(aq)}$$

In net ionic equations, we assume that all strong acids and bases completely dissociate in water. This means that for polyprotic strong acids like H3PO4 and H2SO4, we would write the net ionic equation as the complete dissociation of the acid into its respective ions, as you have correctly done for the reactions involving sulfuric acid.

For the reaction between aluminum chloride and sulfuric acid, the net ionic equation would be Al(3+) + 3Cl(-) + 3H(+) + 3SO4(2-) -> Al(3+) + 3SO4(2-) + 3H(+) + 3Cl(-). This may seem like everything cancels out, but it is still important to include all ions in the equation to show the complete dissociation of the acid and the formation of the products.

Your net ionic equation for the reaction between calcium and water is correct. As you mentioned, calcium hydroxide is a strong base and therefore completely dissociates in water, resulting in the formation of calcium ions and hydroxide ions.

For the reaction between calcium hydroxide and ammonium phosphate, your net ionic equation is also correct. The hydroxide ions remain on the reactant side to balance out the ionic charges, but they are still considered to be in their ionic form.

In the reaction between sulfuric acid and magnesium hydroxide, both of your proposed net ionic equations are correct. The first one, H+ + HSO4(-) + 2OH(-) -> SO4(2-) + 2H2O, shows the complete dissociation of the acid into its ions, while the second one, HSO4(-) + OH(-) -> SO4(2-) + H2O, shows the formation of water from the combination of a hydrogen ion and a hydroxide ion. Both are valid ways of writing the net ionic equation for this reaction.

Finally, your net ionic equation for the reaction between phosphoric acid and magnesium hydroxide is also correct. It shows the complete dissociation of the acid and the formation of the products.

Overall, it is important to remember that in net ionic equations, we assume complete dissociation of strong acids and bases. This may result in seemingly simple equations, but it is important to include all ions in order to accurately represent the

What is a net ionic equation?

A net ionic equation is a chemical equation that shows only the species that participate in a chemical reaction, leaving out spectator ions that do not undergo a change. This type of equation is useful for determining the actual chemical reaction that occurs in a solution.

What is the purpose of writing net ionic equations?

The purpose of writing net ionic equations is to simplify and clarify the chemical reaction that takes place in a solution. By removing spectator ions, the net ionic equation highlights the actual chemical change that occurs and makes it easier to understand and balance the equation.

How do you write a net ionic equation?

To write a net ionic equation, first write the balanced molecular equation for the chemical reaction. Then, identify and cancel out spectator ions on both sides of the equation. Finally, write the remaining ions and their coefficients to form the net ionic equation.

What is the difference between a molecular equation and a net ionic equation?

A molecular equation shows all the reactants and products in a chemical reaction, whereas a net ionic equation only shows the species that participate in the reaction. Molecular equations also include spectator ions, while net ionic equations do not.

When is it necessary to write a net ionic equation?

It is necessary to write a net ionic equation when dealing with reactions in aqueous solutions, as these solutions often contain spectator ions that do not participate in the reaction. Net ionic equations are also useful for identifying the actual chemical change that occurs in a solution.

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