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Course: AP®︎/College Chemistry > Unit 8
Lesson 5: Acid–base reactionsStrong acid–strong base reactions
When a strong acid and a strong base are mixed, they react according to the following net-ionic equation: H₃O⁺(aq) + OH⁻(aq) → 2H₂O(l). If either the acid or the base is in excess, the pH of the resulting solution can be determined from the concentration of excess reactant. If the acid and base are equimolar, the pH of the solution is 7.00 at 25°C. Created by Jay.
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At7:30why is it 2H2O instead of 1H2O?(1 vote)
At7:30, why is it 2H2O?
I know someone asked this question already, but the answer didn't help.
Surely if H3O+ and H+ are interchangeable, then we could have written the equation as H+ + OH- --> H2O
But this would have resulted in half as many H2O molecules.
Can someone explain?(1 vote)
If you take a hydronium ion, H3O^(+), and you deprotonate (remove an H^(+)) that you get a water molecule, H2O. If you take a hydroxide ion, OH^(-), and you protonate (add an H^(+)) that you get another water molecule. Both of these steps happen together so you receive 2 water molecules in total for a single proton exchange.
The reaction where two water molecules are produced is more accurate to what is happening, while the reaction where only a single water is produced is the net ionic equation which simplifies the reaction to the most important chemical species.
Molecular equation: H3O^(+) + OH^(-) → 2H2O
Complete ionic equation: H^(+) + H2O + OH^(-) → H2O + H2O
So we see that there is at least one water molecule on both sides of the reaction, so we can cancel one of the waters like it’s a spectator ion.
Net Ionic equation: H^(+) + OH^(-) → H2O
We use this simplification since it makes the amount of symbols and calculations more concise, but we should keep in mind that it’s only a simplification and not what is actually happening. Especially when we do equilibrium and ICE tables, the amount of water is irrelevant so we can use either equation and get the same answers (so might as well use the simpler one).
Another reason we use the net ionic equation of and acid-base reaction is that the solvation of hydronium (the surrounding of hydronium by water molecules) which actually happens in aqueous solutions creates a variety of interesting and complicated ions. The three main structures (which all form) are the Eigen, Zundel, and Stoyanov ions. The Eigen is a tetrahydrate ion, H3O^(+)(H2O)3. The Zundel cation is a symmetric dehydrate ion, H^(+)(H2O)2. The Stoyanov cation is an expanded Zundel cation which is a hexahydrate: H^(+)(H2O)2(H2O)4. Given the variety and complexity of hydronium ions, we can use the net ionic equation to again make the work much simpler.
Hope that helps.(3 votes)
At3:05, why doesn't he use the net ionic equation as he does in7:05?(1 vote)- It just shows that can you use either the molecular equation or the net ionic equation to do ICE tables.(2 votes)
so if the strong acid and the strong base have the same number of moles, will the ph always be 7?(1 vote)- If we assume that are both are indeed strong and dissociate 100%, then yes.
Hope that helps.(1 vote)
Previously, when we did ICE tables, we used the Molar concentrations of each molecule or ion in the equation. In this video I noticed that he used actual mole amounts rather than Molar concentrations for the ICF table. Is there any reason for this change, or is it up to us to choose whether we use molar concentrations or mole amounts on our ICF and ICE tables?(1 vote)- ICE tables where you want to use the results for equilibrium equation need to have molarities since equilibrium equations utilize molarity. Here we're essentially seeing how much base and acid react with each other in a neutralization reaction and want to know what remains.
Hope that helps.(1 vote)
- When doing the example that had 300 ml of 1 M HCl and 100 mL of 1 M NaOH, why is it that you solved for mols instead of using the M concentration that you were given??(1 vote)
- Jay used an ICF table which uses moles instead of an ICE table which uses molarity. Jay explained this at3:13. He’s using an ICF table because he’s reacting strong acids and strong bases which do not have a significant equilibrium and don’t proceed in the reverse direction to any appreciably amount.
Hope that helps.(1 vote)
- isn't the net ionic equation basically the autoionization of water reversed? why would it be a double arrow in that case but not in this case(1 vote)
7:30Video transcript
- [Instructor] Hydrochloric
acid is an example of a strong acid, and sodium hydroxide is an
example of a strong base. When an aqueous solution
of hydrochloric acid reacts with an aqueous solution
of sodium hydroxide, the products are an aqueous solution of sodium chloride and water. Since the reaction goes to completion, instead of an equilibrium arrow, there's just an arrow going to the right. And this is called an acid-base
neutralization reaction. Let's think about the
ions that are involved in this acid-base neutralization reaction. Since hydrochloric acid is a strong acid, HCl ionizes 100% in solution, and therefore, in solution, HCl consists of H plus
ions and Cl minus anions. Sodium hydroxide is a strong base, and strong bases dissociate
100% in solution. Therefore, an aqueous solution
of sodium hydroxide consists of sodium ions, Na plus, and
hydroxide anions, OH minus. For our products, sodium chloride is a soluble salt and therefore forms an aqueous solution. So in solution, we would
have sodium cations, Na plus, and chloride anions, Cl minus. Since water ionizes only
to a very small extent, we don't write it as the ions, we simply write H2O. To save some time, I went ahead and added
in aqueous subscripts, and for water, it'd be liquid, and some plus signs
and the reaction arrow. This is called the overall ionic equation for our strong acid-strong base reaction. Instead of calling it an
overall ionic equation, we could also call it a
complete ionic equation. And we can use the overall ionic equation to determine the net ionic equation for this strong acid-strong base reaction. To figure out the net ionic equation, we first need to determine
the spectator ions. Remember, spectator ions don't participate in the chemical reaction. Notice how we have a sodium
cation on the reactant side and a sodium cation on the product side, so we can cancel out the sodium cation. We also have a chloride
anion on the reactant side and a chloride anion on the product side. So we can cross that ion out as well. So the sodium cation
and the chloride anion are the spectator ions. And once we take out our spectator ions, we're left with the net ionic equation. So for the net ionic equation,
we would have H plus, plus OH minus yields H2O. So this is one way of writing
our net ionic equation. However, remember that H plus and H3O plus are used interchangeably in chemistry. So instead of writing H plus
in our net ionic equation, we could have also written H3O plus, and when H3O plus, or the hydronium ion, reacts with the hydroxide anion, we still form water. But notice, for this version
of the net ionic equation, we need to put a two as a
coefficient to balance everything. Next, let's think about a situation where we have equal
moles of our strong acid and strong base. For example, let's say we have one mole of HCl and one mole of NaOH. To help us think about what's
happening in this situation, we're gonna use an ICF table, where I stands for the
initial number of moles, C stands for the change in moles, and F stands for the
final amount of moles. The reason why we're using an ICF table instead of an ICE table
is in an ICE table, the E stands for equilibrium, and here we assume the
reaction goes to completion. Therefore, instead of E, we're writing F for final amount of moles. Let's fill out the rest of our ICF table. We have the initial amount
of moles of HCL and NaOH, and if we assume the
reaction hasn't happened yet, the initial amount of moles
of NaCl would be zero. Looking at the balanced equation, one mole of HCl reacts with
one mole of sodium hydroxide. And since the reaction goes to completion, we can go ahead and write,
under the change part here, we're gonna react one mole of HCl, so we're gonna lose that one mole of HCl. And since it's a one-to-one ratio, we're gonna lose one mole of NaOH. And since there's a one as a
coefficient in front of NaCl, under the change part on our ICF table, we would write plus one mole for NaCl. Since we started with one mole
of HCl, and we lost one mole, the final amount of moles
for HCl would be zero. Same thing for sodium hydroxide, the final amount of moles would be zero, because it's one minus one. For NaCl, we started off
with zero and we gained one. So we're ending up with
a final amount of moles of one mole of sodium chloride. So when the reaction goes to completion, the strong acid and the strong base have completely neutralized each other. And essentially, we
have an aqueous solution of sodium chloride. At 25 degrees Celsius,
the pH of water is seven, which is neutral, and the sodium cations
and the chloride anions do not react with water, so they don't affect the pH. Therefore, for the situation
where you have equal amount of moles of a strong
acid and a strong base, the pH of the resulting
solution will be seven. Let's do another strong
acid-strong base calculation. However, this time, the moles of strong acid and
strong base are not equal. So let's say we react 300 milliliters of a 1.0 molar solution of HCl with 100 milliliters
of a 1.0 molar solution of sodium hydroxide. And our goal is to calculate the pH of the resulting solution. And I should point out
these are aqueous solutions of HCl and NaOH. The first step is to
calculate moles of strong acid and moles of strong base. And the equation for molarity is equal to moles over liters. So for our strong acid,
HCl, the molarity is 1.0, and the volume is 300 milliliters, which is equal to .300 liters. So solving for x, we find that x is equal to .30 moles of HCl. For our strong base, sodium hydroxide, the concentration is one molar, and the volume is 100 milliliters, which is equal to .100 liters. So solving for x, we find that x is equal
to .10 moles of NaOH. Going back to HCl, since HCl is strong
acid that ionizes 100%, if we have .30 moles of HCl, we have .30 moles of H plus ions, or we could say .30 moles
of hydronium ions, H3O plus. And for sodium hydroxide, since sodium hydroxide is a strong base that dissociates 100%, if we have .10 moles of sodium hydroxide, we have .10 moles of sodium ions and also .10 moles of
hydroxide ions, OH minus. And if we think about
our net ionic equation, the .30 moles of hydronium
ions will react with the .10 moles of hydroxide ions. So here's one way to write
our net ionic equation. The hydronium ion plus the
hydroxide ion forms 2H2O. And to help us find the pH, we're gonna use another ICF table. The initial moles of hydronium ions, we calculated to be .30, and the initial moles of hydroxide ions, we calculated to be .10. Because we don't have
equal amounts of moles and the mole ratio is one to one. In this case, we're gonna have a limiting
reactant and an excess reactant. Since we only have .10
moles of hydroxide ions, all of the hydroxide ions will react. And since it's a one-to-one mole ratio, .10 moles of hydroxide ions
will react with .10 moles of hydronium ions, so we can write minus .10
under hydronium as well. Therefore, when the
reaction goes to completion, for the hydroxide ions, we'll have used all of them up. We started with .10
and we're using up .10, therefore, we'll have zero moles. For the hydronium ion, we started with .30 moles
minus .10 gives us .20 moles. So the hydroxide ions were
our limiting reactant, and the hydronium ions
where our excess reactant. So the strong base neutralized
some of the strong acid that was present, and we can calculate the pH
of the resulting solution by the number of moles
of excess hydronium ions. Next, we calculate the concentration of hydronium ions in solution. Since the number of moles
of hydronium ions is .20, we plug that into our
equation for molarity, so molarity is equal to moles over liters, and the total volume of our solution will be 300 milliliters
plus 100 milliliters, which is 400 milliliters, or .400 liters. So let me go and write this in here. So .20 moles divided by .400 liters is equal to a concentration of .50 molar. And because the pH is
equal to the negative log of the concentration of hydronium ions, if we plug in our concentration
of hydronium ions of .50, the pH is equal to the
negative log of .50, which is equal to .30. Having a low pH for our resulting solution at 25 degrees Celsius
makes a lot of sense, because we ended up
with an excess of acid.