Bases - Online Tutor, Practice Problems & Exam Prep

Group 1A metals (excluding hydrogen and francium) like lithium, sodium, and potassium, and certain Group 2A metals (excluding beryllium, magnesium, and radium) such as calcium and strontium, form strong bases when combined with basic anions like hydroxide (OH^-), hydride (H^-), amide (NH₂^-), and oxide (O₂^-). Strong bases are strong electrolytes that completely dissociate in water and are potent proton acceptors, meaning they readily accept H⁺ ions. Conversely, weak bases, like neutral amines, only partially dissociate and are weak proton acceptors, favoring the reactant side in equilibrium. Amines with a positive charge are weak acids. Understanding the behavior of strong and weak bases is crucial in predicting the outcomes of acid–base reactions and the strength of electrolytes in aqueous solutions.

Strong Bases

When certain group 1A and 2A metals combine with any of the following anions, they form strong bases. These anions are our basic anions. They include hydroxide ion which is OH-, hydride ion which is H-, guamide ion which is NH2- and then finally your oxide ion which is O2-. Remember when it comes to group 1A and 2A, Group 1A metals have a charge of plus one, and group 2A metals have a charge of two.

In Group 1A we shade out the hydrogen. It's not a metal, and the group 1A medals that we're talking about are lithium, sodium, potassium, rubidium and cesium. Realize here that franxium, which is below cesium, is not present because it's radioactive. It's kind of unpredictable in which it behaves, so we don't include it as one of the group 1A medals. In Group 2A, we Gray out beryllium and magnesium. They're not included as the certain types of group 2A metals. The certain types of Group 2A metals include calcium, strontium and barium.

Realize just like francium we have radium which is below barium. It's not included because again it's so large it's a little bit unpredictable. So we don't include it in our definition of bases. Now here we could choose any of these group 1A and 2A metals that I've written and combine them with one of these four basic anions to create a strong base. So let's say I chose lithium here this is +1. This is -1. When the numbers and the charges are the same, they just cancel out. So this is just lithium hydroxide.

Then let's say I chose strontium. Here the numbers in the charges are different. When they're different, they crisscross. Just worry about the numbers, don't worry about the charge. So this would be strontium hydride here. If we used NA plus one with the amide ion, do so give us sodium amide and then finally here I could use calcium with the oxide. The numbers are the same so they cancel out. So this just becomes calcium oxide. So just remember, these are the certain types of Group 1A metals and Group 2A metals that can combine with our four basic anions to create a strong base.

Bases Example

Which of the following represents a strong base? Now remember, certain group 1A and 2A metals need to be combined with a basic anion. We take a look at the options. Beryllium and magnesium are part of the certain types of group 1A or 2A metals. So A wouldn't be an answer, D wouldn't be an answer.

If we look at our remaining options, we have sodium, lithium and potassium. Yes, those are these certain types of group 1A metals that could potentially create a strong base if they combine with the right basic anion here. Sodium, if it combined with the oxide ion could make a strong base. But we have a problem. Remember, the numbers are different, so they crisscross 2 comes here. One comes here. Sodium oxide would actually be NA2O. This is not sodium oxide. This is actually sodium superoxide. It's not a strong base.

For the next one, we have lithium ion and it's combining with the amide ion, they give us lithium amide. So this is a strong base. It's the right group 1A metal with one of the basic anions in the form of the amide ion. And then finally E, we have potassium, which is good, but it's connected to hypoiodous acid. Hypoiodous acid does not represent one of our four basic anions, so this will not make a strong base.

So here the only answer that gives a strong base is option C in the form of lithium amide.

Strong Base vs Weak Base

Now following acids, we're going to say that strong bases are strong electrolytes and weak bases are weak electrolytes. Now with strong bases, we're going to say because they're strong electrolytes did associate or ionize completely in water and our strong proton acceptors remember when we say the term proton, we just mean H⁺, so they readily accept an H₂O.

Now bases on the other hand, because they're weak, they only partially dissociate. They don't completely ionize into ions and they are weak proton acceptors and they favor reactants. If we take a look here, we have sodium hydroxide which is a good example of a strong base, and then as a weak base we have ammonia.

Here sodium hydroxide completely ionizes when we throw it into water. So it breaks up to form 100% of sodium ion and hydroxide ion it associates completely. We're going to say here that it is a strong proton acceptor. This, OH^- that's created could gain an H from the water surrounding it. We'll talk about that in greater detail later on. And then here since we make 100% of our products, that means we'll have no reactants left. So this reaction heavily favors the formation of products.

Now for a weak base, ammonia is a weak base, it's a positive amine. Here because it's a weak base, very little of these ions are formed. So we're going to have a small arrow pointing towards the product side. A vast majority of it will exist in its molecular form in NH₃. So we show a larger arrow pointing in the opposite direction. So here it only partially dissociates. So we make way less than 100% of these ions. It is a weak proton acceptor and we're heading towards the reactant side to the reactants are more favored than the products.

So keep this in mind when we're looking at the differences, at least in terms of dissociation of strong bases versus weak bases.

Bases Example

Which of the following bases will partially dissolve when placed in water? So partially dissolves means it partially dissociates or ionizes. So we're looking for the weak base out of all the options. If we were to take a look, remember strong bases are formed when certain group 1A and 2A metals combined with a basic anion.

If we take a look at the options, lithium, sodium, cesium and potassium, are those certain types of group 1A metals? So they could potentially make strong bases if they combine with the right basic anion. Aluminum. Aluminum on the other hand, is not one of those certain types of group 1A and 2A metals. In fact, it's not even found in Group 1A or 2A. It's a group 3A metal.

And because of that, it's not going to constitute a strong base. It's going to be a weak base. It's still a base since it's aluminum, a metal cation connected to hydroxide ion. A basic ion OK. Since it's weak, it'll only partially dissolve if we take a look at the other options. If we look, we have lithium here connected to hydroxide ion. This combination can make a strong base.

Here we have sodium ion connected to the amide ion. This also can make a strong base. Here we have cesium oxide. So cesium here which is plus one, is connected to the oxide ion. O₂^- this combination can make a strong base. And then finally we have potassium hydroxide. So potassium is K⁺ Hydroxide is OH^-. So here this combination can make a strong base.

So out of all the options, only option C makes a weak base which only partially dissolve when placed in water.

Amines

Now recall that an amine is a covalent compound containing nitrogen and hydrogen or carbon, nitrogen, and hydrogen. It doesn't have to be in that order as long as it contains carbon, nitrogen, and hydrogen in some way. Now we're going to say that amines up to this point, we're going to take a look at neutral amines and positively charged amines.

Here we're going to say that neutral amines represent weak bases. So here we have three types of amines within the structure. And if we look at positive amines, those with positive charges, they represent weak acids. And here we have the positive charge associated with each of these amines.

So amines are pretty unique because they can exist as either acids or bases. So it's always important to take a look. Does your amine have a charge of positive? If it does, it's going to be a weak acid. Is it neutral? It'll be a weak base.

Now, we've also talked about another type of amine, amide ion, which represents one of the four strong basic anions. That's something else entirely. But realize here that yes, technically amines can have all three types of situations where they're positive, neutral, and negative. But for right now, just remember, the neutral ones are weak bases. The positively charged ones are weak acids.

Bases Example

Here it says to identify the amine that will likely accept a proton. Remember, a proton is H₊ when in the presence of an acid. All right, so first of all it says in a mean. Let's see which ones don't represent a means. A means themselves possess nitrogen and hydrogen or carbon, nitrogen and hydrogen in some way.

If we take a look, C is not an amine because of the presence of sulfur, and it doesn't even have nitrogen. E is also not an amine because here there's no nitrogen present. There's carbon and hydrogen, but where's the nitrogen next? It says, well weekly accept a proton when in the presence of an acid, weakly except a proton. Remember that's a definition of a weak base.

Here AB and D are all means, but remember it's the neutral means that represent weak basis. Positively charged A means represent weak acids. So here this is a weak asset. Since it's positively charged, this is a weak asset since it's possibly charged. The answer here would be option B that represents A neutral mean. Therefore it's a weak base and A weakly accept a proton from an acid.

Which of the following bases would more greatly favor the product side of a chemical reaction? 

a) BeH₂                         b) H₂Se                         c) SrH₂                          d) Pb(OH)₄                    e) HF

• A. BeH₂
• B. H₂Se
• C. SrH₂
• D. Pb(OH)₄
• E. HF

Which of the following compounds would be found as mostly molecules when placed into water? 

I. Be(OH)₂                     II. HNO₃                        III. LiOH                        IV. (CH­₃)₂NH                 V. CaO

a) I, IV, V

b) I only

c) I and IV

d) IV only

e) None of the above

• A. I, IV, V
• B. I only
• C. I and IV
• D. IV only
• E. None of the above

Here’s what students ask on this topic:

How do you rank the strongest to weakest base?

To rank bases from strongest to weakest, you need to consider their ability to accept protons (H⁺ ions) in an aqueous solution. A strong base is one that completely dissociates in water, meaning it donates hydroxide ions (OH^-) readily. Examples of strong bases are group 1 hydroxides like sodium hydroxide (NaOH) and potassium hydroxide (KOH), as well as some group 2 hydroxides like barium hydroxide (Ba(OH)₂).

Weak bases do not fully dissociate in water and have a lower ability to accept protons. They include substances like ammonia (NH₃) and its derivatives, or organic compounds like amines.

To rank bases, look at their dissociation constant (K_b) values. The higher the K_b, the stronger the base. Also, consider the position in the periodic table; bases formed from elements lower down in the group are typically stronger because their ions are larger and can more easily donate an OH^- ion.

In summary, rank bases from strongest to weakest by considering their dissociation in water, their K_b values, and their position in the periodic table.

What determines the strength of a base?

The strength of a base is determined by its ability to donate a hydroxide ion (OH^-) and accept a proton (H⁺) in a solution. Strong bases are characterized by their complete dissociation in water, meaning they release all of their hydroxide ions into the solution. This complete dissociation results in a higher pH, indicating a stronger base.

The strength of a base can also be measured by its basicity constant (K_b), which is an equilibrium constant that reflects the extent to which a base reacts with water to form hydroxide ions. A larger K_b value indicates a stronger base.

Another factor is the molecular structure of the base. For example, bases with a negative charge or with atoms that have a larger atomic radius tend to be stronger because they can more easily donate a hydroxide ion.

Lastly, the solvent in which the base is dissolved can influence its strength. In water, the solvent itself can stabilize or destabilize the base, affecting its ionization and, consequently, its strength.

How can base strength be determined from structure?

Base strength can be determined from its molecular structure by examining several key factors:

1. Electronegativity: In a base, the atom that donates the pair of electrons (usually a nitrogen or oxygen) should have a lower electronegativity so it can more easily share its electrons.
2. Charge: A negatively charged species (anions) are generally stronger bases than their neutral counterparts because the extra electron density increases their ability to donate electrons.
3. Resonance: If the negative charge on the base can be delocalized through resonance, the base is typically stronger because the charge is more stabilized.
4. Atom Size: For bases in the same group of the periodic table, the larger the atom, the stronger the base. This is because larger atoms can spread out the negative charge over a larger volume, making them more stable.
5. Inductive Effect: Electron-donating groups attached to the base can increase base strength by pushing electron density towards the atom that donates the electron pair, while electron-withdrawing groups can decrease base strength.
6. Hybridization: The s-character of the atom that bears the negative charge also affects basicity. Atoms with more s-character (like sp-hybridized atoms) hold onto their electrons more tightly and are less likely to share them, making them weaker bases.