A Lewis base is a substance that donates a pair of electrons to form a covalent bond. So, a Lewis acid-base reaction is represented by the transfer of a pair of electrons from a base to an acid. A hydrogen ion, which lacks any electrons, accepts a pair of electrons. Thus, either NH3 or H2O can act as an acid or a base. Which one is a strong base — ammonia or sodium hydroxide? Sodium Hydroxide is stronger base than Ammonia.
Ammonia NH3 dissolved in water, is a weak base. Because the hydrogen and the nitrogen are attracted to the same electrons, they are pulled together. As it happens, ammonia NH3 is not a strong acid; it is a weak base. The nitrogen of ammonia is surrounded by four pairs of electrons. Solution : Be OH 2is the weakest base, since alkali metal hydroxides are stronger base than alkaline earth metal hydroxides.
Also, basic character of hydroxides increases on moving down the group. Hence, Be OH 2 is the weakest base. Begin typing your search term above and press enter to search. Hydrobromic acid, for example, gives hydrobromide salts.
Dextromethorphan, an ingredient in many cough medicines, is dispensed as dextromethorphan hydrobromide. The accompanying figure shows another medication lidocaine as a hydrochloride salt.
According to the Bronsted-Lowry theory of acids and bases, an acid is a proton donor and a base is a proton acceptor. Once an acid has given up a proton, the part that remains is called the acid's conjugate base. This species is a base because it can accept a proton to re-form the acid. The conjugate base of HF first example below is fluoride ion, F -. Similarly, the part of the base that remains after a base accepts a proton is called the base's conjugate acid.
This species is an acid because it can give up a proton and thus re-form the base. The conjugate acid of fluoride ion, F - first example below is HF. The table below lists conjugate acid-base pairs for your reference so that you can figure out the strategy of identifying them.
For any given acid or base, you should be able to give its conjugate base or conjugate acid. In the reaction of ammonia with water to give ammonium ions and hydroxide ions, ammonia acts as a base by accepting a proton from a water molecule, which in this case means that water is acting as an acid. In the reverse reaction, an ammonium ion acts as an acid by donating a proton to a hydroxide ion, and the hydroxide ion acts as a base. In the forward reaction, the parent acid is H 2 O and and the parent base is NH 3 shown in the illustration below.
The base OH - gains a proton, to produce its conjugate acid H 2 O. When hydrogen fluoride HF dissolves in water and ionizes, protons are transferred from hydrogen fluoride parent acid molecules to water parent base molecules, yielding hydronium ions conjugate acid of water and fluoride ions conjugate base of HF :.
Similarly, in the reaction of acetic acid with water, acetic acid donates a proton to water, which acts as the base. The use of conjugate acid-base pairs allows us to make a very simple statement about relative strengths of acids and bases. The stronger an acid, the weaker its conjugate base , and, conversely, the stronger a base, the weaker its conjugate acid. Write the chemical equation for the reaction that occurs when cocaine hydrochloride C 17 H 22 ClNO 4 dissolves in water and donates a proton to a water molecule.
That's not a problem as far as the definition of amphiprotic is concerned - but the reaction as an acid is. The aluminium oxide doesn't contain any hydrogen ions to donate! But aluminium oxide reacts with bases like sodium hydroxide solution to form complex aluminate ions. You can think of lone pairs on hydroxide ions as forming dative covalent coordinate bonds with empty orbitals in the aluminium ions.
The aluminium ions are accepting lone pairs acting as a Lewis acid. So aluminium oxide can act as both an acid and a base - and so is amphoteric. But it isn't amphiprotic because both of the acid reaction and the base reaction don't involve hydrogen ions. I have gone through odd years of teaching in the lab, and via books and the internet without once using the term amphiprotic!
I simply don't see the point of it. The term amphoteric takes in all the cases of substances functioning as both acids and bases without exception. The term amphiprotic can only be used where both of these functions involve transference of hydrogen ions - in other words, it can only be used if you are limited to talking about the Bronsted-Lowry theory.
Personally, I would stick to the older, more useful, term "amphoteric" unless your syllabus demands that you use the word "amphiprotic". It is easiest to see the relationship by looking at exactly what Bronsted-Lowry bases do when they accept hydrogen ions. Three Bronsted-Lowry bases we've looked at are hydroxide ions, ammonia and water, and they are typical of all the rest. The Bronsted-Lowry theory says that they are acting as bases because they are combining with hydrogen ions.
The reason they are combining with hydrogen ions is that they have lone pairs of electrons - which is what the Lewis theory says. The two are entirely consistent. So how does this extend the concept of a base? At the moment it doesn't - it just looks at it from a different angle. But what about other similar reactions of ammonia or water, for example? On the Lewis theory, any reaction in which the ammonia or water used their lone pairs of electrons to form a co-ordinate bond would be counted as them acting as a base.
Here is a reaction which you will find talked about on the page dealing with co-ordinate bonding. Ammonia reacts with BF 3 by using its lone pair to form a co-ordinate bond with the empty orbital on the boron. As far as the ammonia is concerned, it is behaving exactly the same as when it reacts with a hydrogen ion - it is using its lone pair to form a co-ordinate bond.
If you are going to describe it as a base in one case, it makes sense to describe it as one in the other case as well. Note: If you haven't already read the page about co-ordinate bonding you should do so now. You will find an important example of water acting as a Lewis base as well as this example - although the term Lewis base isn't used on that page.
Lewis acids are electron pair acceptors. In the above example, the BF 3 is acting as the Lewis acid by accepting the nitrogen's lone pair. On the Bronsted-Lowry theory, the BF 3 has nothing remotely acidic about it. What about more obviously acid-base reactions - like, for example, the reaction between ammonia and hydrogen chloride gas?
What exactly is accepting the lone pair of electrons on the nitrogen. Textbooks often write this as if the ammonia is donating its lone pair to a hydrogen ion - a simple proton with no electrons around it. That is misleading! You don't usually get free hydrogen ions in chemical systems. They are so reactive that they are always attached to something else.
There aren't any uncombined hydrogen ions in HCl. There isn't an empty orbital anywhere on the HCl which can accept a pair of electrons. Why, then, is the HCl a Lewis acid? Chlorine is more electronegative than hydrogen, and that means that the hydrogen chloride will be a polar molecule. The electrons in the hydrogen-chlorine bond will be attracted towards the chlorine end, leaving the hydrogen slightly positive and the chlorine slightly negative.
Note: If you aren't sure about electronegativity and bond polarity it might be useful to follow this link. The lone pair on the nitrogen of an ammonia molecule is attracted to the slightly positive hydrogen atom in the HCl. As it approaches it, the electrons in the hydrogen-chlorine bond are repelled still further towards the chlorine. Eventually, a co-ordinate bond is formed between the nitrogen and the hydrogen, and the chlorine breaks away as a chloride ion.
This is best shown using the "curly arrow" notation commonly used in organic reaction mechanisms. Note: If you aren't happy about the use of curly arrows to show movements of electron pairs, you should follow this link. The whole HCl molecule is acting as a Lewis acid. It is accepting a pair of electrons from the ammonia, and in the process it breaks up.
Lewis acids don't necessarily have to have an existing empty orbital. If you are a UK A' level student, you might occasionally come across the terms Lewis acid and Lewis base in textbooks or other sources.
0コメント