Characteristics of potassium and its interaction with water

Why it cannot be kept in the open air

[Deposit Photos]

Potas­si­um is the 19th el­e­ment on the pe­ri­od­ic ta­ble, and is an al­ka­line met­al. It is a sim­ple sub­stance which in nor­mal con­di­tions is in a sol­id ag­gre­gate state. Potas­si­um boils at a tem­per­a­ture of 761 de­grees Cel­sius. The melt­ing tem­per­a­ture of the el­e­ment is 63 de­grees Cel­sius. Potas­si­um has a sil­very-white col­or, and a metal­lic shine.

The chem­i­cal prop­er­ties of potas­si­um

Potas­si­um is a chem­i­cal el­e­ment with a high chem­i­cal ac­tiv­i­ty, so it can­not be kept in the open air – the al­ka­line met­al im­me­di­ate­ly en­ters into a re­ac­tion with sur­round­ing sub­stances. This chem­i­cal el­e­ment be­longs to the 1st group and 4th pe­ri­od of the pe­ri­od­ic ta­ble. Potas­si­um has all of the prop­er­ties char­ac­ter­is­tics for met­als.

Potas­si­um in­ter­acts with sim­ple sub­stances, such as halo­gens (bromine, chlo­rine, flu­o­rine, io­dine), and also phos­pho­rous, sul­fur, ni­tro­gen and oxy­gen. The in­ter­ac­tion of cal­ci­um with oxy­gen is called ox­i­da­tion. In this chem­i­cal re­ac­tion, oxy­gen and potas­si­um are con­sumed in the molec­u­lar ra­tio of 4 parts to 1, as a re­sult of which two moles of potas­si­um ox­ide form. This in­ter­ac­tion can be ex­pressed by the equa­tion:

4К + О₂ = 2К₂О

When potas­si­um burns, it has a bright pur­ple flame. This re­ac­tion is con­sid­ered qual­i­ta­tive for the de­ter­mi­na­tion of potas­si­um. The re­ac­tion of potas­si­um with halo­gens is named in ac­cor­dance with the names of the chem­i­cal el­e­ments: flu­o­ri­na­tion, io­d­i­na­tion, bromi­na­tion and chlo­ri­na­tion. These in­ter­ac­tions are called com­bi­na­tion re­ac­tions, as the atoms of two dif­fer­ent sub­stances unite into one. For ex­am­ple, the re­ac­tion be­tween potas­si­um and chlo­rine, which forms potas­si­um chlo­ride. To car­ry out this re­ac­tion, take two moles of potas­si­um and one mole of chlo­rine. As a re­sult, two moles of the potas­si­um com­pound form:

2К + СІ₂ = 2КСІ

Potassium-chloride-3D-ionic [Wikimedia]

With ni­tro­gen, potas­si­um forms a com­pound when it burns in the open air. Potas­si­um and ni­tro­gen are con­sumed in this re­ac­tion in the molec­u­lar ra­tio of 6 parts to 1, as a re­sult of which two moles of potas­si­um ni­tride form:

6К + N₂ = 2K₃N

The com­pound con­sists of crys­tals of a green­ish-black col­or. Potas­si­um re­acts with phos­pho­rous ac­cord­ing to the same prin­ci­ple. If you take 3 moles of potas­si­um and 1 mole of phos­pho­rous, you get 1 mole of phos­phide:

3К + Р = К₃Р

Potas­si­um re­acts with hy­dro­gen, form­ing a hy­dride

2К + Н₂ = 2КН

All the com­bi­na­tion re­ac­tions take place at high tem­per­a­tures.

In­ter­ac­tion of potas­si­um with com­plex sub­stances

Com­plex sub­stances with which potas­si­um en­ters into a re­ac­tion in­clude wa­ter, salts, acids and ox­ides. As potas­si­um is an ac­tive met­al, it forces the hy­dro­gen atoms out of their bonds. For ex­am­ple, the re­ac­tion be­tween potas­si­um and hy­drochlo­ric acid. Take 2 moles of potas­si­um and 2 moles of acid. As a re­sult of the re­ac­tion 2 moles of potas­si­um chlo­ride and 1 mole of hy­dro­gen form:

2К + 2НСІ = 2КСІ + Н₂

We should look in more de­tail at the in­ter­ac­tion of potas­si­um with wa­ter. Potas­si­um is an ac­tive met­al which re­acts vi­o­lent­ly with wa­ter. The el­e­ment moves over the sur­face of the wa­ter, and is pushed by the hy­dro­gen re­leased:

2K + 2H₂O = 2KOH + H₂↑

Reaction of potassium and water [Wikimedia]

Dur­ing the course of the re­ac­tion, in one unit of time a great deal of heat is re­leased, which leads to the ig­ni­tion of the potas­si­um and the re­leased hy­dro­gen. It is in­ter­est­ing to ob­serve this process – the potas­si­um in­stan­ta­neous­ly ig­nites on con­tact with wa­ter, a pur­ple flame crack­les and quick­ly moves on the sur­face of the wa­ter. At the end of the re­ac­tion there is a flash, spat­ter­ing drips of burn­ing potas­si­um and re­ac­tion prod­ucts. The main end prod­uct of the re­ac­tion of potas­si­um with wa­ter is potas­si­um hy­drox­ide (an al­ka­li). The for­mu­la is

4K + 2H₂O + O₂ = 4KOH

Warn­ing! Don’t try to re­peat this ex­per­i­ment with­out a pro­fes­sion­al su­per­vi­sion!

If the ex­per­i­ment show­ing the in­ter­ac­tion of potas­si­um with wa­ter is con­duct­ed un­suc­cess­ful­ly, then you may get burned by the al­ka­line. To con­duct the re­ac­tion, we usu­al­ly use a crys­tal­liz­er with wa­ter, in which we place a piece of al­ka­line met­al (potas­si­um in this case). Dur­ing this ef­fec­tive ex­per­i­ment, many ob­servers try to get close to the lab­o­ra­to­ry ta­ble, and as soon as the hy­dro­gen stops burn­ing, they look into the crys­tal­liz­er. At this mo­ment, the fi­nal stage of the re­ac­tion of potas­si­um with wa­ter takes place, ac­com­pa­nied by a small ex­plo­sion and the splat­ter­ing of the hot al­ka­li that has formed. So for safe­ty rea­sons, you should re­main at a cer­tain dis­tance from the lab­o­ra­to­ry ta­ble, un­til the re­ac­tion has fin­ished com­plete­ly. Here you’ll find the safest ex­per­i­ments to do at home with your kids.

The struc­ture of potas­si­um

[Deposit Photos]

The potas­si­um atom con­sists of a nu­cle­us con­tain­ing pro­tons and neu­trons, and elec­trons which re­volve around it. The num­ber of elec­trons is al­ways equal to the num­ber of pro­tons in the nu­cle­us. When an elec­tron breaks away or when one joins an atom, the atom ceas­es to be neu­tral and turns into an ion. Ions are di­vid­ed into cations and an­ions. Cations have a pos­i­tive charge, and an­ions a neg­a­tive charge. When an elec­tron joins an atom, the atom turns into an an­ion, while if one of the elec­trons leaves its or­bit, a neu­tral atom turns into a cation.

The atom­ic num­ber of potas­si­um in the pe­ri­od­ic ta­ble is 19, and so there are also 19 pro­tons in the nu­cle­us of the el­e­ment. Con­clu­sion: there are 19 elec­trons re­volv­ing around the nu­cle­us. The num­ber of pro­tons con­tained in the struc­ture of the atom is de­ter­mined as fol­lows: sub­tract the atom­ic num­ber of the el­e­ment from the atom­ic mass. Con­clu­sion: there are 20 pro­tons in the nu­cle­us of potas­si­um. Potas­si­um be­longs to the 4th pe­ri­od, and has 4 “or­bits”, and elec­trons are ar­ranged even­ly on the or­bits, which are in con­stant move­ment. The di­a­gram of potas­si­um looks as fol­lows: in the first “or­bit” there are 2 elec­trons; there are eight in both the sec­ond and third, and on the fourth and last “or­bit”, there is one elec­tron. This ex­plains the high lev­el of chem­i­cal ac­tiv­i­ty of potas­si­um – its last “or­bit” is not com­plete­ly filled, so the el­e­ment strives to bond with oth­er atoms, as a re­sult of which the elec­trons of the last or­bits of two el­e­ments be­come shared.