How hydrochloric acid reacts with aluminum. Formulas and description of the process

Alu­minum is a mal­leable, light, sil­very-white met­al. It is a good elec­tri­cal con­duc­tor. It can re­act with both acids and bases. Com­bin­ing alu­minum with an acid re­sults in a typ­i­cal sin­gle dis­place­ment re­ac­tion, form­ing alu­minum salt and gaseous hy­dro­gen. This can be seen from a sim­ple ex­am­ple – how hy­drochlo­ric acid re­acts with alu­minum.

With al­ka­lis, the re­ac­tion pro­ceeds dif­fer­ent­ly: in ad­di­tion to a hy­dro­gen re­lease, the re­ac­tion forms MeAlO₂ alu­mi­nate (where Me is the cation of a met­al from the al­ka­li) and a com­plex com­pound with the for­mu­la Me[Al(OH)₄] in the so­lu­tion.

How alu­minum re­acts with hy­drochlo­ric acid

Alu­minum re­acts with di­lut­ed hy­drochlo­ric acid at room tem­per­a­ture. The met­al dis­solves in hy­drochlo­ric acid, yield­ing alu­minum chlo­ride and col­or­less hy­dro­gen gas. This re­ac­tion is ir­re­versible, as the fi­nal prod­ucts will not re­act with each oth­er. The re­ac­tion be­tween metal­lic alu­minum and hy­drochlo­ric acid is what is known as an ox­i­da­tion-re­duc­tion re­ac­tion. Alu­minum acts as the re­duc­ing agent, giv­ing up elec­trons:

Al⁰ - 3e = Al³⁺

Cations of hy­drochlo­ric acid take these elec­trons and are re­duced to molec­u­lar hy­dro­gen:

2H⁺ + 2e = H₂

The com­plete ion­ic re­ac­tion equa­tion reads:

2Al⁰ + 6H⁺ + 6Cl⁻ = 2Al³⁺ + 6Cl⁻ + 3H₂↑

Net-ion­ic form:

2Al⁰ + 6H⁺ = 2Al³⁺ + 3H₂↑

In molec­u­lar form, the re­ac­tion looks as fol­lows:

2Al + 6HCl = 2Al­Cl₃ + 3H₂↑

Metal­lic alu­minum is not the only sub­stance ca­pa­ble of re­act­ing with hy­drochlo­ric acid – many met­al com­pounds pos­sess this prop­er­ty. An ex­change re­ac­tion oc­curs with the salts, when ions or re­ac­tive groups of both reagents “change places.” In or­der for the re­ac­tion with alu­minum or its com­pounds to be ir­re­versible, the reagents must form a gas, a pre­cip­i­tate, or a poor­ly-sol­u­ble sub­stance. The nec­es­sary reagent quan­ti­ties must be cal­cu­lat­ed with pre­ci­sion.

Re­ac­tions of alu­minum hy­drox­ides and ox­ides with hy­drochlo­ric acid

Al(OH)₃ is an am­pho­ter­ic base, which is a white jel­ly-like pre­cip­i­tate that dis­solves poor­ly in wa­ter.

Alu­minum hy­drox­ide en­ters into a neu­tral­iza­tion re­ac­tion with hy­drochlo­ric acid (the hy­drox­ide must be fresh­ly-pre­cip­i­tat­ed for the re­ac­tion to pro­ceed re­li­ably):

Al(OH)₃ + 3HCl = Al­Cl₃ + 3H₂O

One can ob­serve the dis­so­lu­tion of the white pre­cip­i­tate of alu­minum hy­drox­ide (alu­minum chlo­ride Al­Cl₃ dis­solves well in wa­ter). With alu­minum ox­ide, the re­ac­tion yields salt and wa­ter ac­cord­ing to the fol­low­ing equa­tion:

Al₂O₃ + 6HCl = 2Al­Cl₃ + 3H₂O

Re­ac­tions of salts, hy­drides, and alu­minum com­plex­es with hy­drochlo­ric acid

Hy­drochlo­ric acid also re­acts with many oth­er alu­minum com­pounds.

With alu­minum car­bide

Al₄C₃ + 12H­Cl = 4Al­Cl₃ + 3CH₄↑

(alu­minum car­bide dis­solves when treat­ed with an ex­cess of hy­drochlo­ric acid)

With alu­minum ac­etate

(CH₃­COO)₃Al + 3HCl = Al­Cl₃ + 3CH₃­COOH

With alu­minum ni­tride

AlN + 4HCl = Al­Cl₃ + NH₄­Cl

(hot con­cen­trat­ed acid is used; the re­ac­tion takes place slow­ly)

With alu­minum sul­fide

Al₂S₃ + 6HCl = 2Al­Cl₃ + 3H₂S↑

With alu­minum phos­phide

AlP + 3HCl = Al­Cl₃ + PH₃↑

(The re­ac­tion pre­sup­pos­es the treat­ment of phos­phide with hot con­cen­trat­ed acid)

With alu­minum phos­phate

AlPO₄ + 3HCl = Al­Cl₃ + H₃PO₄

With lithi­um alanate (tetrahy­droa­lu­mi­nate)

Li[AlH₄] + 4HCl = Al­Cl₃ + LiCl + 4H₂↑

(The re­ac­tion is car­ried out at a low tem­per­a­ture)

With sodi­um alu­mi­nate

NaAlO₂ + 4HCl = NaCl + Al­Cl₃ + 2H₂O

With sodi­um tetrahy­drox­oa­lu­mi­nate

Na[Al(OH)₄] + 4HCl = Al­Cl₃ + NaCl + 4H₂O

Alu­minum sul­fates and ni­trates do not re­act with hy­drochlo­ric acid, as all the com­pounds in the mix­ture are sol­u­ble – no pre­cip­i­tate forms, no poor­ly-sol­u­ble sub­stances form, and gas is not re­leased.

Click here to learn more about alu­minum and its prop­er­ties.

How mix­tures of met­als re­act with hy­drochlo­ric acid

If you take a mix­ture of sev­er­al met­als and treat them with hy­drochlo­ric acid, each met­al will re­act sep­a­rate­ly. For ex­am­ple, if you add HCl to a mix­ture of alu­minum and iron shav­ings, the re­ac­tion will pro­ceed as fol­lows:

Fe + 2HCl = Fe­Cl₂ + H₂↑

2Al + 6HCl = 2Al­Cl₃ + 3H₂↑

As di­lut­ed hy­drochlo­ric acid is a weak ox­i­diz­er, iron is only re­duced to a +2 ox­i­da­tion state.

Prod­ucts of the re­ac­tion be­tween alu­minum and hy­drochlo­ric acid, and their ap­pli­ca­tion

Al­most all re­ac­tions of hy­drochlo­ric acid and alu­minum (or its com­pounds) re­sult in the for­ma­tion of alu­minum chlo­ride (Al­Cl₃). The salt dis­solves well in or­gan­ic sol­vents (ni­troben­zene, dichloroethane, ace­tone) and wa­ter. In aque­ous so­lu­tions, one can ob­serve the hy­drol­y­sis of Al­Cl₃, as this salt is formed by the strong acid HCl and the weak base Al(OH)₃.

Al­Cl₃ is used as a cat­a­lyst in or­gan­ic syn­the­sis. For in­stance, it is uti­lized in the iso­mer­iza­tion of paraf­fins, ini­ti­a­tion of alky­la­tion re­ac­tions, acy­la­tion, and the break­down of oil into frac­tions. Alu­mini­um chlo­ride hex­ahy­drate Al­Cl₃・6H₂O is used to treat tim­ber ma­te­ri­als, pu­ri­fy waste­water, and man­u­fac­ture an­tiper­spi­rants.

The re­ac­tion of alu­minum with a so­lu­tion of hy­drochlo­ric acid can be used as a lab­o­ra­to­ry method for ob­tain­ing hy­dro­gen (but metal­lic zinc is more com­mon­ly used for these pur­pos­es).