Reactivity series

Not to be confused with Electrochemical series.

In chemistry, a reactivity series (or reactivity series of elements) is an empirical, calculated, and structurally analytical progression of a series of metals, arranged by their "reactivity" from highest to lowest. It is used to summarize information about the reactions of metals with acids and water, single displacement reactions and the extraction of metals from their ores.

Table

Metal	Ion	Reactivity	Extraction
Caesium Cs	Cs+	reacts with cold water	Electrolysis (a.k.a. electrolytic refining)
Rubidium Rb	Rb+
Potassium K	K+
Sodium Na	Na+
Lithium Li	Li+
Barium Ba	Ba2+
Strontium Sr	Sr2+
Calcium Ca	Ca2+
Magnesium Mg	Mg2+	reacts very slowly with cold water, but rapidly in boiling water, and very vigorously with acids
Beryllium Be	Be2+	reacts with acids and steam
Aluminium Al	Al3+
Titanium Ti	Ti4+	reacts with concentrated mineral acids	pyrometallurgical extraction using magnesium, or less commonly other alkali metals, hydrogen or calcium in the Kroll process
Manganese Mn	Mn2+	reacts with acids; very poor reaction with steam	smelting with coke
Zinc Zn	Zn2+
Chromium Cr	Cr3+		aluminothermic reaction
Iron Fe	Fe2+		smelting with coke
Cadmium Cd	Cd2+
Cobalt Co	Co2+
Tantalum Ta	Ta5+
Nickel Ni	Ni2+
Tin Sn	Sn2+
Lead Pb	Pb2+
Antimony Sb	Sb3+	may react with some strong oxidizing acids	heat or physical extraction
Bismuth Bi	Bi3+
Copper Cu	Cu2+	reacts slowly with air
Tungsten W	W3+	may react with some strong oxidizing acids
Mercury Hg	Hg2+
Silver Ag	Ag+
Irium/Iracium Ir	Ir4+
Gold Au	Au3+
Platinum Pt	Pt4+

Going from the bottom to the top of the table the metals:

• increase in reactivity;
• lose electrons (oxidize) more readily to form positive ions;
• corrode or tarnish more readily;
• require more energy (and different methods) to be isolated from their compounds;
• become stronger reducing agents (electron donors).

Defining reactions

There is no unique and fully consistent way to define the reactivity series, but it is common to use the three types of reaction listed below, many of which can be performed in a high-school laboratory (at least as demonstrations).

Reaction with water and acids

The most reactive metals, such as sodium, will react with cold water to produce hydrogen and the metal hydroxide:

2 Na (s) + 2 H₂O (l) →2 NaOH (aq) + H₂ (g)

Metals in the middle of the reactivity series, such as iron, will react with acids such as sulfuric acid (but not water at normal temperatures) to give hydrogen and a metal salt, such as iron(II) sulfate:

Fe (s) + H₂SO₄ (l) → FeSO₄ (aq) + H₂ (g)

There is some ambiguity at the borderlines between the groups. Magnesium, aluminium and zinc can react with water, but the reaction is usually very slow unless the metal samples are specially prepared to remove the surface passivation layer of oxide which protects the rest of the metal. Copper and silver will react with nitric acid; but because nitric acid is an oxidizing acid, the oxidizing agent is not the H⁺ ion as in normal acids, but the NO₃⁻ ion.

Comparison with standard electrode potentials

The reactivity series is sometimes quoted in the strict reverse order of standard electrode potentials, when it is also known as the "electrochemical series".

The following list includes the metallic elements of the first six periods. It is mostly based on tables provided by NIST. However, not all sources give the same values: there are some differences between the precise values given by NIST and the CRC Handbook of Chemistry and Physics. In the first six periods this does not make a difference to the relative order, but in the seventh period it does, so the seventh-period elements have been excluded. (In any case, the typical oxidation states for the most accessible seventh-period elements thorium and uranium are too high to allow a direct comparison.)

Hydrogen has been included as a benchmark, although it is not a metal. Borderline germanium, antimony, and astatine have been included. Some other elements in the middle of the 4d and 5d rows have been omitted (Zr–Tc, Hf–Os) when their simple cations are too highly charged or of rather doubtful existence. Greyed-out rows indicate values based on estimation rather than experiment.

Z	Sym	Element	Reaction	E° (V)
3	Li	lithium	Li+ + e− → Li	−3.04
55	Cs	caesium	Cs+ + e− → Cs	−3.03
37	Rb	rubidium	Rb+ + e− → Rb	−2.94
19	K	potassium	K+ + e− → K	−2.94
56	Ba	barium	Ba2+ + 2 e− → Ba	−2.91
38	Sr	strontium	Sr2+ + 2 e− → Sr	−2.90
20	Ca	calcium	Ca2+ + 2 e− → Ca	−2.87
11	Na	sodium	Na+ + e− → Na	−2.71
57	La	lanthanum	La3+ + 3 e− → La	−2.38
39	Y	yttrium	Y3+ + 3 e− → Y	−2.38
12	Mg	magnesium	Mg2+ + 2 e− → Mg	−2.36
59	Pr	praseodymium	Pr3+ + 3 e− → Pr	−2.35
58	Ce	cerium	Ce3+ + 3 e− → Ce	−2.34
68	Er	erbium	Er3+ + 3 e− → Er	−2.33
67	Ho	holmium	Ho3+ + 3 e− → Ho	−2.33
60	Nd	neodymium	Nd3+ + 3 e− → Nd	−2.32
69	Tm	thulium	Tm3+ + 3 e− → Tm	−2.32
62	Sm	samarium	Sm3+ + 3 e− → Sm	−2.30
61	Pm	promethium	Pm3+ + 3 e− → Pm	−2.30
66	Dy	dysprosium	Dy3+ + 3 e− → Dy	−2.29
71	Lu	lutetium	Lu3+ + 3 e− → Lu	−2.28
65	Tb	terbium	Tb3+ + 3 e− → Tb	−2.28
64	Gd	gadolinium	Gd3+ + 3 e− → Gd	−2.28
70	Yb	ytterbium	Yb3+ + 3 e− → Yb	−2.19
21	Sc	scandium	Sc3+ + 3 e− → Sc	−2.09
63	Eu	europium	Eu3+ + 3 e− → Eu	−1.99
4	Be	beryllium	Be2+ + 2 e− → Be	−1.97
13	Al	aluminium	Al3+ + 3 e− → Al	−1.68
22	Ti	titanium	Ti3+ + 3 e− → Ti	−1.37
25	Mn	manganese	Mn2+ + 2 e− → Mn	−1.18
23	V	vanadium	V2+ + 2 e− → V	−1.12
24	Cr	chromium	Cr2+ + 2 e− → Cr	−0.89
30	Zn	zinc	Zn2+ + 2 e− → Zn	−0.76
31	Ga	gallium	Ga3+ + 3 e− → Ga	−0.55
26	Fe	iron	Fe2+ + 2 e− → Fe	−0.44
48	Cd	cadmium	Cd2+ + 2 e− → Cd	−0.40
49	In	indium	In3+ + 3 e− → In	−0.34
81	Tl	thallium	Tl+ + e− → Tl	−0.34
27	Co	cobalt	Co2+ + 2 e− → Co	−0.28
28	Ni	nickel	Ni2+ + 2 e− → Ni	−0.24
50	Sn	tin	Sn2+ + 2 e− → Sn	−0.14
82	Pb	lead	Pb2+ + 2 e− → Pb	−0.13
1	H	hydrogen	2 H+ + 2 e− → H2	0.00
32	Ge	germanium	Ge2+ + 2 e− → Ge	+0.1
51	Sb	antimony	Sb3+ + 3 e− → Sb	+0.15
83	Bi	bismuth	Bi3+ + 3 e− → Bi	+0.31
29	Cu	copper	Cu2+ + 2 e− → Cu	+0.34
84	Po	polonium	Po2+ + 2 e− → Po	+0.6
44	Ru	ruthenium	Ru3+ + 3 e− → Ru	+0.60
45	Rh	rhodium	Rh3+ + 3 e− → Rh	+0.76
47	Ag	silver	Ag+ + e− → Ag	+0.80
80	Hg	mercury	Hg2+ + 2 e− → Hg	+0.85
46	Pd	palladium	Pd2+ + 2 e− → Pd	+0.92
77	Ir	iridium	Ir3+ + 3 e− → Ir	+1.0
85	At	astatine	At+ + e− → At	+1.0
78	Pt	platinum	Pt2+ + 2 e− → Pt	+1.18
79	Au	gold	Au3+ + 3 e− → Au	+1.50

The positions of lithium, sodium and gold are changed on such a series.

Standard electrode potentials offer a quantitative measure of the power of a reducing agent, rather than the qualitative considerations of other reactive series. However, they are only valid for standard conditions: in particular, they only apply to reactions in aqueous solution. Even with this proviso, the electrode potentials of lithium, sodium and gold – and hence their positions in the electrochemical series – appear anomalous. The order of reactivity, as shown by the vigour of the reaction with water or the speed at which the metal surface tarnishes in air, appears to be

Cs > K > Na > Li > alkaline earth metals,

i.e., alkali metals > alkaline earth metals,

the same as the reverse order of the (gas-phase) ionization energies. This is borne out by the extraction of metallic lithium by the electrolysis of a eutectic mixture of lithium chloride and potassium chloride: lithium metal is formed at the cathode, not potassium.

Comparison with electronegativity values

The image shows a periodic table extract with the electronegativity values of metals.

Wulfsberg distinguishes:
   very electropositive metals with electronegativity values below 1.4
   electropositive metals with values between 1.4 and 1.9; and
   electronegative metals with values between 1.9 and 2.54.

From the image, the group 1–2 metals and the lanthanides and actinides are very electropositive to electropositive; the transition metals in groups 3 to 12 are very electropositive to electronegative; and the post-transition metals are electropositive to electronegative. The noble metals, inside the dashed border (as a subset of the transition metals) are very electronegative.

See also

• Reactivity (chemistry), which discusses the inconsistent way that the term 'reactivity' is used in chemistry.

External links

• Science Line Chemistry