Сборник текстов на казахском, русском, английском языках для формирования навыков по видам речевой деятельности обучающихся уровней среднего образования



жүктеу 7.37 Mb.
бет51/67
Дата13.09.2017
өлшемі7.37 Mb.
1   ...   47   48   49   50   51   52   53   54   ...   67

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).[4]

Reaction with water and acids[edit]

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:

An iron nail placed in a solution of copper sulfate will quickly change colour as metallic copper is deposited and the iron is converted into iron(II) sulfate:

Fe (s) + CuSO4 (aq) → Cu (s) + FeSO4 (aq)

Similarly, magnesium can be used to extract titanium from titanium tetrachloride, forming magnesium chloride in the process:

2 Mg (s) + TiCl4 (l) → Ti (s) + 2 MgCl2 (s)

However, other factors can come into play, such as in the preparation of metallic potassium by the reduction of potassium chloride with sodium at 850 °C. Although sodium is lower than potassium in the reactivity series, the reaction can proceed because potassium is more volatile, and is distilled off from the mixture.

Na (g) + KCl (l) → K (g) + NaCl (l)

Comparison with standard electrode potentials[edit]

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

Li > K > Sr > Na > Ca > Mg > Al > Mn > Zn > Cr(+3) > Fe > Cd > Co > Ni > Sn > Pb > H > Cu > Hg > Ag > Pd > Ir > Pt > Au

The positions of lithium and sodium are changed on such a series; gold and platinum are in joint position and not gold leading, although this has little practical significance as both metals are highly unreactive.

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 and sodium – 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

potassium > sodium > lithium > 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.[6]


In a reactivity series, the most reactive element is placed at the top and the least reactive element at the bottom. More reactive metals have a greater tendency to lose electrons and form positive ions.

A reactivity series of metals could include any elements. For example:



A good way to remember the order of a reactivity series of metals is to use the first letter of each one to make up a silly sentence. For example: People Say Little Children Make A Zebra Ill ConstantlySniffing Giraffes.

Observations of the way that these elements react with water, acidsand steam enable us to put them into this series.

The tables show how the elements react with water and dilute acids:



Element

Reaction with water

Potassium

Violently

Sodium

Very quickly

Lithium

Quickly

Calcium

More slowly




Element

Reaction with dilute acids

Calcium

Very quickly

Magnesium

Quickly

Zinc

More slowly

Iron

More slowly than zinc

Copper

Very slowly

Silver

Barely reacts

Gold

Does not react

Note that aluminium can be difficult to place in the correct position in the reactivity series during these experiments. This is because its protective aluminium oxide layer makes it appear to be less reactive than it really is. When this layer is removed, the observations are more reliable.

Каталог: uploads -> doc -> 0a52
doc -> Пєн атауы: Математика
doc -> Сабаќтыњ тарихы: ХІХ ѓасырдыњ 60-70 жылдарындаѓы ќазаќ халќыныњ отарлыќ езгіге ќарсы азаттыќ к‰ресі
doc -> 1 -сынып, аптасына сағат, барлығы 34 сағат Кіріспе (1 сағат)
doc -> Сабақтың тақырыбы: XVIII ғасырдың бірінші ширегіндегі Қазақ хандығының ішкі және сыртқы жағдайы Сабақтың мақсаты
doc -> Сабақтың тақырыбы: XVIII ғасырдың бірінші ширегіндегі Қазақ хандығының ішкі және сыртқы жағдайы
0a52 -> Жантақ өсімдігі туралы жалпы сипаттама
0a52 -> Отбасылық ТӘрбие, оның тарихи негіздері
0a52 -> Өз білімін жетілдіру


Достарыңызбен бөлісу:
1   ...   47   48   49   50   51   52   53   54   ...   67


©kzref.org 2019
әкімшілігінің қараңыз

    Басты бет