Lecture 13sf

Lecture 13sf - The Alkali Metals Group 1A, consisting of...

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The Alkali Metals Group 1A, consisting of Li, Na, K, Rb, Cs is known as the alkali metals. They all have 1 valence electron with a low ionization energy, and are all chemically very reactive. Physically, they are all soft metals, easily cut with a knife, with low melting points and low densities. Li and Na are less dense than water. Cs will melt in your hand. Some typical chemical reactions are: Reaction with halogens: 2M(s) + Cl 2 (g) → 2MCl(s) where M represents any of the alkali metals. Cl 2 is used to represent the halogens. Reaction with water: 2M(s) + 2H 2 O( ) → 2M + (aq) + 2OH - (aq) + H 2 (g) These reactions can be very vigorous, sometimes even explosive. When a small piece of Na is added to H 2 O, the sodium floats on the water (it’s less dense than water), and dances around on the surface, being propelled by the H 2 (g) being given off by the reaction. The Na also turns into a ball, as the heat of the reaction melts the sodium (it has a relatively low melting point). Cs, the alkali metal with the lowest ionization energy, is the most reactive with water. But the reason for this is not as easy as it seems. Li, the alkali metal with the highest ionization energy, is actually more easily oxidized to its aqueous ion than Cs, due to the stronger hydration bond between the small lithium ion and water. The low melting point of Cs makes it react much faster with water than lithium due to a higher surface area of contact with the water. Lithium, the smallest alkali metal, reacts directly with nitrogen to form nitrides, as does magnesium, the alkaline earth metal diagonally below it. 6Li + N 2 → 2Li 3 N 3Mg + N 2 → Mg 3 N 2 Similarities in properties between an element and one below it diagonally to the right, are called diagonal relationships.
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Hydrogen Hydrogen is usually listed in group IA, as it has one valence electron like the alkali metals. However, it is not a metal, and has a much higher ionization energy and electronegativity than the alkali metals. Ionization energy (kJ/mol) Electronegativity Hydrogen 1312 2.1 Lithium 520 1.0 Sodium 496 0.9 Potassium 419 0.8 Rubidium 403 0.8 Cesium 376 0.7 The much-higher ionization energy and electronegativity of hydrogen, compared to the alkali metals, shows that it holds onto its single electron more tightly and also it attracts electrons better. Similar to the alkali metals, hydrogen will react with the halogens. Alkali metal halides are ionic compounds, such as NaCl, while hydrogen halides consist of polar covalent bonds and are acids, such as HCl. Typical chemical equations are: Na(s) + ½ Cl 2 (g) → NaCl(s) H 2 (g) + Cl 2 (g) → 2HCl(g) NaCl is an ionic solid with a very high melting point and boiling point (m.p. = 801 o C, b.p. = 1415 o C), while HCl is a gas at room temperature (m.p. = -115 o C, b.p. = -85 o C). As a gas, HCl is called hydrogen chloride, but we most commonly encounter it when dissolved in water, then called hydrochloric acid. Hydrogen can not only react with non metals, such as halogens or oxygen, but can also react with
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Lecture 13sf - The Alkali Metals Group 1A, consisting of...

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