5.2 Trends in Chemical Reactivity with Oxygen, Water and Dilute Acids

In the previous sections, we learned that the reactivity of Group 2 elements increases down the group due to the decrease in ionization energy. Now, we will look at the specific chemical reactions that demonstrate this trend. The Group 2 metals react with oxygen, water, and dilute acids, and the vigor of these reactions increases as you move down the group from beryllium to barium.

Main Idea

The main idea is that the increasing reactivity of Group 2 metals down the group is evident in their reactions with oxygen, water, and dilute acids. The reactions become more vigorous, and the products formed reflect the increasing ease with which the metals lose their two valence electrons to form M²⁺ ions.

Core Concept

The core concept is that the ease of oxidation of the metal determines its reactivity. A metal that loses electrons easily (low ionization energy) will react more readily and vigorously. Down Group 2, the ionization energy decreases, so the metals become stronger reducing agents. This means they are more easily oxidized by oxygen, water, or acids, and the reactions proceed more rapidly and exothermically.

Detailed Explanation

The textbook describes the trends in reactivity with oxygen, water, and dilute acids.

(a) Reaction with Oxygen / Formation of Basic Oxide

The Group 2 metals burn in air, and more rapidly in oxygen, forming white solid oxides. Strontium and barium also form peroxides (MO₂). For example, magnesium ribbon burns with a bright white flame once ignited in a Bunsen flame: 2Mg_{(s)} + O_{2(g)} → 2MgO_{(s)}

- Solubility of Oxides: Beryllium and magnesium oxides are insoluble in water, while the solubility of the remaining oxides increases down the group, as they form soluble hydroxides. - Basic Character: All oxides of Group 2 elements are basic in character except BeO, which is amphoteric in nature (it reacts both with acid and base). Basic character increases on moving down the group. BeO_{(s)} + 2HCl_{(aq)} → BeCl_{2(aq)} + H_2O_{(l)} BeO_{(s)} + 2NaOH_{(aq)} → Na_2BeO_{2(aq)} + H_2O_{(l)} - Reactivity Trend: The Group 2 metals get more reactive with oxygen going down the group. The larger atoms lose their outer two electrons more readily than the smaller atoms in the group. The greater reactivity of barium metal is illustrated by the fact that it must be stored under oil to keep it out of contact with air. - Covalent Character of BeO: BeO is covalent in character while the remaining oxides are basic in nature. This is due to the small size of the Be^{2+} ion. The polarizing power of Be^{2+} is greater because of which the oxide ion can easily be polarized. Hence, electrons are shared by the nuclei of both atoms. That is why BeO is covalent in character.

Flame Test

Some of the Group 2 metals burn with characteristic flame colours. It is the 2+ ions formed in the reaction that cause the colours. We can test for calcium, strontium, and barium in compounds using flame tests. A nichrome wire, cleaned with concentrated hydrochloric acid, is dipped into a sample of the salt to be tested and heated in a non-luminous Bunsen flame:

• Calcium compounds: give a brick-red colour.
• Strontium compounds: give a scarlet/red colour.
• Barium compounds: give an apple-green colour.

(b) Reaction with Water

The reactivity of Group 2 metals with water also increases down the group.

- Beryllium: Does not react with water because of the protective layer of BeO on its surface. - Magnesium: Does not react with cold water. However, it burns in steam to form magnesium oxide and hydrogen gas: Mg_{(s)} + H_2O_{(g)} → MgO_{(s)} + H_{2(g)} - Calcium: Reacts more readily than magnesium with water: Ca_{(s)} + 2H_2O_{(l)} → Ca(OH)_{2(aq)} + H_{2(g)} - Strontium and Barium: React even more vigorously with cold water, producing hydrogen gas and the corresponding hydroxide.

The trend is clear: the reactivity with water increases from beryllium (no reaction) to barium (vigorous reaction).

(c) Reaction with Dilute Acids

Group 2 metals react with dilute acids (like hydrochloric acid) to produce hydrogen gas and a salt. The reactivity increases down the group, as evidenced by the rate at which hydrogen gas is evolved.

• General Reaction:
• Trend: The bubbles of hydrogen gas are given off much faster as you go down the group. For example, barium reacts much more vigorously with dilute HCl than magnesium. This is because barium loses its valence electrons more easily, making it a stronger reducing agent.

Intermediate Level

The trend in reactivity of Group 2 elements with oxygen, water, and dilute acids is a direct consequence of the decreasing ionization energy down the group. As the atomic radius increases, the valence electrons are less tightly held, and the metals become stronger reducing agents. This means they can more easily donate electrons to oxygen, water, or hydrogen ions (from acids).

• With oxygen: All metals form basic oxides (except BeO, which is amphoteric). The reactivity increases down the group, with barium being so reactive it must be stored under oil.
• With water: The reactivity increases from beryllium (no reaction) to magnesium (reacts with steam) to calcium and the heavier metals (react with cold water). The products are the metal hydroxide and hydrogen gas.
• With dilute acids: All metals react to produce hydrogen gas and a salt. The rate of reaction increases down the group.

Important Points

• - Reactivity with Oxygen: Increases down the group. All form basic oxides except BeO (amphoteric).
• - Reactivity with Water: Increases down the group. Be (no reaction), Mg (steam only), Ca and heavier (cold water).
• - Reactivity with Acids: Increases down the group. All produce hydrogen gas.
• - Flame Tests: Ca (brick-red), Sr (scarlet/red), Ba (apple-green).
• - Reason for Trend: Decreasing ionization energy and increasing atomic radius down the group.
• - Barium: Most reactive; stored under oil.

Remember

• - Group 2 reactivity increases down the group.
• - With oxygen: Basic oxides (except BeO, amphoteric). Barium stored in oil.
• - With water: Be (no reaction), Mg (steam), Ca (cold water).
• - With acids: Produce hydrogen gas. Rate increases down the group.
• - Flame test: Ca = brick-red, Sr = scarlet, Ba = apple-green.
• - Reason: Ionization energy decreases down the group.

Common Mistakes

• - Thinking that all Group 2 metals react with water in the same way. Beryllium does not react with water, magnesium only reacts with steam, and calcium and the heavier metals react with cold water.
• - Forgetting that BeO is amphoteric. Unlike the other Group 2 oxides, BeO can react with both acids and bases.
• - Confusing the flame test colors. Make sure to remember: Ca = brick-red, Sr = scarlet/red, Ba = apple-green.
• - Assuming that the reactions with acids are equally vigorous. The rate of reaction increases down the group.
• - Ignoring the protective oxide layer. The reason beryllium and magnesium are less reactive is due to the protective oxide layer that forms on their surface.

Quick Revision

The chemical reactivity of Group 2 elements with oxygen, water, and dilute acids increases down the group. The reactions become more vigorous, and the metals are more easily oxidized due to the decrease in ionization energy. With oxygen, they form basic oxides (except BeO, which is amphoteric). With water, the reactivity increases from beryllium (no reaction) to barium (vigorous reaction). With dilute acids, they all produce hydrogen gas, but the rate increases down the group. The flame test colors are also characteristic: calcium (brick-red), strontium (scarlet), and barium (apple-green).

60-Second Revision

The reactivity of Group 2 metals with oxygen, water, and dilute acids increases down the group. This is because the ionization energy decreases, making it easier to lose valence electrons. With oxygen, they form basic oxides (except BeO, amphoteric). With water, Be doesn't react, Mg reacts with steam, and Ca and heavier metals react with cold water. With acids, they produce hydrogen gas, and the reaction rate increases down the group. Flame tests: Ca (brick-red), Sr (scarlet), Ba (apple-green).

Frequently Asked Questions (FAQs)

Q1. Why does the reactivity of Group 2 elements increase down the group? Answer: The reactivity increases down the group because the ionization energy decreases. As the atoms get larger, the outer electrons are further from the nucleus and are shielded by more inner shells. This makes it easier to remove the two valence electrons and form M²⁺ ions, which is the key step in their reactions.

Q2. How do Group 2 elements react with oxygen? Answer: Group 2 metals burn in air or oxygen to form white solid oxides. For example: 2Mg + O_2 → 2MgO The reactivity increases down the group. Barium is so reactive that it must be stored under oil to prevent reaction with oxygen. The oxides are basic in nature, except BeO, which is amphoteric.

• Q3. Describe the trend in reactivity of Group 2 elements with water.
• Answer: The reactivity with water increases down the group:
• - Beryllium: Does not react with water due to a protective oxide layer.
• - Magnesium: Does not react with cold water but reacts with steam to form magnesium oxide and hydrogen.
• - Calcium: Reacts readily with cold water to form calcium hydroxide and hydrogen.
• - Strontium and Barium: React even more vigorously with cold water.
• The general reaction is:
• M + 2H_2O → M(OH)_2 + H_2

Q4. Why is beryllium oxide amphoteric? Answer: Beryllium oxide (BeO) is amphoteric because it can react with both acids and bases. This is due to the small size and high polarizing power of the Be^{2+} ion, which gives BeO some covalent character. It reacts with acids to form beryllium salts and with bases to form beryllate salts. BeO + 2HCl → BeCl_2 + H_2O BeO + 2NaOH → Na_2BeO_2 + H_2O

• Calcium: Brick-red
• Strontium: Scarlet/red
• Barium: Apple-green

Q6. Why does the reaction of Group 2 metals with dilute acids become more vigorous down the group? Answer: The reaction with dilute acids becomes more vigorous down the group because the metals become stronger reducing agents. They lose electrons more easily due to the decrease in ionization energy, so they react with hydrogen ions (from the acid) more readily to produce hydrogen gas.

Q7. Why is barium stored under oil? Answer: Barium is stored under oil because it is a very reactive metal that reacts vigorously with oxygen in the air. The oil acts as a protective barrier, preventing contact with air and moisture.

• Q8. What is the general reaction of Group 2 metals with dilute hydrochloric acid?
• Answer: The general reaction is:
• M + 2HCl → MCl_2 + H_2
• The metal is oxidized to M²⁺, and hydrogen ions are reduced to hydrogen gas.

Q9. How do the oxides of Group 2 elements change in solubility down the group? Answer: The solubility of Group 2 oxides (and hydroxides) increases down the group. Beryllium and magnesium oxides are insoluble in water, while calcium, strontium, and barium oxides are more soluble, forming alkaline solutions of their hydroxides.

Q10. What is the trend in basic character of Group 2 oxides? Answer: The basic character of Group 2 oxides increases down the group. This is because the metal-oxygen bond becomes more ionic as the metal ion gets larger and less polarizing. BeO is amphoteric, while the other oxides are basic.

Multiple Choice Questions (MCQs)

• Q1. Which of the following is the correct order of reactivity of Group 2 elements with water?
• A. Be > Mg > Ca > Sr > Ba
• B. Ba > Sr > Ca > Mg > Be
• C. Mg > Be > Ca > Sr > Ba
• D. Ca > Mg > Be > Sr > Ba
• Correct Answer: B
• Explanation: Reactivity with water increases down the group, from beryllium (no reaction) to barium (vigorous reaction).

• Q2. Which Group 2 element does not react with water?
• A. Magnesium
• B. Calcium
• C. Barium
• D. Beryllium
• Correct Answer: D
• Explanation: Beryllium does not react with water due to a protective layer of beryllium oxide on its surface.

• Q3. What is the product of the reaction between magnesium and steam?
• A. Magnesium hydroxide and hydrogen
• B. Magnesium oxide and hydrogen
• C. Magnesium oxide and oxygen
• D. Magnesium hydroxide and oxygen
• Correct Answer: B
• Explanation: Magnesium reacts with steam to form magnesium oxide and hydrogen gas:
• Mg + H_2O → MgO + H_2

• Q4. Which of the following oxides is amphoteric?
• A. MgO
• B. CaO
• C. BeO
• D. BaO
• Correct Answer: C
• Explanation: Beryllium oxide (BeO) is amphoteric, meaning it can react with both acids and bases. The other Group 2 oxides are basic.

• Q5. What is the flame test color for calcium?
• A. Brick-red
• B. Scarlet/red
• C. Apple-green
• D. Blue
• Correct Answer: A
• Explanation: Calcium compounds give a brick-red color in a flame test.

• Q6. What is the flame test color for barium?
• A. Brick-red
• B. Scarlet/red
• C. Apple-green
• D. Blue
• Correct Answer: C
• Explanation: Barium compounds give an apple-green color in a flame test.

• Q7. Which Group 2 element is stored under oil to prevent it from reacting with air?
• A. Magnesium
• B. Calcium
• C. Strontium
• D. Barium
• Correct Answer: D
• Explanation: Barium is so reactive that it must be stored under oil to prevent it from reacting with oxygen in the air.

• Q8. What gas is produced when Group 2 metals react with dilute hydrochloric acid?
• A. Oxygen
• B. Chlorine
• C. Hydrogen
• D. Carbon dioxide
• Correct Answer: C
• Explanation: Group 2 metals react with dilute HCl to produce hydrogen gas.

• Q9. Why does the reactivity of Group 2 elements increase down the group?
• A. Because the atomic mass increases.
• B. Because the ionization energy decreases.
• C. Because the density increases.
• D. Because the melting point decreases.
• Correct Answer: B
• Explanation: Reactivity increases because the ionization energy decreases, making it easier for the metals to lose their valence electrons.

• Q10. Which of the following reactions becomes more vigorous down Group 2?
• A. Reaction with water
• B. Reaction with dilute HCl
• C. Reaction with oxygen
• D. All of the above
• Correct Answer: D
• Explanation: The reactivity of Group 2 metals increases down the group, so their reactions with water, dilute HCl, and oxygen all become more vigorous.

Multiple Choice Questions (MCQs)

• Q1. What happens when magnesium is reacted with steam?
• A. It forms magnesium hydroxide and hydrogen.
• B. It forms magnesium oxide and hydrogen.
• C. No reaction occurs.
• D. It forms magnesium oxide and oxygen.
• Correct Answer: B
• Explanation: Magnesium reacts with steam to form magnesium oxide and hydrogen gas.

• Q2. Which Group 2 element is the least reactive with water?
• A. Magnesium
• B. Calcium
• C. Strontium
• D. Barium
• Correct Answer: A
• Explanation: Among the options, magnesium is the least reactive with water (it requires steam). Beryllium is the least reactive overall but is not listed.

• Q3. What is the product of the reaction between calcium and cold water?
• A. Calcium oxide and hydrogen
• B. Calcium hydroxide and hydrogen
• C. Calcium hydroxide and oxygen
• D. Calcium oxide and oxygen
• Correct Answer: B
• Explanation: Calcium reacts with cold water to form calcium hydroxide and hydrogen gas:
• Ca + 2H_2O → Ca(OH)_2 + H_2

• Q4. Which of the following is NOT a characteristic of the reaction of Group 2 metals with oxygen?
• A. They form white solid oxides.
• B. The reactivity increases down the group.
• C. Barium is stored under oil.
• D. All oxides are amphoteric.
• Correct Answer: D
• Explanation: Not all Group 2 oxides are amphoteric. Only BeO is amphoteric; the others are basic.

• Q5. What is the flame test color for strontium?
• A. Brick-red
• B. Scarlet/red
• C. Apple-green
• D. Blue
• Correct Answer: B
• Explanation: Strontium compounds give a scarlet/red color in a flame test.

• Q6. Why does beryllium not react with water?
• A. Because it is a noble metal.
• B. Because it has a protective layer of BeO.
• C. Because it has the highest ionization energy.
• D. Because it is the least dense metal.
• Correct Answer: B
• Explanation: Beryllium does not react with water because of the protective layer of beryllium oxide (BeO) on its surface.

• Q7. Which gas is produced when barium reacts with dilute HCl?
• A. Oxygen
• B. Chlorine
• C. Hydrogen
• D. Carbon dioxide
• Correct Answer: C
• Explanation: Barium reacts with dilute HCl to produce hydrogen gas:
• Ba + 2HCl → BaCl_2 + H_2

• Q8. Why does the reactivity with dilute acids increase down Group 2?
• A. Because the metals become stronger reducing agents.
• B. Because the metals become weaker reducing agents.
• C. Because the acids become stronger.
• D. Because the temperature increases.
• Correct Answer: A
• Explanation: The metals become stronger reducing agents down the group because their ionization energy decreases, making it easier to lose electrons.

• Q9. Which of the following is the correct reaction of magnesium with oxygen?
• A. Mg + O_2 → MgO_2
• B. 2Mg + O_2 → 2MgO
• C. Mg + O_2 → MgO + O
• D. Mg + 2O_2 → MgO_2
• Correct Answer: B
• Explanation: Magnesium burns in oxygen to form magnesium oxide:
• 2Mg + O_2 → 2MgO

Q10. What is the trend in basic character of Group 2 oxides down the group? A. It decreases. B. It increases. C. It remains the same. D. It first increases then decreases. Correct Answer: B Explanation: The basic character of Group 2 oxides increases down the group. BeO is amphoteric, while the others are basic, with basicity increasing from MgO to BaO.

Multiple Choice Questions (MCQs)

• Q1. What is the trend in reactivity of Group 2 elements with water?
• A. Increases down the group.
• B. Decreases down the group.
• C. Remains constant.
• D. Is unpredictable.
• Correct Answer: A
• Explanation: Reactivity with water increases down the group, from beryllium (no reaction) to barium (vigorous reaction).

• Q2. Which Group 2 element reacts with steam but not cold water?
• A. Beryllium
• B. Magnesium
• C. Calcium
• D. Barium
• Correct Answer: B
• Explanation: Magnesium does not react with cold water but reacts with steam to form magnesium oxide and hydrogen.

• Q3. Which Group 2 oxide is amphoteric?
• A. MgO
• B. CaO
• C. BeO
• D. BaO
• Correct Answer: C
• Explanation: Beryllium oxide (BeO) is amphoteric, meaning it reacts with both acids and bases.

• Q4. What is the flame test color for barium?
• A. Brick-red
• B. Scarlet/red
• C. Apple-green
• D. Blue
• Correct Answer: C
• Explanation: Barium compounds give an apple-green color in a flame test.

• Q5. Which Group 2 element is stored under oil?
• A. Beryllium
• B. Magnesium
• C. Calcium
• D. Barium
• Correct Answer: D
• Explanation: Barium is stored under oil because it is very reactive with oxygen in the air.

• Q6. What gas is produced when Group 2 metals react with dilute acids?
• A. Oxygen
• B. Chlorine
• C. Hydrogen
• D. Carbon dioxide
• Correct Answer: C
• Explanation: Group 2 metals react with dilute acids to produce hydrogen gas.

• Q7. Why does beryllium not react with water?
• A. It has a protective oxide layer.
• B. It is a noble metal.
• C. It has the highest ionization energy.
• D. It is the least dense metal.
• Correct Answer: A
• Explanation: Beryllium does not react with water because of the protective layer of beryllium oxide (BeO) on its surface.

• Q8. Which of the following reactions is characteristic of calcium?
• A. Reacts with cold water to form calcium hydroxide and hydrogen.
• B. Reacts with steam to form calcium oxide and hydrogen.
• C. Does not react with water.
• D. Reacts with oxygen to form a peroxide.
• Correct Answer: A
• Explanation: Calcium reacts with cold water to form calcium hydroxide and hydrogen gas.

• Q9. What is the trend in the basic character of Group 2 oxides?
• A. Increases down the group.
• B. Decreases down the group.
• C. Remains constant.
• D. Is unpredictable.
• Correct Answer: A
• Explanation: The basic character of Group 2 oxides increases down the group, with BeO being amphoteric and the rest being basic.

• Q10. Which Group 2 element has the highest reactivity with oxygen?
• A. Beryllium
• B. Magnesium
• C. Calcium
• D. Barium
• Correct Answer: D
• Explanation: Barium is the most reactive with oxygen among the Group 2 elements, which is why it is stored under oil.