Starter.......

1. If M can be any element what group do you think MO is the general formula for?

Why?

2. Write equations for M reacting with water and oxygen.

3. Write any other general formulas for M.

Group 2 elements

Thursday, 23 April 2020

Group 2 Elements : Redox Reactions

Lesson Objectives:

• Describe the redox reactions of the Group 2 Mg → Ba with oxygen and with water .

elements • Explain the trend in reactivity group.

of Group 2 elements down the Key Words: periodicity, redox reactions, reacitvity.

• •

Specification links Today’s lesson: Redox reactions of Group 2 metals

(a) describe the redox reactions of the Group 2 elements Mg→Ba: – (i) with oxygen, – (ii) with water; (b) explain the trend in reactivity of Group 2 elements down the group due to the increasing ease of forming cations, in terms of atomic size, shielding and nuclear attraction; • • • •

Next lesson: Reactions of Group 2 compounds

(c) describe the action of water on oxides of elements in Group 2 and state the approximate pH of any resulting solution; (d) describe the thermal decomposition of the carbonates of elements in Group 2 and the trend in their ease of decomposition; – (No explanation of thermal decomposition required.) (e) interpret and make predictions from the chemical and physical properties of Group 2 elements and compounds; (f) explain the use of Ca(OH)2 in agriculture to neutralise acid soils; the use of Mg(OH)2 in some indigestion tablets as an antacid.

• • • • •

Group 2 reactions with water

Add magnesium to water in a test tube What happens?

Collect the gas produced by putting another tube over the top and test it with a lit spill (keep the test tube upside down) Test the solution that is left with universal indicator.

What colour does it go?

• Do the same with calcium.

• • • Which is more reactive?

How do you know.

What products are made?

Group 2 Elements : Physical Properties

• • •

High melting point Light metals with low densities They form colourless compounds

Group 2 Elements : Electron Configuration

Element

Be Mg Ca Sr Ba Ra

Electron Configuration

[He]2s 2 [Ne]3s 2 [Ar]4s 2 [Kr]5s 2 [Xe]6s 2 [Rn]7s 2

1 st I.E. /kJ mol -1

900 736 590 548 502 509 • • • Highest energy electrons in an s sub-shell Each element has two more electrons than the preceding noble gas two electrons in the outer shell . (form 2+ ions )

Group 2 Elements: Reactivity

• Reactive metals that are strong reducing agents .

• They are oxidised in reactions – loose 2 electrons M  M 2+ + 2e (+2 oxidation state) • Reactivity increases down the group • Reactivity increases because of the same factors that cause 1 st I.E. to decrease. Reducing agents reduce (add electrons to) another species Oxidation is the loss of electrons

•

Group 2 Elements: Reaction with Oxygen

Group 2 metals react vigorously with oxygen • It is a redox reaction • Produces an ionic oxide with general formula MO e.g. 2Ca(s) + O 2 (g)  2CaO(s)

0 0 +2 -2 Oxidation Numbers – Ca – Oxidised O – Reduced

An oxidation number is a measure of the number of electrons that an atom uses to bond with atoms of another element.

Group 2 Elements: Reaction with Water

• Group 2 metals react with water M(OH) 2 to form hydroxides – • Hydrogen gas is also formed e.g. Ca(s) + 2H 2 O(l) 

0 +1 -2

Ca(OH) 2 (aq) + H 2 (g)

+2 -2 +1 0 Oxidation Numbers – Ca – Oxidised H – Reduced

Only one H atom in each water has been reduced – The other doesn’t change Mg reacts slowly with water – on descending the groups reactions get more vigorous.

Questions

1. Mg + 2HCl  MgCl being reduced?

2 + H 2 is a redox reaction a) Identify the changes in oxidation number b) Which species are being oxidised and which are c)Which species are the oxidising and reducing agents? 2. a) Write the equations for these reactions; i) Barium with water ii) Strontium with oxygen b) Using oxidations numbers, identify what has been oxidised and reduced From page 89 of text book

Now look at the past exam questions

Complete the summary table below for group 2 Bonding Structure Ions Redox character Reactivity

Complete the summary table below for group 2 Bonding Metallic Structure Ions Redox character Reactivity Giant metallic structure; strong forces between positive ions and negative electrons M 2+ Reducing agent reacts by losing electrons M  M 2+ + 2e Increases down the group

• •

Specification links Today’s lesson: Redox reactions of Group 2 metals

(a) describe the redox reactions of the Group 2 elements Mg→Ba: – (i) with oxygen, – (ii) with water; (b) explain the trend in reactivity of Group 2 elements down the group due to the increasing ease of forming cations, in terms of atomic size, shielding and nuclear attraction; • • • •

Next lesson: Reactions of Group 2 compounds

(c) describe the action of water on oxides of elements in Group 2 and state the approximate pH of any resulting solution; (d) describe the thermal decomposition of the carbonates of elements in Group 2 and the trend in their ease of decomposition; – (No explanation of thermal decomposition required.) (e) interpret and make predictions from the chemical and physical properties of Group 2 elements and compounds; (f) explain the use of Ca(OH)2 in agriculture to neutralise acid soils; the use of Mg(OH)2 in some indigestion tablets as an antacid.