Chem 2010_4 Group 1 Alkali Metals

8/11/2019 Chem 2010_4 Group 1 Alkali Metals

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Group

One:The Alkali

Metals



The Alkali Metals

The alkali metals are:

Lithium – Li



The Alkali Metals


Lithium – Li

Sodium – Na



The Alkali Metals


Lithium – Li

Sodium – NaPotassium – K



The Alkali Metals


Lithium – Li


Rubidium – Rb



The Alkali Metals


Lithium – Li


Rubidium – Rb

Caesium – Cs (Francium – Fr )



The Alkali Metals

Properties: The Group 1 metals are good conductors of electricity, malleable

and their freshly cut surface has a typical metallic lustre. The

melting points and densities of the Group 1 metals are

summarised in the table below:

Element Melting Point / C Density / g cm-3

Lithium – Li 180.0 0.53

Sodium – Na 97.8 0.97

Potassium – K 63.7 0.86

Rubidium – Rb 38.9 1.53

Caesium – Cs 28.7 1.90



The Alkali Metals

Question:Look at the melting point of caesium. What would you observe if

you held a small piece of caesium in your hand?


Lithium – Li 180.0 0.53

Sodium – Na 97.8 0.97






The Alkali Metals

Answer:The body temperature of a healthy human being is 37.0 C. This

is greater than the melting point of caesium, and so the caesium

metal would melt if you were to hold it in your hand.


Lithium – Li 180.0 0.53

Sodium – Na 97.8 0.97






The Alkali Metals

Question:What are the trends or patterns in the melting points and densities

of the Group 1 metals?


Lithium – Li 180.0 0.53

Sodium – Na 97.8 0.97






The Alkali Metals

Answer:On descending Group 1 of the Periodic Table, the melting points

of the elements decrease and the densities of the elements

increase.


Lithium – Li 180.0 0.53

Sodium – Na 97.8 0.97






The Alkali Metals

Question:Look at the densities of lithium sodium and potassium. What

would you observe if you were to add a small piece of lithium,

sodium or potassium to a trough of water?

Hint: the density of water is 1.0 g cm-3.


Lithium – Li 180.0 0.53

Sodium – Na 97.8 0.97






The Alkali Metals

Answer:Lithium, sodium and potassium are all less dense than water

(their densities are all less than 1.0 g cm-3) and so the first three

Group 1 metals will all float on the surface of water.


Lithium – Li 180.0 0.53

Sodium – Na 97.8 0.97






The Alkali Metals

Question:Would you expect the Group 1 metals to be hard or soft? Briefly

explain your answer.


Lithium – Li 180.0 0.53

Sodium – Na 97.8 0.97






The Alkali Metals

Answer:The Group 1 metals are very soft compared to other metals such

as iron. The Group 1 metals have low melting points, which

indicates that their metallic bonds are relatively weak.


Lithium – Li 180.0 0.53

Sodium – Na 97.8 0.97






The Alkali Metals

Reactions of the Group 1 Metals with Water Watch the reaction of lithium reacting with cold water, and then

answer the following questions:

What do you observe when lithium is added to a trough of cold

water?

Suggest an identity for the gas that is produced when lithium reacts

with cold water? What is the chemical test for this gas?

Is the solution formed at the end of the reaction acidic or alkaline?

Write a word equation for the reaction between lithium and cold

water.

Write a balanced chemical equation for the reaction between lithium

and cold water.



The Alkali Metals

Reactions of the Group 1 Metals with Water The reaction of lithium with cold water:

Lithium floats on the surface of the water and moves about on the

surface of the water. A gas is produced (effervescence). A “hissing”

sound is heard.

The reaction between lithium and water produces a solution of

lithium hydroxide which is alkaline. This alkaline solution will turn

universal indicator solution blue / purple.

The reaction between lithium and water produces hydrogen gas.Hydrogen gas extinguishes a burning splint with a squeaky “pop”.

lithium + water lithium hydroxide + hydrogen

2Li(s) + 2H2O(l) 2LiOH(aq) + H2(g)



The Alkali Metals

Reactions of the Group 1 Metals with Water Watch the reaction of sodium reacting with cold water, and then


What do you observe when sodium is added to a trough of cold

water?

Suggest an identity for the gas that is produced when sodium

reacts with cold water? What is the chemical test for this gas?


Write a word equation for the reaction between sodium and cold

water.

Write a balanced chemical equation for the reaction between

sodium and cold water.





The Alkali Metals

Reactions of the Group 1 Metals with Water Watch the reaction of potassium reacting with cold water, and then


What do you observe when potassium is added to a trough of cold

water?

Suggest an identity for the gas that is produced when potassium

reacts with cold water? What is the chemical test for this gas?


Write a word equation for the reaction between potassium and coldwater.

Write a balanced chemical equation for the reaction between

potassium and cold water.



The Alkali Metals

Reactions of the Group 1 Metals with Water The reaction of potassium with cold water:

Potassium floats on the surface of the water and moves about on the

surface of the water. A gas is produced (effervescence). A “hissing”

sound is heard. Sparks are observed. A lilac flame is observed.

The reaction between potassium and water produces a solution of

potassium hydroxide which is alkaline. This alkaline solution will turn

universal indicator solution blue / purple.

The reaction between potassium and water produces hydrogen gas.Hydrogen gas extinguishes a burning splint with a squeaky “pop”.

potassium + water potassium hydroxide + hydrogen

2K(s) + 2H2O(l) 2KOH(aq) + H2(g)



The Alkali Metals

Reactions of the Group 1 Metals with WaterIn what ways are the reactions between lithium, sodium, potassium

and water the same? In what ways are the reactions between lithium,

sodium, potassium and water different?

Same Different



The Alkali Metals

Reactions of the Group 1 Metals with WaterIn what ways are the reactions between lithium, sodium, potassium

and water the same? In what ways are the reactions between lithium,

sodium, potassium and water different?

Same Different

Li, Na and K float on water.

Li, Na and K move about on the

surface of the water. Effervescence is observed.

“Hissing” sound is heard.

Alkaline solution produced.

For Li, no flame is observed.

For Na, the hydrogen gas burns with

a yellow flame. The reaction is fasterthan the reaction between Li and

water.

For K, the hydrogen gas burns with a

lilac flame. The reaction is faster than

the reaction between Na and water.



The Alkali Metals

Reactions of the Group 1 Metals with Water

Use your observations from the previous experiments to

arrange the Group 1 metals in order of reactivity:

Least reactive: __________

__________

Most reactive: __________

sodium

potassium

lithium



The Alkali Metals


Question:

What is the trend or pattern in the reactivity of the Group 1

metals?

Answer:

The reactivity of the Group 1 metals increases while

descending the Group.



The Alkali Metals


Question:

What would you expect to see when the Group 1 metal

caesium is added to a trough of cold water?

Answer:

A very vigorous reaction. An explosion!



The Alkali MetalsReactivity of the Group 1 Metals


descending the Group. The Group 1 metals all react by

losing their single valence electron, resulting in the formation

of an ion with a single positive charge.





descending the Group. The Group 1 metals all react by

losing their single valence electron, resulting in the formation

of an ion with a single positive charge.




While descending Group 1, the valence shell is removed

further and further from the nucleus of the atom due to the

addition of a complete, new electron shell.




The further the valence shell is from the nucleus of the

atom, the weaker the force of attraction between the

positive nucleus and the single negative valence electron.




This means that, while descending Group 1, it becomes increasingly

more difficult for the positive nucleus to attract and hold on to the single

negative electron in the valence shell of the atom. While descending

Group 1, less energy is required to remove the single valence electron

from an atom of the Group 1 metal, and the reactivity of the Group 1metals consequently increases.




In addition to this, while descending Group 1, the single

negative valence electron becomes more shielded from the

attractive force of the positive nucleus by the increasing

number of electron shells in between the nucleus and the

valence shell.




This also makes it more difficult for the positive nucleus to

attract and hold onto the single negative electron in the

valence shell of the atom. Less energy is required to

remove the single negative valence electron and the

reactivity of the Group 1 metals consequently increases on

descending the Group.




Reactivity

Increases

LithiumLeast Reactive

The valence shell is close to the nucleus

and is not very well shielded. The force of

attraction between the positive nucleus

and single valence electron is strong. It isdifficult to remove the single valence

electron from the atom.

Sodium

PotassiumMost Reactive

The valence shell is far from the nucleus

and is well shielded. The force ofattraction between the positive nucleus

and single valence electron is weak. The

single valence electron is easily removed

from the atom.

Rubidium

Caesium




ElementFirst Ionization

Energy / kJ mol –1

Lithium 519

Sodium 494

Potassium 418

Rubidium 402

Caesium 376

The table on the left gives the first

ionization energy for the elements in

Group 1 of the Periodic Table. First

ionization energy is the energy

required to convert 1 mole of atomsin the gas phase to one mole of ions,

each with a single positive charge, in

the gas phase:

E(g)

E+

(g)

+ e –

The lower the first ionization energy,

the more easily an electron is

removed from the valence shell, and

so the more reactive the Group 1

metal

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Chem 2010_4 Group 1 Alkali Metals

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