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Lecture 4

CHEM 2501 Lecture 4: Topic 4: Group 1: Hydrogen and the Alkali Metals


Department
Chemistry
Course Code
CHEM 2501
Professor
Maria De Rosa
Lecture
4

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Hydrogen: Where does it belong on the periodic table?
Alkali metals
Halogens
Pros:
Forms a monopositive ion H+
Has a single s electron
Pros:
Is a non
-
Forms a diatomic molecule
Cons:
Is not a metal (but it can be under extreme conditions (see
slide 24)
Does not react with water
Cons:
Rarely forms H
-
(but does occasionally, see slide 16 and
beyond)
Is comparatively nonreactive
Common oxidation states are 0 (H
2
), +1 (Proton, acid/base chemistry) and
-
1 (hydride chemistry)
-
It has two important isotopes: Deuterium (D, 1 extra neutron) and Tritium (T, 2 extra neutrons)
-
Hydrogen has unique properties that set it apart in the periodic table
Deuterium and oxygen forms heavy water, which can be isolated from regular water by electrolysis or fractional
distillation.
-
It has higher melting (3.8°C) and boiling (101.4°C) than water
-
3
T à
4
He +
0
e
Tritium is radioactive has a half
-
life of 12 years:
-
Small difference by having the extra neutrons
You can separate heavy by fractional, distillation and hydrolysis.
-
Deuterium and Tritium
H
+
(called a proton) is never seen under ordinary conditions
-
In reality, the hydrated proton
[H
3
O]
+
is the well known species in aqueous solution
-
Other crystalline acid hydrates contain ions suck as [H
5
O
2
]
+
and [H
9
O
4
]
+
-
The simplest ion: Proton or H+
The easiest method of preparation is electrolysis:
-
H
2
O à H
2
+ ½ O
2
E =
-
0.36 V
Notice E is negative, so current has to be applied for this to go to products
-
Zn + 2HCl
--
> ZnCl
2
+ H
2
In the lab, typically you make it:
-
What‘s happening? The Zn is reducing the H+ to H2. (zinc is a strong enough reducing agent
see slide 10)
-
Preparation of Dihydrogen
Redox potentials are relative to the normal hydrogen electrode:
-
2H
+
(aq)
+ 2e
-
H
2(g)
If a system accepts electrons from a normal hydrogen electrode, it is a halfcell that is defined as having a positive redox
potential
-
If a system donates electrons to the hydrogen electrode, it is defined as having a negative redox potential.
-
E° is measured in millivolts (mV). A high positive E° indicates an environment that favors oxidation reaction such as free
oxygen. A low negative E° indicates a strong reducing environment, such as free metals.
-
Positive =oxidizing
-
Aside: REDOX Potentials
Topic 4: Group 1: Hydrogen and the Alkali Metals
February 19, 2015 8:55 PM
Inorganic Chemistry Page 1

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Positive =oxidizing
-
Negative= reducing
-
CO + H
2
O
CO
2
+ H
2
CH
4
+ H
2
O
CO + 3H
2
(over Ni, 800°C)
Or, you buy it from a chemical company and THEY make it:
-
(over iron
-
chromium oxide, 400°C)
-
Preparation of Dihydrogen (cont’d)
Due to the strength of the H
-
H bond (436 kJ/mol), not very many reactions occur with dihydrogen :
-
O-H is 464 kJ/mol: H2reacts with O=O to form H2O
-
H
2
reacts at high T with many metal oxides to make pure metal (high T to overcome E
a
)
-
Due to the strength of the H
-
H bond (
436 kJ/mol
), not very many reactions occur with dihydrogen:
-
H2 reacts strongly with F
-
F, and the reactivity decreases down Group 17
H
-
F is
565 kJ/mol
:
-
H2 reacts with carbon double bonds (C=C) in the presence of a Pt or Pd catalyst to form CH
-
CH
C
-
H is
411 kJ/mol
:
-
Reactivity of Dihydrogen
Hydrogen adsorbs on a catalyst surface
Hydrogen fuel cell vs. Battery
-
electric cars
Toyota, Hyundai, Honda
-
Needs hydrogen refueling stations (in
its infancy)
-
Hydrogen
Battery
Tesla, Nissan , Chevy, etc.
-
Electricity comes from the battery instead of from the fuel cell
-
charged at a
charging station
-
Li ion Tesla battery
-
The general class of hydrogen bound to other atoms are called the hydrides:
-
-
Hydrides
Inorganic Chemistry Page 2

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The general class of hydrogen bound to other atoms are called the hydrides:
-
This important class of hydrogen containing materials have hydrogen in a
1 oxidation state.
-
Group 1 or 2 cations (CaH
2
, NaH)
-
Ionic (Saline) Hydrides
Their properties:
NaH + H
2
O
NaOH + H
2
CaH
2
+ O
2
CaO + H
2
O
1) Very reactive with water and oxygen
SiCl4+ 4NaH  SiH4+ 4NaCl
2) Excellent reducing agents:
Hydrogen has access to its three possible oxidation states:
-
+1: this is typical acid-base chemistry and leads to hydrogen bonding H2O, NH3, HF
0: the majority of covalent hydrides are covalent compounds hydrocarbons, phosphine (PH
3
), tin hydride (SnH
4
)
-
1: the typical example is boron hydride (diborane)
Covalent (molecular) Hydrides
The bridging hydrogen participates in a 3
-
centre, 2
-
electron bond, sometimes called a banana bond
-
This bonding convention defies description by Lewis dot structure analysis
-
Diborane Bonding
Due to the band structures of metallic bonding, metal hydrides are generally seen as hydrogen atoms dissolved in the
metallic matrix
-
Metal alloys that can accommodate high hydrogen content are considered good candidates for H2storage and
stabilization media (they release H
2
on heating.
-
H atoms are small enough to fit in the holes in a metal lattice
-
Metallic Hydrides
Use a hydrogen absorbing nickel alloy
-
Rechargeable, used in portable electronic devices
-
At least 2 times the capacity of a standard nickel cadmium battery
-
More "green" (no toxic cadmium)
-
They are sealed in a such a way that we won't have it evolve into hydrogen later.
-
This is a metal hydride that can be used for a lot of things
-
Example: Nickel Metal Hydride Batteries
Shiny, silver metals when in the 0 oxidation state
-
Soft, low
-
melting, easily atomized, low density (Weakly bonded metals)
-
High electrical and thermal conductivity
-
Group 1 Alkali Metals: Properties and Trends
Melting Points of the Alkali Metals decrease as you descend the group
Element
Melting Point (°C)
Heat of atomization, (kJ/mol)
Li
180
162
Na
98
108
K
64
90
Inorganic Chemistry Page 3
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