Ch#8 : Chemical Reactivity Flashcards
Learn about the chemical reactions and properties of elements.
Differ between metals and non metals.
Metals : Non metals
- They form cations : They form anions
- They lose electrons : They gain electrons
- They have high melting points : They have low melting points.
- They are good conductors of heat and electricity : They are bad conductors of heat and electricity.
- Metals reflect light : They absorb light
- They are s , d and f block elements : p block elements
- They can be turned into wires and sheets : They cant be turned into wires and sheets as they are brittle.
- They have low ionization energies : They have high ionization energies.
What is electropositivity? Explain its trends.
Electropositivity : The tendency of a metal to lose electrons
Metals have large atomic sizes and low Ionization energies. As ionization energy decreases down the group , electropositivity increases. So Na is more electropositive than Li. The tendency to lose electrons increases as we move from left to right in a period. For instance , in the 2nd period Li and Be are metals , B is metalloid , and C , N , O , F and Ne are non metals. Be is less metallic than Li.These trends are reflected in the chemical reactivity of metals. In a group , the reactivity of metals with oxygen increases. Alkali metals show increased reactivity with O as we move down the group.
Li forms normal oxide: 4Li + O² ---> 2Li²O Na forms per-oxide: 2Na + O² ---> Na²O² K forms super oxide: K + O² ---> KO²
In periods as we move from left to right , the reactivity of metals with O decreases. For instance , in the 3rd period Na reacts with O readily to form sodium peroxide , Mg reacts with oxygen on ignition and Al reacts with oxygen on heating. It is superficially oxidized at room temperature to form Al²O³ layer which prevents further reaction.
We can understand trends in electropositivity from the nature of the oxides. Metal oxides are generally basic in character.This is because they produce bases in water. Non metal oxides are generally acidic because they produce acids in water.The basic character of oxides decreases as we move along a period.This indicates a decrease in metalic character.
Elements of the third period form the following oxides , Na²O(Strongly basic) , MgO(Basic) , Al²O³(Amphotric) , SiO²(Weakly acidic) , P⁴O¹⁰(Acidic) , SO²(Strongly acidic) , Cl²O⁷(Very strongly acidic).
What are Oxides? , state the types of oxides. Explain basic , acidic and Amphotric oxides
Oxides: | |- Normal Oxide(-2) |- Per-oxide(-1) |- Super oxide(-1/2) |- Sub oxide(unstable)
Oxides : Binary compounnds of oxygen are called oxides.
Normal Oxides: | |- Basic Oxides |- Acidic Oxides |- Amphotric Oxides |- Normal Oxides
Types of normal oxides:
Basic Oxides : They are compounds of oxygen formed by reacting with metals of group 1A and 2A.
When dissolved in water , they form bases so they are called basic oxides.
Examples :
Formation:
1. 4Na + O² —> 2Na²O
2. 2Ca + O² —> 2CaO
Dissolving in water:
- Na²O + H²O —> 2NaOH
- CaO + H²O —> Ca(OH)²
Acidic Oxides : When oxygen reacts with non metals , it forms acidic oxides.
They are called acidic oxides because they form acids when dissolved in water.
Examples:
C + O² —> CO²
S + O² —> SO²
Dissolving in water:
CO² + H²O —> H²CO³
SO² + H²O —> S²CO³
Amphotric Oxides: When oxygen reacts with a less electropositive metal , amphotric oxides are formed. These oxides have dual character. In presence of an acid , they behave like a base and in presence of a base , they because like an acid.
Examples :
4Al + 3O² —> 2Al²O³
2Zn + O² —> 2ZnO
Differ between alkal metals and alkaline earth metals.
Alkali metals : Alkaline Earth metal
- Group 1A elements : Group 2A elements
- Includes Li , Na and K : Includes Be , Mg , Ca
- ns¹ valence electron config : ns²
- More metallic : Less metallic
Explain the ionization energies of alkali metals and alkaline earth metals and how they affect reactivity?
Alkali Metals : Alkali metals have low ionization energies so they have a great tendency to lose their single valence electron and form cations. They are so reactive that they are never found in the free state. The alkali metals show increased reactivity down the group. This is due to the decrease in Ionization energy down the group. This means Na is more reactive than Li but less reactive than K. They are all so reactive that they are kept under a liquid such as kerosene oil.
Alkaline Earth Metals : They also have low ionization energies , so they have great tendency to lose both of their valence electrons to form dipositive cations. They are less reactive than alkali metals because they have higher ionization energies. These metals also show increased reactivity down the group. Mg is more reactive than Be but less reactive than Ca. They are also kept under a liquid like kerosene oil to prevent contact with moist air.
Explain the properties of sodium , explain why we shouldnt touch sodium with our bare hands and the 3 uses of sodium
The most important alkali metal is sodium. Iron is the most commonly used metal. It is hard and strong so when the term metal comes to our mind , we think it to be hard and strong but sodium is a soft metal which can be cut with a knife. It melts at 97.8°C and boils at 881.4°C. The reason is that in iron , the metal atons are packed tightly together due to small size. They are held by strong metalic bonds due to the many electrons in the valence shell. Whereas sodium consists of bigger atoms. Because of large size and only one electron , sodium atoms have weak metallic bonds. So sodium is a soft metal.
Why it shouldn’t be picked with bare hands :
Sodium is a silvery white soft white metal.it is a extremely reactive metal. It reacts with water violently to form sodium hydroxide and liberates hydrogen gas.
2Na + 2H²O —> 2NaOH + H²
This reaction is highly exothermic and proceeds with a light explosion. For this reason it shouldnt be picked with fingers.
In a limited supply of oxygen , sodium burns to form sodium oxide Na²O but in excess oxygen it forms sodium peroixde Na²O².
2Na + O² —> Na²O²
Uses:Sodium mixed with potassium forms a liquid alloy which is used as a coolant in nuclear reactors. Sodium is also used in sodium lamps to illuminate highways. The petroleum industry uses sodium for the production of antiknock compounds for gasoline.
Explain the reactivity and uses of magnesium and calcium.
Magnesium and calcium are the most common alkaline earth metals.Metallic bonding in these elements involves 2 electrons. So they are relatively soft but much harder than alkali metals. They are relatively reactive but less reactive than alkali metals.Magnesium reacts with steam but calcium reacts with cold water.
Mg + H²O —> MgO + H²
Ca + 2H²O —> Ca(OH)² + H²
Magnesium reacts with oxygen at high temperature while calcium reacts at room temperature.
2Mg + O² –heat–> 2MgO
2Ca + 2O² —> 2CaO + O²
Ca is more reactive than Mg
Uses of magnesium: Magnesium has relatively low density so it is used in making light weight alloy with aluminum that are used to make the frames of automobiles , aircraft and spaceship. Since magnesium burns brilliantly , it is used in photo flash guns.Mg(OH)² called milm of magnesia is commonly used as antacid.
Uses of calcium : Calcium is an important component of teeth and bones.Calcium ions are found in all living cells.A proper Ca+2 and K+ ion balance is required for normal heart function.
Explain noble metals and their inertness. What is aquaregia?
The chemistry of metals is characterized by their ability to lose electrons to form cations. Some metals such as copper , silver , gold and platinum are relatively difficult to oxidize so they are called noble metals. Gold and Platinum mostly exist as free elements in nature. Copper and solver are found in free and combined states.All active metals react with HCl but noble metals do not.Copper and Silver react with strong oxidizing agents like conc. HNO³ and HClO⁴(per chloric acid). Gold and Platinum react only with aquaregia. Aquaregia is a mixture of 3 parts by volume conc. HCl and 1 part by volume of conc. HNO³.
HNO³ + 3HCl —> NOCl + 2H²O + Cl²
Why are noble metals important?
Silver and gold have been admired for thousands of years and are still used in jewelry , flatware and ornaments. These metals have beautiful luster and high malleability so they can be easily transformed into different shapes.However both are too soft to be used for those purposes so they are alloyed with copper to make them harder. In the past , gold and silver were used to make coins but nowadays copper-nickel alloys are universally used to make coins
Silver is an excellent conductor of heat and electricity. The light sensitive silver halides are used in photograhic processses.
Gold alloys are extensivel used in dentistry.Some compounds of gold have useful therapeutic applications. For example , some salts of gold are used to treat rheumatoid arthritis.
Platinum is widely used as a catalyst for many types of industral processes. For example 100% pure sulphuric acid is prepared by contact process(2SO² + O² –Pt–> 2SO³) . In this process , platinum is used as a catalyst. A cataclyst speeds up a reaction without participating in it.Platinum is used as a electrode as a part of the hydrogen electrode.Automobile exhaust is a major source of pollution so most new cars are equipped with catalytic converters. These converters contain platinum. These converters catalyze the complete combustion of CO and hydrocarbons.A platinum compound called cis-platin is useful as an anti cancer agent.
Catalytic converter :
CO + NO –Pt–> CO² + N²
What are non metals and metalliods?
Non-metals : Elements that conduct heat and electricity poorly.
They appear in the upper right part of the periodic table. They are p block elements.Non-metals are generally dull , brittle solids at room temperature with the exception of bromine which is liquid.
Several elements near the stepped line on the periodic table have the mixed properties of metals and non metals. These elements are called metalloids.They play an importsnt role in electronics For example : silicon and germanium are metalliods.These elements are used in the manufacture of computer chips and solar cells.
What is electronegative character? Explain its trends briefly.
Electronegative character is the reverse of electropositive character. It is the tendency to gain electrons to form anions.
As we move from left to right across a period , the tendency to gain electrons increases. This is due to the increase in nuclear charge and decrease in atomic size , which binds the electron more tightly to the nucleas. But in a group going from top to bottom , the electronegative character decreases due to the increase in atomic size. Halogens are the most electronegative elements in their respective periods. Non metal oxides are generally acidic because they yeild acids in water. The acidic strength increases as we move from left to right in a period.
Explain the formation of halides , hydrohalic acids and the displacement reaction which occurs between free halogens and halides.
The elements of group 7A are called halogens. The name halogen is derived fron the greek words “halous” meaning salt and “gen” meaning former. Halogens include flourine , chorine , bromine , iodine and astatine. Astatine is radioactive and little is known about its properties. All halogens are non metals. They exist as diatomic molecules.
F² —> pale yellow gas
Cl² —> greenish yellow gas
Br² —> Reddish brown liquid
I² —> Bluish black solid.
All halogens react with metals to form salts called halides. In these reactions , metals are oxidized. So halogens act as oxidizing agents. Flourine is the most reactive elements known. Chlorine is less reactive but it combines vigorously with metals. Sodium burns in chlorine gas to form sodium chloride.
2Na + Cl² —> 2NaCl
Bromine and Iodine react with metals less vigorously. The reactivity of metals with halogens decreases as we move down a group.
Halogens react with hydrogen to form hydrogen halides.
H² + X² = 2HX
Where X = F , Cl , Br , I
Reaction of flourine with hydrogen is explosive. The hydrogen halides dissolve in water to form hydrohalic acids. All hydrohalic acids are strong besides HF.HI is the strongest.
The oxidizing power of F² is the highest and that of I² is the lowest.Due to the relative strength as an oxidizing agent , it is possible for a free halogen to oxidize the ion of a halogen next to it in the group. This means F² can oxidize all halide ions to free halogen.
F² + 2KCL —> 2KF + Cl²
F² + 2NaI —> 2NaF + I²
Cl² + 2KBr —> 2KCl + Br²
I cannot oxidize any halide ion.
