05b chemical equations

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by by CHEMISTRY DEPARTMENT CHEMISTRY DEPARTMENT CHEM-111 CHEM-111 General Chemistry General Chemistry Unit five (b) Unit five (b) Chemical Equations Chemical Equations

Transcript of 05b chemical equations

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CHEMISTRY DEPARTMENTCHEMISTRY DEPARTMENT

CHEM-111CHEM-111General ChemistryGeneral Chemistry

Unit five (b)Unit five (b)Chemical EquationsChemical Equations

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Unit Five

• Oxidation and reduction Reactions

• Oxidizing agent• Reducing agents• Oxidation Numbers• Biological Oxidation-Reduction

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CHEMICAL EQUATIONS AND REACTIONS

The evolution of light and heat is an indication that a chemical reaction is taking place.

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CHEMICAL REACTION A process in which nature & chemical composition of a substance is changed.

Examples: food digestion, burning of coal, rust of iron nails, burning of methane etc,

Hydrochloric acid is added to a

solution of sodium carbonate.

Among the products is the gas

CO2, Which is seen bubbling

from the reaction mixture.

The decomposition (decay) of ammonium dichromate

proceeds rapidly, releasing energy in the form of light and heat

Bubbling of CO2

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Types of Chemical ReactionsI. Combination Reaction

Two or more elements or simple compounds bond together to form one product.

+A B BA

+

S + O2 SO2

II. Decomposition ReactionA reaction in which a single reactant splits into two or

more simpler products + BBA A

CaCO3 CaO + CO2

ZnCO3 ZnO + CO2

Δ

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III. Replacement ReactionElements in a compound are replaced by other

elements

a. Single ReplacementOne element replaces another element in the

compound

BCACBA + +

Zn + 2HCl ZnCl2 + H2

b.Double Replacement Reaction

Two elements replace each other in compounds

BA DC A BD C

BaCl2 + Na2SO4 BaSO4 + 2NaCl

+ +

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IV- Combustion (burning) Reactiona reaction in which an element or compound reacts with oxygen to form oxide products

Burning of Methane

Reactants Products

1 C atom = 1 C atom

4 H atoms = 4 H atoms

4 O atoms = 4 O atoms

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Reactants

Products

Equation-1

C (s) +O2 (g) Δ CO2 (g)

++

2H2 + O2 2H2O

Writing a Chemical Reaction

Equation-2

Reactants Products

+

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A short hand way of describing a chemical reactions Examples

1. C (s) + O2 (g) CO2 Carbon + Oxygen Carbon

dioxide.

Chemical Equation

2. H2SO4 + CuO CuSO4 + H2O

sulphuric acid + copper oxide copper sulphate + water

3. Fe + S FeS Iron + Sulphur Iron sulphide

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Balanced Equation

If the number of atoms of each element are the same on reactant & product sides, it is called a balanced equation

Reactants Products

2H2 + O2 2H2O Na2CO3 + 2HCl 2NaCl + H2O + CO2

2C2H5OH + 6O2 4CO2 + 6H2

CH4 + 2O2 CO2 + 2 H2O

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Balancing Simple Chemical Equations

• A balanced equation is one which has the same number of atoms of each element on either side of the equation.

• Matter can neither be created nor destroyed in a chemical reaction.

• The mass of the products of a reaction must be equal to the mass of the reactants. This is the bases of writing a balanced chemical equation. Examples:Balanced equation

ZnS + 2HCl ZnCl2 + H2S

Unbalanced equationZnS + HCl ZnCl2 + H2S

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STEPS FOR BALANCING EQUATIONStep 1.Use correct Formulas for reactants & products:

CH4 (g) + O2 (g) CO2 + H2O

Step 2.Determine If equation is balanced:

CH4 (g) + O2 (g) CO2 + H2O

Reactants Products Remarks 1C atom 1C atom balanced 4H atoms 2H atoms Not balanced 2O atoms 3O atoms Not balanced

Balancing Equation by Inspection Method

(by Trial & Error)

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Step 3 Balance the equation one element at a

time: Work with one element at a time, starting with

relatively small number. Adjust coefficients of reactants & products on both sides.

First Hydrogen atoms by placing coefficient 2 with H2O.

CH4(g) + O2(g) CO2 + 2H2O

Then Oxygen atoms by placing 2 with O2

CH4(g) + 2O2(g) CO2 + 2H2O

STEPS FOR BALANCING EQUATION

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Step 4Check if the equation is balanced :

CH4(g) + 2O2(g) CO2 + 2H2O

Reactants Products Remarks1C atom 1C atom balanced4H atoms 4H atom balanced

4O atoms 4O atoms balanced

STEPS FOR BALANCING EQUATION

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Q: Balance the following equation:

Al + Cl2 AlCl3

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Answer: Balance the following equation:

Al + Cl2 AlCl3 Step 1

Write the correct Formulas in the equation Al (s) + Cl2(g) AlCl3 (Not balanced)

Step 2.Check the number of atoms in each side.

Al (s) + Cl2(g) AlCl3

Reactants Products Remarks1 Al atom 1 Al atom balanced2 Cl atoms 3 Cl atoms Not balanced

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Steps For Balancing Equations

Step 3 Balance the equation one element at a time place coefficient 2 in front of AlCl3.

Al (s) + Cl2(g) 2AlCl3

Then place coefficient 3 in front of Cl2

Al (s) + 3Cl2(g) 2AlCl3

Then place coefficient 2 in front of Al 2Al (s) + 3Cl2(g) 2AlCl3

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Steps For Balancing Equations

Step 4. Now recheck if the equation is balanced

2Al(s) + 3Cl2(g) 2AlCl3

Reactants Products 2Al atoms 2Al atoms 6Cl atoms 6Cl atoms

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Q: Balance the following equation:• HgO Hg + O2

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The many reactions that involve the transfer of electrons from one species to another are called oxidation-reduction reactions, or redox reaction.

Oxidation: is the loss of electrons.Reduction: is the gain of electrons.

OXIDATION-REDUCTION (REDOX) REACTIONS:

Oxidation (loss of electrons)

A AB B

oxidized reduced

Reduction (gain of electrons)

Electron e-Electron e-

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Oxidation1. Addition of oxygen

S + O2 SO2 Addition of oxygen

2. Or loss of electrons by substance

Mg Mg2+ + 2e- loss of electrons

3.Or Removal of hydrogen or any

electropositive element from a compound.

CR

OH

H

R CR R

Ooxidation Removal of hydrogen

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Reduction 1. Removal of oxygen 2.or gaining of electrons.

3.Combination of hydrogen or any electropositive element from a compound.

i. CuO + C Cu + CO Removal of oxygen

(Copper oxide is reduced to copper and

carbon is oxidized to carbon monoxide)

ii. Cl2 + H2 2HCl

CR R

O

CR

OH

H

Rreduction

iii.

Combination

of

hydrogen

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Oxidizing AgentThe species that oxidize other

substances or gain electrons. e.g. O2

Reducing agent The species that reduce other

substances or lose electrons. e.g. Mg

Example: 2Mg + O2 2MgO reducing oxidizing agent agent

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Mg Mg2+ + 2e- loss of two electrons (Oxidation)

2Mg 2Mg2+ + 4e- loss of four electrons (Oxidation)

O2 + 4e- 2O2- gain of four electrons ( Reduction)

The product MgO is ionic which is formed by transfer of electrons.

A. 2Mg(s) + O2(g) 2MgO(s)

Oxidation – Reduction Reactions

Reducing agent

Oxidizing Agent

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Oxidation- Reduction Reactions

Ca + S Ca2+ + S2- = CaS

Ca Ca2+ + 2e- Oxidation

S + 2e- S2- Reduction

Reducing Agent

Oxidizing Agent

Reducing Agent

Oxidizing Agent

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Oxidation Number Oxidation number of an element is a number

which indicates the degree of oxidation or reduction suffered by an element.

1. Oxidation number of an element in simple mono atomic ion is equal to its electrical charge.

Fluorine has oxidation number of -1 in its compounds.

Hydrogen has oxidation number of +1 and Oxygen shows -2.The position of element in the periodic table

helps to assign its oxidation state.Example: Group IA elements have oxidation

number of +1. Group IIA elements have oxidation number of +2.

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Oxidation Number

2. The sum of oxidation number in a molecule must be 0.

E.g. Na = +l, Cl = -1 therefore NaCl = 0

3. In a polyatomic ion, the sum of the oxidation numbers of the constituent atoms is equal to the charge on the ion.

E.g. NH4+, H3O+, OH-, CN-,NO3

-,CO32- etc.

4. The oxidation number of the atoms in any free, uncombined element state is zero

E.g. H2, O2, O3, P4 = 0

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BIOLOGICAL OXIDATION-REDUCTION REACTIONS

The flow of electrons in oxidation-reduction reactions is responsible, directly or indirectly, for all of the work done by living organisms.

The source of electrons is reduced compounds (food).

The path of electron flow in metabolism is complex.

Electrons move from various metabolic intermediates to specialized electrons carriers in

enzyme- catalyze reactions. Those carriers in turn donate electrons to acceptors with higher electron affinities, with the released of energy.

Cells contain a variety of molecular energy transducers, which convert the energy of

electron flow into useful work.

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Balancing Equation1. Balance the following equations and identify the type of reaction:

(a) ZnS + HCl ZnCl2 + H2S

(b) HCl + Cr CrCl2 + H2

(c) Al + Fe3O4 Al2O3 + Fe

(d) H2 + Br2 HBr

(e) Na2S2O3 + I2 NaI + Na2S4O6

2. Identify each of the following as an oxidation or a reduction reaction.

a) Zn2+ + 2e- Zn

b) Al Al3+ + 3e-