chemical reaction [read-only] [compatibility mode]

7/23/2019 Chemical Reaction [Read-Only] [Compatibility Mode]

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Setia Budi

Gaya elektrostatik-Wenggita Maulani (3215126574)

Jurusan Kimia, Universitas Negeri Jakarta

Chemical ReactionsChemical ReactionsChemical ReactionsChemical Reactions



Water, The Common Solvent

One of the most valuable properties of water is its

ability to dissolve many different substances. The shared electrons in water molecule tend to spend

more time close to the oxygen than to either of the

hydrogens.

Because of this unequal charge distribution, water is

said to be a polar molecule.

The polarity gives water its great ability to dissolve

compounds.

11/14/2012sb/chemical reaction2



Hydration A schematic of an ionic solid dissolving in water




Strong electrolyte




Weak electrolyte In a solution containing 0.1 mole of NH3 per liter, for every 100 molecules of

NH3 originally dissolved, only one NH4 ion and one OH ion are produced; 99

molecules of NH3 remain unreacted




Electrolytes




Nonionic substances Water also dissolve many nonionic substances

The illustration shows us why ethanol soluble in water




Types of Solution ReactionsTypes of Solution ReactionsTypes of Solution ReactionsTypes of Solution Reactions1. Precipitation reactions

2. Acid–base reactions3. Oxidation–reduction reactions




Precipitation reactions

When two solutions are mixed, an insoluble substance sometimes forms; that is, a

solid forms and separates from the solution. Such a reaction is called a

precipitation reaction,

The solid that forms is called a precipitate.




Precipitation reactionsThe reaction of K2CrO4(aq) and Ba(NO3)2(aq).

a) The molecular-level “picture” of the mixed solution before any reaction has occurred.

b) The molecular-level “picture” of the solution after the reaction has occurred to formBaCrO4(s). Note: BaCrO4(s) is not molecular. It actually contains Ba2 and CrO4 2 ions

packed together in a lattice.

c) A photo of the solution after the reaction has occurred, showing the solid BaCrO4 on

the bottom.




Precipitation Reactions Photos and accompanying molecular-level representations illustrating the reaction of

KCl(aq) with AgNO3(aq) to form AgCl(s).

Note that it is not possible to have a photo of the mixed solution before the reaction

occurs, because it is an imaginary step that we use to help visualize the reaction. Actually,

the reaction occurs immediately when the two solutions are mixed.




Determining the mass product formedDetermining the mass product formedDetermining the mass product formedDetermining the mass product formed




Determining the mass product formedDetermining the mass product formedDetermining the mass product formedDetermining the mass product formed

Exercise




Acid Base Reactions Discussed in Acids and Bases topic




Volumetric Analysis A technique for determining the amount of a certain

substance by doing a titration.

Titration involves delivery (from a buret) of a measured

volume of a solution of known concentration (the titrant) into

a solution containing the substance being analyzed (the

analyte).

Equivalence point or the stoichiometric point: the

point in the titration where enough titrant has been added to

react exactly with the analyte Equivalence point point is often marked by an indicator




Volumetric Analysis Indicator: a substance added at the beginning of the

titration that changes color at (or very near) the equivalence

point.

The point where the indicator actually changes color is called

the endpoint of the titration


The titration of an acid with a base.

a) The titrant (the base) is in the buret, and the

flask contains the acid solution along with a

small amount of indicator.

b) As base is added drop by drop to the acid

solution in the flask during the titration, the

indicator changes color, but the color

disappears on mixing.

c) The stoichiometric (equivalence) point is

marked by a permanent indicator color

change. The volume of base added is the

difference between the final and initial buret

readings



Oxidation-Reduction Reactions

The reaction of solid natrium and gaseous chlorin to from solid

natrium chloride11/14/2012sb/chemical reaction18



4Fe(s) + 3O2(g) → 2Fe2O3(s)

Fe Fe3+loses 3 electrons

Rusting is an oxidation process



Rusting is an oxidation process

Fe Fe3+loses 3 electrons

Early chemists talked about

“reducing” compounds

into pure metals longbefore they understood the

chemistry of what was

happening

Pure metal

gains 3 electrons

4Fe(s) + 3O2(g) → 2Fe2O3(s)



Redox reactions

Zn(s)

CuSO4(aq)

Cu(s) deposit

Zn(s) + CuSO4 → ZnSO4(aq) + Cu(s)



Oxidation: Loss of electrons. The oxidized element becomes more positive.

Reduction: Gain of electrons. The reduced element becomes more negative.

Redox reactions

Zn(s) + CuSO4 → ZnSO4(aq) + Cu(s)

We can’t actually observe the transfer of electrons, but we know that:

Cu2+ was reduced back to Cu

Zn was oxidized by losing electrons and becoming Zn2+



1) How can we determine if a reaction is a redox reaction or not?

2) If it is, how can we determine which elements are reduced, and

which elements are oxidized?










Track the transfer of electrons



oxidation number: the number of electrons that an

element has lost or gained in forming a chemical bond

with another element.

The oxidation number of Cl is –1

because Cl “pulls” an electron from H

The oxidation number of H is +1because HCl is neutral overall

Cl2 + H2 → 2HCl



Cl2 + H2 → 2HCl




The oxidation number of Cl is –1

because Cl “pulls” an electron from H

The oxidation number of H is +1because HCl is neutral overall

Cl is reduced because it gains an electron

H is oxidized because it lost an electron






The oxidation number is different from the real charge

of the atom



Oxidation number rulesRule 1 Oxidation number of an atom in a pure element is 0

Ex: in Cl2, nCl = 0n = 0

Rule 2Sum of oxidation numbers in a neutral molecule is 0

Ex: nCO2 = nC + 2(nO) = 0 n = 0

Rule 3 Sum of oxidation numbers in an ion is equal to the

charge of the ion. Ex: n for SO42– = –2

n = charge

of ion

Rule 4 Metals have positive oxidation numbers according totheir group. Ex: Group 1A metals (Na, K) have n = +1

Group 2A metals (Mg, Ca) have n = +2

n = +(groupnumber)

Rule 5 For non-metals:

- Fluorine (F)- Hydrogen (H), except in hydrides like LiH or NaH

- Oxygen (O)

- Group 7A (Cl, Br), group 6A (S, Se), and

group 5A (N, P), respectively

n = –1n = +1

n = –2

n = –1, –2,

and –3






Ex: nCO2 = nC + 2(nO) = 0 n = 0



n = charge

of ion



n = +(groupnumber)


- Fluorine (F)

- Hydrogen (H), except in hydrides like LiH or NaH

- Oxygen (O)



n = –1

n = +1

n = –2

n = –1, –2,

and –3






Ex: nCO2 = nC + 2(nO) = 0 n = 0


charge of the ion. Ex: n for SO42– is –2

n = charge

of ion



n = +(groupnumber)


- Fluorine (F)


- Oxygen (O)



n = –1

n = +1

n = –2

n = –1, –2,

and –3






Ex: nCO2 = nC + 2(nO) = 0 n = 0



n = charge

of ion



n = +(groupnumber)


- Fluorine (F)


- Oxygen (O)



n = –1

n = +1

n = –2

n = –1, –2,

and –3






Ex: nCO2 = nC + 2(nO) = 0 n = 0



n = charge

of ion



n = +(groupnumber)


- Fluorine (F)


- Oxygen (O)



n = –1

n = +1

n = –2

n = –1, –2,

and –3

The oxidation number rules

apply in the order they

appear on the list



Oxidation number rulesFind the oxidation number of each element in potassium peroxide (K2O2).

Rule 4: nK = +1

Rule 5: nO = –2

= 2(nK) + 2(nO)

= 2(+1) + 2(–2)

= +2 – 4

= –2

But according to Rule 2:

the sum of oxidation numbers in a neutral molecule is zero

What steps do we follow to make sure the rule works?



Oxidation number rules

1) Rule 2: Sum of oxidation numbers in a neutral molecule is 0

2(nK) + 2(nO) = 0

2) Rule 4: Metals have positive oxidation numbers according to their group.

K belongs to Group 1A, so nK = +1

3) Solve: 2(nK) + 2(nO) = 0

2(+1) + 2(nO) = 0

nO = –2 / 2 = –1

Answer: nK = +1 and nO = –1

Find the oxidation number of each element in potassium peroxide (K2O2).



Find the oxidation number of each element in carbon dioxide (CO2).




nC + 2(nO) = 0





nC + 2(nO) = 0

2) Rule 5: nO = –2




1) Rule 2: Sum of oxidation numbers in a neutral molecule is 0nC + 2(nO) = 0

2) Rule 5: nO = –2

3) Solve: (nC) + 2(nO) = 0

(nC) + 2(–2) = 0

nC = +4




1) Rule 2: Sum of oxidation numbers in a neutral molecule is 0nC + 2(nO) = 0

2) Rule 5: nO = –2

3) Solve: (nC) + 2(nO) = 0

(nC) + 2(–2) = 0

nC = +4

Answer: nC = +4 and nO = –2




Notice that the oxidation number for an element can be different from

one compound to another

In K2O2, nO = –1

In CO2, nO = –2



1) Rule 3: Sum of oxidation numbers in an ion is equal to the charge of theion

nN + 2(nO) = –1

2) Rule 5: nO = –2

3) Solve: nN + 2(nO) = –1

nN + 2(–2) = –1

nN = +4 – 1 = +3

Answer: nN = +3 and nO = –2

Find the oxidation number of each element in the nitrite ion (NO2 –).



Find the oxidation number of each element in the compound C3H4.

1) Rule 2: Sum of oxidation numbers in a neutral molecule is 03(nC) + 4(nH) = 0

2) Rule 5: nH = +1

3) Solve: 3(nC) + 4(nH) = 0

3(nC) + 4(+1) = 0

nC = –4/3

Answer: nC = –4/3 and nH = +1

Oxidation numberscan be fractions!




We can use oxidation numbers to answer these questions






If there is a change in oxidation numbers from reactants to

products, it is a redox reaction







If there is a change in oxidation numbers from reactants to

products, it is a redox reaction



which elements are oxidized?If the oxidation number increases, that element is reduced

If the oxidation number decreases, that element is oxidized



4Fe(s) + 3O2

(g) → 2Fe2

O3

(s)

Find the element that is oxidized and the element that is reduced in the

reaction of iron with oxygen resulting in rust (iron oxide, Fe2O3):



4Fe(s) + 3O2

(g) → 2Fe2

O3

(s)

Find the element that is oxidized and the element that is reduced in the

reaction of iron with oxygen resulting in rust (iron oxide, Fe2O3):

Asked: The element that is oxidized, and the element that is reduced

Given: The balanced equation, and the oxidation number rules

Relationships: The oxidation number rulesSolve: 1) Find the oxidation numbers for each element in the

reactants

2) Find the oxidation numbers for each element in the product

3) Find for which element the oxidation number increased or

decreased



4Fe(s) + 3O2(g) → 2Fe2O3(s)



Relationships: The oxidation number rules

Solve: 1) Find the oxidation numbers for each element in the

reactants

2) Find the oxidation numbers for each element in the product3) Find for which element the oxidation number increased or

decreased

Rule 1: The oxidation number of an atom in a pure element is zero

nFe = 0 and nO = 0



4Fe(s) + 3O2(g) → 2Fe2O3(s)





reactants


decreased

Rule 2: Sum of oxidation numbers in a neutral molecule is zero

2(nFe) + 3(nO) = 0Rule 5: nO = –2

Solve: 2(nFe) + 3(–2) = 0 so nFe = +6 / 2 = +3

nFe = 0

nO = 0



4Fe(s) + 3O2(g) → 2Fe2O3(s)





reactants


decreased

3O2(g) + 4Fe(s) → 2Fe2O3(s)oxidation

numbers 0 0 +3 –2oxidation

reduction



Fe is oxidized by OO is the oxidizing agent

oxidation

numbersoxidation

reduction

3O2(g) + 4Fe(s) → 2Fe2O3(s)

0 0 +3 –2



oxidation

numbers

Fe is oxidized by O

O is the oxidizing agent

O is reduced by Fe

Fe is the reducing agent

oxidation

reduction

3O2(g) + 4Fe(s) → 2Fe2O3(s)

0 0 +3 –2



oxidation

numbersoxidation

reduction

3O2(g) + 4Fe(s) → 2Fe2O3(s)

0 0 +3 –2

Where there is a reduction reaction,there is also an oxidation reaction,

and vice versa.

This is because electrons that are lostby one element must be gained by

another element



Find the reducing and the oxidizing agents in:

2Mg(s) + O2(g) → 2MgO(s)




2Mg(s) + O2(g) → 2MgO(s)

Relationships: The balanced chemical reaction and the oxidation number rules

Solve: O2(g) + 2Mg(s) → 2MgO(s)

oxidation

reduction

0 0 +2 –2

oxidation

numbers




2Mg(s) + O2(g) → 2MgO(s)

Relationships: The balanced chemical reaction and the oxidation number rules

Solve: O2(g) + 2Mg(s) → 2MgO(s)

oxidation

reduction

0 0 +2 –2

oxidation

numbers

Answer: Mg is oxidized by O O is the oxidizing agent

O is reduced by Mg Mg is the reducing agent



Rules to assign oxidation numbers:

- the rules must be followed in the order they are presented

- the oxidation number of an element can be different fromone compound to another

- the oxidation number of an element can be a fraction



Rules to assign oxidation numbers:

- the rules must be followed in the order they are presented

- the oxidation number of an element can be different fromone compound to another

- the oxidation number of an element can be a fraction

Assigning oxidation numbers helps to:

- determine if a chemical reaction is a redox reaction or not

- determine which elements are reduced, and which elements

are oxidized

- determine which elements or compounds are oxidizing agents,

and which are reducing agents



Changes in SubstancesChanges in SubstancesChanges in SubstancesChanges in Substances Physical change – altered appearance but same composition

Water to steam

Glass bottle to broken glass

Chemical change or reaction – results in a different chemical

composition

Coal to ashes Milk to sour milk


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