class12 chemistryg12 notes and homework

8/4/2019 Class12 ChemistryG12 Notes and Homework

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GRADE12CHEMISTRY

OLYMPIADSCHOOL

WEDNESDAY2:304:30

FRIDAY4:456.45


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Oxidation-Reduction Reactions

Oxidation is the loss of electrons. Reduction is the gain ofe ec rons.

In the reaction of zinc atoms with co er II ions the zinc atomslose electrons and undergo oxidation. In other words, the zinc

atoms are oxidized. e copper ons ga n e ectrons an un ergo re uct on. n

other words, the copper(II) ions are reduced.

,

known as an oxidation-reduction reaction or redox reaction.


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Notice that electrons are transferred from zinc atoms to copper(II) ions. The

copper ons are respons e or e ox a on o e z nc a oms. reac anthat oxidizes another reactant is called an oxidizing agent. The oxidizing agentaccepts electrons in a redox reaction. In this reaction, copper(II) is the oxidizing

. .reactant that reduces another reactant is called a reducing agent. The reducing

agent gives or donates electrons in a redox reaction. In this reaction, zinc is the.

A redox reaction can also be defined as a reaction between an oxidizing agentand a reducing agent.


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Half-Reactions ,

represent the oxidation and reduction separately. A half-reaction is a balanced equation that shows the number of

electrons involved in either oxidation or reduction. Because a redox reaction involves both oxidation and reduction, two

half-reactions are needed to represent a redox reaction. One half-reaction shows oxidation, and the other half-reaction shows reduction.

In most redox reactions, one substance is oxidized and a differentsubstance is reduced. In a disproportionation reaction, however, asingle element undergoes both oxidation and reduction in the same

reac on. For example, a copper(I) solution undergoes disproportionation in thefollowing reaction: 2Cu+(aq) Cu(s) + Cu2+(aq)

In this reaction, some copper(I) ions gain electrons, while othercopper(I) ions lose electrons.

The two half-reactions are as follows.Oxidation: Cu+ (aq) Cu2+ (aq) + 1e

Reduction: Cu+(aq) + 1e- Cu(s) a -reac ons a ways come n pa rs: an ox a on a -reac on s

always accompanied by a reduction half-reaction, and vice versa.


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Oxidation Numbers Oxidation numbers are actual or hypothetical charges, assigned

using a set of rules.

ey are use o escr e re ox reac ons w cova en

reactants or products. They are also used to identify redox, .

Consider Lewis Structures. Oxygen (electronegativity 3.44) is

The electronegativity difference is less than 1.7, so the twohydrogen-oxygen bonds are polar covalent, not ionic. In eachbond, the electrons are more strongly attracted to the oxygen

atom than to the hydrogen atom.


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To assign oxidation numbers to the atoms in a water molecule,

more electronegative oxygen atom. Thus, each hydrogen atomin a water molecule is considered to have no electrons, ashydrogen would in a hydrogen ion, H+.

Therefore, the element hydrogen is assigned an oxidation. ,water molecule is considered to have a filled octet of electrons,as oxygen would in an oxide ion, O2. Therefore, the element

oxygen is assigned an oxidation number of 2 in water.

(Note: These are not ionic charges, since water is a covalent

molecule. Also, note that the plus or minus sign in an oxidation, , .

minus sign in an ionic charge, such as 2-, is written after the


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Balancing Half-Reactions for Acidic

Step 1 Write an unbalanced half-reaction that shows the.

Step 2 Balance any atoms other than oxygen and hydrogen first.

.

Step 4 Balance any hydrogen atoms by adding hydrogen ions.

ep a ance e c arges y a ng e ec rons.


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Balancing Half-Reactions for Basic

Step 1 Write an unbalanced half-reaction that shows the formulas ofthe given reactant(s) and product(s).

Step 2 Balance any atoms other than oxygen and hydrogen first. ep a ance any oxygen an y rogen a oms as e con ons

are acidic.

number of hydroxide ions as the number of hydrogen ions alreadypresent.

Step 5 Simplify the half-reaction by combining the hydrogen ions andhydroxide ions on the same side of the equation into water molecules.

half-reaction.


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S f B l i b h


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Steps for Balancing by the

- Step 1 Write an unbalanced net ionic equation, if it is not alreadygiven.

Step 2 Divide the unbalanced net ionic equation into an oxidationhalf-reaction and a reduction half-reaction. To do this, you may need

to assign oxidation numbers to all the elements in the net ionic. Step 3 Balance the oxidation half-reaction and the reduction half-

reaction independently.

electrons in the oxidation half-reaction and the reduction half-reaction. Ste 5 Use coefficients to write each half-reaction so that it includes

the LCM of the numbers of electrons. Step 6 Add the balanced half-reactions that include the equal

numbers of electrons. Step 7 Remove the electrons from both sides of the equation. Step 8 Remove any identical molecules or ions that are present on

both sides of the equation. tep you requ re a a ance c em ca equat on, nc u e any

spectator ions in the chemical formulas. Step 10 If necessary, include the states.

B l i R d R ti i A idi


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Balancing a Redox Reaction in Acidic

Problem Write a balanced net ionic equation to show the, -,

acidic solution to produce chloride ions and nitrate ions.

What Is Re uired? You need to write a balanced net ionicequation for the given reaction.

What Is Given? You know the identities of two reactants andtwo pro ucts, an t at t e react on ta es p ace n ac csolution.


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The Oxidation Number Method

for Balancing Equations ep r e an un a ance equa on, s no

given.

reaction by assigning an oxidation number to eachelement wherever it appears in the equation.

Step 3 If the reaction is a redox reaction, identify theelement(s) that undergo an increase in oxidation

in oxidation number.

Step 4 Find the numerical values of the increase andthe decrease in oxidation numbers.


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ep e erm ne e sma es w o e-num er ra o othe oxidized and reduced elements so that the total

increase in oxidation numbers e uals the totaldecrease in oxidation numbers.

Step 6 Use the smallest whole-number ratio toa ance t e num ers o atoms o t e e ement s

oxidized and the element(s) reduced

,possible.

Step 8 For reactions that occur in acidic or basicsolutions, include water molecules, hydrogen ions,or hydroxide ions as needed to balance the equation.


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