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Chapter 6 Chemical Reactions

6.5 Oxidation-Reduction Reactions

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Oxidation and reduction Are an important type of

reaction. Provide us with energy

from food. Provide electrical energy

in batteries. Occur when iron rusts.

4Fe + 3O2 2Fe2O3

Oxidation and Reduction

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An oxidation-reduction reaction involvesthe transfer of electrons from one reactantto another.

In oxidation, electrons are lost. Zn Zn2+ + 2e- (loss of electrons) In reduction, electrons are gained.

Cu2+ + 2e- Cu (gain of electrons)

Electron Loss and Gain

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Half-Reactions for Oxidation-Reduction

In the oxidation-reduction reaction of zinc andcopper(II) sulfate, the zinc is oxidized and theCu2+ (from Cu2+ SO4

2-) is reduced.Zn Zn2+ + 2e- oxidationCu2+ + 2e- Cu reduction

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Identify each of the following as an

1) oxidation or a 2) reduction:

__A. Sn Sn4+ + 4e-

__B. Fe3+ + 1e- Fe2+

__C. Cl2 + 2e- 2Cl-

Learning Check

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Identify each of the following as an

1) oxidation or a 2) reduction:

1 A. Sn Sn4+ + 4e-

2 B. Fe3+ + 1e- Fe2+

2 C. Cl2 + 2e- 2Cl-

Solution

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In a balanced oxidation-reduction equation,the loss of electrons is equal to the gain ofelectrons.Zn + Cu2+ Zn2+ + Cu

The loss and gain of two electrons is shownin the separate oxidation and reductionreactions.Zn Zn2+ + 2e- oxidationCu2+ + 2e- Cu reduction

Balanced Red-Ox Equations

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In light-sensitive sunglasses, UV light initiatesan oxidation-reduction reaction. uv light

Ag+ + Cl- Ag + ClA. Which reactant is oxidized?

1) Ag+ 2) Cl- 3) AgB. Which reactant is reduced?

1) Ag+ 2) Cl- 3) Cl

Learning Check

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In light-sensitive sunglasses, UV light initiatesan oxidation-reduction reaction.

uv light

Ag+ + Cl– Ag + ClA. Which reactant is oxidized

2) Cl– Cl– Cl + e–

B. Which reactant is reduced?1) Ag+ Ag+ + e– Ag

Solution

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Write the separate oxidation and reductionreactions for the following equation.

2Cs + F2 2CsF

Learning Check

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Write the separate oxidation and reductionreactions for the following equation.

2Cs + F2 2CsF

Cs Cs+ + 1e– oxidation

F + 1e– F- reduction

Solution

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An early definition of oxidation is theaddition of oxygen O2 to a reactant.

A metal or nonmetal is oxidized while theO2 is reduced to O2-.

4K + O2 2K2O

C + O2 CO2

2SO2 + O2 2SO3

Oxidation with Oxygen

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In organic and biological reactions,oxidation involves the loss of hydrogenatoms and reduction involves a gain ofhydrogen atoms.oxidation = Loss of Hreduction = Gain of HCH3OH H2CO + 2H (loss of H)Methanol Formaldehyde

Gain and Loss of Hydrogen

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Summary

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Learning Check

Identify the substances that are oxidized andreduced in the following reactions.

A. 4Fe + 3O2 2Fe2O3

B. 6Na + N2 2Na3N

C. 2K + I2 2KI

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Solution

3+ 2-

A. 4Fe + 3O2 2Fe2O3

Fe oxidized; O2 reduced1+ 3-

B. 6Na + N2 2Na3N Na oxidized: N2 reduced

1+ 1-

C. 2K + I2 2KI K oxidized: I2 reduced

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Chapter 6 Chemical Reactions

6.6 Energy in Chemical Reactions

6.7 Rate of Reaction

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Reaction Conditions

A chemical reaction occurs when thereacting molecules collide.

Collisions between molecules must havesufficient energy to break the bonds in thereactants.

Once the bonds between atoms of thereactants are broken, new bonds can form togive the product.

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Chemical Reactions

In the reaction H2 + I2 2 HI, the bonds ofH2 and I2 must break, and bonds for HI mustform.

H2 + I2 collision bonds break 2HI

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Activation Energy

The activation energyis the minimumenergy needed for areaction to take place.

When a collision hasthe energy that isequal to or greaterthan the activationenergy, reaction canoccur.

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Exothermic Reactions

The heat of reaction is thedifference in the energy ofthe reactants and theproducts.

An exothermic reactionreleases heat because theenergy of the products isless that the reactants.

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Endothermic Reactions

In anendothermicreaction, heat isabsorbedbecause theenergy of theproducts isgreater that thatof the reactants.

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Learning Check

Identify each reaction as

1) exothermic or 2) endothermic

A. N2 + 3H2 2NH3 + 22 kcal

B. CaCO3 + 133 kcal CaO + CO2

C. 2SO2 + O2 2SO3 + heat

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Solution

Identify each reaction as

1) exothermic or 2) endothermic

1 A. N2 + 3H2 2NH3 + 22 kcal

2 B. CaCO3 + 133 kcal CaO + CO2

1 C. 2SO2 + O2 2SO3 + heat

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Rate of Reaction

The rate of a reaction is the speed at which productforms.

Adding more of the reactants speeds up a reactionby increasing the number of collisions that occur.

Raising the temperature speeds up a reaction byproviding the energy of activation to more collidingmolecules.

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Factors that Increase Rate

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Effect of Catalysts

A catalyst speeds up therate of a reaction bylowering the energy ofactivation.

Then more collisionscan result in reactionand the formation ofproducts.

A catalyst is not usedup during the reaction.

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Learning Check

State the effect of each on the rate of reaction.

1) increases rate 2) decreases rate

3) does not change the rate

A. Increasing the temperature.

B. Removing some of the reactants.

C. Adding a catalyst.

D. Placing the reaction flask in ice.

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Solution

State the effect of each on the rate of reaction.

1) increases rate 2) decreases rate

3) does not change the rate

1 A. Increasing the temperature

2 B. Removing some of the reactants

1 C. Adding a catalyst

2 D. Placing the reaction flask in ice

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Chapter 6 Chemical Reactions

6.8 Chemical Equilibrium

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Reversible Reactions

A reversible reaction has both forward and reverse reactions. If SO2 and O2 are present initially, they collide and the

forward reaction begins.2SO2 + O2 2SO3

As the amount of product increases, SO3 molecules collideand form the reactants. This is shown as a double arrow.

forward2SO2 + O2 2SO3

reverse

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Chemical Equilibrium

In chemical equilibrium The rate of the forward reaction becomes equal to

the rate of the reverse reaction. There is no further change in the amounts of

reactant and product. Reactions continue at equal rates in both

directions.

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Reaching Chemical Equilibrium

A container filled with SO2 and O2 or only SO3eventually contains mostly SO3 and smallamounts of O2 and SO3.

Equilibrium is reached in both situations.

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Equilibrium can Favor Product

If equilibrium is reached after most of theforward reaction has occurred, the systemfavors the product.

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Equilibrium Can Favor Reactant

If equilibrium is reached when very little ofthe forward reaction has occurred, thesystem favors the reactants.

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LeChâtelier’s Principle

For a system at equilibrium, a change inthe amounts of reactants or products or thetemperature causes stress.

LeChâtelier’s principle states that theequilibrium will shift to relieve the stress.

That means that the rate of the forwardand reverse reaction will change until theyare equal again.

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Effect of Adding Reactant

Consider the equilibrium system for thereaction H2(g) + F2(g) 2HF(g)

If one of the reactants (H2 or F2 ) is added,there is an increase in the number of collisions.

To remove this stress, the rate of the forwardreaction increases and forms more HF product.

Because more HF is produced, the effect ofadding a reactant shifts the equilibriumtowards the products.

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Effect of Adding Product

If more of the product HF is added, there is anincrease in collisions of HF molecules.

To remove this stress, the rate of the reversereaction increases and forms more H2 and F2reactants.

The effect of adding a product shifts theequilibrium towards the reactants.

H2(g) + F2(g) 2HF(g)

Adding HF

Adding H2

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Effect of Removing Reactant

If some reactant, H2 or F2, is removed, thereare fewer collisions between reactingmolecules.

The rate of the forward reaction decreases. Removing a reactant shifts the equilibrium

towards the reactants.H2(g) + F2(g) 2HF(g)

Removing H2 orF2

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Effect of Removing Product

If some product HF is removed, the rate of thereverse reaction decreases.

Removing some product shifts the equilibriumtowards the products.

H2(g) + F2(g) 2HF(g)

Removing HF

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Effect of Temperature

When the temperature for an exothermic reaction islowered (heat removed), the equilibrium shifts towardsthe products.

When the temperature of an exothermic reaction israised (heat added), the equilibrium shifts towards thereactants.

N2(g) + 3H2(g) 2NH3(g) + 22 kcal

Adding heat

Removing heat

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Effect of Temperature

When the temperature is lowered (heat removed) foran endothermic reaction, the equilibrium shiftstowards the reactants.

When the temperature is raised (heat added) for anendothermic reaction, the equilibrium shifts towardsthe products.

CaCO3 + 133 kcal CaO + CO2

Removing heat

Adding heat

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Summary of Changes onEquilibrium

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Learning Check

Indicate the effects of the changes on equilibrium forN2(g) + O2(g) + heat 2 NO(g)1) Shifts towards the products2) Shifts towards the reactants

A. Adding NOB. Adding N2C. Raising the temperatureD. Removing O2

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Solution

Indicate the effects of the changes on equilibrium forN2(g) + O2(g) + heat 2 NO(g)1) Shifts towards the products2) Shifts towards the reactants

2 A. Adding NO1 B. Adding N21 C. Raising the temperature2 D. Removing O2