Y EAR 12 C HEMISTRY Term 2 - 2012. R EVERSIBLE R EACTIONS Some chemical reactions are reversible....

YEAR 12 CHEMISTRYTerm 2 - 2012

REVERSIBLE REACTIONS

Some chemical reactions are reversible.

Water can become liquid, solid or gas depending on the circumstances.

We saw in the previous unit that the reaction to form an ester could either move forward to produce the ester and water or in the right conditions move backward to produce the alcohol and the carboxylic acid.

This reaction is known as a reversible reaction.


Without a something push it to completion this reaction will reach a point where it contains both products and reactants.

When the reaction reaches this point both reactions are occurring at equal rates. This means there is no overall change in the amount of each item present. This is called dynamic equilibrium.

We write this reaction as:Reactant ↔ Product

EQUILIBRIUM

Equilibrium is the term given to an object in a state of balance.

Static equilibrium is like a tug of war where both sides are equal strength. The rope does not move because the force on both sides is equal

Dynamic equilibrium is like a sports game, there is always a certain number of players on the field but the players themselves can change.


Chapter 1 Page 203

REVERSIBLE REACTIONS For reversible reactions we have:

Forward reactions - the reaction going as written from left to right.

Reverse or backward reactions - the reaction going opposite to the way it is written from right to left.

What is the product of the reverse reaction for the following reversible reaction?

Pb(NO3)2(s) ↔ Pb2+ + 2NO3

-(aq)


Reversible Reaction - A reaction which can go forward or backward depending on the circumstances.

Dynamic Equilibrium - The state where the concentration of products and reactants in a reaction remains stable, the forward and reverse reactions are occurring at the same rate.

Forward Reaction - A reversible reaction which is occurring from left to right.

Backward / Reverse Reaction - A reversible reaction which is occurring from right to left.

TYPES OF REVERSIBLE REACTIONS

All precipitation reactions are reversible

Potassium chromate ↔ Potassium dichromate

Esters

Gases

HOW DOES EQUILIBRIUM OCCUR?

Ag+(aq) + Fe2+(aq) ↔ Fe3+ + Ag(s)

When the reaction above begins we have large concentrations of Ag+ and Fe2+.

Reactants decrease as products increase

As products increase they begin to react

Eventually reach state of equilibrium where both reactions are occurring at the same rate.

Br2 (g) + 2NO(g) ↔ 2NOBr (g)

CATALYSTS

How do catalysts affect equilibrium? Draw graph:

EXERCISES

CARBON DIOXIDE EQUILIBRIUM

Read page 207- 208.

Write down equation of dissolution of CO2 in H2O.

What is the relationship between solubility and pressure for gases in liquids.

Does the reaction lie to the right or left?

SOLUBILITY AND PRESSURE FOR GASES IN LIQUIDS

As the pressure increases ↑ solubility increases ↑

As pressure decreases ↓ solubility decreases ↓

As solubility increases ↑ temperature decreases ↓

As solubility decreases ↓ temperature increases ↑

This is the opposite of solids and liquids in which temperature normally increases ↑ as solubility increases ↑.

EQUILIBRIUM POSITION

The extent to which the reaction has gone in the forward or reverse direction.

Lies to the leftReverse reaction is favouredMost of reactant is still resentSmall amount of product

Lies to the rightForward reaction is favouredMost of the reactants has converted to

product

LE CHATELIER’S PRINCIPLE

If a system in equilibrium is disturbed, the system adjusts itself so as to minimize the disturbance

LE CHATELIER’S PRINCIPLEDisturbance to a system may include:

Concentration Pressure Temperature

Note a system at equilibrium is not disturbed by adding more solid to it. This is because the concentration of ions is not changed when a solid is added.

Concentration not amount effects equilibrium

CARBON DIOXIDE AND CARBONIC ACID

Read page 210 – 211.Predict the direction of the

equilibrium if:a) The plunger was pulled creating

more volumeb) The temperature of the system

decreasedc) Adding sodium hydroxide to the

mix.

FORCING REACTIONS TO COMPLETION

Water liquid and gas equilibrium:


H2O(l) H2O(g)Water in a terrarium reaches

equilibrium because it is a closed system water evaporates while vapour condenses.

Water in wet clothes attempts to establish equilibrium however as liquid evaporates, it is carried by wind and diffuses into the atmosphere and so dryness will eventually occur.


When synthesizing chemicals, chemists may wish to push a reversible reaction to completion to obtain the maximum amount of a product.

A common way to force reactions to completion is to remove a product as it is produced. Addition of an excess of cheap or common reactant is another way.

EXERCISES PAGE 212

CONDITION FOR EQUILIBRIUM

There is a quantitative relationship between reactants and products at equilibrium.

Using

When the reaction is at equilibrium at a constant temperature the expression [I3-] / [I2] [I-] has a constant value.

CONDITION FOR EQUILIBRIUM

See table 2.1 page 240.

CONDITIONS FOR EQUILIBRIUM

Table shows a clear relationship between the reactants and products in an equilibrium reaction.

For any equilibrium reaction there is a function of the concentration of the species which has a constant value at equilibrium

This constant is given the symbol K and known as the equilibrium constant.

EQUILIBRIUM CONSTANT

In an equilibrium reaction where aA + bB ↔ cC + dD

When the reaction is at equilibrium the expression:

Has a constant value, regardless of the starting concentrations of the substances involved.

This is called the equilibrium constant K

EQUILIBRIUM CONSTANT VS REACTION QUOTIENT

= K

The above equation at constant temperature is known as the equilibrium expression

Q=

Alternatively the above equation at constant temperature is known as the reaction quotient


The equilibrium constant [K] is the relationship between the products and reactants at equilibrium.

The reaction quotient [Q] is the relationship between the products and reactants at any given point during the reaction.

When Q = K the reaction is at equilibrium. If Q ≠ K the reaction is not at equilibrium.


It is possible to use Q to determine which direction a reaction if occurring.

If Q < K the reaction goes from left to right until Q = K

If Q > K the reaction goes from right to left until Q = K

If Q = K the reaction is at equilibrium.

REACTION QUOTIENT

RULES FOR WRITING EQUILIBRIUM EXPRESSIONS

Both Q and K are always written with the products in the numerator and the reactants in the denominator.

K or Q =

K or Q =

RULES FOR WRITING EQUILIBRIUM EXPRESSIONS

Coefficients of products or reactants are written as powers for that product or reactant.

eg:

UNITS FOR EQUILIBRIUM CONSTANTS

Reaction quotients have units.

mol/L

To calculate the value of the unit:

Cancel units above and below the division symbol. If the remainder is below the division symbol the units will be in mol/L-1 etc. If the remainder is above the division symbol units will be in mol/L1 etc.

EXERCISES

Page 244

Questions 1 - 4

TEMPERATURE AND EQUILIBRIUM CONSTANT

The equilibrium constant K only remains stable if temperature is stable. If temperature changes then the value of the equilibrium constant will change.

If reaction is exothermic and temperature increases:

Reaction will move from right to leftK value will decrease.

If reaction is endothermic and temperature increases:

Reaction will move from left to rightK value will increase.

TEMPERATURE AND EQUILIBRIUM CONSTANT

For an exothermic reaction:

K decreases as temperature increases.

For an endothermic reaction:

K increases at temperature increases.

EXERCISES

Questions 16 and 17

Page 252.

NOTES FOR EQUILIBRIUM

Always use coefficients as written.

2H2O + O2 ↔ 2H2O2

Will have a difference equilibrium constant to:

1H2O + ½O2 ↔ 1H2O2

If concentrations are not given they must be in mol/L

EQUILIBRIUM IN GASES

Recall: In gas at constant temperatures pressure is proportional to concentration.

PV = nRT Where P = pressure

V = volumen = number of molesR = The gas constantT = temperature.


PV = nRT This can be rearranged: P = RT = concentration of the gas.

If we were to compress gas in a system to ½ the volume then we would have double the pressure and double the concentration.

i.e. same moles of gas in half the volume


To increase the pressure in a gas we can:

Adding more gas to a given volume or Keeping amount of gas stable and decreasing

volume.

Therefore:

As pressure increases, concentration increases and volume decreases.

As pressure decreases, concentration decreases and volume increases

GAS EQUILIBRIUM AND LE CHATELIER

If pressure increases then equilibrium is disturbed and the reaction will go in the direction which minimises the disturbance.

If there is a decrease in volume the reaction will go in the direction which produces less moles of gas. e.g.

If pressure is increased which direction will the following reaction go?

PCl3(g) + Cl2(g) ↔ PCl5(g)

GAS EQUILIBRIUM AND LE CHATELIER

If moles of gas of products and reactants are equal, then changing pressure will not disrupt equilibrium.

Concentration of products and reactants will remain the same.

CO(g) + NO2(g) ↔ CO2(g) + NO(g)

CALCULATIONS USING K

It is difficult to measure equilibrium constant.

Why?

If we remove substances the reaction quickly moves to minimise the disturbance because of LeChatelier.

To counter this chemists often use the absorption of light or pH meters to measure Q and K values as these measure without disturbing the system.

CALCULATIONS USING K

Do we need to know the concentrations of every species to be able to calculate K?

MEASUREMENT OF EQUILIBRIUM CONSTANTS

Example 1

2SO2 (g) + O2 (g) 2SO3 (g)

At equilibrium at 900K concentration of SO3 was 9 times the concentration of SO2 when the equilibrium concentration of oxygen was 0.068 mol/L. Calculate the equilibrium constant.

Set out the information that you know from the problem.

At equilibrium:

[O2] = 0.068 mol/L

[SO3]/[SO2] = 9 so

[SO3] = 9

[SO2] = 1

EQUILIBRIUM EXPRESSION:

K = ][][

][

22

2

23

OSO

SO

K =

]068.0[]1[

]9[2

2

K = 1.2 x 103 (mol/L)-1

Example 2

PCl5 (g) PCl3 (g) Cl2 (g)

0.0100 mole phosphorus pentachloride was placed in a 1.00L flask at 523K. At equilibrium the concentration of chlorine was 0.0083 mol/L. Calculate the equilibrium constant for the reaction.

Take the information you know from the equation:

The ratio for the equation is 1:1:1. At completion 1 mole of PCl5 forms 1 mole of PCl3 and 1 mole of Cl2. At equilibrium if we have 0.0083 mole of Cl2

then we also have 0.0083 mole of PCl3 .

Cl2 = PCl3 = 0.0083 mole

The PCl3 and Cl2 came from the PCl5 therefore 0.0083 mole of PCl5 must have been used. If we initially had 0.0100 mole of PCl5 then at equilibrium we have:

0.0100 – 0.0083 = 0.0017 mole

SETTING OUT

PCl5 (g) PCl3 (g) Cl2 (g)

Initially: 0.0100 mol/L 0 0

At equilibrium: 0.0100 – 0.0083 0.0083 0.0083

0.0017 0.0083 0.0083

][

]][[

5

23

PCl

ClPCl

]0017.0[

]0083.0][0083.0[

K =

K =

K = 0.041 mol/L

USE OF K

K tells us the position of equilibrium and the concentration of species at equilibrium.

If K is large e.g. > 103 equilibrium lies to the right and the reaction favours the products.

If K is small e.g. < 10-3 equilibrium lies to the left and the reaction favours the reactants.

If K is in the centre there are similar amounts of products and reactants.

USING Q AND K

N2O4(g) ↔ 2NO2

The equilibrium constant is 0.48 at 100°C. 0.1mol N2O4 and 0.25 mol NO2 were placed in a 1.0L flask at 100°C. Is the mixture at equilibrium? If not which direction will the reaction proceed.

USING Q AND K Just like any mathematical expression the

expression for K can be rearranged.

If we know K and some other concentration in the expression we can rearrange it to determine the unknown values.

USING Q AND K At 1000K the equilibrium constant for the

decomposition of phosgene(COCl2) into carbon monoxide and chlorine is 0.40.

A sample of phosgene was placed in an evacuated container and heated to 1000K. When equilibrium was reached, the concentration of carbon monoxide was 0.24 mol/L. Calculate the equilibrium concentration of phosgene.

USING Q AND K At 1000K the equilibrium constant for the

decomposition of phosgene(COCl2) into carbon monoxide and chlorine is 0.40.

In a second experiment another sample of phosgene was brought to equilibrium at 1000K, the equilibrium concentration of phosgene was 0.18mol/L. Calculate the equilibrium concentration of chlorine.

EXERCISES

Questions 6 - 14Page 249-250

SOLUBILITY EQUILIBRIA Remember when a solid ionic compound is in

solution with that solid equilibrium occurs.

If the solid has low solubility there is a relationship between the concentration of ions at equilibrium.

e.g. PbSO4(s) ↔ Pb2+ + SO42-

(aq)

At equilibrium:

[Pb2+] [SO42-] = constant

SOLUBILITY EQUILIBRIA[Pb2+] [SO4

2-] = constant

Note that there is no term for the solid as the concentration for a solid is constant.

When a pure substance, solid or liquid is used its concentration its concentration is not included in the equilibrium constant expression for the reaction.

An increase in the amount of pure solid does not effect equilibrium

SOLIDS IN KSP

If = Ksp

And the concentration of the solid is constant than

= Ksp and

Solution = [Ksp][solid]

SOLUBILITY EQUILIBRIA The equilibrium constant in this case is called

the solubility product and written as Ksp

[Pb2+] [SO42-] = Ksp

The reaction for Ksp is always written with the solid on the left and the solution on the right. Why?

Solid ↔ Solution

As with any equilibrium expression coefficients are written as powers for the relevant terms.

SOLUBILITY EQUILIBRIA Solubility expression

MxAy(s) ↔ xMb+(aq) + yAc-

(aq)

[Mb+]x [Ac-]y = Ksp

Ionic product expression[Mb+]x [Ac-]y = Q

If IP < Ksp solid dissolves until =

If IP > Ksp solid precipitates until =

If IP = Ksp sys is at equilibrium

EXERCISES

Page 259

Exercises 26 - 29

REVISION

Concepts we have covered so far: Dynamic equilibrium Reversible reactions Catalysts Kc

Ksp Q Le Chatelier Temperature and Equilibrium Equilibrium in Gases

INTRODUCTION TO ACIDS AND BASES

Acid is a substance which in solution produces H+ ions or H3O+ ions (this is the more correct terminology)

Base is a substance which contains the O2- ion or OH- ion or in solution produces the OH- ion.

Soluble base is called an alkali

PROPERTIES OF ACIDS

Acids have a sour taste

Acids sting or burn the skin

In solution acids conduct electricity

Acids turn blue litmus red

PROPERTIES OF BASES

Bases have a soapy feel

Bases have a bitter taste

In solution bases are good conductors of electricity

Bases turn red litmus blue

ACID BASE REACTIONS

Acids react with bases in neutralisation reactions to form salts.

A salt is an ionic compound formed when a base reacts with an acid. They are not limited to NaCl

A neutralisation reaction occurs when the H+ ions in the acid react with the OH- ions in the base to produce H2O

ACID-BASE REACTIONS

We can write an acid base reaction in 3 ways:

Neutral species equation:HCl + NaOH H2O + NaCl

Complete ionic equation:H+ + Cl- + Na+ + OH- H2O + Na+ + Cl-

Net ionic equation:H+ + OH- H2O

ACID-BASE REACTIONS

Because the Na+ and the Cl- do not actually take part in the reaction we call them spectator ions and do not always have to include them in the reaction when writing it.

In general in a neutralisation reaction

Acid + base salt + water

There are some exceptions to this rule.

NAMING SALTS

When naming salts there are some general rules:

Cation is named first anion then comes from the name of the acid.

Hydrohalic acids e.g. HCl become halide salts eg NaCl

Oxyacids e.g. H2SO3 (carbonic acid) the ic at the end of the name becomes ate carbonic acid calcium carbonate CaCO3

Ous acids e.g. nitrous HNO2 become ites e.g. sodium nitrite NaNO2

Anions formed from oxyacids are oxy anions.

TABLE OF ACIDS AND ANIONS PG 217

QUESTIONS

Page 217

Questions 8 - 13

ACIDIC AND BASIC OXIDES

Oxides are compounds that can show either acidic or basic properties.

An acidic oxide either: Reacts with water to form an acid or Reacts with bases to form salts or Both

A basic oxide: Reacts with acids to form salts Does not react with alkali solutions (NaOH or

KOH)

ACIDIC AND BASIC OXIDES

Acidic oxides are oxides of non-metals. They are covalent compounds generally found at the top right of the periodic table.

Basic oxides are oxides of metals. They are ionic compounds generally found at the left of the periodic table.

QUESTIONS

Page 219

Questions 14 and 15

Y EAR 12 C HEMISTRY Term 2 - 2012. R EVERSIBLE R EACTIONS Some chemical reactions are reversible....

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