y ear 12 c hemistry term 2 - 2012. r eversible r eactions some chemical reactions are reversible....
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YEAR 12 CHEMISTRYTerm 2 - 2012
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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.
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REVERSIBLE REACTIONS
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
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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.
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REVERSIBLE REACTIONS
Chapter 1 Page 203
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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)
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REVERSIBLE REACTIONS
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.
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TYPES OF REVERSIBLE REACTIONS
All precipitation reactions are reversible
Potassium chromate ↔ Potassium dichromate
Esters
Gases
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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.
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Br2 (g) + 2NO(g) ↔ 2NOBr (g)
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CATALYSTS
How do catalysts affect equilibrium? Draw graph:
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EXERCISES
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EXERCISES
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EXERCISES
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EXERCISES
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EXERCISES
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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?
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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 ↑.
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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
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LE CHATELIER’S PRINCIPLE
If a system in equilibrium is disturbed, the system adjusts itself so as to minimize the disturbance
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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
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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.
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FORCING REACTIONS TO COMPLETION
Water liquid and gas equilibrium:
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FORCING REACTIONS TO COMPLETION
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.
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FORCING REACTIONS TO COMPLETION
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.
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EXERCISES PAGE 212
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EXERCISES PAGE 212
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EXERCISES PAGE 212
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EXERCISES PAGE 212
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EXERCISES PAGE 212
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EXERCISES PAGE 212
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EXERCISES PAGE 212
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EXERCISES PAGE 212
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EXERCISES PAGE 212
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EXERCISES PAGE 212
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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.
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CONDITION FOR EQUILIBRIUM
See table 2.1 page 240.
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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.
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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
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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
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EQUILIBRIUM CONSTANT VS 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.
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EQUILIBRIUM CONSTANT VS REACTION QUOTIENT
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.
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REACTION QUOTIENT
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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 =
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RULES FOR WRITING EQUILIBRIUM EXPRESSIONS
Coefficients of products or reactants are written as powers for that product or reactant.
eg:
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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.
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EXERCISES
Page 244
Questions 1 - 4
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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.
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TEMPERATURE AND EQUILIBRIUM CONSTANT
For an exothermic reaction:
K decreases as temperature increases.
For an endothermic reaction:
K increases at temperature increases.
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EXERCISES
Questions 16 and 17
Page 252.
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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
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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.
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EQUILIBRIUM IN GASES
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
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EQUILIBRIUM IN GASES
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
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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)
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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)
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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)
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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.
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CALCULATIONS USING K
Do we need to know the concentrations of every species to be able to calculate K?
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MEASUREMENT OF EQUILIBRIUM CONSTANTS
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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.
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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
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EQUILIBRIUM EXPRESSION:
K = ][][
][
22
2
23
OSO
SO
K =
]068.0[]1[
]9[2
2
K = 1.2 x 103 (mol/L)-1
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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.
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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
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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
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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
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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.
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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.
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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.
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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.
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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.
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EXERCISES
Questions 6 - 14Page 249-250
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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
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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
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SOLIDS IN KSP
If = Ksp
And the concentration of the solid is constant than
= Ksp and
Solution = [Ksp][solid]
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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.
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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
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EXERCISES
Page 259
Exercises 26 - 29
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REVISION
Concepts we have covered so far: Dynamic equilibrium Reversible reactions Catalysts Kc
Ksp Q Le Chatelier Temperature and Equilibrium Equilibrium in Gases
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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
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PROPERTIES OF ACIDS
Acids have a sour taste
Acids sting or burn the skin
In solution acids conduct electricity
Acids turn blue litmus red
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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
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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
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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
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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.
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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.
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TABLE OF ACIDS AND ANIONS PG 217
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QUESTIONS
Page 217
Questions 8 - 13
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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)
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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.
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QUESTIONS
Page 219
Questions 14 and 15
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