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Example of calculating average speedWhen Dr Collings did the Lima marathon last year (42 km), he completed the race in 4 hrs and 13 minutes. He passed the first quarter mark after 57 minutes, the halfway mark after 1hr 55 minutes, the three quarter mark after 3 hours.
What was his average speed throughout the race?Which was faster, his average speed in the first half or the second half?Which quarter of the race was he fastest in?
What is the relationship between Speed and rate?
A rate is a measure of how something changes in a given amount of time
Speed is an example of a rate
In a chemical reaction, the reaction rate is expressed as the change in the amount of a reactant or product over time
Reaction progressAs a reaction progresses, the number of reactants decreases and the number of products increases
Collision Theory● Collision theory
relates kinetic energy and reaction rate
● When atoms, ions or molecules collide
● When the particles have enough kinetic energy, they can react and form products
● When they do not have enough energy, they do not react
Activation energy● This is the minimum amount of energy that colliding particles need to react● A barrier the reactants must cross before the product can form● Activated complex - an unstable arrangement of atoms that exists at the top
of the activation-energy barrier
What can affect the rate of a reaction?● Temperature - Raising the
temperature speeds up the rate of reaction, lowering the temperature slows it down - why?
● At higher temperature particles move faster and collide more often
○ More have enough energy to get over the activation energy barrier
Reaction rates continued…..
Concentration - More particles in a confined space leads to increased numbers of collisions, and a higher reaction rate
Effect of particle size
The greater the surface area, the faster the reaction rate
A higher surface area maximises the rate at which collisions take place at the surface of reactants
This can be done through either reducing particle size or dissolving reactants
What is a catalyst?A catalyst increases the rate of reaction without being used up in the reaction - by lowering the activation energy
This is often more effective than increasing temperature
Enzymes are natural catalysts found in the human body
Inhibitors block the actions of the catalyst
Rate lawsThe rate of a reaction depends in part on the concentrations of the reactants.Suppose there were a reaction with only one reactant and one product.A -> B
The rate at which A forms B can be expressed as the change in A (ΔA) with time
Rate laws continuedThe rate of disappearance of A is proportional to the concentration of A.
The proportionality can be expressed as the concentration of A, [A], multiplied by a constant, k.
This equation is a rate law, an expression for the rate of a reaction in terms of the concentration of the reactants.
What about if there is more than 1 reactant?The specific rate constant (k) for a reaction is a proportionality constant relating the concentrations of reactants to the rate of the reaction.
aA + bB -> cC + dD
For the reaction of A with B, the rate of reaction is dependent on the concentrations of both A and B.
The specific rate constant● In the rate law, the concentrations are multiplied by the specific rate constant
(k)● K relates the concentrations of reactants to the concentrations of products● The value of k depends on the reaction and the experimental conditions, such
as temperature● It can only be determined experimentally● If products form quickly k is large, ● If products form slowly k is small
What does a chemical equation tell you? What does it leave out?An equation does not tell you the path of a reaction from start to finish
Most chemical reactions are made up of multiple steps - the reaction mechanism
We can show the route reactions take using reaction progress curves
Are they always so straight forwards?● No, most reactions are not one step
reactions● In a complex (multistep) reaction,
there are peaks for each activated complex, and a valley for each intermediate
● An intermediate is a product of one one step in a reaction mechanism, and a reactant in the next
● Intermediates do not appear in the chemical equation
The rate determining stepIn a multistep reaction, one step will be slower than the others
The overall rate of reaction depends on the rate of the slowest step
Do reactions always occur in one direction?● Based on reactions that you have been exposed to, you may think that this is
the case, but in actual fact this is not the case● Some reactions are reversible● A Reversible reaction is one where reactants change to products and
products change to reactions at the same time● It is important to pay attention to the direction of the arrow, to determine which
is the forward reaction and which is the reverse reaction● 2SO2(g) + O2(g) → 2SO3(g)● 2SO2(g) + O2(g) ← 2SO3(g)
Establishing EquilibriumWhat happens then when you mix a sulfur dioxide and oxygen in a sealed container?
Establishing EquilibriumThe forward reaction begins, and initially the rate of the reverse reaction is zero as no sulfur trioxide has forme. Once the sulfur trioxide begins to form the reverse reaction begins, but is slow at first.
As more sulfur trioixde forms the reverse reaction speeds up, and the rate of the forward reaction slows down because sulfur dioxide and oxygen are being used up
Eventually the rates of the forward and reverse reactions are equal, and the reaction has reached a state of balance: Chemical Equilibrium
What happens at Equilibrium? ● At equilibrium the reactions have not
stopped● An equilibrium is dynamic, meaning
the reactions continue taking place● The rates of the forward and reverse
reactions are equal● When a store opens, for example -
you only get people moving one way, yet later in the day, people move in and out at an equal rate - just like an equilibrium
Are concentrations equal at equilibrium? ● No - not necessarily!● Equilibrium only means equal rates of reaction, not equal concentrations● The Equilibrium Position is the relative concentrations of reactants and
products at equilibrium○ It tells you which reaction is more likely to take place○ If a mixture is 1 % A and 99 % B, then the formation of B is favored, yet f the mixture is 99% A
and 1 % B, then the formation of A is favored
● This can be represented by different length equilibrium arrows ● Almost all reactions are reversible under the right conditions, yet often one
reaction direction is so favored that the other set cannot be measured● When no reactants are measured the reaction has gone to completion● When no products are measured, no reaction has taken place
So what happens at equilibrium?At Chemical equilibrium, both the forward and reverse reactions continue . Because the rates are equal, no net change occurs in the concentration of the reaction components.
Le Chatelier’s Principle● Equilibrium systems have a delicate balance
○ Changing conditions can upset this balance
● Once disturbed, the system will work to get back to equilibrium - this equilibrium can be in a different position; a shift in equilibrium occurs
● Le Chatelier’s principle states that when a stress is applied to a system in equilibrium, the system changes in a way that relieves the stress
● Examples of stresses: ○ Changes in concentration○ Changes in temperature○ Changes in pressure
The effect of changing concentration
● If more CO2 is added, the reverse reaction speeds up. As more H2CO3 forms the forward reaction then speeds up until equilibrium is restored at a new position
● Adding a product pushes the equilibrium in the direction of the reactant● If CO2 is removed the equilibrium will move in the direction of the product● If you complete removing a product the reaction wlil go to completion
In summary...
H2CO3(aq) CO2(aq) + H2O(l)
Add CO2Direction of shift
H2CO3(aq) CO2(aq) + H2O(l)Remove CO2
Direction of shift
The effect of temperatureIncreasing the temperature causes the equilibrium position of a reaction to shift in the direction that absorbs heat.
In this example the reverse reaction is endothermic, and therefore works to reduce the stress in the system - equilibrium shifts in the direction of the reactants
N2(g) + 3H2(g) 2NH3(g) + heatAdd heat
Direction of shift
Remove heat (cool)Direction of shift
The effect of pressureEquilibrium systems in which some reactants and products are gases can be affected by a change in pressure.
A shift will occur only if there are an unequal number of moles of gas on each side of the equation.
N2(g) + 3H2(g) 2NH3(g)Add pressure
Direction of shift
Reduce pressureDirection of shift
The effect of pressure….
● You can predict which way the equilibrium position will shift by comparing the number of molecules of reactants and products.
● When two molecules of ammonia form, four molecules of reactants are used up.● A shift toward ammonia (the product) will reduce the number of molecules.
N2(g) + 3H2(g) 2NH3(g)
The effect of catalystsCatalysts speed up the time it takes for a system to reach equilibrium
Catalysts do not affect the amounts of reactants and products at equilibrium
The energy path for a reverse reaction is the opposite of the energy path for the forward reaction
Adding a catalyst lowers the energy path by the same amount for both reactions
Equilibrium constants● The equilibrium position is expressed as a number, which relates the number
of products to the number of reactants at equilibrium● The equilibrium constant is the ratio of the concentration of products to the
concentration of reactants at equilibrium● Each concentration is is raised to a power equal to the number of moles of
that substance in the balanced equation
aA + bB cC + dD
Keq = [C]c x [D]d[A]a x [B]b
Values of Keq
● The concentration of products is always the numerator
● The concentration of reactants is always the numerator
● As units can sometimes cancel out, equilibrium constants are reported without a unit
● A large value for the equilibrium constant means the equilibrium mixture is mostly product
● A small value for the equilibrium constant means the mixture is mostly reactant
● If the temperature of a system changes the Keq values will also change
Keq = [C]c x [D]d[A]a x [B]b
The solubility product constant● Something that dissolves in water is described as being soluble● Something that does not dissolve in water is insoluble● Compounds containing carbonate or phosphate ions are generally
insoluble● Some general rules exist for whether something will be soluble in
water….
Compounds of the alkali metals and of ammonium ions
InsolubleCarbonates, phosphates, and sulfites
Alkali metal sulfides and hydroxides are soluble. Compounds of Ba, Sr, and Ca are slightly soluble.
Most are insoluble
Sulfides and hydroxides
Compounds of Ag and some compounds of Hg and Pb
SolubleChlorides, bromides, and iodides
Compounds of Pb, Ag, Hg, Ba, Sr, and CaSolubleSulfates
Few exceptionsSolubleEthanoates, nitrates, chlorates, and perchlorates
Some lithium compoundsSolubleSalts of Group 1A metals and ammonia
ExceptionsSolubilityCompounds
Solubility of Ionic Compounds in Water
How do you describe solubility?Scientists use the solubility product constant (Ksp) to compare the solubilities of ionic compounds
Ksp is essentially an equilibrium constant based only on the concentration of dissolved ions on a solution
Ksp = [A]a × [B]b
ExampleSilver chloride is an example of compound that is very slightly soluble in water
An equilibrium is established between the solid and the dissolved ions in the saturated solution.
In this equation, the coefficient for each ion is 1. So the Ksp expression for the dissociation is written as:
Ksp = [Ag+] x [Cl-]
AgCl(s) Ag+(aq) + Cl–(aq)
KSp Examples
1.8 × 10–14
1.2 × 10–12
ChromatesPbCrO4Ag2CrO4
4.5 × 10–9
9.3 × 10–10
1.0 × 10–10
8.15 × 10–12
5.0 × 10–9
CarbonatesCaCO3SrCO3ZnCO3Ag2CO3BaCO3
4.0 × 10–20
8.0 × 10–37
8.0 × 10–51
3.0 × 10–23
8.0 × 10–19
1.0 × 10–27
3.0 × 10–28
SulfidesNiSCuSAg2SZnSFeSCdSPbS
3.0 × 10–34
3.0 × 10–16
6.5 × 10–6
7.1 × 10–12
7.9 × 10–16
6.3 × 10–7
1.1 × 10–10
2.4 × 10–5
1.8 × 10–10
5.0 × 10–13
8.3 × 10–17
1.7 × 10–5
2.1 × 10–6
7.9 × 10–9
3.6 × 10–8
3.9 × 10–11
HydroxidesAl(OH)3Zn(OH)2Ca(OH)2Mg(OH)2Fe(OH)2
SulfatesPbSO4BaSO4CaSO4
HalidesAgClAgBrAgI
PbCl2PbBr2PbI2PbF2CaF2
KspIonic compoundKspIonic compoundKsp
Ionic compound
Solubility Product Constants (Ksp) at 25oC
How are Kspvalues used?The smaller the value of the Ksp the lower the solubility of the compound
There is a wide range in possible Ksp values
The common ion effectCommon Ion - An ion that is found in more than one ionic compound in a solution
Common Ion effect - The lowering of the solubility of an ionic compound due to the addition of a common ion
What does this all mean?
Example….In a saturated solution of lead(II) chromate, an equilibrium is established between the solid lead(II) chromate and its ions in solution.
What would happen if you added some lead nitrate to this solution?
PbCrO4(s) Pb2+(aq) + CrO42–(aq)
How do we apply Le Chatelier's principle● Lead(II) nitrite is soluble in water, so adding Pb(NO3)2 causes the
concentration of lead ion to increase.● The lead(II) ion is a common ion in both ionic compounds in a solution● The addition of lead ions is a stress on the equilibrium.● Applying Le Châtelier’s principle, the stress can be relieved if the reaction
shifts to the left - PbCrO4 precipitates out● Adding the common ion essentially lowered the solubility of the lead(II)
chromate○ The common ion effect○ This requires the new ion to be more soluble than the original compound in solution
The yellow solid in the test tube, which is PbCrO4, cannot dissolve because the solution is saturated with Pb2+ and CrO4
2– ions.
When some Pb(NO3)2 is added, the excess lead ions combine with the chromate ions in solution to form additional solid PbCrO4.
Why is all this useful? ● Combining all of this, it is possible to predict whether a precipitate will form
when two solutions are mixed● A precipitate will form if the product of the concentrations of two ions in the
mixture is greater than the Ksp value for the compound formed from the ions.● Example - Does a precipitate form when 0.5 L of 0.008 M Na2SO4 is mixed
with 0.5 L of 0.002 M Ba(NO3)2? The Ksp for BaSO4 is 1.1 × 10–10.● As you combine the two, the volume has doubled, therefore the concentration
of Ba2+ and SO42+ have halved.
● 0.001 x 0.004 is 4 x 10-6, greater than the Kspfor BaSO4 so a precipitate forms
Can something spontaneously combust? Sometimes, this is possible, if there are the correct conditions in the system
What is meant by the term free energy? ● Free energy is the energy in a chemical reaction that is available to do
work○ This does not mean that it is all used to do work, just that it can be○ For example - in an internal combustion engine, 30 % of free energy is used to move the car,
whereas 70 % is lost as heat and friction○ This is therefore not very efficient
● An efficient process uses the least amount of energy, time or money to produce a result
○ No process is ever 100 % efficient
Spontaneous vs Nonspontaneous reactions● There are two types of reactions: ● Spontaneous reactions: a reaction that favors the formation of products
at the stated conditions○ These release free energy
● Nonspontaneous reactions: A reaction that does not favor the formation of products at the stated conditions
○ This type of reaction produces very little energy
How does this relate to reversible reactions?In nearly all reversible reactions one reaction is favored over the other:
Do spontaneous reactions occur quickly? ● Not necessarily● Spontaneous and nonspontaneous are terms that have nothing to do with
rate ● Some spontaneous reactions are so slow that they appear to not be taking
place○ Example - table sugar and oxygen○ Without heat takes thousands of year, with heat produces carbon dioxide and water
● Therefore changing the conditions not only affects reaction rate, but whether a reaction can take place or not
● Conditions determine whether a reaction is spontaneous or nonspontaneous
Can an endothermic system/process be spontaneous?
Ice melting is an endothermic process
Enthalpy alone does not determine whether a change is spontaneous or nonspontaneous
The other factor is Entropy - a measure of the disorder in a system
The law of disorderThis states that the natural tendency is for systems to move in the direction of increasing disorder or randomness
Reactions in which entropy increases as reactants form products tend to be favored
How to increase entropy in a systemThere are 4 ways that changes to a system can increase the entropy
1: State of matter - gas has more entropy than liquid or solid
Number of molecules ENtropy increases when the total number of product molecules is greater than the number of reactant molecules
Relationship between Enthalpy and Entropy● The size and direction of enthalpy and entropy changes determines
whether a reaction is spontaneous or not● If one change is favorable, and one change is not, whether a reaction occurs
depends upon whether the unfavorable change is offset by the favorable one● When ice melts, it is endothermic - an unfavorable change● Yet, the change in entropy is favorable - reaching states of increased
disorder, therefore ice melting is spontaneous ● An endothermic reaction that leads to a decrease in energy is never
spontaneous
NoDecreasesIncreases (endothermic)
Only if unfavorable entropy change is offset by favorable enthalpy change
Decreases (less disorder in products than in reactants)
Decreases (exothermic)
Only if unfavorable enthalpy change is offset by favorable entropy change
IncreasesIncreases (endothermic)
YesIncreases (more disorder in products than in reactants)
Decreases (exothermic)
Is the reaction spontaneous?Entropy changeEnthalpy change
How Enthalpy Changes and Entropy ChangesAffect Reaction Spontaneity
Gibbs free energyThis is the term often used to describe free energy - named after the scientist who discovered this property
The symbol is G
Free energy can be either released or absorbed, and is calculated by the following equation:
A positive ΔG value = nonspontaneous reaction
A negative ΔG value = spontaneous reaction
ΔG = ΔH – TΔS