Nature of chemical reactions

Atoms

i) Origin of Elements

Atoms come together to form molecules

What is a molecule?A molecule is the smallest amount of something, particularly of a chemical substance, that can exist on its own. (Webster's New World College Dictionary)

Molecules are made up of atoms that are held together by chemical bonds.

What is a molecule?

A molecule is an assembly of nuclei and electrons which prepare to coexist in a certain configuration and with considerable stability.

Alan Carrington (FRS)

NB: free radicals and unstable species are also classified as molecules

Chemical Change and Physical Change

Physical change: the appearance of a substance is altered, but not its composition.Chemical change: the reacting substances change into new substances that have different properties.

To React or Not To React

This is the question?

Chemical reactionsDemo

Reaction occurs No reaction

Why???

X

To React or Not To React

This is the question?

1. Types of chemical reactions2. Spontaneity of chemical reactions3. Rate of chemical reactions

What is a chemical reaction ?

What is a chemical reaction ?

Chemical reaction occurs when one or more chemicals or atoms react to become different chemicals.

Chemical reaction is characterized by the re-arrangement of atoms from the reactant side of the equation to the product side.

Chemical reaction occurs whenever bondsare formed or broken between molecules.

Reaction of elements

For example: 2 H2 (g) + O2 (g) 2 H2O (g)

Types of chemical reactions

Types of Reactions

Combination Reaction

Decomposition Reaction

Displacement ReactionSingle displacementDouble displacement

Combination ReactionTwo or more elements or compounds bond together to form one product

Decomposition ReactionA compound breaks up into two or more separate substances

E.g. CuCO3 (s) CuO (s) + CO2 (g)

Displacement ReactionElements in a compound are replaced by other elementSingle displacement: an uncombined element displaces another element from a compound

E.g. Cu (s) + 2AgNO3 (aq) Cu(NO3)2 (aq) + 2Ag (s)

Displacement ReactionDouble displacement: two sets of atoms are replaced

E.g. Na2SO4 (aq) + BaCl2 (aq) 2 NaCl (aq) + BaSO4 (s)

Or AB + CD AC + BD

To React or Not To React

This is the question?

2.Spontaneity of chemical reactions3.Rate of chemical reactions

Two major concerns for chemical reactions

What is (are) the most stable product(s) that nature prefers? (Thermodynamics)

SpontaneityHow long does it take for a chemical

reaction to proceed to finishing?(Kinetics )

Rate

Two major concerns for chemical reactions

What is (are) the most stable product(s) that nature prefers? (Thermodynamics)

Spontaneity

Chemical reactions

Spontaneitygeneral to all reactions

Energy changes Exothermic reaction (-H) :

The energy contents in products are lower than the reactants

(-H)

Energy changes

Endothermic reaction (+H) :The energy contents in products are higher

than the reactants

(+H)

The lower the energy of the products, the more stable they are.

H is the change in energy, it is the energy difference between the products and the reactants

**Nature prefers to have the lowest energy (stable state)

Entropy (S)Entropy is a measure of the randomness in a chemical systemThe greater the disorder in a system, the greater the entropy it has

** Nature prefers to have greater randomness ie. Higher entropy

When hydrogen gas and oxygen

gas are put together, they tend

to mix up and become a random

mixture.

Factors affecting entropy in a system1. When comparing the same or similar substances, entropies of gases are much larger than those for liquids, and entropies of liquid are larger than those for solids.

Standard molar entropy,So, of water: 69.9 JK-1 mol-1

Standard molar entropy, So, of steam: 188.8 JK-1 mol-1

Entropy

2. Larger molecules have higher entropy than smaller molecules, and molecules with more complex structures have higher entropies than simple molecules

Standard molar entropy of methane: 186.61 JK-1 mol-1

Standard molar entropy of ethane: 229.69 JK-1 mol-1

Spontaneity of a chemical reaction

Gibbs free energy :G = H - TS

Where,G = change in Gibbs free energy,H = change in energy, T = absolute temperature, S = change in entropy.

G > 0 for non-spontaneous reaction, G < 0 for spontaneous reaction

Both energy and entropy should be considered

Meet the Scientist

Josiah Willard GibbsAmerican scientist who made important theoretical contributions to physics, chemistry, and mathematics.

He is one of the scientists who created the field of statistical mechanics.

1839 1903(aged 64)

Spontaneity of a chemical reactionTo summarize:

Gibbs free energy :G = H - TS

G > 0 for non-spontaneous reaction,

G < 0 for spontaneous reaction

Spontaneity of a chemical reaction

Gibbs free energy :

G = H - TS

What happen when G = 0 ?

Condition for chemical equilibrium

SpontaneityWhat do you know about this?

Diamonds are forever ?!

Case 1: Diamond Graphite (S = 0.003363 kJK-1 mol-1, H = -1.895 kJ mol-1)

At 25 oC (298K), G = -2.90 kJ mol-1

G < 0, this reaction is spontaneous! Your diamond is changing to carbon spontaneously

Surprising result!!!!

G = H - TS = -1.895 T(0.0033)

Case 2: Hydrogen + Oxygen Water (S = - 0.887 kJK-1 mol-1, H = - 483.64 kJ mol-1)

At 25 oC (298K), G = - 219.32 kJ mol-1G < 0, this reaction is spontaneous!

However, hydrogen and oxygen gases in air do not react spontaneously to form water!!

HydrogenMolecule

H2

OxygenMolecule

O2

Why wouldnt water become steam?As we mentioned that nature prefers greater entropy and steam has a greater entropy than water, why not all water exist as steam? (S =188.8-69.9 = 118.9 JK-1 mol-1)

This is because the absorption of heat of vaporization (latent heat) is needed for water to vaporize to steam. Since under normal condition, such heat cannot be obtained simultaneously, not all water will change to steam. (H = 40.79 kJmol-1)

G = H - TS = 40790 T(118.9) > 0

When T goes to 373 K, G < 0

Rate of reaction

DemonstrationIodine clock reaction

Why would theIodine clock reaction

oscillate?

Briggs-Rauscher Reaction

(The rate is different for two competing reactions)

Rate of reactionSince nature prefers the most stable products, why don't we get graphite from diamond?

Diamond is forever!?!

Revisiting the case that the spontaneous reaction of converting diamond to graphite, the activation energy of the reaction is so high that the rate constant is extremely small. Hence the reaction rate is extremely slow so that we would not notice it (it is neglected).

The fact that diamond is turning into graphite is true, but it will be in a very very slow rate (long time).

Diamond is not forever!!!

Factors affecting rate of reaction

1.Concentrations [c]2.Temperature T k3.Catalyst (different pathway)

Rate of reactionChange in concentration

Change in time ________________________________

Change in concentration can be:a decrease in [ Reactant ]

or an increase in [Product ]

Rate of reaction

k is called the rate constant

Factors affecting rate of reaction1.Concentrations [c]A higher concentration of reactants leads to more effective collisions per unit time, which leads to an increasing reaction rate (except for zero order reactions).

Similarly, a higher concentration of products tends to be associated with a slower reaction rate.

Factors affecting rate of reaction2. Temperature T kAn increase in temperature is accompanied by an increase in the reaction rate.

Temperature is a measure of the kinetic energy of a system, so higher temperature implies higher average kinetic energy of molecules and more collisions per unit time.

k is the rate constant

Kinetic-energy distribution of moleculesAt a given temperature, the energy of atoms or molecules follows a Maxwell-Boltzmann Distribution:

Fraction of molecule which possess energy greater

than the activation energy

Temperature T k

Larger fraction of molecules react at higher temperature

T2 is higher than T1

James Clerk Maxwell (18311879)Scottish physicist and mathematician who made great contribution in establishing electromagnetism.(Maxwell Equations)

Ludwig Eduard Boltzmann (1844-1906)Austrian physicist famous for his founding contributions in the fields of statistical mechanics and statistical thermodynamics.

Meet the Scientists

Factors affecting rate of reaction

3. CatalystThe presence of a catalyst increases the reaction rate (positive catalyst) (in both the forward and reverse reactions) by providing an alternative pathway with a lower activation energy.

Activation energy (Ea) Molecules or atoms require some minimum energy to react

The energy barrier is the activation energy

If the barrier (Ea) is low, i.e. the energy required is low

more reactant molecules have sufficient energy to react,

the reaction rate is fast

If the barrier (Ea) is high, i.e. the energy required is high

only few reactant molecules have sufficient energy to react

the reaction rate is slow

Effect of catalysts on reaction rateCatalyst changes the mechanism (pathways) of the reaction. The overall energy barrier has been greatly lowered.

(Recall the Arrhenius equation, k = Ae-Ea/RT, as Eadecreases, k increases.)

Ea

Svante Arrhenius (1859-1927)Swedish scientist originally a physicist, but often referred to as a chemist.

He was one of the founders of the field of science : physical chemistry. He received the Nobel Prize for Chemistry in 1903.

Meet the Scientists

In fact, the rate of reaction and the activation energy is linked by the Arrhenius Equation :

A and R are constants, T is the absolute temperature, Ea is the activation energy, k is the rate constant which affects the rate of reaction. The higher the value of k, the faster the reaction proceeds.

Note that an increase in T, Ea / RT is smaller k is faster

e- Ea / RT Ea / RT = e1/

Activation energy (Ea) Molecules or atoms require some minimum energy to react

The energy barrier is the activation energy

In the example of combining oxygen and hydrogen to water, the activation energy is so high that even if we put the two gases together, there will not be any reaction.

A spark provides the energy for activation, which is needed to ignite the gases. Once the reaction starts, the heat released in the reaction will enable the reaction to go on until completion.

To React or Not To React

This is the question?

Spontaneity of chemical reactions nature of the reactants and products

Rate of chemical reactions the time needed for reactions to proceed

Femtochemistry a study of atoms and molecules in motion during a chemical reaction

A. Zewail made use of the fastest camera in the worldto take the photo of a reaction which is as fast as femtosecond scale (One femtosecond = 10-15 seconds, i.e. 0.000000000000001 seconds).

In 10-15 sec, light only travels a distance of 0.0004 mm

Meet the Scientist

In 1999, Ahmed H. Zewail was awarded the Nobel Prize in Chemistry for studying how atoms in a molecule break during a chemical reaction (rate of decomposition).

The first experimentshe studied the disintegration of iodocyanide:

ICN + hv ---> I*-CN ---> I + CN His team was able to observe a transition state exactly when the I-C bond was about to break: the whole reaction took place in 200 femtoseconds (2 x 10-13 seconds)

Arrow of time in chemistry and biology - steps over a century of development

FemtochemistryThis area of physical chemistry has been named femtochemistry. This provides a way to study what happens in a chemical reaction in detail.

Conditions for chemical reaction to occur

reacting molecules must collide with one another (animation)reacting molecules must collide with sufficient energy to break bonds (animation)reacting molecules must collide in a right orientation (animation)

Chemical Reaction

E.g. NO3(gas) + CO(gas) NO2(gas) + CO2(gas)

Relationships between Energy & Entropy

Laws of thermodynamics

Laws of thermodynamics1st law: basically says that energy or matter can neither be created nor destroyed. (The Law of Conservation of energy or matter).

2nd law: essentially says that it is impossible to obtain a process where the unique effect of converting energy from one form to another without an increase in entropy (Law of Increasing Entropy). Such increase in entropy makes the conversion less efficient.

3rd law: means that as the temperature of a system approaches absolute zero, its entropy approaches a constant (zero).

Laws of thermodynamicsC.P. Snow, the British scientist and author has offered up an easy and funny way to remember the Three Laws. He says they can be translated as:

(1) you cannot win (you cant get something for nothing because matter and energy are conserved.)(At most you can break even)

(2) You cannot break even (you cannot return to the same energy state because entropy always increases)

(3) you cannot get out of the game (because absolute zero is not attainable).