Chemical reactions

1. By
Dr KHALED ALGARIRI
CAMS-Qassim University
September 2020

2. What is a Chemical Reaction?
 A chemical reaction is a process in which at least one new substance is
produced as a result of chemical change. It is a chemical change in
which one or more substances are destroyed and one or more new
substances are created.
BEFORE
H2 gas
and
O2 gas
AFTER
H2O liquid

3. Parts of a Chemical Reaction
Reactants  Products
Reactants: Substances that are destroyed by the chemical change
(bonds break).
Products: Substances created by the chemical change (new bonds
form).
The arrow () is read as “yields”.

4. Evidence for a Chemical Reaction
1) Evolution of light or
heat.
2) Temperature change (increase or
decrease) to the surroundings.

5. Evidence for a Chemical Reaction
3) Formation of a gas (bubbling
or an odor) other than boiling.
4) Color change (due to the formation
of a new substance).

6. Evidence for a Chemical Reaction
5) Formation of a precipitate (a new solid forms) from the
reaction of two aqueous solutions.

7. TYPES OF CHEMICAL REACTIONS
1. COMBINATION REACTIONS
2. DECOMPOSITION REACTIONS
3. SINGLE-REPLACEMENT REACTIONS
4. DOUBLE REPLACEMENT REACTIONS
5. COMBUSTION REACTIONS.

8. COMBINATION REACTIONS
 A combination reaction is a chemical reaction in which a single
product is produced from two (or more) reactants. The general
equation for a combination reaction involving two reactants is
X + Y XY
 The reactants X and Y can be elements or compounds
or an element and a compound. The product of the
reaction (XY) is always a compound.

9. EXAMPLES OF COMBINATION REACTIONS
Element A + Element B Compound
Na(s) + Cl2 (g)  2NaCl(s)
a)Element + Compound A  Compound B
O2(g) + 2SO2(g)  2SO3(g)
a)Compound A + Compound B  Compound C
CaO(s) + H2O(l)  Ca(OH)2 (s)
When a hot nail is stuck into a pile of
zinc and sulfur, a fiery combination
reaction occurs and zinc sulfide forms.
Zn + S ZnS

10. Decomposition Reactions
A decomposition reaction is a chemical reaction in which a single reactant is
converted into two (or more) simpler substances (elements or compounds). Thus a
decomposition reaction is the opposite of a combination reaction. The general
equation for a decomposition reaction in which there are two products is
XY X + Y
Although the products may be elements or compounds, the reactant is
always a compound. At sufficiently high temperatures, all compounds
can be broken down (decomposed) into their constituent elements.

11. EXAMPLES OF DECOMPOSITION REACTIONS
In organic chemistry, decomposition reactions are often called
elimination reactions.
Decomposition Reactions

12. Single-Replacement Reactions
 A single-replacement reaction is a chemical reaction in which an atom or
molecule replaces an atom or group of atoms from a compound. There are
always two reactants and two products in a single-replacement reaction.
The general equation for a single replacement reaction is
X + YZ XZ + Y
A common type of single-replacement reaction is one in which
an element and a compound are reactants, and an element and
a compound are products.

13. EXAMPLES OF SINGLE-REPLACEMENT REACTIONS
In organic chemistry, replacement reactions ( both single
and double) are often called substitution reactions.

14. Double-Replacement Reactions
 A double-replacement reaction is a chemical reaction in which
two substances exchange
 parts with one another and form two different substances. The
general equation for a double replacement reaction is
When the reactants in a double-replacement reaction are
ionic compounds in solution, the parts exchanged are the
positive and negative ions of the compounds present.
XA + YB XB + YA

15. In most reactions of this type, one of the
product compounds is in a different
physical state (solid or gas) from that of
the reactants . Insoluble solids formed
from such a reaction are called
precipitates; AgCl and PbI2 are
precipitates in the reactions.
A double-replacement reaction involving solutions of potassium iodide and
lead(II) nitrate (both colorless solutions) produces yellow, insoluble lead(II)
iodide as one of the products.
2KI(aq)Pb(NO3)2(aq) 2KNO3(aq)PbI2(s)
Double-Replacement Reactions

16. Combustion Reaction
A combustion reaction is a chemical reaction between a substance and
oxygen (usually from air) that proceeds with the evolution of heat and light
(usually from a flame).
In hydrocarbon combustion, the carbon of the hydrocarbon combines
with the oxygen of air to produce carbon dioxide (CO2). The hydrogen
of the hydrocarbon also interacts with the oxygen of air to give water
(H2O) as a product. The relative amounts of CO2 and H2O produced
depend on the composition of the hydrocarbon.

18. Redox and Nonredox Reactions
 Chemical reactions can also be classified, in terms of whether
transfer of electrons occurs, as either oxidation–reduction (redox)
or nonoxidation–reduction (nonredox) reactions.
 An oxidation–reduction (redox) reaction is a chemical reaction in
which there is a transfer of electrons from one reactant to another
reactant.
 A nonoxidation–reduction (nonredox) reaction is a chemical
reaction in which there is no transfer of electrons from one
reactant to another reactant

19. oxidation number
An oxidation number is a number that represents the charge that an atom
appears to have when the electrons in each bond it is participating in are assigned to
the more electronegative of the two atoms involved in the bond.
There are several rules for determining oxidation numbers.
1. The oxidation number of an element in its elemental state is zero. For example, the
oxidation number of copper in Cu is zero, and the oxidation number of chlorine in
Cl2 is zero.
2. The oxidation number of a monatomic ion is equal to the charge on the ion. For
example, the Naion has an oxidation number of +1, and the S2 ion has an
oxidation number of -2.
3. The oxidation numbers of Groups IA and IIA metals in compounds are always +1,
and +2, respectively.
4. The oxidation number of hydrogen is 1 in most hydrogen-containing compounds.

20. 5. The oxidation number of oxygen is -2 in most oxygen-containing
compounds.
6. In binary molecular compounds, the more electronegative element
is assigned a negative oxidation number equal to its charge in binary
ionic compounds. For example, in CCl4 the element Cl is the more
electronegative, and its oxidation number is 1 (the same as in the
simple Clion).
7. For a compound, the sum of the individual oxidation numbers is
equal to zero; for a polyatomic ion, the sum is equal to the charge on
the ion.
oxidation number

21. Many elements display a range of oxidation numbers in their various
compounds.
For example, nitrogen exhibits oxidation numbers ranging from 3 to
5. Selected examples are

22. oxidation number
To determine whether a reaction is a redox reaction or a nonredox reaction, we look for
changes in the oxidation number of elements involved in the reaction. Changes in
oxidation number are a requirement for a redox reaction. The reaction between calcium
metal and chlorine gas is a redox reaction.
The oxidation number of Ca changes from zero to +2,
and the oxidation number of Cl changes from zero to -1.
The decomposition of calcium carbonate is a
nonredox reaction. It is a nonredox reaction
because there are no changes in oxidation number

24. Oxidation and Reduction Reactions
Oxidation is the process whereby a reactant in a chemical reaction loses
one or more electrons.
Reduction is the process whereby a reactant in a chemical reaction
gains one or more electrons.
Oxidation and reduction are complementary processes that always
occur together.
When electrons are lost by one species, they do not disappear: rather,
they are always gained by another species. Thus electron transfer always
involves both oxidation and reduction.

25. Electron loss (oxidation) always leads to an increase in oxidation number.
Conversely, electron gain (reduction) always leads to a decrease in oxidation
number.
Electron loss produces positive ions (increase in oxidation number), and
electron gain produces negative ions (decrease in oxidation number).

26.  An oxidizing agent is the reactant in a redox reaction that causes
oxidation of another reactant by accepting electrons from it.
 A reducing agent is the reactant in a redox reaction that causes
reduction of another reactant by providing electrons for the
other reactant to accept. Thus the reducing agent and the
substance oxidized are one and the same, as are the oxidizing
agent and the substance reduced.
 Substance oxidized = reducing agent
 Substance reduced = oxidizing agent

28. Exothermic and Endothermic Reactions
An exothermic reaction is a chemical reaction in which energy is released as the
reaction occurs. The burning of a fuel (reaction of the fuel with oxygen) is an
exothermic process. An
endothermic reaction is a chemical reaction in which a continuous input of
energy is needed for the reaction to occur. The photosynthesis process that
occurs in plants is an example of an endothermic reaction. Light is the energy
source for photosynthesis
An exothermic reaction (release of energy) occurs when the energy required to
break bonds in the reactants is less than the energy released by bond formation in
the products. The opposite situation applies for an endothermic reaction

29. Factors That Influence Reaction Rates
1-Physical Nature of Reactants:
The physical nature of reactants includes not only the physical state of each
reactant (solid, liquid, or gas) but also the particle size. In reactions where
reactants are all in the same physical state, the reaction rate is generally faster
between liquid-state reactants than between solid-state reactants and is fastest
between gaseous-state reactants

30. Factors That Influence Reaction Rates
2-Reactant Concentrations
An increase in the concentration of a reactant causes an increase in the rate
of the reaction.
Combustible substances burn much more rapidly in pure oxygen than in air
(21% oxygen)
Increasing the concentration of a reactant means that there are more
molecules of that reactant present in the reaction mixture; thus collisions
between this reactant and other reactant particles are more likely.

31. Factors That Influence Reaction Rates
3-Reaction Temperature
Reaction rate increases as the temperature of the reactants increases.
An increase in the temperature of a system results in an increase in the
average kinetic energy of the reacting molecules. The increased molecular
speed causes more collisions to take place in a given time.

32. Factors That Influence Reaction Rates
4- Presence of Catalysts
A catalyst is a substance that increases a chemical reaction rate without
being consumed in the chemical reaction. Catalysts enhance reaction rates
by providing alternative reaction pathways that have lower activation
energies than the original, uncatalyzed pathway.
Catalysts lower the activation energy for
chemical reactions. Reactions proceed
more rapidly with the lowered activation
energy.

33. Factors That Influence Reaction Rates

34. Chemical Equilibrium
 When product build up does occur, reactions do not go to completion.
This is because product molecules begin to react with one another to re-
form reactants. With time, a steady-state situation results where in the
rate of formation of products and the rate of re-formation of reactants
are equal.
 Chemical equilibrium is the state in which forward and reverse
chemical reactions occur simultaneously at the same rate.

35. Chemical Equilibrium
 Suppose equal molar amounts of gaseous H2 and I2 are mixed together in a closed
container and allowed to react to produce gaseous HI.
H2 + I2 2HI
Initially, no HI is present, so the only reaction that can occur is that between H2 and I2.
However, as the HI concentration increases, some HI molecules collide with one another in a
way that causes a reverse reaction to occur:
2HI H2 + I2
The initially low concentration of HI makes this reverse reaction slow at first, but as the
concentration of HI increases, the reaction rate also increases. At the same time that the
reverse-reaction rate is increasing, the forward-reaction rate (production of HI) is decreasing
as the reactants are used up.

36. Chemical Equilibrium
. Eventually, the concentrations of H2, I2, and HI in the reaction mixture reach a
level at which the rates of the forward and reverse reactions become equal. At
this point, a state of chemical equilibrium has been reached. Thus the reaction
between H2 and I2 at equilibrium is written as
At chemical equilibrium, forward and reverse reaction rates are equal.
Reactant and product concentrations, although constant, do not have to
be equal.

37. Chemical Equilibrium

40. References

41. Thank you
for
your
attention!