Chemical reactions and equations

2. 

Chemical Reactions are associated with chemical
change.
Whenever a chemical change occurs chemical
reaction is said to take place.
 Chemical reactions are usually irreversible and a
new product is formed.

3. Signs of Chemical Reactions
There are five main signs that indicate a chemical reaction has taken place:
Change in color Evolution of a gas
Change in
temperature
Change in state
Formation of
precipitate

4.  A WORD EQUATION describes chemical change
using the names of the reactants and products.
The word equation for the reaction of methane gas with
oxygen gas to form carbon dioxide and water is follows :-
Product
CO2 + 2 H2O
Reactant
CH4 + 2O2
methane + oxygen carbon dioxide + water

5.  The simplest form of description of a
chemical reaction in a shorter form is by
writing it in the form of an equation. E.g.-
2Mg + O2 2MgO
 The reactants are written on the left side
and product is written on the right side.
 An arrow is placed between them to
show the direction of reaction.

6.  The equation must represent known
facts.


The equation must contain the correct
formulas for the reactants and products.
The law of conservation of mass must
be satisfied.

7. Showing Phases
in Chemical
Equations
Solid Phase – the substance is relatively rigid and has a
definite volume and shape. NaCl(s)
Liquid Phase – the substance has a definite volume, but is
able to change shape by flowing. H2O(l)
Gaseous Phase – the substance has no definite volume or
shape, and it shows little response to gravity. Cl2(g)
H2O(s) H2O(l) H2O(g)

8. Cl
Cl
H
H
H
Cl
Cl H H
Cl Cl
H
H
H
Cl
2 2
2 2
2 1
1 1
H
Cl
Unbalanced and Balanced Equations

9. Reactants
1 C atom
4 H atoms
4 O atoms
Products
1 C atom
4 H atoms
4 O atoms

10. 1. Combination reaction
2. Decomposition reaction
3. Single-displacement reaction
4. Double-displacement reaction
5. Redox reaction

11.  These type of reactions occur when two
reactants combine to form one or more
products. E.g. –
1. CaO + H2O
2. C + O2
Ca(OH)2
CO2
 They are generally exothermic reactionswhich
involve evolution of heat during reaction.

12. Decomposition Reaction
• The types of reaction in which a single reactant
breaks down to give simpler products are called
decomposition reaction. E.g.-
• When a decomposition reaction is carried out by
heating, it is known as thermal decomposition.
2 H2O 2 H2 + O2

13. electricity
2 H2O2 2 H2O + O2
Hydrogen Peroxide
Electrolysis of water
2 H2O 2 H2 + O2
AB A + B
General Form
Nitrogen triiodide
2NI3 N2 + 3 I2

14. • The reaction in which a more reactive
element displaces a less reactive element
from its solution is known as displacement
reaction.
• They are of two types:-
1.Single Displacement Reaction
2.Double Displacement Reaction

15. Double-replacement reaction
CaCO3 + 2 HCl  CaCl2 + H2CO3
General form:
AB + CD  AD + CB
Single-replacement reaction
Mg + CuSO4  MgSO4 + Cu
General form:
A + BC  AC + B

16. Double Displacement Reaction
K2CO3 (aq)
Potassium carbonate
BaCl2 (aq)
Barium chloride
2 KCl (aq)
Potassium chloride
BaCO3 (s)
Barium carbonate
+ +

17. Ca
Foiled again –
Aluminum loses to Calcium
Element Reactivity

18. • If a substance gains oxygen during a reaction,
it is said to be oxidised.
• If a substance loses oxygen during a reaction,
it is said to be reduced.
• Reactions in which this type of change
occurs is known as Oxidation and Reduction
reactions or Redox reactions. E.g. –
Reduction
CuO + H2 Cu + H2O
Oxidation

19. What is Rancidity and how it occurs?
Rancidity is a very general term and in its most general
meaning, it refers to the spoilage of a food in such a way
that it becomes undesirable (and usually unsafe) for
consumption.
It is characterized by an unpleasant smell and taste. The
term “rancid” particularly applies to oils and fats. When
food scientists talk about rancidity, they are
often talking about a specific type of rancidity
Involving oxygen damage to foods, and this
type of rancidity is called "oxidative rancidity.”
Oxidation of fats (rancidity) is caused by a
biochemical reaction between fats and oxygen.
In this process the long-chain fatty acids are
degraded and short-chain compounds are formed.
One of the reaction products is butyric acid,
which causes the typical rancid taste.

20. Factors causing Rancidity
1.Temperature – Rancidity rate increases with increase in
temperature.
2.Duration – For the more time you keep a thing
unattended or without using it, it has more
chances to turn rancid.
3.Oxygen – Oxygen promotes the decomposition
of food.
4. Light - In the presence of oxygen, light
promotes the decomposition of unsaturated fatty acids.
5.Microorganisms and fungi (molds) – They are the most common
reason for the food to become rancid . They use their enzymes on the
food material and destroy its chemical
composition.
6.Moisture - Moist air helps the microbes in
their activity upon the food material.
7.Trace elements – Trace elements like Fe
and Zn also increase the rate of rancidity.

21. Some food items that are likely to turn
rancid
1.
2.
3.
- Various Vegetable oils.
- Clarified butter (Ghee).
- Butter

22. Some food items that are likely to turn
rancid
4.
5.
6.
- Fishes and
crustaceans.
- Bread.
- Pickle

23. Some food items that are likely to turn
rancid
7.
8.
9.
- Baked Potato.
- Shell eggs.
- Meat

24. Preventive measures
1. Adding Inert gases – Inert gas can be added to the packet or the
container like nitrogen, which does not react with oxygen. Example –
Chips packets are flushed with nitrogen gas, so they don’t become
rancid.
2. Adding antioxidants- Antioxidants are added to some foods to
slow down or eliminate oxidative deterioration. Examples-
* BHA – Butylated Hydroxy Anisole.
* BHT – Butylated Hydroxy Toluene.
3. Refrigeration – Refrigerators reduce the temperature and hence
don’t allow the microbes to continue their processes.
4. Vacuum packaging – Vacuum packaging is done to keep oxygen out.
5. By using oxygen scavengers- Oxygen scavengers or oxygen
absorbers are added to enclosed packaging to help remove or
decrease the level of oxygen in the package. They are used to help
maintain product safety and extend shelf life.
6. Keeping food in air tight containers – Less air too prevents
rancidity.
7. By storing food in dark place.

25. Preventive measures

26. Harmful effects of consuming rancid food
Consuming rancid food products is unlikely to cause immediate
illness or harm. Rancidification can reduce the nutritional value
of food, and some vitamins are highly sensitive to degradation.
In addition, rancidification can produce potentially toxic
compounds associated with long-term harmful health effects
concerning advanced aging and neurological disorders. Rancid
fats have also been implicated in increased rates of heart
disease, atherosclerosis and are carcinogenic (cancer causing).
Rancid food can cause
cancer
Rancid food can cause heart
diseases

27. What is Corrosion and how it occurs?
Corrosion is the oxidization of metals in the presence of air(oxygen)
and moisture to form compounds of the metals. Corrosion converts
refined metal to their more stable oxide. It is the gradual destruction
of materials (usually metals) by chemical reaction with their
environment.
Corrosion can occur in two general ways:
Generalized Corrosion: Typically never
Happens , aside from in acidic conditions. This
uniform corrosion over the entire surface of
the metal is rare and leads to overall thinning
which has little effect outside of fatigue and
stress conditions.
Localized Corrosion: The most common, and most detrimental, form of
localized corrosion is pitting. Pitting is when the attack happens in one
single location on the surface and creates a pit, or small cavity, in the
metal. This type of corrosion attack is hard to prevent, engineer
against, and often times difficult to detect before structural failure is
met due to cracking. Pipes are often compromised due to pitting.

28. Ill-effects of Corrosion
Losses are economic and safety:
• Reduced Strength
• Downtime of equipment
• Decrease in weight
• Lost surface properties
• Reduced value of goods
The consequences of corrosion are many and varied and
the effects of these on the safe, reliable and
efficient operation of equipment or structures are
often more serious than the simple loss of a mass of
metal. Failures of various kinds and the need for
expensive replacements may occur even though the
amount of metal destroyed is quite small.

29. Corrosion of different metals and alloys
1.
2.
3.
- Iron (Fe) (Rusting)
- Copper (Cu)
- Silver (Ag)

30. Corrosion of different metals and alloys
4.
5.
6.
- Lead (Pb)
- Gold (Au)
- Platinum (Pt)

31. Corrosion of different metals and alloys
7.
8.
9.
- Steel
- Brass
- Zinc (Zn)

32. Preventive measures
1. Painting - Paints can protect metals from the degrading
effect of environmental gases.
2. Galvanization – It is the process of applying a
protective zinc coating to steel or iron, to prevent rusting.
The most common method is hot-dip galvanization, in which
parts are submerged in a bath of molten zinc.
3. Electroplating – This helps in avoiding the metal to have a
direct contact with the environment.
4. Alloying - Corrosion can also be prevented by making
alloys.
5. Anodization – It is a surface treatment which prevents
corrosion.
6. Cathodic protection (CP) - It is a technique to control
the corrosion of a metal surface by making that surface
the cathode of an electrochemical cell.
7. Enameling – It too helps in preventing corrosion.

33. Preventive measures