CHEMISTRY 10th CLASS

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CHEMICAL REACTIONS AND EQUATIONS

• CHEMISTRY 10th CLASS

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INTRODUCTION

• We commonly observe many chemical reactions in
  our surroundings but we rarely notice them. We
  are so much used to them. Infact millions of
  chemical reactions take place in our own body.

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BRAIN EXERCISE

• What happens to milk when it is left at room
  temperature for long time?

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BRAIN EXERCISE

• What happens to iron tawa/pan/nail, when they are
  left exposed to humid atmosphere?

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• grapes get fermented.

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food is cooked
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Burning of a magnesium ribbon in air and
collection of magnesiumoxide
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• In all the above situations, the nature and the
  identity of the initial
• substance have somewhat changed.

Whenever a chemical change occurs, we can say
that a chemical reaction has taken place
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CHEMICAR REACTION IS ACCOMPANIED WITH

• change in state
• change in color
• evolution of a gas
• change in temperature

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CHEMICAL EQUATIONS

• when a magnesium ribbon is burnt in
• oxygen, it gets converted to magnesium oxide.
• This statement can be written as a word equation

Magnesium Oxygen ? Magnesium oxide (Reactants)
(Product)
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• A word-equation shows change of reactants to
  products through an
• arrow placed between them.
• The reactants are written on the left-hand side
  (LHS) with a plus sign () between them.

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• Similarly, products are written on the right-hand
  side (RHS) with a plus sign () between them.
• The arrowhead points towards the products, and
  shows the direction of the reaction.

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WRITING A CHEMICAL EQUATION

• There is still a shorter way to represent a
  chemical reaction by the use of chemical
  formulae.
• It can be written as shown below

Mg O2 ? MgO
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Skeletal chemical equation

• Count and compare the number of atoms of each
  element on the LHS and RHS of the arrow in the
  previous equation.

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• If the number of atoms of each element on both
  the sides is not equal, then the equation is
  unbalanced because the mass is not the same on
  both sides of the equation.
• Such a chemical equation is a skeletal chemical
  equation for a reaction.

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Balanced chemical equation

• the number of atoms of each element remains the
  same, before and after a chemical reaction.
• Hence, we need to balance a skeletal chemical
  equation.

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Balancing a chemical equation

• Zinc Sulphuric acid ? Zinc sulphate Hydrogen
• The above word-equation may be represented by the
  following
• chemical equation
• Zn H2SO4 ? ZnSO4 H2

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TYPES OF CHEMICAL REACTIONS

• atoms of one element do not change into those of
  another element. Nor do atoms disappear from the
  mixture or appear from elsewhere.
• chemical reactions involve the breaking and
  making of bonds between atoms to produce new
  substances.

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COMBINATION REACTION
Calcium oxide reacts vigorously with water to
produce slaked lime (calcium hydroxide) releasing
a large amount of heat. CaO(s) H2O(l) ?
Ca(OH)2(aq) (Quick lime) (Slaked lime)
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• In this reaction, calcium oxide and water combine
  to form a single product, calcium hydroxide.
• Such a reaction in which a single product is
  formed from two or more reactants is known as a
  combination reaction.

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• Other examples of combination reactions
• (i) Burning of coal
• C(s) O2(g) ? CO2(g)
• (ii) Formation of water from H2(g) and O2(g)
• 2H2(g) O2(g) ? 2H2O(l)

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• large amount of heat is evolved in these
  reactions.
• This makes the reaction mixture warm.
• Reactions in which heat is released along with
  the formation of products are called exothermic
  chemical reactions.

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Other examples of exothermic reactions

• Burning of natural gas
• CH4(g) 2O2 (g) ? CO2 (g) 2H2O (g)

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• respiration is an exothermic process
• We need energy to stay alive. We get this energy
  from the food we eat. During digestion, food is
  broken down into simpler substances. These
  carbohydrates are broken down to form glucose.
  This glucose combines with oxygen in the cells of
  our body and provides energy.

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• The special name of this reaction is respiration,
• C6H12O6(aq) 6O2(aq) 6H2O(l) ? 6CO2(aq)
  12H2O(l) energy

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DECOMPOSITION REACTION
a single reactant breaks down to give simpler
products. This is a decomposition reaction.
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• 2FeSO4(s) ? Fe2O3(s) SO2(g) SO3(g)

• Ferrous sulphate crystals (FeSO4, 7H2O) lose
  water when heated and the color of the crystals
  changes.
• It then decomposes to ferric oxide (Fe2O3),
  sulphur dioxide (SO2) and sulphur trioxide (SO3).
• Ferric oxide is a solid, while SO2 and SO3 are
  gases.

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ELECTROLYSIS OF WATER
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Other examples are

• 2Pb(NO3)2(s) ?2PbO(s) 4NO2(g) O2(g)

2AgCl(s) ?2Ag(s) Cl2(g)
2AgBr(s) ?2Ag(s) Br2(g)
CaCO3(s) ? CaO(s) CO2(g)
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DISPLACEMENT REACTION

• Chemical reaction in which a less reactive
  element is replaced in a compound by a more
  reactive one.

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• When iron nail is kept in copper sulphate
  solution iron nail becomes brownish in color and
  the blue color of the solution fades.
• This is so because of the reaction taking place
  both of them.
• This is an example of displacement reaction.

Fe(s) CuSO4(aq) ? FeSO4(aq) Cu(s)
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EXAMPLES

• Zn(s) CuSO4(aq)? ZnSO4(aq) Cu(s)
• Pb(s) CuCl2(aq) ? PbCl2(aq) Cu(s)

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DOUBLE DISPLACEMENT REACTION

• The reaction in which two compounds react by an
  exchange of ions to form two new compounds.

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OXIDATION

• If a substance gains oxygen during a reaction, it
  is said to be oxidised.Then this process is
  called oxidation

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BRAIN EXERCISE

• The surface of copper powder becomes coated with
  black copper oxide. Why has this black substance
  formed?

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REDUCTION REACTION

• If a substance loses oxygen during a reaction, it
  is said to be reduced .During this reaction ,the
  copper oxide is losing oxygen and is being
  reduced. The process is called reduction.

ZnO C ? Zn CO
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EFFCTS OF OXIDATION REACTIONSEVERYDAY LIFE?
E

• CORROSION
• RANCIDITY

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CORROSION

• Iron articles are shiny when new, but get coated
  with a reddish brown powder when left for some
  time. This process is commonly known as rusting
  of iron. When a metal is attacked by substances
  around it such as moisture, acids, etc., it is
  said to corrode and this process is called
  corrosion.

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• General Rules For Preventing Galvanic Corrosion
• Use metals below the water that are as close to
  each other as possible on the galvanic scale.
  Don't mix metals.
• Fasteners must always be more noble than the
  fitting on which they're used.

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• With a simple boat, or a wood boat of any kind,
  strictly for the sake of corrosion protection it
  will be best to electrically isolate all
  underwater metal fittings from each other, and
  then.....
• Put a zinc on it...!  But don't put too much zinc
  on it! Weld the zinc on if possible, or bolt the
  zinc directly to the piece.

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• Painting the cathodic metals is beneficial.
• Never paint a zinc anode!
• Never use graphite-bearing lubricants. Graphite
  is noble to almost everything!

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• When bringing AC shore power aboard, it should
  always first pass through a true marine grade
  isolation transformer.
• Battery chargers must be a marine quality
  isolation transformer type.

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• Use bonding if necessary, if you have a highly
  complicated electrical system, in order to reduce
  the electrical shock hazard.
• Do everything possible to avoid stray currents in
  the water, and to prevent them aboard.

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BRAIN EXERCISE

• Have you noticed the color of the coating formed
  on copper and silver?

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THANK YOU...!!!