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Tuesday, 30 June 2015

CHEMICAL REACTION AND EQUATIONS notes 2


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CHEMICAL REACTION AND EQUATIONS 



Chemical reactions are the processes in which new substances with new properties are formed.
Only a rearrangement of atoms takes place in a chemical reaction.
(i)The substances while take part in a chemical reaction are called reactants.
(ii)The new substances produced as a result of chemical reaction are called products.
The burning of magnesium in air to form magnesium oxide is an example of a chemical reaction.
Before burning in air, the magnesium ribbon is cleaned by rubbing with a sand paper. This is done to remove the protective layer of basic magnesium carbonate from the surface of magnesium ribbon.

Formation of curd from milk, Cooking of food, Digestion of food in our body, Process of respiration, Formation of grapes, Rusting of iron (when left expose to humid atmosphere), Burning of fuels (like wood ,coal, kerosene, petrol and LPG),Burning of candle wax, and Ripening of fruits, are all chemical changes which involve chemical reactions.

Characteristics of chemical reactions

The important characteristics of chemical reactions are:
  1. Evolution of gas,
  2. Formation of precipitation,
  3. Change in colour,
  4. Change in temperature,
  5. Change in state,

1.) Evolution of gas
Some chemical reactions are characterized by the evolution of gas.
The chemical reaction between zinc and dilute sulphuric acid is characterized by the evolution of hydrogen gas.
The chemical reaction between sodium carbonate and dilute hydrochloric acid is characterised by evolution of carbon dioxide gas.
A precipitation is a ‘solid product’ which separates out from the solution during a chemical reaction.

2.) Formation of precipitate
Some chemical reactions are characterised by the formation precipitate.
The chemical reaction between potassium iodide and lead nitrate is the characterized by the formation of a yellow precipitate of lead iodide.
The chemical reaction between sulphuric acid and barium chloride solution is the characterised by the formation of a white precipitate of barium sulphate.


3.) Change in colour
Some chemical reactions are characterised by the change in colour.
The chemical reaction between citric acid purple coloured potassium permanganate solution is characterised by a change in colour from purple to colourless.
The chemical reaction between sulphur dioxide gas acidified potassium dichromate solution is characterized by a change in colour from orange to green.

4.) Change in temperature
Some chemical reactions are characterized by a change in temperature.
The chemical reaction between quicklime water to form slaked lime is characterized by a change in temperature (which is rise in temperature).
The chemical reaction between zinc granules and dilute sulphuric acid is also characterised by change in temperature (which is rise in temperature).
The chemical reaction between barium hydroxide and ammonium chloride to form barium chloride, ammonia and water is the characterised by a change in temperature (which is fall in temperature).

5.) Change in state
Some chemical reactions are characterized by a change in state.
The combustion reaction of candle wax is characterised by a change in state from solid to liquid and gas (because wax is a solid, water formed by the combustion of wax is a liquid at room temperature whereas carbon dioxide produced by the combustion of wax is a gas).There are some chemical reactions which can show more than one characteristics.


CHEMICAL EQUATIONS

The method of presenting a chemical reaction with the help of symbols and formulae of the substances involved in it is known as a chemical equation.
Zinc metal reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas.
This reaction can be written in word as:
Zinc + Sulphuric acid ---> Zinc sulphate + Hydrogen
Using symbol and formula we write
Zn + H2SO4 -- ZnSO4 + H2
The substances which combine or react are known as reactants.
The new substances produced in a reaction are known as products.

A chemical equation is a short-hand method of representing a chemical reaction.

Balanced and unbalanced chemical equations

  1. A balanced equation has an equal number of atoms of different elements in the reactants and products.
  2. An unbalanced chemical equation has an unequal number of atoms of one or two elements in the reactants and products.
“Matter can neither be created nor destroyed in a chemical reaction” This is law of conservation of mass.The chemical equations are balanced to satisfy the law of conservation of mass in chemical reactions.
H2 + O -- H2O
We should never change formula an element or a compound to balance a equation.

Balancing of chemical Equations

The process of making the number of different types of atoms equal on both the sides of an equation is called balancing of equation.

To Make Information More Informative

The equation which gives more information about the chemical reaction is known as more informative or information giving equation. The chemical equations can be made more informative in three ways:

1.) To indicate the physical states of reactants and products in the equation.
Solid state is indicated by the symbol (s)
Liquid state is indicated by the symbol (l)
Gaseous state is indicated by the symbol (g)
Aqueous solution (solution made in water) is indicated by the symbol (aq)

2.) To indicated heat changes in an equation

(i ) Those reactions in which is evolved are known as exothermic reaction. An exothermic reaction is indicated by writing “+ Heat” or “Heat Energy” or just “Energy” on the products’ side of equation.
(ii) Those reactions in which heat is absorbed are known as endothermic reactions. An endothermic reaction is usually indicated by writing “+Heat” or “Heat Energy” or just “Energy” on the reactants’ side of an equation.

3.) To indicate the conditions under which the reaction takes place

If heat required for a reaction to take place, then the heat sign delta (Δ) is put over the arrow of the equation. If the reaction takes place in the presence of a catalyst, then the symbol or formula of the catalyst is written above or below the arrow sign in the equation.


TYPES OF CHEMICAL REACTION

Some of the important types of chemical reactions are:
  1. Combination reaction
  2. Decomposition reaction
  3. Displacement reaction
  4. Double displacement reaction
  5. Oxidation and Reduction reaction

1.) COMBINATION REACTION

Those reaction in which two or more substances combine to form a single substance, are called combination reactions.
Magnesium and oxygen combine, when heated, to form magnesium oxide:
Combination
2Mg(s) + O2(g) ---------------------- > 2MgO(s)
Here two elements combine to form a single compound. In some combination reactions, however, two or more compounds combine together to form a new compound.

2.) DECOMPOSITION REACTION

Those reactions in which a compound splits up into two or more simpler substances are known as decomposition reactions.
When calcium carbonate is heated, it decomposes to give calcium oxide and carbon dioxide: heat
CaC03(s) ------------- > CaO(s) + CO2(g)
A decomposition reaction is just the opposite of a combination reaction.
When decomposition reaction is carried out by heating, it is called ‘thermal decomposition’.

3.) DISPLACEMENT REACTIONS

Those reactions, in which one element takes the place of another element in a compound, are known as displacement reactions.
When a strip of zinc metal is placed in copper sulphate solution, then zinc sulphate solution and copper are produced:
CuSO4(aq) + Zn(s) ---- ZnSO4(aq) +Cu(s)
This displacement reaction takes place because zinc is more reactive than copper.

4.) DOUBLE DISPLACEMENT REACTION

Those reactions in which two compounds react by an exchange of ions to form two new compounds are called double displacement reactions.
When silver nitrate solution is added to sodium chloride solution, then a white precipitation of silver chloride is formed along with sodium nitrate solution:
AgNO3(aq) + NaCl(aq) --- AgCl(s) + NaNO3(aq)
Silver nitrate sodium chloride silver chloride sodium nitrate

In this double displacement reaction, two compound, silver nitrate and sodium chloride reacts to form two new compounds, silver chloride and sodium nitrate. An exchange of ions takes place in this reaction.

Any reaction in which an insoluble solid (called precipitate) is formed that separates from the solution is called a precipitation reaction.

5.) OXIDATION AND REDUCTION REACTIONS

Oxidation: (i) The addition of oxygen to a substance is called oxidation.
(ii)The removal of hydrogen from a substance is also called oxidation.
Reduction: (i) The addition of hydrogen to a substance is called reduction.
(ii) The removal of oxygen from a substance is called reduction.
Oxidation and reduction takes place together

Oxidizing agent: (i) The substances which gives oxygen for oxidation is called an oxidizing agent.
(ii) The substance which removes hydrogen is also called an oxidizing agent.
Reducing agent: (i) The substance which gives hydrogen for reduction is called a reducing agent.
(ii) The substance which removes oxygen is also called a reducing agent.

The oxidation and reduction reaction is also called redox reactions.

When copper oxide is heated with hydrogen, then copper metal and hydrogen formed:
CuO + H2 --- Cu + H2O
    1. In this reaction CuO is changing into Cu. Oxygen is being removed from copper oxide. Removal of oxygen from a substance is called reduction, so copper oxide is being reduced to copper.
    2. In this reaction H2 is changing to H2O.Oxigen is being added to hydrogen. Addition of oxygen to a substance is called oxidation, so hydrogen is being oxidised to water.
(a)The substance which gets oxidised is the reducing agent.
(b) The substance which gets reduced is called oxidizing agent.


EFFCTS OF OXIDATION REACTION IN EVERYDAY LIFE

There are two common effects of oxidation reaction in daily life.
  1. Corrosion of metals
  2. Rancidity of food

Corrosion:
Corrosion is the process in which metals are eaten up gradually by the action of air, moisture or a chemical (such as an acid) on their surface.
Rusting of iron metal is the most common form of corrosion.
During corrosion of iron, iron metal is oxidized by the oxygen of air in the presence of water to form hydrate iron (III)oxide called rust:
4Fe + 3O2 +2xH2O ---- 2Fe2O3xH2O
The rusting of iron is a redox reaction.
Corrosion weakens the iron and steel objects and structures such as railings car bodies, bridges and ships, etc., and cuts short their life.

Rancidity
The condition produced by aerial oxidation of fats and oils in foods marked by unpleasant smell and taste is called rancidity.
The development of rancidity of food can be prevented
  1. Rancidity can be prevented by adding anti-oxidation to foods containing fats and oils.
  2. Rancidity can be prevented by packaging fat and oil containing foods in nitrogen gas.
  3. Rancidity can be retarded by keeping food in a refrigerator.
  4. Rancidity can be retarded by storing food in air-tight containers.
  5. Rancidity can be retarded by storing foods away from light.
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Questions for practice:
  1. What is meant by a chemical reaction? Give one example of chemical reaction.
  2. State one characteristic each of a chemical reactions which take place when :
    1. Dilute hydrochloric acid is added to sodium carbonate.
    2. Lemon juice is added gradually to potassium permanganate solution.
    3. Dilute sulphuric acid is added to barium chloride solution.
    4. Quicklime is treated with water.
  3. Give two characteristics of chemical reaction which take place when dilute sulphuric acid is poured over zinc granules.
  4. State two characteristics of the chemical reaction which occurs on adding potassium iodide solution to lead nitrate solution.
  5. What is the chemical equation? Explain with an example.
  6. What is a balanced chemical equation? Why chemical equations should be balanced.
  7. State whether the following statement is true or false (yes/no).
A chemical equation can be balanced easily by altering the formula of a substance.
8. How will you indicate the following effects in a chemical equation?
(a) A solution is made in water.
(b) Exothermic reaction.
(c) Endothermic reaction
9. What do you understand by exothermic and endothermic reactions? Explain with
example.
  1. Which of the following are exothermic reaction and which are endothermic?
(a) Burning of natural gas (b) Photosynthesis
(c) Electrolysis of water (d) Respiration
(e) Decomposition of calcium carbonate
  1. Explain why respiration is considered as exothermic reaction?
  2. Why photosynthesis is considered as exothermic reaction?
  3. Write balanced chemical equation with state symbols for the following reaction:
Sodium hydroxide solution reacts with hydrochloric acid solution to produce sodium chloride solution and water.
  1. Aluminum hydroxide reacts with sulphuric acid to form aluminum sulphate and water. Write a balanced equation for this reaction.
  2. Potassium chlorate (KClO3) on heating forms potassium chloride and oxygen. Write a balanced equation for this reaction and indicate the evolution of gas.
  3. When potassium nitrate is heated, it decomposes into potassium nitrite and oxygen. Write a balanced equation for this reaction and add the state symbol of the reactants and products.
  4. Write balanced chemical equation from the following information.
An aqueous calcium hydroxide solution (lime water) reacts with carbon dioxide gas to produce a solid calcium carbonate precipitate and water.
  1. What is meant by a decomposition reaction? Explain with two examples.
  2. Name the five types of chemical reactions.
  3. When hydrogen burns in oxygen, water is formed and when water is electrolysed, then hydrogen and oxygen are produced. What type of reaction takes place:
  1. In the first case?
  2. In the second case?
  1. State an important use of decomposition reactions.
  2. (a) Give an example of a decomposition reaction which is carried out with electricity.
(b) Give one example of a decomposition reaction which is carried out by applying heat.
  1. What type of chemical reaction is used to extract several metals from there naturally occurring compounds like oxides and chlorides?
  2. A colorless lead salt, when hated, produce a yellow residue and brown fumes.
    1. Name of lead salts.
    2. Name the brown fumes.
    3. Write a chemical equation for the reaction involved.
  3. What happens when silver chloride is exposed to sunlight? Write a chemical equation for this reaction. Also give one use of such a reaction.
  4. Why decomposition reactions are called the opposite of combination reactions? Explain with equations of these reactions.
  5. What happens when a zinc strip is dipped into a copper sulphate solution?
    1. Write the equation for the reaction that takes place.
    2. Name the type of reaction involved
  6. What happens when a piece of iron metal is placed in copper sulphate solution? Name the type of reaction involved.
  7. What happens when silver nitrate solution is added to sodium chloride solution?
    1. Write the equation for the reaction which takes place.
    2. Name the type of reaction involved.
  8. Express the following facts in the form of a balanced chemical equation:
“When a strip of copper metal is placed in a solution of silver nitrate, metallic silver is precipitated and a solution containing copper nitrate is formed”.
  1. What is the difference between displacement and double displacement reaction? Write equations for these reactions.
  2. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
  3. Write a chemical equation to show the process of corrosion (or rusting) of iron?
  4. Name any three objects (or structures) which are gradually damaged by the corrosion of iron and steel.
  5. Explain the term ‘rancidity’.
  6. What is the general name of the chemicals which are added to fat and oil containing foods to prevent the development of rancidity?
  7. What are anti-oxidants? Why are they added to fat and oil containing foods?
  8. Explain why, food products containing fats and oils 9like potato chips) are packaged in nitrogen.
  9. Name two anti-oxidants which are usually added to fat and oil containing foods to prevent rancidity.
  10. Fill the following blank with suitable word:
Anti-oxidants are often added to fat containing foods to prevent……..due to oxidation.


Chemistry for class 10....
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