Friday 26 June 2020

Chapter 2 acides bases and salt

Acids Bases and Salt notes

Properties of Acids:


– Produce hydrogen ions [H+] in H2O.
– Sour taste.
– Turn blue litmus red.
– Act as electrolytes in Solution.
– Neutralize solutions carrying hydroxide ions.
– React with several metals releasing Hydrogen gas.
– React with carbonates releasing CO2 (g)
– Destroy body tissues.
– corrode metal surface quickly.


On the basis of origin, acids are classified as :


a.
Organic acids: Acids derived from living organisms like plants and animals . For example: citric acid is present in fruits, acetic acid present in vinegar, oxalic acid present in tomato, tartaric acid present in tamarind, lactic acid present in sour milk and curd.


b.
Mineral acids: They are also called inorganic acids. They are dangerous Example sulphuric acid (H2SO4), hydrochloric acid (HCl) etc.


➣  On the basis of their strength, acids are classified as :


a. Strong acids: Completely dissociate into its ions in aqueous solutions.
Example: Nitric acid (HNO3), sulphuric acid (H2SO4), hydrochloric acid (HCl).


b. Weak acids: Weak acids are those acids which do not completely dissociate into
its ions in aqueous solutions. For example: carbonic acid (H2CO), acetic acid (CH3COOH).


➣    On the basis of their concentration, acids are classified as :


a. Dilute acids: Have a low concentration of acids in aqueous solutions.
b. Concentrated acids: Have a high concentration of acids in aqueous solutions.


➣     On the basis of number of hydrogen ion, acids can be classified as :


Monoprotic acid –
Such type of acid produces one mole of H+ ions per mole of acid, e.g., HCl , HNO3

Diprotic acid – They can produce two moles of H+ ions per mole of acid, e.g., H2SO4.

Triprotic acid – They produce three moles of H+ ions per mole of acid, e.g., H3PO4.

Polyprotic – They can produce more than three H+ ions per mole of acid.


➣    Properties of Base:


– Produce hydroxide ions [OH –] in H2O.
– Water soluble bases are called alkalies.
– Bitter Taste
– Turn Red Litmus blue.
– Act as electrolytes in Solution.
– Neutralize solutions containing H+ ions.
– Have a slippery, ‘soapy’ feel.
– Dissolve fatty material.


➣    On the basis of their strength, bases are classified as:


a. Strong bases: Strong bases are those bases which completely dissociate into its ions in aqueous solutions. Example: sodium hydroxide (NaOH), potassium hydroxide (KOH).

b. Weak bases: Weak bases are those bases which do not completely dissociate into its ions in aqueous solutions. For example: ammonium hydroxide (NH4OH).


➣    On the basis of their concentration, bases are classified as:


a.
Dilute bases: Have a low concentration of alkali in aqueous solutions.
b. Concentrated bases: Have a high concentration of alkali in aqueous solutions.


➣    Strength Of Acid Or Base Solutions:


A scale for measuring hydrogen ion concentration in a solution, called pH scale has been developed. The p in pH stands for ‘potenz’ in German, meaning power.
p= potential or Power         H = Hydrogen


pH =7 Neutral Solution H3O+ = OH
pH>7 Basic Solution H3O+ < OH
pH<7 Acidic Solution H3O+ > OH


Range of pH is from 1 to 14


Range of pH is from 1 to 14


➣    pH Sensitivity of Plants & Animals:


• Human body works in a narrow range of pH 7 to 7.8. Acidity can be lethal for plants and animals.
• pH of Digestive System: Stomach secretes HCl to kill bacteria in the food. The inner lining of stomach protects vital cells from this acidic pH.
• pH and tooth decay: Lower pH because of sour food and sweet food can cause tooth decay. The pH of mouth should always be more than 5.5.
• pH as self defense mechanism in plants & animals: Certain animals like bee and plants like nettle secrete highly acidic substance for self defense.


  Properties of salts:


• Salts form by the combination of acid and base through neutralization reaction.
• The acidic and basic nature of salts depends on the acid and base combined in neutralization reaction.


properties of salts


• The most common salt is sodium chloride or table salt which forms by the combination of sodium hydroxide (base) and hydrochloric acid.
• Other examples include Epsom salts (MgSO4) used in bath salts, ammonium nitrate (NH4NO3 ) used as fertilizer, and baking soda (NaHCO3) used in cooking.
• The pH of salts solution depends on the strength of acids and base combined in neutralization reaction.
• Indicators – Indicators are substances which indicate the acidic or basic nature of the solution by their colour change.


The colour of some acid – base indicators in acidic and basic medium are given below :


S. No.

Indicators

Colour in

Colour in

acidic medium

basic medium

1

Litmus solution

Red

Blue

2

Methyl Orange

Pink

Orange

3

Phenolphthalein

Colourless

Pink

4

Methyl red

Yellow

Red


Chemical properties of acids:


i)
Acids react with active metals to give hydrogen gas.
Zn + H2SO3 →ZnSO4 + H2

ii) Acids react with metal carbonate and metal hydrogen carbonate to give carbon dioxide.
NaHCO3 +HCl → NaCl + H2O + CO2

iii) Acids react with bases to give salt and water. This reaction is called as neutralization reaction.NaOH + HCl → NaCl +H2O

iv) Acids react with metals oxides to give salt and water.
CuO + H2SO4 → CuSO4 + H2O


Addition of Acids or Bases to Water


The process of dissolving an acid, specially nitric acid or sulfuric acid or a base in water is a highly exothermic one. As a rule: Always add acid to water and never the other way! The acid must be added slowly to water with constant stirring. If one mixes the other way by adding water to a concentrated acid, the heat generated causes the mixture to splash out and cause burns.


➣   Chemical properties of Bases:


i) Reaction with Metals -
Certain reactive metals such as Zinc, Aluminium, and Tin react with alkali solutions on heating and hydrogen gas is evolved. 2NaOH + Zn →  Na2ZnO2 +H2


ii) Reaction with acids -Bases react with acids to form salt and water. KOH +HCl → KCl +H2O


iii) Reaction with Non -metallic oxides –
These oxides are generally acidic in nature. They react with bases to form salt and water. 2NaOH + CO2 → Na2CO3 + H2O


 ➣   Some Important Chemical Compounds:•   


• Common Salt (NaCl
)


Sodium chloride is known as common salt. Its main source is sea water. It is also exists in the form of rocks and is called rock salt.
Common salt is an important component of our food. It is also used for preparing sodium hydroxide, baking soda, washing soda etc.


 Sodium hydroxide (NaOH)


Prepared by Chlor Alkali process :Electricity is passed through an aqueous solution of Sodium chloride (called brine). Sodium chloride decomposes to form sodium hydroxide. Chlorine gas is formed at the anode, and hydrogen gas at the cathode. Sodium hydroxide solution is formed near the  cathode. 2NaCl(aq) + 2 H2O (l) → 2NaOH(aq) + Cl2(g) + H2(g)


Bleaching powder:


Bleaching powder is represented as CaOCl2, though the actual composition is quite complex.
Bleaching powder is produced by the action of chlorine on  dry slaked lime. Ca(OH)2 + Cl2 → CaOCl2+ H2O


• Baking soda:
Sodium hydrogen carbonate (NaHCO3) Preparation: NaCl + H2O + CO2+ NH3 → NH4Cl + NaHCO3


Washing soda: Sodium carbonate N2CO3.10H2 In the first step, sodium carbonate is obtained by heating baking  soda. 2 NaHCO3(heat) →Na2CO3 + H2O + CO2


Then washing sod a is produced by recrystallisation of sodium  carbonate Na2CO3 + 10H2O →  Na2CO3 .10H2O


•  Plaster of Paris:
Calcium sulphate hemihydrate CaSO4. ½ H2O Prepared by heating Gypsum at 373K. CaSO4. 2H2O(Heat at 373K) → CaSO4. ½ H2O + 1½ H2O

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Thursday 25 June 2020

Chapter 1 Chemical equation and reaction

Chapter 1 of class 10th

Introduction to Chemical Reactions and Equations

Physical and chemical changes

Chemical change – one or more new substances with new physical and chemical properties are formed.
Chemical Reactions and Equations-1
Here, when copper sulphate reacts with iron, two new substances, i.e., ferrous sulphate and copper are formed.
Chemical-Reactions-and-Equations-2
Physical change – change in colour or state occurs but no new substance is formed.
Example: Water changes to steam on boiling but no new substance is formed(Even though steam and water look different when they are made to react with a piece of Na, they react the same way and give the exact same products). This involves only a change in state (liquid to vapour).

Observations that help determine a chemical reaction

A chemical reaction can be determined with the help of any of the following observations:
a) Evolution of a gas
b) Change in temperature
c) Formation of a precipitate
d) Change in colour
e) Change of state

Chemical reaction

Chemical reactions are chemical changes in which reactants transform into products by making or breaking of bonds(or both) between different atoms.

Types of chemical reactions

Taking into consideration different factors, chemical reactions are grouped into multiple categories.
Few examples are:
● Combination
● Decomposition
● Single Displacement
● Double displacement
● Redox
● Endothermic
● Exothermic
● Precipitation
● Neutralisation

Chemical Reactions and Equations I

Word equation

A  word equation is a chemical reaction expressed in words rather than chemical formulas. It helps identify the reactants and products in a chemical reaction.
For example,
Sodium + Chlorine → Sodium chloride
The above equation means: “Sodium reacts with chlorine to form sodium chloride.”

Symbols of elements and their valencies

A symbol is the chemical code for an element. Each element has one or two-letter atomic symbol, which is the abbreviated form of its name.
Valency is the combining capacity of an element. It can be considered as the number of electrons lost, gain or shared by an atom when it combines with another atom to form a molecule.

Writing chemical equations

Representation of a chemical reaction in terms of symbols and chemical formulae of the reactants and products is known as a chemical equation.

 Chemical Reactions and Equations-3
• For solids, the symbol is “(s)”.
• For liquids, it is “(l)”.
• For gases, it is “(g)”.
• For aqueous solutions, it is “(aq)”.
• For gas produced in the reaction, it is represented by “(↑)”.
• For precipitate formed in the reaction, it is represented by “(↓)”.

Balancing of a Chemical Reaction

Conservation of mass

According to the law of conservation of mass, no atoms can be created or destroyed in a chemical reaction, so the number of atoms for each element in the reactants side has to balance the number of atoms that are present in the products side.
In other words, the total mass of the products formed in a chemical reaction is equal to the total mass of the reactants participated in a chemical reaction.

Balanced chemical equation

The chemical equation in which the number of atoms of each element in the reactants side is equal to that of the products side is called a balanced chemical equation.

Steps for balancing chemical equations

Hit and trial method: While balancing the equation, change the coefficients (the numbers in front of the compound or molecule) so that the number of atoms of each element is same on each side of the chemical equation.

Short-cut technique for balancing a chemical equation

Example:

aCaCO3+bH3PO4→cCa3(PO4)2+dH2CO3

Set up a series of simultaneous equations, one for each element.

Ca: a=3c

C: a=d

O: 3a+4b=8c+3d

H: 3b=2d

P: b=2c

Let’s set c=1

Then a=3 and

d=a=3

b=2c=2

So a=3; b=2; c=1; d=3

The balanced equation is

3CaCO3+2H3PO4→Ca3 (PO4)2+3H2CO3

Chemical Reactions and Equations II

Types of chemical reactions

Taking into consideration different factors, chemical reactions are grouped into multiple categories.
Few examples are:
● Combination
● Decomposition
● Single Displacement
● Double displacement
● Redox
● Endothermic
● Exothermic
● Precipitation
● Neutralisation

Combination reaction

In a combination reaction, two elements or one element and one compound or two compounds combine to give one single product.
Chemical Reactions and Equations-4

Decomposition reaction

A single reactant decomposes on the application of heat or light or electricity to give two or more products.
Types of decomposition reactions:
a. Decomposition reactions which require heat – thermolytic decomposition or thermolysis.

Chemical-Reactions-and-Equations-5

                                               Thermal decomposition of HgO

b. Decomposition reactions which require light – photolytic decomposition or photolysis.

Chemical-Reactions-and-Equations-6

                                       Photolytic decomposition of H2O2

c. Decomposition reactions which require electricity – electrolytic decomposition or electrolysis.

 

Electrolytic decomposition of H2O

Displacement reaction

More reactive element displaces a less reactive element from its compound or solution.

 Chemical Reactions and Equations-7

Double displacement reaction

An exchange of ions between the reactants takes place to give new products.
For example, Chemical Reactions and Equations-8

Precipitation reaction

An insoluble compound called precipitate forms when two solutions containing soluble salts are combined.
Chemical Reactions and Equations- 9

Chemical-Reactions-and-Equations-10
Redox reaction

Oxidation and reduction take place simultaneously.
Oxidation: Substance loses electrons or gains oxygen or loses hydrogen.
Reduction: Substance gains electrons or loses oxygen or gains hydrogen.
Oxidising agent – a substance that oxidises another substance and self-gets reduced.
Reducing agent – a substance that reduces another substance and self-gets oxidised.

Chemical Reactions and Equations-11

Endothermic and exothermic reaction

Exothermic reaction – heat is evolved during a reaction. Most of the combination reactions are exothermic.

Al+Fe2O3→Al2O3+Fe+heat
CH4+2O2→CO2+2H2O+heat

Endothermic – Heat is required to carry out the reaction.

6CO2+6H2O+Sunlight→C6H12O6+6O2

Glucose
Most of the decomposition reactions are endothermic.

Corrosion

Gradual deterioration of a material, usually a metal, by the action of moisture, air or chemicals in the surrounding environment.

Rusting:

 4Fe(s)+3O2(from air)+xH2O(moisture)→2Fe2O3.xH2O(rust)
Corrosion of copper:
Cu(s)+H2O(moisture)+CO2(from air)→CuCO3.Cu(OH)2(green)
Corrosion of silver:
Ag(s)+H2S(from air)→Ag2S(black)+H2(g)

Rancidity

It refers to the oxidation of fats and oils in food that is kept for a long time. It gives foul smell and bad taste to food. Rancid food causes stomach infection on consumption.
Prevention:
(i) Use of air-tight containers
(ii) Packaging with nitrogen
(iii) Refrigeration
(iv) Addition of antioxidants or preservatives


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