Chemical Reaction and Equation
• Whenever a chemical change occurs, we can say that a chemical reaction takes place i.e Chemical reactions involve chemical changes.
Activity 1.1
• When the solution of lead nitrate [Pb(NO₃)₂ ] and potassium iodide (KI) are mixed, they react with each other to form yellow prepitate of lead iodide (PbI₂) and and potassium nitrate (KNO₃) which remains in the solution.
• When dilute hydrochloric (HCl) or sulphuric (H₂SO₄) acid is added to zinc (Zn) grannules in a test tube they react with each other to forms bubbles of hydrogen gas and resepective salts i.e (Zinc chloride- ZnCl₂) or (Zinc sulphate-ZnSO₄) with rise in temperature.
3Fe (s) + H₂O (g) → Fe₃O₄ (s) + H₂ (g)
3Fe (s) + 4H₂O (g) → Fe₃O₄ (s) + H₂ (g)
3Fe (s) + 4H₂O (g) → Fe₃O₄ (s) + 4H₂ (g)
• When water is added to calcium oxide (CaO-Quick lime) in a beaker it reacts vigorously with water to produce calcium hydroxide [Ca(OH)₂-slaked lime] releasing a large amount of heat.
• When a ferrous sulphate (FeSO₄) crystal is heated in a dry boiling tube then after sometime green colour of the ferrous sulphate crystal changes to brown due to the formation of ferric oxide (Fe₂O₃) and a characteristic odour (smell) of burning sulphur is observed due to the formation of sulphur dioxide (SO₂) and sulphur trioxide (SO₃) gas.
• When electric current is passed through acidified water (H₂O), it decomposes into its constituents i.e hydrogen (H₂) and oxygen (O₂) gas.
• Hydrogen (H₂) gas is collected at cathode (-ve electrode) and oxygen (O₂) is collected at anode (+ve electrode).
• When white color of silver chloride (AgCl) powder is exposed (kept) in sunlight, then after sometime it turns grey due to decomposition of silver chloride (AgCl) into silver (Ag) metal and chlorine (Cl₂) gas by light.
• When yellow color of silver bromide (AgBr) powder is exposed (kept) in sunlight, then after sometime it turns grey due to decomposition of silver bromide (AgBr) into silver (Ag) metal and bromine (Br₂) gas by light.
• These reaction are used in black and white photography.
• When an iron (Fe) nail emerged (dipped) in blue color of copper sulphate (CuSO₄) solution, then after sometime the blue colour of copper sulphate (CuSO₄) solution fades into light green color due to the formation of iron sulphate (FeSO₄) solution and the iron (Fe) nail become coated by brownish colour substance due to deposition of copper (Cu) on it.
• When sodium sulphate (Na₂SO₄) solution is mixed with barium chloride (BaCl₂) solution in a test tube, they react with each other to from a white precipitate of barium sulphate (BaSO₄) which is insoluble and sodium chloride (NaCl) salt which is soluble (remains) in the solution.
• In this reaction carbon (C) is oxidised to carbon monoide (CO) and zinc oxide (ZnO) is reduced to zinc (Zn).
• In this reaction hydrochloric acid (HCl) is oxidised to chlorine (Cl₂) whereas manganese oxide (MnO₂) is reduced to manganese chloride (MnCl₂).
• When a china dish containing copper (Cu) powder is heated, after sometime the brown surface of copper powder truns (becomes) coated with a black colour substance of copper (II) oxide (CuO).
• The above reaction is a redox reaction because copper oxide (CuO) is reduced to copper (Cu) and Hydrogen( H₂) is reduced to water (H₂O).
Chemical Change :
• If the identity, composition or nature of a chemical substance change and can not be reserved by any physical process, it is called chemical change.
Chemical Reaction:
• The process involving a chemical change is called chemical reaction.
• The process in which one or more substance as reactants gets converted (transformed) into new substances with new properties in product as a result of chemical change is called chemical reaction.
Reactants:
• The substances that undergo chemical change or takes part in a chemical reaction is called reactants.
Products:
• The new substances produced (formed) during the reaction as a result of chemical change are called products.
• Chemical reactions involves the breaking and making of bonds between atoms i.e in a chemical reaction old bonds are broken to form a new bond.
• During a chemical reaction atoms of one element do not change into those of another element.
• Neither atoms disappear from the mixture nor appear from elsewhere, only rearrangement of atoms takes place in a chemical reaction.
A large variety of chemical reactions takes place around us in our daily life, some of these are :
▪︎ Milk is left at room temperature during summers (Suoring of Milk).
▪︎ Iron tawa/pan/nail is left exposed to humid atmosphere (Rusting of Iron).
▪︎ Grapes get fermented (Fermentation of Grapes).
▪︎ Food is cooked (Cooking of Food).
▪︎ Food gets digested in our body (Digestion of food).
▪︎ We respire (Respiration).
When a chemical reaction occurs following observations takes place :
i) Change in state
C (s) + O₂ (g) → CO₂ (g)
Solid Carbon is changes into carbon dioxide gas.
ii) Change in colour
iii) Evolution of a gas
iv) Change in temperature
v) Formation of precipitate
These are the characteristic of chemical reactions.
• When a magnesium (Mg) ribbon is heated, it react with the oxygen present in air and burns with a dazzling white flame and changes/gets converted into a white powder substance called magnesium oxide (MgO) also produces large amount of heat.
Magnesium + Oxygen → Magnesium oxide
Mg (s) + O₂ (g) → MgO (s) + Heat
In this reaction change in temperature is observed.
Note : Before burning, the magnesium ribbon is cleaned by rubbing with a sand paper to remove the protective layer of magnesium oxide from its surface.
Activity 1.2
Lead nitrate + Potassium iodide → Lead iodide + Potassium nitrate
Pb(NO₃)₂ (aq) + KI (aq) → PbI₂ (s) + KNO₃ (aq)
(Yellow ppt)
• In this reaction formation of precipite is observed.
Activity 1.3
i) Zinc + Hydrochloric acid → Zinc Chloride + Hydrogen gas
Zn (s) + HCl (aq) → ZnCl₂ (aq) + H₂ (g)
ii) Zinc + Sulphuric acid → Zinc sulphate + Hydrogen gas
Zn + H₂SO₄ → ZnSO₄ (aq) + H₂ (g)
• In these reactions change in temperature and evolution of gas is observed.
Representation of Chemical reactions :
• A chemical reaction can be written (represented) by using words, symbols or chemical formula.
CHEMICAL EQUATIONS :
• A chemical equation represents a chemical reaction.
• The method of representing a chemical reaction with the help of symbols and formulae of the substances involved in it is known as chemical equation.
• A complete chemical equation represents the reactants, products and their physical states symbolically.
Fe (s) + H₂O (g) → Fe₃O₄ (s) + H₂ (g)
Zn (s) + HCl (aq) → ZnCl₂ + H₂ (g)
• The gaseous, liquid, aqueous and solid states of reactants and products are represented by the notations (g), (l), (aq) and (s), respectively.
• The word aqueous (aq) is written if the reactant or product is present as a solution in water.
Note : The symbol (g) is used with H2O to indicate that in the reaction water is used in the form of steam.
• Sometimes the reaction conditions, such as temperature, pressure, catalyst, etc., for the reaction are indicated above and/or below the arrow in the equation.
Word-equation:
• A word-equation shows change of reactants to products through an arrow placed between them.
Magnesium + Oxygen → Magnesium oxide
(Reactants) (Products)
• The reactants are written on the left-hand side (LHS) with a plus sign (+) between them.
• 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.
Balanced and Unbalanced chemical equation:
• A chemical equation in which the numbers of atoms of each elements on both sides i.e reactant as well as product side or (LHS and RHS of the arrow) are equal/same is called balanced chemical equation.
For ex: 3Fe (s) + 4H₂O (g) → Fe₃O₄ (s) + 4H₂ (g)
• We can see from the table that number of atoms of each element in reactant is equal to that of product and hence above chemical equation is a balanced chemical equation.
• If not, then the equation is called unbalanced chemical equation because the mass is not same on both sides of the equation.
For ex: Fe (s) + H₂O (g) → Fe₃O₄ (s) + H₂ (g)
• We can see from the table that number of atoms of each element in reactant and product side are not equal and hence above chemical equation is unbalanced chemical equation.
Skeletal Chemical Equation:
• Unbalanced chemical equation is known as skeletal chemical equation for a reaction.
Que : Why a chemical reaction should be balance?
Ans : A chemical equation must always be balanced so that the numbers of atoms of each type involved in a chemical reaction are the same on the reactant and product sides of the equation.
• Also a chemical reaction should be balance to satisfy the law of conservation of mass.
Que : What is law of conservation of mass?
Ans : "This law states that mass can neither be created nor destroyed in a chemical reaction" i.e the total mass of the elements present in the reactant of a chemical reaction is equal to the total mass of the elements present in the product.
• In other words, the number of atoms of each element remains the same, before and after a chemical reaction.
Balancing of a chemical reaction :
• The process of equalising the number of atoms of each element on reactants and product sides of a chemical equation is called balancing of a chemical equation.
Steps involve in writing a balance chemical equation or steps involves to balance an unbalance chemical equation :
Step 1: Write down the chemical formula and symbols of the reactants and products for the reaction.
Step 2: Check the number of atoms of each element on both side of the equation.
Step 3: Balance the equation by placing numbers in front of formula and symbols of the substance in the equation.
Step 4: Include the states symbols in the equation.
For ex: Fe (s) + H₂O (g) → Fe₃O₄ (s) + H₂ (g)
Step 1:
Step 2:
Step 3:
TYPES OF CHEMICAL REACTIONS:
1.Combination Reaction:
• The reaction in which two or more substances (elements or compounds) in reactant side combine together to form a new single substance as product is called combination reactions.
A + B → AB
Note : Combination reaction is also known as synthesis reaction.
Examples of combination reactions:
(i) Burning of coal
C (s) + O₂ (g) → CO₂ (g)
(ii) Formation of water (H₂O) from H₂ (g) and O₂ (g).
H₂ (g) + O₂ (g) → H₂O (l)
Activity 1.4
Calcium Oxide + Water → Calcium Hydroxide + Heat
Quick lime + Water → Slaked lime + Heat
CaO (s) + H₂O (l) → Ca(OH)₂ (aq) + Heat
• In the above reaction calcium oxide and water combine to produce (from) a single product calcium hydroxide hence it a combination reaction.
Do you know?
• A solution of slaked lime [calcium hydroxide [Ca(OH)₂] is used for white washing walls.
• Calcium hydroxide [Ca(OH)₂] reacts slowly with the carbon dioxide (CO₂) in air to form a thin shiny layer of calcium carbonate (CaCO₃-limestone) on the walls.
Calcium hydroxide + Carbon dioxide → Calcium Carbonate + Water
Calcium hydroxide + Carbon dioxide → Lime Stone + Water
Ca(OH)₂ (aq) + CO₂ (g) → CaCO₃ (s) + H₂O (l)
Note: The chemical formula for marble is also CaCO₃.
Exothermic reactions:
• Reactions in which heat is released (given out) along with the formation of products are called exothermic chemical reactions.
For Example:
(i) Burning of natural gas (Methane)
CH₄ (g) + O₂ (g) → CO₂ (g) + H₂O (g) + Heat
(ii) Respiration.
C₆H₁₂O₆ (aq) + O₂ (aq) → CO₂ (aq) + H₂O (l) + energy
(Glucose)
• During digestion carbohydrates are broken down into simpler substance to form glucose (C₆H₁₂O₆) which combines with oxygen and provides energy, this process is called respiration.
(iii) The decomposition of vegetable matter into compost.
(iv) Burning of magnesium (Mg) ribbon.
Mg (s) + O₂ (g) → MgO (s) + Heat
2. Decomposition Reaction:
• The reaction in which a single substance as reactant breaks down (decomposes) to give two or more simpler substance as products is called decomposition reaction.
AB → A + B
• Decomposition reactions are opposite to combination reactions.
• Decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants.
For example:
• Decomposition of calcium carbonate (CaCO₃) to calcium oxide (CaO-lime or quick lime) and carbon dioxide (CO₂) on heating.
CaCO₃ (s) → CaO(s) + CO₂ (g)
Thermal decomposition reqction :
• When a decomposition reaction is carried out by heating, then it is called thermal decomposition reaction.
Activity 1.5
Ferrous sulphate → Ferric oxide + Sulphur dioxide + Sulphur trioxide.
FeSO₄ (s) → Fe₂O₃ (s) + SO₂ (g) + SO₃ (g)
(Green) (Brown)
• The above reaction is an example of thermal decomposition reaction.
• Ferrous sulphate crystals contains seven water molecules called hydrated ferrous sulphate (FeSO₄.7H₂O), which is green in colour, when heated, it lose all water molecules and changes into white crystal of anhydrous ferrous sulphate and then decomposes to ferric oxide sulphur dioxide and sulphur trioxide.
FeSO₄.7H₂O → FeSO₄ + 7H₂O
(Green) (White)
Activity 1.6
• When lead nitrate [Pb(NO₃)₂] powder in a boiling test tube is heated it breaks down and evolve brown fumes of nitrogen dioxide (NO₂) gas, also a yellow colour of lead oxide (PbO) and oxygen (O₂) gas is formed.
Lead nitrate → Lead oxide + Nitrogen dioxide + Oxygen
Pb(NO₃)₂ (s) → PbO (s) + NO₂ (g) + O₂ (g)
(Yellow) (Brown fumes)
Activity 1.7
• The amount of hydrogen (H₂) gas is collected at cathode (-ve electrode) is double the amount of oxygen (O₂) gas collected at anode (+ve electrode) because the ratio of hydrogen (H) and oxygen (O) element in water (H₂O) is 2:1.
Electrolysis :
• When a substance in molten state or in aqueous solution is decomposes by passing electric current in it is called electrolysis or electrolytic decomposition reaction.
• Electrolysis of water (H₂O) is an example of decomposition reaction.
Activity 1.8
Photochemical decomposition reaction :
• When a substance is decomposed by absorbing light energy or in the presence of light then the reaction is called photochemical decomposition reaction.
• The decomposition of silver chloride (AgCl) or bromide (AgBr) into silver (Ag) metal and chlorine (Cl₂) or bromine (Br₂) gas by light is an example of photochemical decomposition reaction.
Endothermic reaction :
• Reactions in which energy is absorbed during a chemical reaction is called endothermic reactions.
For example:
• The chemical reaction between barium hydroxide [Ba(OH)₂] and ammonium chloride (NH₄Cl) to form barium chloride (BaCl₂), ammonia (NH₃) and water (H₂O) is characterised by a change in temperature (which is fall in temperature). It is an endothermic reaction (which means heat absorbing reaction).
Barium hydroxide + Ammonium chloride → Barium chloride + Ammonia + Water.
Ba(OH)₂ + NH₄Cl → BaCl₂ + NH₃ + H₂O
3. Displacement Reaction :
• The reaction in which one element displaced or removed or takes the position (place) of another element from its compound is known as displacement reaction.
• These reaction occurs mostly is solution form.
• Generally a more reactive elements displace less reactive elements from its compound or salt solution.
For example :
Zn (s) + CuSO₄ (aq)→ ZnSO₄ (aq) + Cu (s)
Pb (s) + CuCl₂ (aq) → PbCl₂ (aq) + Cu(s)
Note : Zinc (Zn) and lead (Pb) are more reactive elements than copper (Cu).
Activity 1.9
Iron + Copper sulphate → Iron sulphate + Copper
Fe (s) + CuSO₄ (aq) → FeSO₄ (aq) + Cu (s)
(Blue) (light green) (Brown)
• In this reaction iron (Fe) displaces copper (Cu) from its salt solution and from iron sulphate (FeSO₄) so we can say that zinc (Zn) is more reactive than copper (Cu) and also it is and example of displacement reaction.
4. Double Displacement Reaction :
• The reactions in which there is an exchange of ions (i.e cation and anion) between the reactants takes place are called double displacement reactions.
AB + CD → AD + CB
(ppt)
• Two different atoms or groups of atoms (ions) are exchanged in double displacement reactions.
• A double displacement reaction usually occurs in solution and may one of the product from is a precipitate (an insoluble solid substance or Salt or Compound) and the other is soluble in water.
Precipitate :
• A insoluble substance formed in a double displacement reaction is known as a precipitate.
Precipitation reaction :
• The reaction which produces a precipitate can be called as precipitation reaction
• Precipitation reactions produce insoluble salts (ppt).
For example:
Pb(NO₃)₂ (aq) + KI (aq) → PbI₂ + KNO₃
Activity 1.10
• The white precipitate of BaSO4 is formed by the reaction of SO42- and Ba2+ and NaCl is formed by the reaction of Na+ and Cl- ions.
Sodium sulphate + Barium chloride → Bariumsulphate + Sodium chloride
Na₂SO₄ (aq) + BaCl₂ (aq) → BaSO₄ (s) + NaCl (aq)
(White ppt)
Oxidation :
• If a substance gains oxygen or loses hydrogen during a reaction, it is oxidised and the process involved is called oxidation.
Mg (s) + O₂ (g) → MgO (s)
• In this reaction magnesium (Mg) oxidised to magnesium oxide (MgO).
Oxidising agent :
• If a substance oxidised the other substance and itself gets reduced is called oxidising agent.
Reduction :
• If a substance loses oxygen or gains hydrogen during a reaction, it is said to be reduced and the process involved is called reduction.
Reducing agent :
• If a substance reduces the other substance and itself gets oxidised is called reducing agent.
Redox reactions :
• The reaction in which one reactant gets oxidised while the other gets reduced is called oxidation-reduction reactions or redox reactions.
ZnO + C → Zn + CO
MnO₂ + HCl → MnCl₂ + H₂O + Cl₂
Activity 1.11
Copper + Oxygen → Copper Oxide
Cu + O₂ → CuO
• In this reaction copper is oxidised to copper oxide.
• If hydrogen (H₂) gas is passed over this heated copper oxide (CuO), the black coating on the surface is removed and it again turns brown as the reverse reaction takes place due formation of copper (Cu) which is responsible for reddish-brown colour.
Copper Oxide + Hydrogen → Copper + Water
CuO +H₂ → Cu + H₂O
EFFECTS OF OXIDATION REACTIONS IN EVERYDAY LIFE :
• Oxidation has damaging effect on metals as well as on food containing fats and oil.
• There are two common effects of oxidation reactions which we observe in daily life :
1. Corrosion :
• When a metal is attacked by air, moisture, acids, etc., it is said to corrode and this process is called corrosion.
• The process of are eaten up of metals gradually by the action of air, moisture or a chemical (such as an acid) on their surface is called corrosion.
• The black coating on silver and the green coating on copper are the examples of corrosion.
• Corrosion of Iron is called Rusting.
• When an Iron articles left (exposed) for some time, its shine gets turnished and gets coated with a reddish brown layer/flasky substance called rust and the process is commonly known as rusting of iron.
Iron + Oxygen + Water → Hydrated ferric oxide
Fe + O₂ + xH₂O → Fe₂O₃.xH₂O
(Rust)
• Some other metals also get tarnished in this manner.
• Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metals, specially those of iron.
Methods of Prevention of Corrosion : (Chapter-3).
2. Rancidity :
• The oxidation of Fat/oil containing food materials results in unpleasant smell and bad taste is called rancidity
• They become rancid when left for a long time and their smell and taste change because fats and oils are oxidised.
Methods of Prevention of Rancidity :
1) Rancidity can be prevent by adding (antioxidants) to the foods containing fats and oil.
• Antioxidants are the reducing agent which prevents oxidation.
For Example:
BHA (Butylated Hydroxy-Anisole)
BHT (Butylated Hydroxy-Toluene).
2) Keeping food in air tight containers helps to slow down oxidation.
3) Rancidity can be prevented by packaging fat and oil containing foods in nitrogen gas.
4) Rancidity can be slow down by keeping food in a refrigerator.
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