Chapter 1 Chemical Reactions and Equations

Combustion of Liquefied Petroleum Gas (LPG) is not a physical change. It is a chemical change because combustion of LPG produces new substances. A lot of heat is produced along with carbon dioxide and water vapour during this reaction. It is also irreversible in nature.

Rest three are physical changes as no new products are formed; the only state of the substance changes.

Note:

During a physical change, the physical state of substance changes. A physical change does not produce a new substance although the initial and final substances appear different. A physical change is reversible.

During a chemical change the chemical composition of substance changes. A chemical change produces a new substance with new properties. Energy is either released or absorbed during a chemical change. A chemical change is irreversible.

This is a redox reaction because oxidation and reduction both are happening simultaneously. It is also a displacement reaction because H of NH₃ is displaced by oxygen. A displacement reaction is also a redox reaction as it is always followed by oxidation as well as reduction.

Note: When oxygen is added to a compound in a reaction, it is called oxidation. Removal of oxygen from a substance during a reaction is called reduction. Here, NH₃ is changing to NO. So, NH₃ is undergoing oxidation. Addition of hydrogen atom is also reduction. Here, oxygen is getting reduced.

When quick lime reacts with water, calcium hydroxide is produced and a large amount of heat is released. Similarly, when an acid or a base is diluted with water, heat is released making it a highly exothermic reaction.

Note: Exothermic processes release energy in the form of heat along with the formation of products. Such processes are hot because they give out heat.

Chemical reaction in beakers A and B gives out heat. Hence the temperature of the solution increases. Thus, we can call it an exothermic reaction. In beaker C, heat is absorbed from water instead and hence the temperature decreases. Thus, we can call it an endothermic process.

KMnO₄ is an oxidising agent, it oxidises FeSO₄.

Double displacement reactions are such chemical reactions in which one component of each reactant exchange to form products. The cations and anions of two compounds exchange their ions to form two new compounds. Here only in reaction (ii), ions of both reactants are exchanged and two entirely different compounds are formed.

When silver chloride is exposed to sunlight, it is decomposed to form silver. During this reaction, white silver chloride changes into greyish white silver metal. This is also known as photolytic decomposition reaction as it takes place in the presence of sunlight.

Slaking of lime is an exothermic reaction because a large amount heat is produced during the reaction. The heat can be felt by touching the beaker from outside. The resulting compound Ca(OH)₂, which is also called slaked lime, turns red litmus solution to blue and hence proves it to be a basic solution. Thus, the pH of this solution will be more than seven.

This reaction is a double displacement reaction as well as a precipitation reaction. This is a double displacement reaction because one component of each compound has exchanged places with each other. It is also called a precipitation reaction due to the formation of white precipitate of barium sulphate. Rest other products formed are soluble in water except barium sulphate that precipitates out.

The mole ratio of hydrogen and oxygen gases liberated during electrolysis of water is 2:1 by volume.

In a decomposition reaction, a compound is broken down into its simpler forms. During electrolysis water is broken down into oxygen gas and hydrogen gas due to passing of electric current through it.

Sublimation of dry ice and evaporation of water both are endothermic processes. These processes pull heat (energy) from their surroundings to change from one state to another. Hence both these processes produce cooling rather than heating. Dry ice absorbs heat from the environment to convert into gas. This feature makes it a good refrigerant.

Processes of dilution of sulphuric acid and condensation of water vapours produce a large amount of heat. That is why these are exothermic processes.

Lead acetate can be used in place of lead nitrate because lead acetate is also water soluble like lead nitrate.

The chemical equation for this reaction is

When potassium iodide and lead acetate are mixed, a bright yellow precipitate of lead iodide is seen. Lead iodide is unusually soluble in hot water and forms crystals on cooling.

Helium or nitrogen both can be used for storage of fresh sample of an oil for a long time. These gases are called inert gases as they do not react with most elements including oxygen. Thus these gases create an inert environment for the oil and prevent its reaction with any element in the environment that prevent from getting it rancid.

In a decomposition reaction, a compound is broken down into two or more simpler substances. This reaction is a decomposition reaction as KClO₃ is broken down into KCl and O₂. It is also endothermic reaction because during decomposition of KCIO₃ heat is taken in for the formation of desired products.

During a chemical reaction the chemical composition of a substance changes. A chemical reaction produces a new substance with new properties. Energy is either released or absorbed during a chemical reaction. It is an irreversible reaction.

It is because, the standard state for hydrogen and oxygen is gas and for water is liquid at reaction temperature.

A combination reaction is one in which or more reactants combine to form a single product. In reaction (ii), MgO and H₂O combine to from Mg (OH)₂. In reaction (iii), Al and O₂ combine to form Al₂O₃. A combination reaction is also known as synthesis reaction.

Reaction (i) is decomposition reaction and reaction (iv) is displacement reaction.

(a)

This is an example of combination reaction. A combination reaction is one in which or more reactants combine to form a single product.

(b) NaOH (aq)+CH₃COOH()CH₃COONa(aq)+H₂O()

This is an example of neutralisation reaction between a strong base and weak acid. A neutralisation occurs between an acid and a base to form water and salt. This reaction releases H⁺ and OH^- ions. It is also a double displacement reaction. In such reaction cations and anions of both reactants exchange their places.

(c) C₂H₅OH(aq)+CH₃COOH()CH₃COO C₂H₅ (aq)+H₂O()

This is an example of neutralisation reaction as double displacement reaction.

A neutralisation occurs between an acid and a base to form water and salt. This reaction releases H⁺ and OH^- ions. It is also a double displacement reaction. In such reaction cations and anions of both reactants exchange their places.

(d) C₂H₄(g)+3O₂(g) 2CO₂(g) + 2H₂O(g)+Heat + Light

This is an example of oxidation reaction. The reactant burns in oxygen to produce carbon dioxide and water. It is also an exothermic reaction as it evolves a lot of heat. It is also known as oxidation reaction.

(a)

This is an example of displacement reaction. Displacement reaction is one in which more reactive component of a reactant displaces the comparatively lesser reactive component.

(b)

When magnesium is burned in air , then the magnesium reacts with nitrogen in the air and some magnesium nitride is also formed.

(c)

This is an example of displacement reaction. Displacement reaction is one in which more reactive component of a reactant displaces the comparatively lesser reactive component.

(d)

This is an example of combustion reaction. In such a reaction the reactant burns in oxygen to produce carbon dioxide and water. It is also an exothermic reaction as it evolves a lot of heat. It is also known as oxidation reaction.

(a) Pb(NO₃)₂ (aq)+2KI(aq) PbI₂(s)+2KNO₃(aq)

(b) Cu(s)+2AgNO₃ (aq) Cu(NO₃)₂ (aq)+ 2Ag(s)

(c) Zn(s) + H₂SO₄(aq) ZnSO₄(aq)+H₂(g)

(d) CaCO₃ (s) CaO(s)+ CO₂ (g)

(a) Decomposition of ferrous sulphate is an endothermic reaction because heat is absorbed from the environment during the reaction.

(b) Dilution of sulphuric acid is an exothermic reaction because heat is released into the environment during the reaction.

(c) Dissolution of sodium hydroxide in water is an exothermic reaction because heat is released into the environment during the reaction.in water.

(d) Dissolution of ammonium chloride in water is an endothermic reaction because heat is absorbed from the environment during the reaction.

(a) NH₃ is the reducing agent because it gets oxidised to NO by the removal of hydrogen and addition of oxygen. O₂ has been reduced to H₂O by the addition of hydrogen.

(b) H₂O is the reducing agent because of the addition of electronegative F to get oxidised to HOF. It reduces F₂ to HF by the addition of hydrogen.

(c) CO is a reducing agent. Here, CO has been oxidised to CO₂ by the addition of oxygen. Fe₂O₃ has been reduced to Fe by the removal of oxygen.

(d) H₂ is the reducing agent as it gets oxidised to H₂O by the addition of oxygen. O₂ has been reduced to H₂O by the addition of hydrogen.

Note: A substance that donates electrons in a redox reaction is called a reducing agent. It helps in reduction by oxidising itself in a chemical reaction. A reducing agent also losses hydrogen.

(a) Pb₃O₄ is the oxidising agent.
Here, in Pb₃O₄ the oxidation state of Pb is +6. And in PbCl₂, the oxidation state is +2. Since the oxidation state is decreased thus, Pb₃O₄ is the oxidant.

(b) O₂ is the oxidising agent.
Because O₂ is an element and the oxidation state of any element is 0. Thus, the oxidation state of oxygen is 0. Now, in MgO, the oxidation state of oxygen is -2.

Since, the oxidation state of oxygen is decreased from 0 to -2. Thus, O₂ is the oxidising agent.

(c) CuSO₄ is the oxidising agent.
The oxidation state of Cu in CuSO₄ is +2. On the reactants side, the oxidation state of Cu changes to 0, as it is in its elemental form. On the other hand, the oxidation state on Zn changes from 0 to +2. Therefore, Cu has been Reduced and Zn has been oxidised. Cu has gained electrons and hence, CuSO₄ is the Oxidising agent.

(d) V₂O₅ is the oxidising agent.
The oxidation state of Calcium goes from 0 (in its elemental state) to +2 on the products side. On the other hand, the oxidation state of V changes for +5 to 0. Therefore, we can see that V has been reduced (and has gained electrons ). Therefore, V₂O₅ is the oxidising agent.

(e) H₂ O is the oxidising agent.
The oxidation state of Oxygen on the reactants side is -2 whereas on the products side is -3. This gain of electrons leads us to believe that H₂ O is the oxidising agent in the above reaction.

(f) CuO is the oxidising agent.
The oxidising state of Cu has changed from +2 on the reactants side to 0 on the products side. It has been reduced and hence, CuO is the oxidising agent in this reaction.

(a) HCl + Na₂CO₃→ NaCl + NaHCO₃
NaHCO₃ + HCl → NaCl + H₂O + CO₂

However, this reaction can be summed up as.

2HCl + Na₂CO₃→ 2NaCl + H₂O + CO₂

(b) NaHCO₃ + HCl → NaCl + H₂O + CO₂

(c) 2CuSO₄ + 4KI → Cu₂I₂ + I₂ + 2K₂SO₄

When a solution of Potassium Chloride is mixed with Silver Nitrate solution, we get a precipitate of Silver Chloride. Chlorides are usually soluble but one important exception to this rule is Silver Chloride.
The chemical reaction is KCl + AgNO3 → KNO3 (aq)+ AgCl(s)

This type of chemical reaction is called a double displacement reaction.

Note: A double displacement reaction is a type of chemical reaction in which two chemical compounds react and the positive ions and the negative ions of the two compounds switch places.

The thermal decomposition or thermolysis of Ferrous Sulphate results in the formation of Iron (III) Oxide and Sulphur dioxide and sulphur trioxide.

2FeSO₄ + Heat → Fe₂O₃ + SO₂ + SO₃

Sulphur dioxide is the gas that has a distinctive odour of burning sulphur. The type of reaction is Thermal decomposition.

Fireflies have a luminous bag at their lower abdomen region that contain a protein called Luciferin. This luminous bag also secretes a certain enzyme called, Luciferase. This enzyme on reaction with Luciferin, emits light in the presence of Magnesium and oxygen. Therefore, fireflies glow in the night.

When the grapes are attached to the plants, they are still considered as living. They have their immune system intact, due to which they cannot undergo fermentation. After the grapes are plucked, microbes are made to attack them, in the absence of air, to form alcohol. This process is called fermentation and it is a chemical process.

(a) This is a Physical change.
Petrol just changes its state from the liquid form to the gaseous form because of the presence of energy in the form of heat. A change of state is a reversible reaction and hence, this is a Physical change.

(b) This is a Chemical change.
Burning of Liquefied Petroleum Gas releases energy and heat as its by products which cannot be converted back to the original reactant. The chemical composition of LPG has changed. Hence, it is a chemical reaction.

(c) This is a Physical change.
The temperature affects the metal temporarily. On reversing the temperature change, the metal goes back to its previous state. Hence, it is a physical state.

(d) This is a Chemical change.
The process of curdling of milk takes place due to bacteria. It is an irreversible reaction in which the composition of milk is changed and hence, a chemical change.

(e) This is a Physical change.
Energy in the form of heat has prompted solid ammonium chloride to evaporate. On reversing the temperature, it goes back to its previous solid state. Hence, this is a physical change.
A physical change in a substance doesn't change what the substance is. In a chemical change where there is a chemical reaction, a new substance is formed and energy is either given off or absorbed.

(a) Silver is less reactive than Hydrogen and hence, cannot displace Hydrogen in a chemical reaction. ( Silver lies below Hydrogen in the activity series. )

(b) This reaction is exothermic in nature and hence the temperature of the mixture rises.

(c) Sodium is a highly reactive metal and combines with moisture to form Hydrogen gas. This reaction is highly exothermic and quick which leads to Sodium catching fire.

(d) The bubbles are observed due to the formation of Hydrogen gas. The reaction gradually becomes slow due to the formation of a coating of Lead chloride on Lead, which prevents further reaction.
Pb + 2HCl → PbCl₂ + H₂

The element is Calcium and the substance X is Calcium Oxide (CaO). Calcium oxide is used extensively in the cement industry and Calcium is present in our bones in the form of Calcium Phosphate.
CaO + H₂O → Ca(OH)₂
Calcium oxide dissolves in water to form a basic solution which turns red litmus blue.

(a) Pb(CH₃COO)₂ + 2HCl → PbCl₂ + CH₃COOH
This is a Double Displacement reaction.

(b) 2Na + 2C₂H₅OH → 2C₂H₅ONa + H₂
This is a Displacement reaction.

(c) Fe₂O₃ + 3CO → 2Fe + 3CO₂
This is a redox reaction.

(d) 2 H₂S + O₂→ 2 S + 2 H₂O
This is a replacement reaction.

We store Silver Chloride in dark coloured bottles to prevent the breakdown of the salt into its elements. ( Photo decomposition )
When Silver Chloride is exposed to light, then it breaks down into elemental silver and chlorine gas.
2AgCl→2Ag+Cl2
In order to prevent the contamination of Silver chloride with Silver, we store them in dark coloured bottles which block the light.

(a) Mg(s) + Cl₂(g) → MgCl₂(s)
This type of reaction is called a combination reaction or a synthesis reaction.

(b) 2HgO(s) ⎯⎯⎯⎯ (Heat) →2 Hg(l) + O₂(g)
This is an example of thermal decomposition reaction.

(c) 2Na(s) + S(s) ⎯⎯⎯⎯ (Fuse) → Na₂S(s)
This is an example of a Combination reaction.

(d) TiCl₄ (l) + 2Mg(s) → Ti(s) + 2MgCl₂ (s)
This reaction falls under the category of Displacement reactions.
(e) CaO(s) + SiO₂(s) → CaSiO₃(s)
This is a synthesis reaction.

(f) 2H₂O₂ (l) ⎯⎯⎯⎯ UV → 2H₂O(l) + O₂ (g)
This is a decomposition reaction.

(a) X is Magnesium Oxide - MgO and Y is Magnesium nitride - Mg3N2.

(b) MgO + H₂O → Mg(OH)₂

Zinc reacts with hydrochloric acid to liberate Hydrogen gas as it is more reactive than Hydrogen and hence, displace it. Copper is less reactive when compared to Hydrogen and hence cannot displace it.
The same can be understood by looking at the activity series of metals. The activity series of metals refers to the list of metals ranked in the decreasing order of their reactivity in order to displace Hydrogen gas from water or acid solutions. It helps us in predicting which metal will displace which metal in Acidic solutions.
Zinc lies above Hydrogen whereas Copper lies below Hydrogen.

(a) When silver articles are kept in the open, then a layer of silver oxide is formed on the articles due to the reaction of silver with the oxygen present in the air. Silver oxide is black in colour. Finally, after sufficient time, it will form silver sulphide after coming in contact with oxygen and Hydrogen sulphide present in the atmospheric air. Silver sulphide is also black in colour.
4Ag + O₂ + 2H₂S → 2Ag₂S + 2H₂O
This process is termed as tarnishing of silver or corrosion.
As toothpastes consist of several abrasive substances (like, Aluminium hydroxide), this layer of silver sulphide can be removed by rubbing the article with toothpaste.
3Ag₂S + 2Al →6Ag + Al₂S₃
This reaction leaves the silver metal all shiny. This particular cleaning process can be improved by using hot water and sodium bicarbonate.

(b) The black substance that is formed over the silver metal on exposure to atmospheric air is silver sulphide. The chemical formula of silver sulphide is Ag₂S.

(a) 2Cu(NO₃)₂ → 2Cu + 4NO₂ + O₂

(b) The brown gas 'X' has to be nitrogen dioxide.

(c) This type of reaction falls under the category of Decomposition reactions.

(d) The pH of the aqueous solution of gas X would be less than 7.
The oxides of non metals are generally acidic in nature. Therefore, their aqueous solutions would exhibit acidic nature.

(a) When carbon dioxide is made to run through lime water, the gas turns the solution milky. This is unique only to carbon dioxide.
The reaction produces Calcium carbonate which is a white precipitate, thus giving rise to the milky appearance. Lime water is nothing but a solution of Calcium hydroxide.
CO₂ + H₂O → H₂CO₃

H₂CO₃ + Ca(OH)₂→ CaCO₃ + 2H₂O

(b) The unique identifying feature of sulphur dioxide is its odour.It smells like burning of sulphur. Also, Sulphur dioxide turns moist litmus paper from blue to red.

When SO₂ is passed through acidified potassium permanganate solution (pink/purple in colour) , the solution turns green due to the formation of Chromium sulphate. It is a redox reaction where Potassium dichromate acts as an oxidising agent and sulphur dioxide acts as a reducing agent.
Cr₂O₇^2- (aq) + 2H+ (aq) + 3SO₂ (g) → 2Cr³⁺ (aq) + H₂O (l) + 3SO₄^2- (aq)

(c) When any fire element - a matchstick or a candle - is brought into an oxygen atmosphere, it starts burning more brightly.
Oxygen supports combustion.

(d) When a fire element - a matchstick or a candle - is brought into a Hydrogen atmosphere, it burns with a pop sound. This is unique to Hydrogen.

(a) When zinc reacts with a solution of copper sulphate, we obtain Zinc Sulphate and Copper.
Zn + CuSO₄→ Cu + ZnSO₄
Zinc lies above Copper in the activity series.
The activity series of metals refers to the list of metals ranked in the decreasing order of their reactivity in order to displace Hydrogen gas from water or acid solutions. It helps us in predicting which metal will displace which metal in Acidic solutions.

(b) Aluminium chloride is formed and Hydrogen gas is evolved when Aluminium is made to react with dilute Hydrochloric acid.
2Al + 6HCl → 2AlCl₃ + 3H₂

(c) Silver being less reactive than Copper, we see no reaction taking place when silver metal is added to copper sulphate solution.

In the case of the reaction of zinc granules with Sulphuric acid or Hydrochloric acid, a salt is formed - Zinc sulphate or Zinc Chloride respectively - and the Hydrogen gas is released.
Zn + H₂SO₄→ ZnSO₄ + H₂↑
Zn + 2HCl → ZnCl₂ + H₂↑
Similarly, Hydrogen gas and Zinc Nitrate is formed when the Zinc granules are treated with HNO₃ .
Zn + 2HNO₃ → Zn(NO₃ )₂ + H₂↑

In the case of NaCl, Zinc being less reactive than Sodium, no reaction occurs.
When zinc is made to react with NaOH, it forms Sodium zincate and hydrogen gas.
Zn + 2H₂O + 2NaOH ⇨ Na₂Zn(OH)₄ + H₂↑

(a) When Barium chloride solution is added to an aqueous solution of sodium sulphite, we obtain a white precipitate of Barium Sulphite along with Sodium Chloride.
BaCl₂ + Na₂SO₃→ BaSO₃ + 2NaCl

(b) This precipitation reaction can also be categorised as a double displacement reaction.

A double displacement reaction is a type of chemical reaction in which two chemical compounds react and the positive ions(cations) and the negative ions(anions) of the two compounds switch places.

(c) The white precipitate of sodium chloride reacts with hydrochloric acid to form Barium Chloride and water and release sulphur dioxide. Barium chloride is a soluble salt which is why the white precipitate of Barium Sulphite disappears.

The chemical reaction for the above process is BaSO₃ + HCl ⇨ BaCl₃ + SO₂ + H₂O.

The solutions of dilute HCl, ZnCl₂ and H₂O can be kept in the container that is made of Copper. The Aluminium container can hold dilute HNO₃ and H₂O.
We can store a solution in a container with which the solution will not react.
When compared to Aluminium, copper is a less reactive metal. Also, Copper is placed below Hydrogen in the activity series. Therefore, it will not react with dilute HCl or H₂O; due to which we can store them in the copper container. However, dilute HNO₃ being a strong oxidising agent, will react with Copper and hence cannot be stored in a copper container. Copper, being less reactive than Zinc, will not react with ZnCl2.
Aluminium, on the other hand, is highly reactive and can react with the said solutions. However when dilute HNO₃ comes in contact with Aluminium, a protective layer of Aluminium Oxide is formed which prevents further reaction. Therefore, dilute HNO₃ can be stored in an Aluminium container.
Also, Aluminium does not react with water. So, water can be stored in the Aluminium container as well.

Note:
The activity series of metals refers to the list of metals ranked in the decreasing order of their reactivity in order to displace Hydrogen gas from water or acid solutions. It helps us in predicting which metal will displace which metal in Acidic solutions.