Chemical Reactions And Equations

The substances which react are called Reactants of a chemical reaction. The substances which are formed are called Products of a chemical reaction.

“Mass can neither be created nor be destroyed in a chemical reaction”- This is known as the law of conservation of mass.

Skeletal Equation: KBr(aq) + BaI₂(aq) → KI(aq) + BaBr₂(s)

Balanced Equation: 2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(s)

Verification:

There are 2 atoms of K present in LHS and 2 atoms in RHS. Similarly, atoms of I, Br, Ba are also the same in both sides, This is what a balanced equation means.

(i) Mg(OH)₂ + 2HCl → MgCl₂ +2H₂O

(ii) N₂ +3H₂→ 2NH₃

(iii) P₄+5O₂ → 2P₂O₅

Reason: Number of atoms on both sides of the equation should be equal for balanced equations.

Equation B is Balanced.

Reason: In equation, A number of Cl atoms in LHS is 2 But in RHS number of atoms is 4. Hence it is not balanced. But in equation B number of each atom in LHS is equal to number of that atom in RHS.

These symbols refer to the phase of reactants and products which take part in the reaction. (aq) represents aqueous phase which means that this substance is in dissolved in water. (s) represents solid phase.

(i) CH₄(g) + 2O₂ (g) → CO₂ (g) + 2H₂O (g): This is an example of Exothermic reaction because it leads to release of HEAT as can be seen water is given in gaseous form.

(ii) 2NaCI(l) 2Na (s) + Cl₂ (g) : This is the example of Decomposition reaction of common salt using electricity. This reaction is also called Electrolysis of Bryn solution.

(iii) MnO₂ + 4HCI → MnCl₂ + 2H₂O+ Cl₂

This reaction is the example of a redox reaction. Because MnO₂ has donated its oxygen atoms and hence got reduced. Also, Chlorine is converted into Cl^- to form Cl₂ and hence get oxidised. Since both oxidation and reduction has taken place. This is a redox reaction.

When Hydrogen gas is added to copper oxide reduction of copper oxide to metallic copper takes Place. Following is the Reaction:

CuO + H₂ → Cu + H₂O : Reduction of Copper oxide

All reactions which are Exothermic are accompanied by evolution of heat. One such example is burning of methane. Reaction is as Follows:

CH₄(g) + 2O₂ (g) → CO₂ (g)+ 2H₂O (g)

i) Vegetable matter is changed into compost by microorganisms using Decomposition Reactions. The complex organic material of which Vegetable matter is made up of is broken down into simpler molecules during this process.

ii) Burning of natural gas leads to evolution of heat hence it is an example of an exothermic reaction. In this reaction Natural gas is also oxidized into CO₂ Hence it is also an example of redox reaction.

Following four factors are responsible for corrosion of Iron:

i) Moisture: Moisture reacts with iron to form rust(Mixture of Iron oxide and water)

ii) Oxygen: Oxygen reacts with iron to form Iron oxide which on combining with moisture becomes rust

iii) Poor maintenance: Poor maintenance leads to continuous exposure of iron to moisture and air which causes rust.

iv) Poorly built iron substances: If other substances like carbon, are not mixed while making iron substances in proper proportions, It gets rusted.

In the case of Aluminium, Corrosion is not harmful. Because the new layer of corroded aluminium itself act as a prevention for further corrosion.

Breaking of bonds is called as Bond Dissociation [Dissociate means to break]. Bond Dissociation is an endothermic reaction which means that energy is absorbed in breaking of a bond.

Making of a bond is called as Bond Formation. This reaction is exothermic in nature which means that energy is given off or liberated while the formation of bonds.

Carbon Dioxide when dissolved in water leads to the formation of a weak acid which is called carbonic acid.

Reaction is as follows:

CO₂ + H₂O → H₂CO₃

Double Displacement Reaction is a type of chemical reaction in which anions and cations of the reacting compounds exchange their positions to form different compounds.

The precipitation reaction is a type of reaction in which one of the reactants is solvent and one of the products formed is insoluble in the solvent.

So the example of double displacement and precipitation reaction is as follows:

When an electric current is passed through fused lead bromide, lead bromide being an ionic compound gets dissociated into lead and bromide ions.

The reaction is as follows:

Whenever a metal is exposed to air and water for a long time it gets surrounded by a layer due to any type of chemical reactions and gets corroded. This process is called corrosion. So when a silver article gets a black coating, the phenomenon is called as corrosion.

The reaction based on reducing property of water is as follows:

F + H₂O → HF + O₂

In the above reaction fluorine reacts with water to form hydrogen fluoride and oxygen gas is liberated.

The reaction based on oxidizing property of water is as follows:

Al + H₂O → Al₂O₃ + H₂

In the above reaction aluminum reacts with water to aluminum oxide and hydrogen gas is liberated.

(i) This reaction is a type of combination reaction in which reactants combine to form a single product.

(ii) This reaction is a type of displacement type of reaction in which a reactive reactant displaces the less reactive element from the compound.

In the above reaction reactant A displaces B from the compound BC to form a new compound AC.

(iii) This reaction is a decomposition reaction in which compound A gets dissociated or broken down to form simple products B and C.

(iv) Double Displacement Reaction is a type of chemical reaction in which anions and cations of the reacting compounds exchange their positions to form different compounds.

This reaction is a double displacement type of reaction.

Hydrogen Peroxide or H₂O₂ is a highly reactive compound which gets dissociated in the presence of light to form water and oxygen.

To prevent this dissociation reaction, H₂O₂ is stored in coloured bottles to prevent light from entering the bottles.

From the description, it can be concluded that the element of the coin is copper.

On being heated copper gets oxidized due to the presence of oxygen in air which results in the formation of copper oxide (precisely cupric oxide) which is black in color.

The reaction is as follows:

2Cu + O₂→ 2CuO

Taj Mahal is an example of corrosion of marble. The discoloration of its marble is due to corrosion.

Air primarily consists of various pollutant gases like carbon dioxide, nitrogen dioxide, Sulphur dioxide etc which mix with moisture in air to form acids such as carbonic acid, nitric acid and sulphuric acid. This acid comes down as acid rain and causes corrosion of marble statues and monuments. Marble which is composed of calcium carbonate reacts with the different acids which is given as follow:-

CO₂ + H₂O + CaCO₃→ Ca (HCO₃).

CaCO₃ + H₂SO₄→ CaSO₄ + CO + H₂O.

The compound formed are water soluble and are easily washed away. Thus blackening of marble surface is the corrosion of marble surface due to the formation of side products of calcium.

The difference between skeletal equation and balanced chemical equation is as follows:

(i) To get the missing component we have to think that in product we got sulphate salt (BaSO₄) so in reactant there must be some other sulphate must be present so that double displacement reaction could happen.

So, the complete reaction is:

BaCl₂ + ‘Na₂So₄’ → NaCl + BaSO₄↓

(ii) In this reaction, we can clearly see displacement reaction is taking place in which copper displaces silver nitrate and forms copper nitrate and silver.

So, the complete reaction is: Cu + AgNO₃→ Cu(NO₃)₂ + 'Ag'

(iii) We can guess after seeing this reaction that it must be decomposition reaction as a single reactant breaks into 2 products bus there is only one thing missing i.e. heat is not written on the arrow now it is thermal decomposition reaction.

So, the complete reaction is:

The substance which gains oxygen or loses hydrogen during the reaction, it is said to be oxidised and the substance which loses oxygen or gains hydrogen during the reaction, it is said to be reduced.

i) CuO + H₂→ Cu + H₂O

In the following reaction copper oxide loses oxygen and forms copper so copper is reduced and hydrogen gains oxygen and forms water so hydrogen is oxidised.

ii) MnO₂ +4HCl → MnCl₂ + Cl₂+ 2H₂O

In the following reaction HCl is oxidised to Cl₂ as it loses hydrogen and MnO₂ is reduced to MnCl₂ as it loses oxygen.

The reaction mentioned in the question is as follows:

The oxidising agent is the substance which oxidises other substance and gets reduced.

Reducing agent is the substance which reduces other substance and gets oxidised.

i) In the reaction zinc oxide (ZnO) is oxidising agent as it oxidises the carbon to carbon monoxide (CO) and zinc oxide gets reduced to zinc.

ii) Carbon is the reducing agent as it reduces zinc oxide to zinc and gets oxidised to carbon monoxide.

For a chemical reaction to take place some chemical change must occur. So, here is the activity to demonstrate a chemical reaction:

Step-1: Take a magnesium ribbon around 2cm long by rubbing it with sand paper.

Step-2: Hold it with tongs.

Step-3: Burn the ribbon and collect the ash in a watch glass which is white in pair.

Precaution must be taken while performing the experiment to keep the burning ribbon far away from eye. The experiment should be under some expert supervision.

In the activity we observed that magnesium ribbon burns with a white flames and changes into white powder. This powder is magnesium oxide.

There was a change in state, change in colour, change in temperature all of these are chemical changes and this proves that chemical reaction takes places here in which magnesium was converted into magnesium dioxide.

Here we will first find out the gas. It is stated that this gas turns lime water milky so it is sure that the gas is carbon dioxide (CO₂).

When CO₂ reacts with water then carbonic aid is formed (H₂CO₃), which turns blue litmus red.

CO₂ + H₂O → H₂CO₃

When CO₂ is passed through lime water (Ca(OH)₂) it turns it milky and on excess passing of carbon dioxide it forms calcium bicarbonate which forms a colourless solution, so the reaction is:

Ca(OH)₂ + CO₂→ CaCO₃ + H₂O

On passing in an excess amount

CaCO₃ + H₂O + CO₂→ Ca(HCO₃)₂

For the identification of substance X, we know that in product carbon dioxide is there so there must be something of carbon must be there in reactant and X is water-insoluble so it should be calcium carbonate that is insoluble. So the reaction is:

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

X= CaCO₃

Slaked lime is the common name used for calcium hydroxide Ca(OH)_{2 .} It is formed by the reaction of quick lime CaO with water.

(i) The chemical name of compound formed is calcium carbonate.

Explanation: The calcium hydroxide used for white washing the walls reacts with the carbon dioxide (CO₂) present in the air to form Calcium carbonate (CaCO₃) which gives shiny appearance to the walls.

(ii) The chemical reaction occurring during the conversion of slaked lime (calcium hydroxide) to calcium carbonate which gives the shiny appearance to the walls is as follows:

(iii) The Type of reaction is exothermic.

Explanation: A reaction which involves the release of heat when the product formation takes place is known as an exothermic reaction. The reaction of calcium hydroxide (base) with CO₂ (acidic non-metal oxide) is a neutralization reaction in which heat is evolved. Thus it is an exothermic reaction.

When the identity and nature of the product are different from that of the reactants then this change is known as chemical change and whenever a chemical change takes place, we can say that a chemical reaction has occurred. In the first three options, only the physical state of the water, nitrogen and petrol changes thus they are physical changes. But when magnesium wire is heated in the presence of air, formation of a new substance (MgO) takes place. The chemical reaction involved is as follows:

According to law of conservation of mass, the mass of the elements on the product side should be equal to the mass of the elements on reactant side. In simple words, the number of atoms of each element should be same on the both sides. This can be achieved by balancing a chemical reaction.

Unbalanced chemical equation for combustion of ethane:

The balanced chemical equation:

From the balanced chemical equation, we can observe that the coefficient of O₂ is .

The salt (X) is undergoing thermal decomposition reaction as it breaks to give multiple products on heating. Release of brown fumed Nitrogen dioxide (NO₂) gas indicates that the salt is a nitrate salt. Moreover formation of a yellow lead oxide (PbO) residue indicates that the salt is a lead salt. Thus the salt is Lead Nitrate. The thermal decomposition reaction involved is:

2Pb(NO₃)₂→ 2PbO + 4NO₂ + O₂

The only reddish brown colour metal among the given options is copper. When copper powder is heated in a china dish it gets oxidized and turns black due to the formation of copper oxide (CuO). The reaction involved is:

2CuO + O₂→ 2CuO

When H₂ gas is passed over CuO, it gets reduced to Cu and thus regains its original colour.

CuO + H₂→ Cu + H₂O

Dilute sulphuric acid: dil. H₂SO₄

Iron sulphide: FeS

Hydrogen sulphide: H₂S

Ferrous sulphate: FeSO₄

The reaction involved can be written as:

FeS + H₂SO₄→ FeSO₄ + H₂S

We can see that there is the exchange of SO₄^2- and S^2- ions in the reactants. Thus it is a double displacement reaction.

Since the reaction involves the breaking down of a single reactant molecule into multiple products, it is a decomposition reaction. Moreover it involves the absorption of heat, so it is also an endothermic reaction.

Among the given metals, only aluminium has a stable +3 oxidation state. Aluminium reacts with acid to produce a metal salt and H₂ gas. For example: Al reacts with HCl to give AlCl₃ and H₂ gas.

Reaction:

When silver chloride is exposed to sunlight it undergoes decomposition and turns grey due to the formation of silver metal which provides the grey colour. Although Cl₂ gas is also evolved but, it is not responsible for grey colour.

Reaction involved is:

2AgCl → 2Ag + Cl₂