Chemical Reactions and EquationsClass 10 Chemistry NCERT Solutions

Let's analyze the reaction: $2PbO(s) + C(s) \rightarrow 2Pb(s) + CO_2(g)$.

The incorrect statements are (a) and (b). Therefore, the correct option is (i) (a) and (b).

In the given reaction, aluminium ($Al$) is displacing iron ($Fe$) from its compound, iron(III) oxide ($Fe_2O_3$). A more reactive element (aluminium) is displacing a less reactive element (iron) from its oxide. This type of reaction is known as a displacement reaction. Therefore, the correct option is (d) displacement reaction.

When dilute hydrochloric acid ($HCl$) is added to iron fillings ($Fe$), a displacement reaction occurs. Iron, being more reactive than hydrogen, displaces hydrogen from the acid to produce hydrogen gas ($H_2$) and iron(II) chloride ($FeCl_2$). The reaction is: $Fe(s) + 2HCl(aq) \rightarrow FeCl_2(aq) + H_2(g)$. Therefore, the correct option is (a) Hydrogen gas and iron chloride are produced.

A balanced chemical equation is a chemical equation in which the number of atoms of each element is equal on both the reactant side and the product side. The mass of the reactants is equal to the mass of the products.

Chemical equations should be balanced to satisfy the Law of Conservation of Mass. This law states that mass can neither be created nor destroyed in a chemical reaction. For a chemical equation to obey this law, the total mass of the elements present in the reactants must be equal to the total mass of the elements present in the products. This is only possible if the number of atoms of each element remains the same before and after the reaction.

(a) Hydrogen ($H_2$) + Nitrogen ($N_2$) → Ammonia ($NH_3$) Balanced equation: $3H_2(g) + N_2(g) \rightarrow 2NH_3(g)$

(b) Hydrogen sulphide ($H_2S$) + Oxygen ($O_2$) → Water ($H_2O$) + Sulphur dioxide ($SO_2$) Balanced equation: $2H_2S(g) + 3O_2(g) \rightarrow 2H_2O(l) + 2SO_2(g)$

(c) Barium chloride ($BaCl_2$) + Aluminium sulphate ($Al_2(SO_4)_3$) → Aluminium chloride ($AlCl_3$) + Barium sulphate ($BaSO_4$) Balanced equation: $3BaCl_2(aq) + Al_2(SO_4)_3(aq) \rightarrow 2AlCl_3(aq) + 3BaSO_4(s)$

(d) Potassium ($K$) + Water ($H_2O$) → Potassium hydroxide ($KOH$) + Hydrogen ($H_2$) Balanced equation: $2K(s) + 2H_2O(l) \rightarrow 2KOH(aq) + H_2(g)$

(a) The balanced equation is: $2HNO_3 + Ca(OH)_2 \rightarrow Ca(NO_3)_2 + 2H_2O$

(b) The balanced equation is: $2NaOH + H_2SO_4 \rightarrow Na_2SO_4 + 2H_2O$

(c) The equation is already balanced: $NaCl + AgNO_3 \rightarrow AgCl + NaNO_3$

(d) The balanced equation is: $BaCl_2 + H_2SO_4 \rightarrow BaSO_4 + 2HCl$

(a) $Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O$

(b) $Zn + 2AgNO_3 \rightarrow Zn(NO_3)_2 + 2Ag$

(c) $2Al + 3CuCl_2 \rightarrow 2AlCl_3 + 3Cu$

(d) $BaCl_2 + K_2SO_4 \rightarrow BaSO_4 + 2KCl$

(a) Equation: $2KBr(aq) + BaI_2(aq) \rightarrow 2KI(aq) + BaBr_2(s)$ Type of reaction: Double displacement reaction (and also a precipitation reaction).

(b) Equation: $ZnCO_3(s) \rightarrow ZnO(s) + CO_2(g)$ Type of reaction: Decomposition reaction (specifically, thermal decomposition).

(c) Equation: $H_2(g) + Cl_2(g) \rightarrow 2HCl(g)$ Type of reaction: Combination reaction.

(d) Equation: $Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)$ Type of reaction: Displacement reaction.

Exothermic Reactions: These are chemical reactions in which heat is released along with the formation of products. The temperature of the surroundings increases. The general form is: Reactants → Products + Heat.

Endothermic Reactions: These are chemical reactions in which energy (in the form of heat, light, or electricity) is absorbed from the surroundings. The temperature of the surroundings decreases. The general form is: Reactants + Heat → Products.

Respiration is considered an exothermic reaction because energy is released during this process. We obtain energy from the food we eat. During digestion, complex carbohydrates are broken down into simpler substances like glucose. This glucose enters the cells of our body and combines with oxygen (from breathing). This reaction breaks down the glucose into carbon dioxide and water, releasing a significant amount of energy which our body uses for various life processes.

The chemical equation for respiration is: $\underset{\text{(Glucose)}}{C_6H_{12}O_6(aq)} + 6O_2(aq) \rightarrow 6CO_2(aq) + 6H_2O(l) + \text{Energy}$ Since energy is released as a product, respiration is an exothermic process.

Decomposition reactions are called the opposite of combination reactions because of their fundamental nature.

As you can see, the processes are reversed. One involves the synthesis of a single compound from multiple substances, while the other involves the breakdown of a single compound into multiple substances.

Here is one equation for each type of decomposition reaction:

The main difference between displacement and double displacement reactions lies in the number of elements displaced and the nature of the reactants.

The recovery of silver from silver nitrate solution using copper metal is a displacement reaction. Copper is more reactive than silver, so it displaces silver from the silver nitrate solution. Copper goes into the solution as copper(II) nitrate, and solid silver metal is deposited.

The balanced chemical equation for the reaction is: $Cu(s) + 2AgNO_3(aq) \rightarrow Cu(NO_3)_2(aq) + 2Ag(s)$

A precipitation reaction is a type of chemical reaction in which two soluble salts in aqueous solution combine, and one of the products is an insoluble solid that separates from the solution. This insoluble solid is called a precipitate.

These reactions are a common type of double displacement reaction.

Example 1: Formation of Barium Sulphate When a solution of sodium sulphate is mixed with a solution of barium chloride, a white precipitate of barium sulphate is formed. $Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)$ Here, barium sulphate ($BaSO_4$) is the white precipitate.

Example 2: Formation of Lead Iodide When a solution of lead(II) nitrate is mixed with a solution of potassium iodide, a yellow precipitate of lead(II) iodide is formed. $Pb(NO_3)_2(aq) + 2KI(aq) \rightarrow PbI_2(s) + 2KNO_3(aq)$ Here, lead(II) iodide ($PbI_2$) is the yellow precipitate.

(a) Oxidation Oxidation is a chemical process that involves the gain of oxygen by a substance.

(b) Reduction Reduction is a chemical process that involves the loss of oxygen from a substance.

The shiny brown coloured element 'X' is Copper (Cu).

When copper is heated in air, it reacts with oxygen to form a black coloured compound, which is Copper(II) oxide (CuO).

The chemical reaction is: $2Cu(s) + O_2(g) \xrightarrow{\text{Heat}} 2CuO(s)$

We apply paint on iron articles to prevent them from rusting. Rusting is a corrosion process where iron reacts with oxygen and moisture from the atmosphere to form a reddish-brown flaky substance (hydrated iron(III) oxide). The layer of paint acts as a protective barrier, cutting off the contact between the surface of the iron article and the air (oxygen and moisture). This prevents the chemical reaction of rusting from occurring and thus protects the article from damage.

Oil and fat containing food items are flushed with nitrogen to prevent rancidity. Rancidity is the process where fats and oils in food get oxidised when exposed to air (oxygen). This oxidation leads to a change in their smell and taste, making them unpleasant to consume.

Nitrogen is a relatively inert (unreactive) gas. By flushing the food packages with nitrogen, the oxygen is displaced. In the absence of oxygen, the oxidation of fats and oils is slowed down significantly, thus increasing the shelf-life of the food items like potato chips.

(a) Corrosion: Corrosion is the gradual deterioration of a metal by the action of air, moisture, or chemicals (like acids) in its surroundings. The metal is converted into an undesirable compound such as an oxide, sulphide, or carbonate.

(b) Rancidity: Rancidity is the process of slow oxidation of oils and fats present in food materials, resulting in a change in their smell and taste, making them unpalatable. This process is accelerated by exposure to light and air.

A magnesium ribbon should be cleaned before burning in air to remove the protective layer of magnesium oxide ($MgO$) that forms on its surface due to the reaction with oxygen in the atmosphere. This layer is quite stable and can prevent or slow down the burning of the magnesium ribbon. Cleaning it with sandpaper removes this layer, allowing the underlying pure magnesium to react readily with oxygen upon heating.

Activity 1.7 describes the electrolysis of water. The chemical reaction for the electrolysis of water is: $2H_2O(l) \xrightarrow{\text{Electrolysis}} 2H_2(g) + O_2(g)$ From the balanced equation, two molecules of water decompose to produce two molecules of hydrogen gas and one molecule of oxygen gas. Therefore, the mole ratio of hydrogen gas to oxygen gas produced is 2:1. Since the volume of a gas is directly proportional to the number of moles (at the same temperature and pressure), the volume of hydrogen gas collected is double the volume of oxygen gas collected.

The gas collected in double the amount is Hydrogen ($H_2$).

The colour of copper sulphate solution changes because iron is more reactive than copper. When an iron nail is dipped in a blue copper sulphate ($CuSO_4$) solution, iron displaces copper from the solution, forming iron(II) sulphate ($FeSO_4$), which is pale green in colour, and depositing solid copper metal on the iron nail. This is a displacement reaction.

The chemical equation for the reaction is: $\underset{\text{(Blue)}}{Fe(s) + CuSO_4(aq)} \rightarrow \underset{\text{(Pale Green)}}{FeSO_4(aq)} + Cu(s)$ Due to the formation of pale green iron(II) sulphate, the blue colour of the copper sulphate solution fades and changes.

An example of a double displacement reaction, other than the reaction between sodium sulphate and barium chloride (Activity 1.10), is the reaction between lead(II) nitrate and potassium iodide (from Activity 1.2). In this reaction, the two compounds exchange ions to form a yellow precipitate of lead(II) iodide and a solution of potassium nitrate.

The balanced chemical equation is: $Pb(NO_3)_2(aq) + 2KI(aq) \rightarrow PbI_2(s) + 2KNO_3(aq)$