Class X CBSE Chemistry Q&A

Cheatsheet Content

### Carbon and its Compounds #### 1. What are hydrocarbons? Give examples. **Answer:** Hydrocarbons are organic compounds composed solely of hydrogen and carbon atoms. - **Saturated Hydrocarbons:** Contain only single bonds between carbon atoms (e.g., Alkanes like Methane ($\text{CH}_4$), Ethane ($\text{C}_2\text{H}_6$)). - **Unsaturated Hydrocarbons:** Contain double or triple bonds between carbon atoms. - **Alkenes:** Contain at least one carbon-carbon double bond (e.g., Ethene ($\text{C}_2\text{H}_4$)). - **Alkynes:** Contain at least one carbon-carbon triple bond (e.g., Ethyne ($\text{C}_2\text{H}_2$)). #### 2. Explain the terms 'catenation' and 'tetravalency' with respect to carbon. **Answer:** - **Catenation:** The unique property of carbon atoms to form bonds with other carbon atoms, forming long chains, branched chains, and rings. This leads to a large number of carbon compounds. - **Tetravalency:** Carbon has a valency of four, meaning it can form four covalent bonds with other atoms (carbon, hydrogen, oxygen, nitrogen, etc.). This allows carbon to form a wide variety of compounds. #### 3. What are functional groups? Give examples. **Answer:** Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. - **Alcohol:** -OH (e.g., Ethanol, $\text{CH}_3\text{CH}_2\text{OH}$) - **Aldehyde:** -CHO (e.g., Ethanal, $\text{CH}_3\text{CHO}$) - **Ketone:** >C=O (e.g., Propanone, $\text{CH}_3\text{COCH}_3$) - **Carboxylic Acid:** -COOH (e.g., Ethanoic Acid, $\text{CH}_3\text{COOH}$) - **Haloalkane:** -X (where X = F, Cl, Br, I) (e.g., Chloromethane, $\text{CH}_3\text{Cl}$) #### 4. Draw the electron dot structure for methane ($\text{CH}_4$). **Answer:** ``` H | H:C:H | H ``` (Each ':' represents a shared pair of electrons, forming a covalent bond.) #### 5. Why does carbon form covalent compounds? **Answer:** Carbon has 4 valence electrons. - It cannot gain 4 electrons to form $\text{C}^{4-}$ because it would require a large amount of energy to overcome the repulsion from the existing 6 electrons in the nucleus. - It cannot lose 4 electrons to form $\text{C}^{4+}$ because it would require a large amount of energy to remove 4 electrons, leaving a small nucleus with a large positive charge, which would hold the remaining electrons strongly. Therefore, carbon shares its valence electrons with other atoms to form covalent bonds, achieving a stable octet configuration. ### Chemical Properties of Carbon Compounds #### 1. What is a combustion reaction? Give an example. **Answer:** Combustion is a chemical process in which a substance reacts rapidly with oxygen, usually producing heat and light. - **Example:** Combustion of methane $$\text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(g) + \text{Heat} + \text{Light}$$ #### 2. Differentiate between saturated and unsaturated hydrocarbons based on their reactions. **Answer:** | Feature | Saturated Hydrocarbons (Alkanes) | Unsaturated Hydrocarbons (Alkenes, Alkynes) | |-------------------|-----------------------------------------------------------------|----------------------------------------------------------------------| | **Bonds** | Only C-C single bonds | At least one C=C double or C≡C triple bond | | **Reactivity** | Less reactive, undergo substitution reactions | More reactive, undergo addition reactions | | **Combustion** | Burn with a clean, blue flame (complete combustion) | Burn with a sooty, yellow flame (incomplete combustion due to higher % carbon) | | **Bromine Water** | Do not decolorize bromine water | Decolorize reddish-brown bromine water (test for unsaturation) | #### 3. What is an addition reaction? Illustrate with an example. **Answer:** Addition reactions are organic reactions where two or more molecules combine to form a larger one, usually by breaking a $\pi$ (pi) bond in an unsaturated compound. - **Example:** Hydrogenation of ethene $$\text{CH}_2=\text{CH}_2 + \text{H}_2 \xrightarrow{\text{Ni/Pd/Pt, Heat}} \text{CH}_3-\text{CH}_3$$ (Ethene reacts with hydrogen in the presence of a catalyst like Nickel, Palladium, or Platinum to form ethane). This reaction is used in the hydrogenation of vegetable oils. #### 4. What is a substitution reaction? Give an example. **Answer:** Substitution reactions are chemical reactions in which one functional group in a chemical compound is replaced by another functional group. Saturated hydrocarbons (alkanes) typically undergo substitution reactions. - **Example:** Chlorination of methane $$\text{CH}_4 + \text{Cl}_2 \xrightarrow{\text{Sunlight}} \text{CH}_3\text{Cl} + \text{HCl}$$ (Methane reacts with chlorine in the presence of sunlight to form chloromethane and hydrogen chloride). This reaction is slow and can produce multiple products if excess chlorine is present. #### 5. Explain the importance of ethanol and ethanoic acid. **Answer:** - **Ethanol ($\text{CH}_3\text{CH}_2\text{OH}$):** - Used as a solvent in medicines (e.g., tincture of iodine, cough syrups). - Used as a fuel (e.g., in some countries, as an additive to petrol). - Raw material for many organic compounds. - Disinfectant and antiseptic. - **Ethanoic Acid ($\text{CH}_3\text{COOH}$):** - Commonly known as acetic acid. A 5-8% solution in water is called vinegar and is used as a preservative. - Used in the production of esters, which are used in perfumes and flavouring agents. - Used in the preparation of acetone and ethyl acetate. ### Soaps and Detergents #### 1. What are soaps? How are they formed? **Answer:** Soaps are sodium or potassium salts of long-chain carboxylic acids. They are formed by the saponification process, where fats or oils (esters of glycerol and fatty acids) are hydrolyzed with a strong base (like NaOH or KOH). - **Reaction:** $$\text{Fat/Oil} + \text{NaOH/KOH} \xrightarrow{\text{Heat}} \text{Soap} + \text{Glycerol}$$ #### 2. Explain the cleansing action of soap. **Answer:** Soap molecules have two parts: 1. **Hydrophilic (water-loving) head:** Ionic part ($\text{-COO}^-\text{Na}^+$) that dissolves in water. 2. **Hydrophobic (water-hating) tail:** Long hydrocarbon chain that dissolves in oil/grease. When soap is dissolved in water, the hydrophobic tails attach themselves to the oily dirt, while the hydrophilic heads face outwards into the water. This forms spherical structures called **micelles**. The micelles trap the oily dirt at their center. Due to the repulsion between the negatively charged heads of the micelles, they remain suspended in water and can be rinsed away, carrying the dirt with them. #### 3. Why are detergents preferred over soaps for washing clothes in hard water? **Answer:** Hard water contains dissolved salts of calcium and magnesium. - **Soaps:** React with $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ ions in hard water to form an insoluble precipitate called **scum** (calcium or magnesium stearate). This scum adheres to clothes, making them stiff and dull, and reduces the cleansing efficiency of soap. - **Detergents:** Are sodium salts of long-chain benzene sulphonic acids or long-chain alkyl hydrogen sulphates. They do not form insoluble precipitates with $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ ions, remaining effective even in hard water. Therefore, detergents are more effective for washing in hard water.