Science Chapter 4: Carbon and its Compounds

Comprehensive practice flashcards covering bonding in carbon, allotropes, catenation, homologous series, functional groups, reactions of ethanol and ethanoic acid, and soaps and detergents from Chapter 4: Carbon and its Compounds.

What is the percentage of carbon present in the earth's crust and in the atmosphere?

The earth's crust has only $0.02\%$ carbon in the form of minerals (such as carbonates, hydrogen-carbonates, coal, and petroleum), and the atmosphere has $0.03\%$ of carbon dioxide.

Why can carbon neither gain four electrons ($C^{4-}$) nor lose four electrons ($C^{4+}$) to attain a noble gas configuration?

Gaining four electrons would form a $C^{4-}$ anion, which is difficult for a nucleus with $6$ protons to hold on to ten electrons. Losing four electrons would form a $C^{4+}$ cation, which requires a huge amount of energy to remove four electrons leaving a nucleus with $6$ protons holding on to just two electrons.

How does carbon overcome the problem of attaining a noble gas configuration?

Carbon overcomes this problem by sharing its valence electrons with other atoms of carbon or with atoms of other elements, forming covalent bonds.

Why do covalent compounds generally have low melting and boiling points and act as poor conductors of electricity?

They have low melting and boiling points because the intermolecular forces of attraction between molecules are weak. They are poor conductors of electricity because electrons are shared and no charged particles or ions are formed.

What type of chemical bond is formed in a molecule of nitrogen ($N_2$)?

A triple covalent bond, formed by each nitrogen atom contributing three valence electrons, giving rise to three shared pairs of electrons.

Which allotrope of carbon is represented by this rigid, three-dimensional tetrahedral structure?

Diamond, in which each carbon atom is bonded to four other carbon atoms.

Which allotrope of carbon is shown in this diagram featuring hexagonal arrays placed in parallel layers?

Graphite, in which each carbon atom is bonded to three other carbon atoms in the same plane with one double bond, allowing the layers to slide.

Why is graphite a good conductor of electricity unlike diamond and most other non-metals?

In graphite, each carbon atom is bonded to only three other carbon atoms, leaving one free valence electron per carbon atom that can move through the hexagonal layers.

Which carbon allotrope is shown in this image featuring carbon atoms arranged in a football-like shape?

Buckminsterfullerene ($C_{60}$), named after the US architect Buckminster Fuller because its structure resembles a geodesic dome.

How can synthetic diamonds be prepared?

Synthetic diamonds can be synthesised by subjecting pure carbon to very high pressure and high temperature.

What is catenation?

The unique ability of an element to form covalent bonds with other atoms of the same element, giving rise to large chains, branched chains, or rings.

Why is catenation exceptionally prominent and stable in carbon compared to silicon?

Carbon has a very small atomic size, which enables its nucleus to hold on to shared electron pairs tightly, forming very strong and stable $C-C$ bonds. Silicon forms larger, weaker bonds that are much more reactive.

Who disproved the 'vital force' theory of organic compounds and how?

Friedrich Wöhler disproved it in 1828 by synthesizing the organic compound urea from the inorganic compound ammonium cyanate.

What is the fundamental difference between saturated and unsaturated carbon compounds?

Saturated carbon compounds contain only single bonds between carbon atoms, whereas unsaturated carbon compounds contain double or triple bonds between carbon atoms and are more reactive.

Which molecule's electron dot structure is represented in this figure?

Ethane ($C_2H_6$), a saturated hydrocarbon with a single covalent bond between the two carbon atoms.

Which molecule's electron dot structure is shown in this figure?

Ethene ($C_2H_4$), an unsaturated hydrocarbon containing a double covalent bond between the two carbon atoms.

What are structural isomers?

Compounds that have identical molecular formulae but different structural arrangements of atoms (such as butane and isobutane with formula $C_4H_{10}$).

What is the name and formula of this cyclic unsaturated hydrocarbon?

Benzene, with the molecular formula $C_6H_6$.

What is a heteroatom in an organic compound?

An element such as halogen, oxygen, nitrogen, or sulphur that replaces one or more hydrogen atoms in a hydrocarbon chain while satisfying the valency of carbon.

What is a functional group?

An atom or group of atoms attached to a carbon chain that confers specific chemical properties to the compound, regardless of the length and nature of the carbon chain.

What are the formulae of the functional groups for aldehydes, ketones, and carboxylic acids?

Aldehyde: $-CHO$, Ketone: $-CO-$, and Carboxylic acid: $-COOH$.

What is a homologous series?

A series of organic compounds having the same functional group and similar chemical properties, in which successive members differ by a $-CH_2-$ unit.

By what molecular mass do any two successive members of a homologous series differ?

They differ by $14\,u$ (corresponding to one carbon atom of $12\,u$ and two hydrogen atoms of $1\,u$ each).

What are the general formulae for alkanes, alkenes, and alkynes?

Alkanes: $C_nH_{2n+2}$, Alkenes: $C_nH_{2n}$, Alkynes: $C_nH_{2n-2}$.

Why is the bottom of a cooking vessel blackened when heated over a gas stove with blocked air holes?

Blocked air holes limit the supply of oxygen, leading to incomplete combustion of the fuel, which produces a yellow, sooty flame that deposits unburnt carbon (soot).

Why do some substances burn with a flame while others, like charcoal, merely glow without a flame?

A flame is produced only when gaseous substances burn. Charcoal lacks volatile substances that vaporise, so it glows red and releases heat without producing a flame.

Which oxidising agents are used to convert ethanol to ethanoic acid?

Alkaline potassium permanganate ($\text{KMnO}_4$) with heat, or acidified potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$) with heat.

What is the industrial application of the catalytic addition reaction of unsaturated hydrocarbons?

The hydrogenation of vegetable oils (containing unsaturated carbon chains) into saturated fats using a nickel ($Ni$) or palladium ($Pd$) catalyst.

What reaction occurs when methane reacts with chlorine in the presence of sunlight?

A substitution reaction: $\text{CH}_4 + \text{Cl}_2 \xrightarrow{\text{sunlight}} \text{CH}_3\text{Cl} + \text{HCl}$.

What gas is evolved when ethanol reacts with sodium metal?

Hydrogen gas ($H_2$), along with the formation of sodium ethoxide ($2\text{Na} + 2\text{CH}_3\text{CH}_2\text{OH} \rightarrow 2\text{CH}_3\text{CH}_2\text{O}^-\text{Na}^+ + \text{H}_2$).

What product is formed when ethanol is heated at $443\,K$ with excess concentrated sulphuric acid?

Ethene ($\text{CH}_2=\text{CH}_2$), because concentrated sulphuric acid acts as a dehydrating agent that removes water from ethanol.

What is denatured alcohol and why is it prepared?

Ethanol made unfit for drinking by adding poisonous substances like methanol and coloured dyes (such as blue dye) to prevent its misuse for drinking in industrial applications.

What is vinegar and what is glacial acetic acid?

Vinegar is a $5\text{--}8\%$ solution of acetic acid in water. Glacial acetic acid is pure ethanoic acid, named because its melting point is $290\,K$, causing it to freeze in cold climates.

What is an esterification reaction?

A reaction where a carboxylic acid reacts with an alcohol in the presence of an acid catalyst to produce a sweet-smelling ester and water.

What is saponification?

The alkaline hydrolysis of an ester using sodium hydroxide ($\text{NaOH}$) to regenerate the alcohol and yield the sodium salt of the carboxylic acid (soap).

What products are obtained when ethanoic acid reacts with sodium hydrogencarbonate?

Sodium ethanoate (sodium acetate), water, and carbon dioxide gas ($\text{CH}_3\text{COOH} + \text{NaHCO}_3 \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2$).

Describe the dual-nature structure of a soap molecule.

A soap molecule consists of an ionic hydrophilic head that interacts with water and a long hydrocarbon hydrophobic tail that interacts with oil and grease.

What is a micelle and how does it facilitate the cleaning of clothes?

A spherical cluster formed by soap molecules in water where hydrophobic tails entrap oily dirt in the interior and hydrophilic ionic heads face outward into water, forming a stable emulsion that can be rinsed away.

What causes the formation of scum when soap is used in hard water?

The reaction of soap molecules with calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$) ions present in hard water, forming insoluble curdy precipitates known as scum.

Why are detergents effective cleansing agents in hard water unlike soaps?

Detergents are sodium salts of sulphonic acids or ammonium salts with chloride/bromide ions whose charged ends do not form insoluble precipitates with calcium and magnesium ions in hard water.