OCR GCSE Chemistry B (J258) — Material choices
From the DNA in your cells to the plastic in your water bottle, carbon is the backbone of almost everything around you. There are millions of known organic compounds (compounds containing carbon atoms) on Earth. This vast array of natural and synthetic materials exists entirely because of carbon's unique atomic structure and bonding abilities.
To explain why there is such a vast array of organic compounds, we must look at carbon's position in the periodic table:
Because there is no limit to the length of a carbon sequence, the number of possible organic compounds is virtually infinite.
When carbon bonds to itself to form chains and rings, it creates three main structural varieties:
Carbon can also form single (), double (), or triple () bonds. This leads to the formation of isomers, which are compounds that have the exact same molecular formula but a completely different arrangement of atoms. Furthermore, carbon bonds do not sit flat; a molecule like methane () forms a 3D tetrahedral shape with bond angles of approximately .
Because there are millions of organic compounds, chemists group them into families called a homologous series. Every member of a homologous series shares specific characteristics:
Some series are hydrocarbons (containing only carbon and hydrogen atoms). These can be saturated (containing only single bonds) or unsaturated (containing at least one multiple carbon-carbon bond, like a double bond, or an aromatic ring). Note that not all organic compounds are hydrocarbons; many contain other elements like oxygen or nitrogen.
A student is studying a homologous series of alkanes. The first member is methane () and the second member is ethane (). Predict the molecular formula of the fourth member of this series and calculate the difference in relative formula mass between any two successive members.
Step 1: Determine the general formula or the pattern of the series.
Step 2: Predict the formulas for the 3rd and 4th members.
Step 3: Calculate the mass difference of a unit.
Answer:
The incredible flexibility of carbon bonding allows for both complex natural life and advanced modern materials. Natural organic compounds include DNA (a nucleotide polymer), proteins, and starch. In contrast, synthetic organic compounds are man-made and include addition polymers like PVC, condensation polymers like nylon, and synthetic medicines.
Students often incorrectly assume all organic compounds are hydrocarbons. Remember, a hydrocarbon contains ONLY carbon and hydrogen; organic compounds like alcohols and DNA also contain oxygen, nitrogen, and other elements.
In 6-mark explanation questions, examiners explicitly look for the phrase 'carbon bonds to itself to form chains and rings'—this is the OCR-preferred way to describe catenation.
When asked why boiling points increase in longer carbon chains, always use the term 'intermolecular forces'. Never say 'bonds between molecules' or imply that covalent bonds are broken during boiling.
If a question asks you to prove a group of chemicals belong to a homologous series, you must state three things: they have the same general formula, the same functional group, and show a graduation in physical properties.
Organic compound
A chemical compound that contains carbon atoms, encompassing both natural biological molecules and synthetic materials.
Covalent bond
A strong chemical bond formed when two non-metal atoms share pairs of electrons to achieve a full outer shell.
Tetravalency
The ability of a carbon atom to form exactly four strong covalent bonds with other atoms.
Catenation
The unique ability of carbon atoms to form strong covalent bonds with other carbon atoms, resulting in the formation of long chains and rings.
Isomers
Compounds that have the exact same molecular formula but different arrangements of atoms in space.
Homologous series
A family of organic compounds that have the same functional group and general formula, where each successive member differs by a -CH₂- group.
Functional group
An atom or group of atoms in an organic molecule that dictates its characteristic chemical reactions.
Intermolecular forces
The weak forces of attraction between molecules, which must be overcome to melt or boil a substance.
Hydrocarbon
A compound containing exclusively carbon and hydrogen atoms.
Saturated
An organic compound containing only single carbon-carbon bonds, with no double or triple bonds.
Unsaturated
An organic compound containing at least one multiple carbon-carbon bond (such as a C=C double bond) or an aromatic ring.
Put your knowledge into practice — try past paper questions for Chemistry B
Organic compound
A chemical compound that contains carbon atoms, encompassing both natural biological molecules and synthetic materials.
Covalent bond
A strong chemical bond formed when two non-metal atoms share pairs of electrons to achieve a full outer shell.
Tetravalency
The ability of a carbon atom to form exactly four strong covalent bonds with other atoms.
Catenation
The unique ability of carbon atoms to form strong covalent bonds with other carbon atoms, resulting in the formation of long chains and rings.
Isomers
Compounds that have the exact same molecular formula but different arrangements of atoms in space.
Homologous series
A family of organic compounds that have the same functional group and general formula, where each successive member differs by a -CH₂- group.
Functional group
An atom or group of atoms in an organic molecule that dictates its characteristic chemical reactions.
Intermolecular forces
The weak forces of attraction between molecules, which must be overcome to melt or boil a substance.
Hydrocarbon
A compound containing exclusively carbon and hydrogen atoms.
Saturated
An organic compound containing only single carbon-carbon bonds, with no double or triple bonds.
Unsaturated
An organic compound containing at least one multiple carbon-carbon bond (such as a C=C double bond) or an aromatic ring.
