Organic Chemistry (OCR GCSE Chemistry A (Gateway)): Flashcards

An unsaturated hydrocarbon contains at least one C=C double bond, which allows it to react by addition reactions.

The general formula is C_nH_2n, where n is the number of carbon atoms.

True.

The C=C double bond is the functional group of alkenes. It can open up to allow incoming atoms to bond, making alkenes much more reactive than alkanes.

In a hydrogenation reaction, an alkene reacts with hydrogen at 150°C using a nickel catalyst to form an alkane.

Bromine water is an orange solution added to the compound being tested. An alkene decolourises the bromine water to colourless because bromine adds across the C=C bond.

Alkenes have a higher carbon-to-hydrogen ratio than alkanes, so they are more likely to undergo incomplete combustion, producing carbon (soot) and giving a smoky yellow flame.

An addition reaction is one in which atoms from a simple molecule add across the C=C double bond of an alkene, forming a single product with no atoms wasted.

True.

Alkanes have no C=C double bond, so bromine cannot add across it. The bromine remains in solution and the orange colour is unchanged.

Hydrogenation of alkenes is used industrially to manufacture margarine from vegetable oils by partially hydrogenating the C=C bonds.

The molecular formula of ethene is C₂H₄.

The C=C double bond occurs between the 2nd and 3rd carbon atoms from the left of the chain.

A monomer is a small molecule that can join with many other monomers to form a long-chain polymer via covalent bonds.

The monomer must contain a C=C double bond. One bond in the double bond opens to link with the adjacent monomer, forming a chain of single bonds.

Addition polymerisation usually requires high pressures and a catalyst to proceed.

False.

The C=C double bond opens during polymerisation. The resulting polymer chain contains only single C–C bonds.

A repeat unit is the smallest section of a polymer chain that repeats along its length, shown in brackets with continuation bonds and a subscript n.

The prefix poly- is added to the name of the monomer. For example, propene monomer gives the polymer polypropene.

Polyethene is formed by the addition polymerisation of ethene monomers. The monomer formula is C₂H₄.

False.

Nylon is a condensation polymer (a polyamide). Addition polymers are made from alkene monomers only, such as polythene and PVC.

Replace the C=C double bond with a single bond, then add a continuation bond to each end of the unit, place the structure in brackets and write a subscript n to indicate many repeating units.

Condensation polymerisation is the joining of two different monomers to form a polymer, with the elimination of a small molecule (usually water) at each linkage.

Addition polymerisation produces the polymer only. Condensation polymerisation produces the polymer and a small molecule (usually water) for every bond formed.

True.

The -COOH group of the dicarboxylic acid reacts with the -OH group of the diol, forming an ester link (-COO-) and releasing water.

A polyamide is a condensation polymer formed from a dicarboxylic acid and a diamine, linked by amide bonds (-CONH-). Nylon is a well-known example.

In a polyester, the functional group linking the monomers is the ester link (-COO-). In a polyamide, it is the amide link (-CONH-).

A polyamide is made from a dicarboxylic acid (with a -COOH group at each end) and a diamine (with a -NH₂ group at each end).

False.

Terylene is an example of a polyester, not a polyamide. Nylon is the common example of a polyamide.

A white film of nylon forms at the interface between the two solutions. The film can be pulled out continuously as a diamine and a diacid chloride react at the boundary.

In condensation polymerisation, the monomers must have two functional groups, one at each end, so that they can form long alternating chains.

The solutions contain toxic materials and hazardous solvents. Small quantities are used and care must be taken during handling and disposal to minimise risk.

DNA (deoxyribonucleic acid) is a large polymer molecule that stores genetic information as instructions for how an organism develops and functions.

| Abbreviation | Monomer name |\n|---|---|\n| A | adenine |\n| T | thymine |\n| C | cytosine |\n| G | guanine |

True.

Two strands of nucleotides intertwine and are held together by base pairing between complementary bases on each strand, giving the famous double helix shape.

Starch and cellulose are complex carbohydrates made from sugar monomers joined by chemical bonds.

The general formula is C_x(H₂O)_y. Examples include glucose (a monosaccharide).

An amino acid is the monomer from which proteins are built. It contains an amino group (-NH₂) and a carboxylic acid group (-COOH).

Starch and cellulose are polymers made from many sugar monomers joined together in long chains.

True.

Enzymes are proteins — natural polymers made from amino acid monomers — and they speed up biochemical reactions without being consumed.

The genetic information in DNA is stored in the order in which the four bases arrange themselves, acting as a code for an organism's genes.

The hydroxyl group (-OH) is the functional group found in all alcohols. It is responsible for the characteristic reactions of alcohols.

The general formula for alcohols is C_nH_2n+1OH.

False.

Alcohols dissolve in water to form neutral solutions.

The complete combustion of ethanol produces carbon dioxide and water.

Acidified potassium dichromate(VI) is used. The alcohol is heated with this reagent to produce a carboxylic acid.

A homologous series is a family of compounds that share the same general formula and functional group, differing by one -CH₂ unit from one member to the next.

True.

The first four alcohols burn cleanly and are widely used as fuels and solvents. Ethanol is used in spirit burners in school laboratories.

Alcohols use the same stem as the corresponding alkane but replace the final 'e' with 'ol'. For example, ethane becomes ethanol.

Alcohols can act as solvents because they can dissolve both fats and oils as well as most substances that water can.

Writing the -OH group at the end clearly identifies the molecule as an alcohol. It shows the functional group and the type of reactions the molecule undergoes, which is standard chemical practice.

Carboxylic acids are a homologous series of organic compounds containing the -COOH functional group, with the general formula C_nH_2n+1COOH.

The general formula for carboxylic acids is C_nH_2n+1COOH.

False.

Boiling point increases with increased carbon chain length. Longer chains have stronger intermolecular forces, requiring more energy to overcome.

Carboxylic acids are named by replacing the final 'e' of the corresponding alkane name with 'oic' followed by the word acid.

Each successive member differs by one -CH₂ unit in its molecular formula. This gradual change leads to a gradation in physical properties such as boiling point and viscosity.

The -COOH group (carboxyl group) is the functional group in carboxylic acids.

True.

Longer carbon chains increase viscosity because the molecules experience stronger intermolecular forces, making them flow less easily.

The molecular formula of ethanoic acid is CH₃COOH. It is the second member of the carboxylic acid homologous series.

Members of the same homologous series have similar chemical properties but show a gradation in their physical properties.

A hydrocarbon is a compound that contains carbon and hydrogen atoms only.

Crude oil is described as finite because it takes millions of years to form from the remains of plants and animals under high pressure and temperature. It is being used up far faster than it can be replenished.

False.

Fractions with the highest boiling points condense first and are collected at the bottom of the column. Lower boiling point fractions rise higher before condensing.

Crude oil is separated into useful fractions by a process called fractional distillation.

The column is hot at the bottom and cool at the top. Crude oil vapours rise and cool gradually; each fraction condenses at the height where the temperature matches its boiling point and is tapped off as a liquid.

A fraction is a group of hydrocarbons with similar chain lengths and boiling points, collected together during fractional distillation of crude oil.

True.

Larger molecules have stronger intermolecular forces, so more energy is needed to overcome them. This gives larger hydrocarbons higher boiling points.

Most fractions contain mainly alkanes, which are saturated hydrocarbons with only single carbon-carbon bonds.

Crude oil is formed over millions of years from the remains of plants and animals under high pressure and temperature.

Crude oil as a mixture has limited uses. The individual fractions each consist of hydrocarbons with similar properties and boiling points, making them suitable for specific applications such as fuels, solvents and raw materials.

Cracking is the process of breaking long-chain alkane molecules into shorter, more useful molecules including smaller alkanes, alkenes and hydrogen, using heat and a catalyst.

The hydrocarbon is heated to around 600–700°C to vaporise it. The vapours then pass over a hot powdered catalyst of alumina or silica, which causes thermal decomposition.

False.

Cracking produces shorter alkanes, alkenes and hydrogen. Hydrogen is also a product of the process.

Alkanes are saturated compounds because they contain only single bonds. Alkenes are unsaturated compounds because they contain double bonds.

There is greater demand for shorter-chain hydrocarbons such as petrol than for the longer-chain fractions from crude oil. Cracking converts less useful long-chain alkanes into more valuable fuels and alkenes for chemical synthesis.

C₆H₁₄ → C₄H₁₀ + C₂H₄

Hexane cracks to give butane (an alkane) and ethene (an alkene). Check both sides: 6C and 14H on each side.

True.

At higher temperatures and pressures, cracking produces more alkenes and more hydrogen relative to alkanes.

The general formula for alkanes is C_nH_2n+2 and the general formula for alkenes is C_nH_2n.

Thermal decomposition occurs. The catalyst breaks covalent bonds as the vapours contact its surface, splitting the large molecules into smaller ones in a random way.

A simple chemical cell is a source of electrical energy consisting of two electrodes made from metals of different reactivity, immersed in an electrolyte and connected by an external circuit.

The greater the difference in reactivity, the greater the difference in the metals' ability to release electrons. This sets up a larger charge difference between the electrodes, producing a higher voltage.

False.

Hydrogen is oxidised at the anode, where it loses electrons to form hydrogen ions. The cathode is where oxygen gains electrons.

In a simple cell, the more reactive metal forms the negative electrode, giving it a negative charge.

2H₂ (g) + O₂ (g) → 2H₂O (l)

Hydrogen and oxygen combine to produce water. Energy is released as electricity during this process.

A hydrogen fuel cell is an electrochemical cell in which hydrogen donates electrons at the anode and oxygen gains electrons at the cathode, generating electricity and producing water as the only product.

True.

Hydrogen fuel cells are more energy-dense per kilogram than petrol or diesel, which is one of their key advantages.

Hydrogen fuel cells are a continuous process — they keep producing energy as long as fuel is supplied.

1. The materials used to make fuel cells are expensive.

2. High-pressure tanks are needed to store hydrogen and oxygen, which are dangerous and difficult to handle.

At the anode: 2H₂ → 4H⁺ + 4e^- (hydrogen is oxidised, releasing electrons).

At the cathode: 4H⁺ + O₂ + 4e^- → H₂O (hydrogen ions react with oxygen to form water).