Organic Chemistry (OCR GCSE Combined Science A (Gateway): Chemistry): Flashcards
Exam code: J250
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What is a hydrocarbon?
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Cards in this collection (18)
What is a hydrocarbon?
A hydrocarbon is a compound that contains carbon and hydrogen atoms only.
Why is crude oil described as a finite resource?
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, but is being used up far faster than it forms.
True or False?
Crude oil is a pure substance made of a single hydrocarbon compound.
False.
Crude oil is a mixture of many different hydrocarbons with different chain lengths and ring sizes.
Crude oil is separated into useful .......... by the process of fractional distillation.
Crude oil is separated into useful fractions by the process of fractional distillation.
In a fractionating column, where do hydrocarbons with the lowest boiling points collect?
Hydrocarbons with the lowest boiling points collect at the top of the fractionating column. They have short chains and weak intermolecular forces, so they remain as vapour for longest and rise to the highest point before condensing.
Why do larger hydrocarbon molecules have higher boiling points than smaller ones?
Larger hydrocarbon molecules have stronger intermolecular forces, which require more energy to break. This means more heat is needed to turn them into vapour.
True or False?
Most fractions from fractional distillation contain mainly alkanes.
True.
Most fractions contain mainly alkanes, which are hydrocarbons with only single bonds between carbon atoms.
In a fractionating column, crude oil vapours .......... at different heights depending on their boiling points.
In a fractionating column, crude oil vapours condense at different heights depending on their boiling points.
What does the term fraction mean in the context of crude oil separation?
A fraction is a group of hydrocarbons with similar chain lengths and boiling points that are separated together during fractional distillation.
What is cracking in chemistry?
Cracking is an industrial process that uses heat and a catalyst to break long-chain alkane molecules into shorter, more useful molecules including alkenes and hydrogen.
Why is cracking carried out on heavier fractions of crude oil?
Heavier fractions contain long-chain hydrocarbons that are in low demand because they are thick, viscous, and do not burn easily. Cracking breaks them into shorter-chain, more useful molecules (such as petrol-range alkanes and alkenes) that are in higher demand as fuels and chemical feedstocks.
True or False?
Cracking converts alkenes into shorter-chain molecules.
False.
Cracking converts long-chain alkanes into shorter-chain molecules. The products include shorter alkanes, alkenes and hydrogen.
During cracking, hydrocarbon vapours pass over a hot powdered .......... of alumina or silica.
During cracking, hydrocarbon vapours pass over a hot powdered catalyst of alumina or silica.
What type of reaction occurs during cracking when bonds in hydrocarbon molecules are broken?
Cracking involves thermal decomposition reactions, where covalent bonds in the hydrocarbon molecules are broken as the vapours contact the hot catalyst surface.
What is the difference between a saturated and an unsaturated hydrocarbon?
A saturated hydrocarbon contains only single bonds between carbon atoms (e.g. alkanes), whereas an unsaturated hydrocarbon contains at least one double bond (e.g. alkenes).
True or False?
Higher temperatures during cracking produce a higher proportion of alkenes.
True.
At higher temperatures and pressures, cracking produces a higher proportion of alkenes and hydrogen.
Hexane (C6H14) can be cracked to form butane and ...........
Hexane (C6H14) can be cracked to form butane and ethene.
Give one use of ethene produced by cracking.
Ethene produced by cracking can be used as the starting material for the production of alcohol (ethanol) or for making polymers such as poly(ethene).
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