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Chemistry - Carbon Chemistry - Hydrocarbons (Alkanes and Alkenes) and polymers
Terms in this set (13)
Identify and explain the trend of boiling points for alkanes.
As the length of the alkane increases i.e to number of carbon atoms its boiling point also increases. This is because the longer the length of the carbon chain the larger the mass of the hydrocarbon is and therefore the stronger the intermolecular attractions are. So more heat energy is required to overcome these attractions for the molecules in the hydrocarbon to change state from a liquid to a gas. Smaller mass hydrocarbons with fewer carbon atoms have weak intermolecular attractions so require less heat energy to change state. Therefore as the number of carbon atoms in the hydrocarbons increases the boiling point also increases.
In October 2011 a ship, the Rena, grounded on a reed near Tauranga Harbour, causing oil to be spilled into the ocean. It was observed that the oil formed a layer on top of the water, and that it lasted for long time. Give an explanation for both observations by referring to the properties of oil.
Oil is made up of hydrocarbons, hydrocarbons are less dense than water so two layers will be observed as the oil would float on top of the water. Oil is insoluble. For a substance to dissolve, it must exert strong intermolecular attractions on the water molecules in order to overcome the strong attractions that water molecules exert on each other. In this way, they can separate the water molecules and dissolve. Oil has only weak forces of attraction with the water molecules and they are not sufficient enough to overcome the intermolecular attractions between the water molecules so therefore the oil will not dissolve in the water which is why it formed a layer on top of the water instead of dissolving. The reason it lasted for a long time is that oil is un-reactive since all of its electron are used up in covalent bonding, so it is already stable, so it would not react with the air.
The monomer of ethene burns with a smoky flame in a limited oxygen supply. Identify and explain TWO negative effects on human health of the products of this combustion reaction.
Because it burns with a smoky flame in limited oxygen this shows that incomplete combustion is occurring. The products of the incomplete combustion of ethene is carbon monoxide, carbon and water. Carbon monoxide is a poisonous gas that effects human health because it interferes and limits the amount of oxygen transported to the brain and other cells through haemoglobin in the blood and this can lead to death from the lack of oxygen. (Carbon monoxide also reacts with oxides of nitrogen in the air creating a photochemical smog. This smog is toxic and can cause death in humans as well as leaving a visible haze) Carbon is another products that is produced from incomplete combustion and is a fine black particulate which contributes to a visible haze which causes breathing problems and can lead to lung cancer in humans.
Polythene is available in both a low density (LDPE) and high density (HDPE) form. Some properties of LDPE and HDPE are given in the table below.
Mass - Solubility - chemical resistance - flexibility - polymer chain packing
LDPE - light - insoluble - high - more flexible - chains packed loosely together
HDPE - light - insoluble - high - less flexible - chains packed closely together
Explain why LDPE is used to make plastic food wrap and HDPE is used to make plastic drink bottles, by analysing the properties provided above.
LDPE is used to make plastic food wrap as it is un-reactive because all of its valence electrons are used up in covalent bonding so it is stable, which means that it will not react with the food in the wrap. It is also very flexible because the attractions between the long chains of joined monomers is weak so the chains can slide past each other. The chains are packed loosely together so it is easy to mould to fit around the food it is holding, it is also insoluble in water which is useful as it will not dissolve in water. Because it is un-reactive, flexible to fit around the found and light so it is easily stored and easy to mould - LDPE is used to make plastic food wrap.
HDPE is insoluble which means that it will not dissolve in the drink that is in holding. HDPE is less flexible than LDPE which is good as the bottle will be sturdy and able to properly hold the drink it was instead of bending. Its chains are packed more tightly together which shows that the individual molecules in HDPE are very strong and so are the attractive forces between the long molecules. This makes it tough and able to hold it shape and is useful to make into drink bottles. Its mass is also light as plastic drink bottles are not supposed to be difficult to carry. It is also un-reactive which is good because it means that the polythene bottle will not react with the substances inside it. Therefore, the HDPE is used to make drink bottles as it is not too flexible, un-reactive, insoluble, light and has tightly packed chains so is tougher than LDPE.
Explain why alkenes can be used make polymers, but alkanes cannot
Alkenes are hydrocarbons that are unsaturated and contain a double covalent bond between the carbon atoms. This makes alkenes reactive as they have one spare covalent bond. During polymerisation this double covalent bond is broken into single covalent bonds, creating a spare bond that is used to connect to neighbouring alkene molecule. Eventually many small alkenes called monomers are joined to form a long chain, large polymer. This process is called polymerisation. Where many small repeating units of monomers join together to form a large polymer under high temperature, high pressure and the presence of a catalyst. Alkanes cannot be used to make polymers because it is saturated meaning they only have one single covalent bond between the carbon atoms and have no spare bonds (it is already stable). As they have no spare bonds to connect an alkane with another alkane and therefore cannot join to make a polymer.
Polypropylene has many uses that are linked to its chemical and physical properties. Name TWO uses of polypropylene. Link each use to TWO physical and or chemical properties of polypropylene.
- Polypropylene is a polymer made of many propene monomers by a process called polymerisation. It is saturated and has single covalent bonds
- Polypropylene has a low chemical reactivity, high tensile strength, insoluble in water, able to be moulded, insulator, low density/lightweight and floats, recyclable
- One use of polypropylene is to make it into drink bottles. This is because it is very tough and insoluble which would be useful so the drink bottle is able to hold its shape and the bottle will not be dissolved by the liquid inside the bottle or by the air surrounding the bottle. It is also able to be moulded into shapes with moderate heating due to its high melting points of around 160°C. Polypropylene is also lightweight and this would be useful so the drink bottle will be light and easy to carry
- Another use of polypropylene is to make it into ropes that can be used in water. This is because polypropylene has a high tensile strength so will be able to hold substantial amount of weight and it is also insoluble in water which will be useful as it won't dissolve in the water. It is also lightweight so it is able to float on water so the rope will be visible on the surface of the water and not sink to the bottom
Alkanes and alkenes are organic compounds made up of carbon and hydrogen atoms.
- Name the type of bond that occurs between the atoms in these organic compounds and explain your answer
Covalent bonding. Alkanes and Alkenes are up of non-metal atoms of carbon atoms and hydrogen atoms that both need to gain electrons to achieve a full valence shell hence a stable octet. Therefore instead of transferring electrons, they share electrons to form a full valence shell each and chemically combine to form covalent bonding and form organic compound of hydrocarbons.
Explain how the chemical bonding in alkanes and alkenes affects the way they are used
- Give the general formula of alkanes and alkenes
- Describe the similarities and differences in chemical bonding
- Identify ONE common use for each of alkanes and alkenes
- Link the chemical bonding to the property or properties of each type of compound that makes them suitable for the identified use
- Alkanes have the general formula CnH2n+2 and Alkenes have the general formula of CnH2n
- In both alkanes and alkenes, the carbon and hydrogen atoms are covalently bonded to each other. Alkanes are saturated hydrocarbons because each carbon atom has four single covalent bonds. Alkenes are unsaturated hydrocarbons containing mostly single covalent bonds, but also a double covalent bond between the carbon atoms.
- Alkanes do not have a functional group, so they are generally un-reactive/ readily combust. Alkanes are commonly used as fuels, because, while they are mostly un-reactive, they undergo combustion since single bonds do not require as much energy to break. So a lot of energy is released as they combust.
- The double bond in alkenes is a functional group and is easily broken, so alkenes are chemically reactive. Since alkenes are chemically reactive, they can also undergo other chemical reactions, and are therefore commonly used to make other organic molecules such as polymers e.g. polythene used to make food wrap since the double bond easily breaks to form long chains
Butane and propane are both used as fuel in camping burners. Propane has boiling point of -42°C.
- What state would propane be at room temperature (18°C)
- State whether the boiling point of butane will be higher or lower than propane. Give a reason for your answer using your knowledge of the structure and properties of alkanes.
- The boiling point of butane will be higher than propane. Because it has a larger number of carbon atoms compared to propane. This is because the trend for straight carbon chain alkanes is as the length of a hydrocarbon increases i.e. the number of carbon atoms, its molecular mass also increases which means the intermolecular forces of attraction between the molecules are stronger and will require more heat energy to overcome the attractive forces between molecules in the hydrocarbon to change state from a liquid to a gas. So butane will have a higher boiling point than propane because it is a longer hydrocarbon.
Camping burners usually have a warning notice instructing people to always use them in well-ventilated place (plenty of oxygen) otherwise serious injury or death may occur. Elaborate on why this warning is given on camping burners. Use a burner that contains propane as an example
- State the type of combustion reaction that occurs when there is a shortage of oxygen
- Describe the observations that may be seen if there was a shortage of oxygen, and link these to the reaction occurring
- Explain two effects that the combustion products can have on human health when there is a shortage of oxygen
- Write a word equation and a balanced symbol equation for the reaction occurring
- Propane + Oxygen ---> Carbon monoxide + water
C3H8 + 3.5O2 ---> 3CO + 4H2O
- Propane + Oxygen ---> Carbon + water
C3H8 + 2O2 ---> 3C + 4H2O
- When propane is burned in an insufficient amount of energy incomplete combustion occurs and produces carbon soot and/or carbon monoxide gas and water
- When propane is burning without sufficient energy it burns with a yellow flame and produces less energy (less energy efficient) than complete combustion. The carbon solids glow a orange hue, so the sooty flame (contains carbon soot) has the yellow colour.
- Carbon soot is a fine black particulate that is produced and produces a visible haze that can cause breathing problems in humans e.g. asthma and can lead to lung cancer. If carbon soot enters the blood stream, it can also cause blockages in the arteries, which is very dangerous as this can lead to heart diseases
- Carbon monoxide is a poisonous gas that is produced which interferes and limits the amount of oxygen that is transported to the brain and other cells through haemoglobin in blood and can lead to humans feeling dizzy and then eventually dying due to the lack of oxygen in the cells
- What is the type of bonding present in a molecule of propane
- Give a reason for this answer
- Single covalent bonds
- Propane is an alkane which is saturated which means it contains single covalent bonds between carbon atoms. These bonds are covalent because non-metal atoms of carbon and hydrogen both need to gain electrons to achieve a full valence shell and this is achieved by sharing of electrons between hydrogen and carbon. Each carbon atom has 4 single covalent bonds
How does the structure of propene differ to propane
Propane is an alkane so it is saturated and only contains single covalent bonds between the carbon atoms whereas propene is an alkene so it is unsaturated and contains one double covalent bond between the carbon atoms.
Alkanes can be used as fuels, Compare and contrast: the complete combustion of alkanes, which produces carbon dioxide; and the incomplete combustion, which produces carbon monoxide and carbon in addition to carbon dioxide
- Use butane as an example to illustrate your answer
- Give an explanation of an effect on the environment for TWO combustion products
- Include balanced symbol equations for the reactions occurring, in the labelled boxes below
The complete combustion of butane occurs when there is a sufficient amount of oxygen in the air for it to form the products colourless carbon dioxide gas and water. The incomplete combustion of butane occurs when there is an insufficient supply of oxygen in the air and produces carbon monoxide, carbon (in the form of black soot) and water. Complete combustion of alkanes e.g. butane is more energy efficient producer of energy than incomplete combustion. Complete combustions of butane produces carbon dioxide, which is a greenhouse gas and can have a negative impact on the environment as it contributes to the greenhouse effect because it helps to retain heat from the sun in the earths atmosphere instead of radiating into space. This causes the temperature of earth to rise leading to ice caps melting and sea levels rising as well as changes in weather patterns and global warming. The incomplete combustion of butane/alkanes can also negatively impact the environment as the product carbon leaves a surface coating on plants and limits the plant's ability to photosynthesise efficiently and contributing to a visible haze.
Complete combustion of butane
C4H10 + 6.5O2 ---> 4CO2
Incomplete combustion of butane
C4H10 + 3O2 ---> CO + 4C + 5H2O
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