Chemistry

Question: What are the names of the polymers shown in this diagram?

Answer: A = High density poly(ethene) (HDPE) B = Low density poly(ethene) (LDPE) Explanation: HDPE and LDPE are made from the same monomer (ethene) but under different reaction conditions. The branched chains of LDPE cannot pack together as tightly as the unbranched chains in HDPE so they have different properties. LDPE has a lower softening temperature and is less strong compared to HDPE.

Question: What is an alloy?

Answer: A metal made from at least two elements. Explanation: An alloy is a metal made from at least two elements. Pure metals are often soft because atoms in the regular structure can slide over each other easily. Adding atoms of a different size (which can be a metal or non-metal element) disrupts the regular structure of metal ions and prevents the layers sliding over each other so easily. This is why alloys are harder than the pure metal alone.

Question: What type of bonding is present in this water molecule?

Answer: Covalent - simple molecule Explanation: Non-metal atoms react together to form covalent compounds. Electrons in their outer energy levels are shared so that each atom gains the electronic configuration of a noble gas. In this diagram, the hydrogen atom electrons are represented as crosses and the electrons of the oxygen atom are represented as dots.

Question: Why can the nanotube represented here conduct electricity? The spheres represent carbon atoms.

Answer: Delocalised electrons Explanation: Nanoscience is the study of molecules on the nanometre scale, meaning it is the study of molecules between 1 and 100 nm in size which are made up of a few hundred or less atoms. In the carbon nanotube, each carbon atom is covalently bonded to 3 other carbon atoms. So each carbon atom has one free electron which is delocalised around the nanotube (it is free to move around between the carbon atoms). The delocalised electrons are charged and so the nanotube can conduct electricity.

Question: Delocalised electrons hold together layers of positively charged ions in metal. The delocalised electrons can hold the metal ions together and move around at the same time. What does this enable a metal do?

Answer: Distort Explanation: Delocalised, negatively charged electrons hold the positively charged metal ions together and so act like a glue. However, because the electrons are also free to move around, the metal will not break apart when struck. Instead the layers of atoms can slip past each other without breaking up the structure.

Question: Allotropes are different structural versions of the same element, where atoms of the same type bond together in different ways. Which allotrope of carbon is represented in the image below?

Answer: Graphite Explanation: Graphite, diamond and fullerenes are all allotropes of carbon, which means they are all made up of carbon atoms, but the carbon atoms are bonded together in different ways. In graphite, each carbon atom is bonded to 3 other carbon atoms to form layers made out of hexagons. In diamond, each carbon atom is bonded to 4 other carbon atoms in a large network. In fullerenes, each carbon atom is bonded to 3 other carbon atoms to form hexagonal rings which take the shape of spheres, tubes and many other shapes.

Question: Complete the sentence: Substances made up of simple, covalently bonded molecules...

Answer: Have low melting and boiling points Explanation: Most simple molecules exist as liquids or gases at room temperature because they have low melting and boiling points. Boiling and melting points are low because the intermolecular forces holding simple molecules together are weak. It is these forces, not the covalent bonds between the atoms, that are broken when a simple molecular substance melts or boils.

Question: What do the red lines in this diagram represent?

Answer: Intermolecular forces Explanation: The solid blue lines represent the strong covalent bonds formed between the two oxygen atoms to form diatomic oxygen molecules. It takes a lot of energy to break these covalent bonds. The dashed red lines represent the weak intermolecular forces between oxygen molecules. Each molecule is separate from its neighbouring molecules. Because the intermolecular forces are weak, it does not take a lot of energy to overcome the forces between covalently bonded simple molecules - this is why the boiling and melting points of covalent molecules are relatively low.

Question: Why doesn't ammonia (a simple molecule) conduct electricity?

Answer: It does not carry a charge Explanation: Ionic compounds will conduct electricity if they are in a liquid because the ions carry a charge. Covalent molecules are often liquid at room temperature but cannot carry a charge because they are neutral. This can be tested by placing electrodes connected to a bulb into a solution of ethanol molecules - the bulb does not light up.

Question: Why are the melting points and boiling points of ionic compounds high?

Answer: Lots of energy is required to overcome a lattice of ionic bonding Explanation: Ionic compounds are made from millions of oppositely charged ions arranged in a lattice, where strong ionic bonds act in all directions. It takes a lot of energy to overcome the large number of strong bonds, which is why the boiling points of ionic compounds are high.

Question: Out of solid sodium chloride and liquid butane, which one would conduct electricity?

Answer: Neither Explanation: Sodium chloride is an ionic compound. It is made up of millions of charged ions which can carry a charge. However, in a solid they can only vibrate on the spot and so cannot move around to conduct electricity. If the compound were melted or dissolved so the ions were free to move around, the sodium chloride could conduct electricity. Although the molecules of butane are free to move around in a liquid, butane cannot conduct electricity because it is a neutral, simple covalent molecule which cannot carry a charge.

Question: What elements make up the shape memory alloy nitinol?

Answer: Nickel and titanium Explanation: After a shape memory alloy is bent and then cooled, it will exert force to return to its original shape if it is heated. Dentists use nitinol in braces in this way to help pull teeth in the right position. Shape memory alloys are also used to pull parts of badly broken bones back together.

Question: Liquid potassium iodide conducts electricity but liquid ethanol does not. Why is this?

Answer: Potassium iodide contains charged ions but ethanol does not Explanation: Ethanol cannot conduct electricity in any state because its simple covalent molecules are neutral and so cannot carry a charge. Solid potassium iodide, which is an ionic compound, cannot conduct electricity, because although the ions are charged they are not free to move around when they are in the solid. However, in molten potassium iodide, the ions are free to move around and so can conduct electricity.

Question: Why is pure iron so soft?

Answer: Regular layers of iron ions slide over each other easily Explanation: Ions in metals, for example iron, are held together in giant metallic structures. Metal ions are packed together in tight layers, and between them the free electrons can travel around the layers so are known as delocalised electrons. Because the layers are packed together so tightly, they can slide easily past each other. To make a metal harder we can add atoms of a different element (which are a different size) to form an alloy. The differently sized atom makes the structure less regular so that layers do not slide over each other so easily.

Question: A substance is a liquid at room temperature and does not conduct electricity. What structure does the substance likely have?

Answer: Simple covalent structure Explanation: Many simple molecules exist as liquids or gases at room temperature because they have low melting and boiling points. Simple molecules can also be solids with low melting points. Simple covalent molecules cannot conduct electricity as they do not carry a charge.

Question: Which factors determine the properties of a polymer?

Answer: The monomer and the reaction conditions Explanation: Polymers are made of chemicals extracted from crude oil. Small molecules called monomers are joined together to make polymers in a reaction called polymerisation. There are two types of poly(ethene) - low density (LD) and high density (HD) poly(ethene). They both have the same monomer (ethene) but are made under different reaction conditions.

Question: In an experiment, a student adds different volumes of borax solution to warm PVA glue and measures the 'rigidity' of the slime-like substance formed. Which of the statements below is true?

Answer: The more borax added, the more cross links are formed between polymers Explanation: Borax causes covalent cross-links to form between the polymer chains in the PVA glue. When polymer chains are cross-linked, the polymer does not melt when we heat it, so it is called a thermosetting polymer. This is because the polymers can't be separated from each other easily. Cross-linked polymers are also thicker and more rigid than polymers which are not bound together.

Question: What sort of polymers are used to make hairdriers?

Answer: Thermosetting Explanation: Polymer chains in thermosetting polymers are covalently cross linked. Covalent bonds are strong so they do not separate when heated. Thermosetting polymers are therefore used to make appliances which get hot because they do not soften and lose their shape when they are being used.

Question: Is the polymer represented in this diagram a thermosetting or thermosoftening polymer?

Answer: Thermosoftening polymer Explanation: Melting or boiling a substance involves separating the molecules from each other by overcoming the intermolecular forces between them. The covalent cross links between polymer molecules in thermosetting polymers require a lot of energy to break, so they do not soften when they are heated and only char at very high temperatures. Thermosoftening polymers, on the other hand, soften when they are heated, when they can be reshaped, and reset when they are cooled back down. This is because they are made up of individual polymer molecules which are tangled together.

Question: Why are nanoparticles used in cosmetic creams?

Answer: They are absorbed deeper into the skin Explanation: Nanoparticles are substances between 1 and 100 nanometres in size, which means they are made up of just a few atoms. As well as being used in cosmetic products, nanoparticles are also used in sun creams and for the delivery of medicines within the body.

Question: Why do materials behave so differently at the nanometre scale?

Answer: They have a large surface area:volume ratio Explanation: Nanoscience studies really tiny molecules on the nanometre scale - this scale looks at substances which are a few hundred atoms in size. Because of their high surface area:volume ratio the properties of nanoparticles are remarkable.

Question: LDPE and HDPE

Explanation: LDPE and HDPE are two different types of poly(ethene). The type of poly(ethene) formed depends on the conditions used for the polymerisation reaction. When the polymerisation rection is carried out using a catalyst at 50oC and a slightly raised pressure, HDPE is formed, which is made of straight poly(ethene) molecules. When the polymerisation reaction is carried out under very high pressures and trace amounts of oxygen, LDPE is formed, which contains branched poly(ethene) chains which cannot pack as closely together. The lower density of HDPE mean that it is lighter and more flexible than LDPE, but it is not suited for use at higher temperatures.

Question: What are the risks of nano-particles?

Explanation: Nanoscience studies molecules on the nanometre scale - these structures are the size of a few hundred atoms or less. There are many exciting new applications for nanoparticles. However, using them more and more could allow them access into our body which could cause major health problems. Their high surface areas make them good catalysts but could make them explosive.