- Created by: mgcd1998
- Created on: 22-02-14 15:21
Using a Catalyst
Catalysts increase the rate of reaction without being used up. They do this by lowering the activation energy needed. With a catalyst, more collisions result in a reaction, so the rate of reaction increases. Different reactions need different catalysts.
Catalysts are important in industry because they reduce costs.
Effects of catalysts
A catalyst is a substance that can increase the rate of a reaction. The catalyst itself remains unchanged at the end of the reaction it catalyses. Only a very small amount of catalyst is needed to increase the rate of reaction between large amounts of reactants.
Different catalysts catalyse different reactions.
Some common catalysts used in industry and the reactions they catalyse
CatalystReaction catalysed Iron Making ammonia from nitrogen and hydrogen Platinum Making ammonia from nitrogen and hydrogen Vanadium(V) oxide Making sulfuric acid
Modern cars have a catalytic converter to help reduce the production of toxic gases. Catalytic converters use a platinum and rhodium catalyst with a high surface area. This increases the rate of reaction of carbon monoxide and unburnt fuel from exhaust gases with oxygen from the air. The product from this is carbon dioxide and water, which is less harmful to the environment. The catalysts are designed to work best at the high temperatures found in the engine.
What is an ion?
Ions are electrically charged particles formed when atoms lose or gain electrons. This loss or gain leaves a complete highest energy level, so the electronic structure of an ion is the same as that of a noble gas - such as a helium, neon or argon.
Metal atoms and non-metal atoms go in opposite directions when they ionise:
- Metal atoms lose the electron, or electrons, in their highest energy level and become positively charged ions
- Non-metal atoms gain an electron, or electrons, from another atom to become negatively charged ions
How many charges?
There is a quick way to work out what the charge on an ion should be:
- The number of charges on an ion formed by a metal is equal to the group number of the metal
- The number of charges on an ion formed by a non-metal is equal to the group number minus eight
- Hydrogen forms H+ ions
Forming an ionic bond
When metals react with non-metals, electrons are transferred from the metal atoms to the non-metal atoms, forming ions. The resulting compound is called an ionic compound.
Consider reactions between metals and non-metals, for example:
- Sodium + chlorine → sodium chloride
- Magnesium + oxygen → magnesium oxide
- Calcium + chlorine → calcium chloride
In each of these reactions, the metal atoms give electrons to the non-metal atoms. The metal atoms become positive ions and the non-metal atoms become negative ions.
When a non-metal forms a bond the name ending changes. In these reactions the ending is –ide showing only that element is present. If the ending was –ate it means that oxygen is also present as well as the element.
There is a strong electrostatic force of attraction between these oppositely charged ions, called an ionic bond. The animation shows ionic bonds being formed in sodium chloride, magnesium oxide and calcium chloride.
Properties of ionic compounds
- High melting and boiling points - Ionic bonds are very strong - a lot of energy is needed to break them. So ionic compounds have high melting and boiling points.
- Conductive when liquid - Ions are charged particles, but ionic compounds can only conduct electricity if their ions are free to move. Ionic compounds do not conduct electricity when they are solid - only when dissolved in water or melted.
Soluble and insoluble salts
SolubleInsoluble All nitrates None Most sulfates Lead sulfate, barium sulfate and calcium sulfate Most chlorides, bromides and iodides Silver chloride, silver bromide, silver iodide, lead chloride, lead bromide, lead iodide Sodium carbonate, potassium carbonate, ammonium carbonate Most other carbonates Sodium hydroxide, potassium hydroxide, ammonium hydroxide Most other hydroxides
Notice that nitrates and most chlorides are soluble. This is why many of the chemicals you use in the laboratory are nitrates or chlorides. To make an insoluble salt, it is possible to make two soluble salts react together in a precipitation reaction.
Making an insoluble salt
You can see from the table above that silver chloride is insoluble. To make it, you need a soluble silver salt and a soluble chloride salt. Silver nitrate and sodium chloride are both soluble. When mixing their solutions together, the result is insoluble silver chloride and soluble sodium nitrate.
The silver chloride appears as tiny particles suspended in the reaction mixture: it forms a precipitate. The precipitate can be filtered, washed with water on the filter paper and then dried in an oven.
Here are the word and balanced formulae equations for the reaction:
silver nitrate (soluble) + sodium chloride (soluble) → silver chloride (insoluble) + sodium nitrate (soluble)
AgNO3 + NaCl → AgCl + NaNO
Uses of insoluble salts
Barium sulfate is an example of an insoluble salt. It is used with patients in order to help diagnose problems with the intestine. Like bone and metal, barium sulfate shows up on an x-ray. A 'barium meal' is given to a patient and they are then x-rayed. The barium sulfate will show up the shapes of the intestine. Doctors can then tell if there are any problems such as growths or lumps.
Barium sulfate is toxic but it is safe to use because it is insoluble (does not dissolve). This prevents it from entering the blood.
Metals change the colour of a flame when they are heated in it. Different metals give different colours to the flame, so flame tests can be used to identify the presence of a particular metal in a sample. This is how you would carry out a typical flame test:
- Dip a clean flame test loop in the sample solution
- Hold the flame test loop at the edge of a Bunsen burner flame
- Observe the changed colour of the flame, and decide which metal it indicates
- Clean the loop in acid and rinse with water, then repeat steps 1 to 3 with a new sample
Testing for the sulfate ion
You can test to see if a solution contains sulfate ions by using barium chloride. If barium chloride solution is added to a sample of water containing sulfate ions, barium sulfate is formed. Barium sulfate is insoluble in water, and will be seen as a white precipitate.
Barium chloride solution + sodium sulfate solution → sodium chloride solution + solid barium sulfate
Testing for halide ions
The halogens are the elements in group 7 of the periodic table, like chlorine, bromine and iodine. Their ions are called halide ions.
You can test to see if a solution contains chloride, bromide or iodide ions by using silver nitrate. If silver nitrate solution is added to a sample of water containing halide ions the silver halide is precipitated. This is because the silver halides are all insoluble in water.
The results look like this:
- Silver chloride is a white precipitate
- Silver bromide is a cream precipitate
- Silver iodide is a pale yellow precipitate
Testing for carbonate ions
Limewater is used to test for the presence of carbonate ions (CO32-). Acid is added to the test compound. If carbonate ions are present then carbon dioxide gas bubbles off. If this is passed through limewater is turns the limewater from clear to cloudy.
All atoms give off light when heated, although sometimes this light is not visible to the human eye. A prism can be used to split this light to form a spectrum, and each element has its own distinctive line spectrum. This technique is known as spectroscopy.
Chemists use spectroscopy to detect very small amounts of an element.
Scientists have used line spectra to discover new elements. In fact, the discovery of some elements, such as rubidium and caesium, was not possible until the development of spectroscopy. The element helium was discovered by studying line spectra emitted by the Sun.
Low melting and boiling points
The covalent bonds binding the atoms together are very strong but there are only very weak forces holding the molecules to each other (the intermolecular forces). Therefore, only a low temperature is needed to separate the molecules when they are melted or boiled.
Covalent compounds have no free electrons and no ions so they do not conduct electricity.
These contain only a few atoms held together by strong covalent bonds. An example is carbon dioxide (CO2), the molecules of which contain one atom of carbon bonded with two atoms of oxygen.
Properties of simple molecular substances
- Low melting and boiling points - This is because the weak intermolecular forces break down easily.
- Non-conductive - Substances with a simple molecular structure do not conduct electricity. This is because they do not have any free electrons or an overall electric charge.
Hydrogen, ammonia, methane and water are also simple molecules with covalent bonds. All have very strong bonds between the atoms, but much weaker forces holding the molecules together. When one of these substances melts or boils, it is these weak 'intermolecular forces' that break, not the strong covalent bonds. Simple molecular substances are gases, liquids or solids with low melting and boiling points.
Giant molecular compounds
Giant molecular compounds
Giant covalent structures contain a lot of non-metal atoms, each joined to adjacent atoms by covalent bonds. The atoms are usually arranged into giant regular lattices - extremely strong structures because of the many bonds involved.
Properties of giant covalent structures
- Very high melting points - Substances with giant covalent structures have very high melting points, because a lot of strong covalent bonds must be broken. Graphite, for example, has a melting point of more than 3,600ºC.
- Variable conductivity - Diamond does not conduct electricity. Graphite contains free electrons, so it does conduct electricity. Silicon is semi-conductive - that is, midway between non-conductive and conductive.
Graphite and Diamond
Graphite is a form of carbon in which the carbon atoms form layers. These layers can slide over each other, so graphite is much softer than diamond. It is used in pencils, and as a lubricant. Each carbon atom in a layer is joined to only three other carbon atoms. Graphite conducts electricity.
Diamond is a form of carbon in which each carbon atom is joined to four other carbon atoms, forming a giant covalent structure. As a result, diamond is very hard and has a high melting point. It does not conduct electricity.
Silica and buckminsterfullerene
Silica, which is found in sand, has a similar structure to diamond. It is also hard and has a high melting point, but contains silicon and oxygen atoms, instead of carbon atoms.The fact that it is a semi-conductor makes it immensely useful in the electronics industry: most transistors are made of silica.
Buckminsterfullerene is yet another allotrope of carbon. It is actually not a giant covalent structure, but a giant molecule in which the carbon atoms form pentagons and hexagons - in a similar way to a leather football. It is used in lubricants.
The allotropes of carbon: DIAMOND
A diamond is one giant molecule of carbon atoms.
Diamonds are colourless and transparent. They sparkle and reflect light, which is why they are described as lustrous. These properties make them desirable in items of jewellery.
Diamond is extremely hard and has a high melting point. For this reason it is very useful in cutting tools. The cutting edges of discs used to cut bricks and concrete are tipped with diamonds. Heavy-duty drill bits, such as those used in the oil exploration industry to drill through rocks, are made with diamonds so that they stay sharp for longer.
Diamond is insoluble in water. It does not conduct electricity.
Every atom in a diamond is bonded to its neighbours by four strong covalent bonds leaving no free electrons and no ions. This explains why diamond does not conduct electricity. The bonding also explains the hardness of diamond and its high melting point. Significant quantities of energy would be needed to separate atoms so strongly bonded together.
The allotropes of carbon: GRAPHITE
Graphite is formed from carbon atoms in layers.
Graphite is black, shiny and opaque. It is not transparent. It's also a very slippery material. It's used in pencil leads because it slips easily off the pencil onto the paper and leaves a black mark. It is a component of many lubricants, for example bicycle chain oil. Graphite is insoluble in water. It has a high melting point and is a good conductor of electricity, which makes it a suitable material for the electrodes needed in electrolysis.
Each carbon atom is bonded into its layer with three strong covalent bonds. This leaves each atom with a spare electron, which together form a delocalised 'sea' of electrons loosely bonding the layers together. These delocalised electrons can all move along together – making graphite a good electrical conductor. Because the layers are only weakly held together they can easily slip over one another. This explains why graphite is so slippery. Melting graphite is not easy, though. It takes considerable energy to break the strong covalent bonds and separate the carbon atoms.
The allotropes of carbon: BUCKMINSTERFULLERENE
Buckminsterfullerene is one type of fullerene. Fullerenes are made from carbon atoms joined together to make balls, 'cages' or tubes of carbon. The molecules of Buckminsterfullerene are spherical and are also known as 'buckyballs' – formula C60.
Buckminsterfullerene is a black solid although it is coloured in certain solutions eg deep red when in petrol.
The tube fullerenes are called nanotubes which are very strong and are conductors of electricity. Their unusual electrical properties mean that nanotubes are used as semiconductors in electronic circuits. Their strength makes them useful in reinforcing structures where exceptional lightness and strength are needed for example, the frame of a tennis racket. They are also used as a platform for industrial catalysts.
Mixtures of liquids can be separated according to their properties. The technique used depends on whether the liquids dissolve in each other, and so are miscible, or if they are immiscible. Fractional distillation is a technique used to separate liquids according to their boiling points. Chromatography is used to separate mixtures of coloured compounds.
Liquids can be described in two ways – immiscible and miscible. The separation technique used for each liquid depends on the properties of the liquids.
Immiscible means that the liquids don't dissolve in each other – oil and water are an example. It is possible to shake up the liquids and get them to mix but they soon separate. Separating immiscible liquids is done simply using a separating funnel. The two liquids are put into the funnel and are left for a short time to settle out and form two layers. The tap of the funnel is opened and the bottom liquid is allowed to run. The two liquids are now separate.
Fractional distillation of liquid air
About 78 per cent of the air is nitrogen and 21 per cent is oxygen. These two gases can be separated by fractional distillation of liquid air.
Liquefying the air
Air is filtered to remove dust, and then cooled in stages until it reaches –200°C. At this temperature it is a liquid. We say that the air has been liquefied.
Here's what happens as the air liquefies:
- Water vapour condenses, and is removed using absorbent filters
- Carbon dioxide freezes at –79ºC, and is removed
- Oxygen liquefies at –183ºC
- Nitrogen liquefies at –196ºC
The liquid nitrogen and oxygen are then separated by fractional distillation.
Different chromatograms and the separated components of the mixtures can be identified by calculating the Rf value using the equation:
Rf = distance moved by the compound ÷ distance moved by the solvent
The Rf value of a particular compound is always the same - if the chromatography has been carried out in the same way. This allows industry to use chromatography to identify compounds in mixtures.
Chromatography can be used to separate mixtures of coloured compounds. Mixtures that are suitable for separation by chromatography include inks, dyes and colouring agents in food.
Simple chromatography is carried out on paper. A spot of the mixture is placed near the bottom of a piece of chromatography paper and the paper is then placed upright in a suitable solvent, eg water. As the solvent soaks up the paper, it carries the mixtures with it. Different components of the mixture will move at different rates. This separates the mixture out.
The particles in a metal are held together by strong metallic bonds. It takes a lot of energy to separate the particles. That is why they have high melting points and boiling points.
Solid metals are crystalline. This means that the particles are close together and in a regular arrangement.
Metals have loose electrons in the outer shells which form a 'sea' of delocalised negative charge around the close-packed positive ions. There are strong electrostatic forces holding the particles together.
The elements in the centre of the periodic table, between groups 2 and 3, are called the transition metals. Most of the commonly used metals are there, including iron, copper, silver and gold.
The transition metals have the following properties in common:
- They form coloured compounds
- They are good conductors of heat and electricity
- They can be hammered or bent into shape easily
- They are less reactive than alkali metals such as sodium
- They have high melting points - but mercury is a liquid at room temperature
- They are usually hard and tough
- They have high densities