Practical Tests for Cations PDF

# PRACTICAL Tests for Cations **So now you know how to work out what the anion in a mystery compound is. Time to find out what the positive ions (cations) in a compound. Don't say I don't spoil you.** **You Can Use Flame Tests to Identify Metal Ions** Compounds of some metals produce a characteri...

# PRACTICAL Tests for Cations **So now you know how to work out what the anion in a mystery compound is. Time to find out what the positive ions (cations) in a compound. Don't say I don't spoil you.** **You Can Use Flame Tests to Identify Metal Ions** Compounds of some metals produce a characteristic colour when heated in a flame. 1) You can test for various metal ions by putting your substance in a flame and seeing what colour the flame goes. * Lithium, Li+, gives a crimson red flame. * Sodium, Na+, gives a yellow flame. * Potassium, K+, gives a lilac flame. * Calcium, Ca2+, gives a brick red flame. * Copper, Cu2+, gives a blue-green flame. 2) To carry out a flame test in the lab, first clean a nichrome wire loop by dipping it into hydrochloric acid and then rinsing it in deionised water. 3) Then dip the wire loop into a sample of the metal compound and put the loop in the clear blue part of a Bunsen flame (the hottest bit). Record what colour the flame goes. **Some Metal Ions Form a Coloured Precipitate with Sodium Hydroxide** This is also a test for metal ions, but it's slightly more complicated: 1) Many metal hydroxides are insoluble and precipitate out of solution when formed. Some of these hydroxides have a characteristic colour. 2) For this test, you add a few drops of sodium hydroxide solution to a solution of your mystery compound. 3) If a hydroxide precipitate forms, you can use its colour to tell which metal ion was in the compound. | Metal ion | Colour of precipitate | Ionic equation | |---|---|---| | Calcium, Ca2+| White | Ca2+ + 2OH- → Ca(OH)2 | | Copper, Cu2+| Blue | Cu2+ + 2OH- → Cu(OH)2 | | Iron(II), Fe2+| Green | Fe2+ + 2OH- → Fe(OH)2 | | Iron(III), Fe3+| Brown | Fe3+ + 3OH- → Fe(OH)3 | | Zinc, Zn2+| White at first, but then redissolves in excess NaOH to form a colourless solution. | Zn2+ + 2OH- → Zn(OH)2<br>Then: Zn(OH)2 + 2OH- → Zn(OH)42- | **Nobody presses my shirts better than Mister Furryboots...** If you're having trouble remembering that anions are negative ions and cations are positive, try imagining a happy (positive) cat doing the ironing. It might even make you smile in the middle of an exam, which is definitely a plus. **Q1** In a flame test, if a compound gives a brick red flame, what would this tell you? **Q2** A student adds a few drops of sodium hydroxide solution to another solution. A green precipitate forms. What does this tell you about the solution? # Halogen Displacement Reactions The halogens are a pretty competitive lot really. In fact the more reactive ones will push the less reactive ones out of a compound. How uncivilized! Has nobody ever taught them that it's bad manners to push? **A More Reactive Halogen Will Displace a Less Reactive One** 1) The elements in Group 7 take part in displacement reactions. 2) A displacement reaction is where a more reactive element 'pushes out' (displaces) a less reactive element from a compound. 3) For example, chlorine is more reactive than bromine (it's higher up Group 7). If you add chlorine water (an aqueous solution of Cl2) to potassium bromide solution, the chlorine will displace the bromine from the salt solution. 4) The chlorine is reduced to chloride ions, so the salt solution becomes potassium chloride. The bromide ions are oxidised to bromine, which turns the solution orange. 5) The equation for this reaction is shown below: **Cl2 + 2KBr → Br2 + 2KCl** **chlorine + potassium bromide → bromine + potassium chloride** **Displacement Reactions Show Reactivity Trends** You can use displacement reactions to show the reactivity trend of the halogens. 1) Start by measuring out a small amount of a halide salt solution in a test tube. 2) Add a few drops of a halogen solution to it and shake the tube gently. 3) If you see a colour change, then a reaction has happened - the halogen has displaced the halide ions from the salt. If no reaction happens, there won't be a colour change. 4) Repeat the process using different combinations of halide salt and halogen. 5) The table below shows what should happen when you mix different combinations of chlorine, bromine and iodine water with solutions of the salts potassium chloride, potassium bromide and potassium iodide. | Start with: | Potassium chloride solution KCl (aq) colourless | Potassium bromide solution KBr (aq) colourless | Potassium iodide solution KI (aq) colourless | |---|---|---|---| | Add chlorine water Cl2 (aq) colourless | no reaction | orange solution (Br2) formed | brown solution (I2) formed | | Add bromine water Br2 (aq) - orange | no reaction | no reaction | brown solution (I2) formed | | Add iodine water I2 (aq) brown | no reaction | no reaction | no reaction | 6) Chlorine displaces both bromine and iodine from salt solutions. Bromine can't displace chlorine, but it does displace iodine. Iodine can't displace chlorine or bromine. 7) This shows the reactivity trend - the halogens get less reactive as you go down the group. This is to do with how easily they gain electrons. **New information displaces old information from my brain...** If you remember that the halogens get less reactive as you go down the group, you can work out what will happen when you mix any halogen with any halide salt. You need to know the colour changes too. **Q1** Give the balanced symbol equation for the displacement reaction between bromine and sodium iodide. # Group 7 - Halogens Here's a page on another periodic table group that you need to be familiar with - the halogens. **Group 7 Elements are Known as the Halogens** Group 7 is made up of the elements fluorine, chlorine, bromine, iodine and astatine. 1) All Group 7 elements have 7 electrons in their outer shell so they all have similar chemical properties. 2) The halogens exist as diatomic molecules (e.g. Cl2, Br2, I2). Sharing one pair of electrons in a covalent bond (see page 20) gives both atoms a full outer shell. 3) As you go down Group 7, the melting points and boiling points of the halogens increase. 4) This means that at room temperature: * Chlorine (Cl2) is a fairly reactive, poisonous, green gas (it has a low boiling point). * Bromine (Br2) is a poisonous, red-brown liquid, which gives off an orange vapour at room temperature. * Iodine (I2) is a dark grey crystalline solid which gives off a purple vapour when heated. **Reactivity Decreases Going Down Group 7** 1) A halogen atom only needs to gain one electron to form a 1- ion with a stable electronic structure. 2) The easier it is for a halogen atom to attract an electron, the more reactive the halogen will be. 3) As you go DOWN Group 7, the halogens become less reactive - it gets harder to attract the extra electron to fill the outer shell when it's further away from the nucleus (the atomic radius is larger). **The Halogens React With Alkali Metals to Form Salts** The halogens will react vigorously with alkali metals (Group 1 elements, see page 51) to form salts called 'metal halides'. For example: **2Na + Cl2 → 2NaCl** **Sodium + Chlorine → Sodium chloride** **2K + Br2 → 2KBr** **Potassium + Bromine → Potassium bromide** **The Halogens Undergo Displacement Reactions** 1) A more reactive halogen can displace a less reactive one from a salt solution. 2) There's loads more about these reactions coming up on the next page... **Halogens - one electron short of a full shell...** **Q1** The melting point of chlorine (Cl2) is -101.5 **C**. Predict whether bromine (Br2) would be a solid, a liquid or a gas at -101.5 **C**. Explain your answer. **Q2** Write a balanced symbol equation for the reaction between sodium metal and iodine. # Reactivity of Metals Reactive metals tend to do exciting, fizzy things when you drop them into acid or water. If you do the same with an unreactive metal, it'll just sit there. How boring. Here's a bit more detail on reactivity experiments... **How Metals React With Acids Tells You About Their Reactivity** 1) The easier it is for a metal atom to lose its outer electrons and form a positive ion, the more reactive it will be. 2) Here's a classic experiment that you can do to show that some metals are more reactive than others. All you do is to place little pieces of various metals into dilute hydrochloric acid: **Metals Also React With Water** The reactions of metals with water also show the reactivity of metals. This is the basic reaction: **metal + water → metal hydroxide + hydrogen** **(Or: less reactive metal + steam → metal oxide + hydrogen)** 1) Very reactive metals like potassium, sodium, lithium and calcium will all react vigorously with water. 2) Less reactive metals like magnesium, zinc and iron won't react much with cold water, but they will react with steam. You could show this in the lab using this experiment: 3) Copper won't react with either water or steam. **I AM NOT HIGHLY REACTIVE - OK...** **Q1** Give the balanced equation, including state symbols, for the reaction of sodium and water. **Q2** A student has small samples of three metals, A, B and C. He puts them in dilute hydrochloric acid. Metal C fizzes a bit and the gas given off gives a quiet squeaky pop when lit with a burning splint. Metal B fizzes vigorously. The gas given off gives a loud squeaky pop when lit with a burning splint. The same thing happens to Metal A. a) Put the three metals in order, from most reactive to least reactive. b) The metals used were zinc, magnesium and copper. Use this to identify metals A, B and C. # Group 0 - Noble Gases The elements in Group O of the periodic table are known as the noble gases. 'Noble' here is just being used in the old chemistry sense of being unreactive - nothing to do with them being particularly honourable or good. **Group 0 Elements are All Inert, Colourless Gases** Group O elements are called the noble gases. Group O is made up of the elements helium, neon, argon, krypton, xenon and radon. 1) All of the elements in Group O are colourless gases at room temperature. 2) The noble gases are all monatomic - that just means that their gases are made up of single atoms (not molecules). 3) They're also more or less inert - this means they don't react with much at all. The reason for this is that they have a full outer shell of electrons. This means they don't easily either give up or gain electrons. 4) As the noble gases are inert, they're non-flammable - they won't set on fire. 5) These properties make the gases pretty hard to observe - it took a long time for them to be discovered. **There are Patterns in the Properties of the Noble Gases** 1) As with the other groups in the periodic table, there are trends in the properties of the noble gases, for example, boiling point, melting point and density all increase as you go down Group 0. 2) You could be given information about a particular property of the noble gases (or Group 1 and Group 7 elements) and asked to use it to estimate the value of this property for a certain element. For example: **EXAMPLE** Use the densities of helium (0.2 kg/m3) and argon (1.8 kg/m3) to predict the density of neon. Neon comes between helium and argon in the group, so you can predict that its density will be roughly halfway between their densities: (0.2 + 1.8) / 2 = 2.0 / 2 = 1.0 Neon should have a density of about 1.0 kg/m3. **EXAMPLE** The table on the right shows the melting points of the first five noble gases. Predict the melting point of radon. Melting points increase as you go down the group, so radon's melting point must be higher than xenon's. To predict how much higher, look at the gaps between the melting points of the other elements: He to Ne: (-249) - (-272) = 23 Ne to Ar: (-189) - (-249) = 60 Ar to Kr: (-157) - (-189) = 32 Kr to Xe: (-112) - (-157) = 45 The gaps aren't exactly the same, so find the average gap: (23 + 60 + 32 + 45) / 4 = 160 / 4 = 40 °C Now you can just add this average gap to the melting point of xenon. Radon should have a melting point of about (-112) + 40 = -72 °C. 4) You could be asked about how an element reacts too, so remember - elements in the same group react in similar ways as they all have the same number of electrons in their outer shells. And, all you need to do to find which group an element is in is look at the periodic table. Simple. **What's a pirate's favourite element? Arrrrgon...** The noble gases might seem a bit dull, given how unreactive they are, but they're not so bad. They'd be pretty good at hide and seek for a start. And what would helium balloon sellers be without them? Deflated. **Q1** The boiling points of the first four noble gases are: helium = -269 °C, neon = -246 °C, argon = -186 °C and krypton = -153 °C. Predict the boiling point of xenon. # Transition Metals You'll find the transition metals sitting together slap bang in the middle of the periodic table. They've got plenty of different properties that you need to know about - so grab a cup of tea and a biscuit, and read on... **The Transition Metals Sit in the Middle of the Periodic Table** A lot of everyday metals are transition metals (e.g. copper, iron, zinc, gold, silver, platinum) - but there are loads of others as well. If you get asked about a transition metal you've never heard of don't panic. These 'new' transition metals will follow all the properties you've already learnt for the others. **Transition Metals Have Typical Metallic Properties** 1) The transition metals have all the typical properties of metals (see page 24): * they are hard, strong and shiny materials that conduct heat and electricity well. 2) They have high melting points (with the exception of mercury, which is liquid at room temperature). 3) They also have high densities. For example, at room temperature, potassium has a density of 0.9 g/cm3, while copper has a density of 9.0 g/cm3, and iron has a density of 7.9 g/cm3. **Transition Metals and Their Compounds Make Good Catalysts** 1) Iron is the catalyst used in the Haber process for making ammonia (see p.80). 2) Vanadium pentoxide (V2O5) is the catalyst for making sulfuric acid in the Contact Process (see p.82). **Transition Metals Often Have More Than One Ion, e.g. Fe2+, Fe3+** Two other examples are copper, which has Cu+ and Cu2+ ions, and chromium which has Cr2+ and Cr3+ ions. **Their Compounds are Very Colourful** The compounds of transition elements are colourful. What colour they are depends on what transition metal ion they contain - e.g. compounds containing Fe2+ ions are usually light green, ones with Fe3+ ions are orange/brown (e.g. rust, see page 79) and those with Cu2+ ions are often blue. **Transition Metals are Relatively Unreactive** 1) Transition metals are much less reactive than Group 1 and Group 2 metals. 2) For example, most transition metals will react with dilute acids to form metal salts (see next page), but these reactions happen much more slowly than the reaction of metals like sodium and magnesium with dilute acids. **You can't get much more colourful than transition metal ions...** Transition metals are everywhere. They make good catalysts, iron's used to make steel for construction, copper's so unreactive you can use it to make water pipes, and you can even use their pretty compounds to colour stained glass. **Q1** Name one industrial process that uses a transition metal catalyst. Name the catalyst used. **Q2** Rubidium is a Group 1 metal. Palladium is a transition metal. a) Predict which of these two metals will have a higher density. Explain your answer. b) Predict which of these two metals will be more reactive. Explain your answer. # The Reactivity Series and Displacement On the previous page, you covered some reactions that help you work out how reactive a metal is. You can use information like this to put the metals in order of their reactivity. Which is more useful than it sounds, promise. **The Reactivity Series Shows How Reactive Metals Are** A reactivity series is just a table that lists metals in order of their reactivity. Here's an example: **More Reactive Metals Displace Less Reactive Ones** 1) If you put a more reactive metal into a solution of a less reactive metal salt, the reactive metal will "kick out" the less reactive metal from the salt. You end up with a solution of the more reactive metal salt and the less reactive metal. 2) If you put a less reactive metal into a solution of a more reactive metal salt, nothing will happen. The more reactive metal (copper) is already in the salt. 3) You can use displacement reactions to work out where in the reactivity series a metal should go. **Q1** State whether magnesium would displace iron from iron sulfate solution. Explain your answer. **Q2** Tin sits between iron and copper in the reactivity series. State whether tin would displace zinc from zinc sulfate solution and explain your answer. # Chemical Analysis You've just seen loads of tests that you can do by hand in a lab to help you identify gases and ions. Well, you can also get some high tech machines that will analyse mystery substances for you. **You Can Use Machines to Analyse Substances** 1) You can analyse and identify elements and compounds using instrumental methods - this just means using machines (rather than a person doing chemical analysis). 2) Instrumental analysis is used in science research labs, but also in medical labs, police forensics work, environmental analysis and testing products in industry. 3) There are a number of advantages to instrumental methods: * They are very sensitive - they can detect even the tiniest amounts of a substance. * They are very fast (and tests can even be automated). * They are very accurate - they don't involve human error, as manual analysis does. 4) There are lots of different ways that chemists use machines to analyse samples and identify chemicals. Here are some examples: * **INFRARED SPECTROSCOPY**: This technique produces a graph showing which frequencies of infrared radiation a molecule will absorb (or transmit). You can use the absorbance pattern on the graph to identify the molecule. * **ULTRAVIOLET (UV) SPECTROSCOPY**: Similar to infrared spectroscopy, but using ultraviolet light. * **GAS CHROMATOGRAPHY (see page 30)**: Used to separate out the chemicals in a mixture. This technique produces a graph (chromatogram) with one peak for each chemical. The time that each chemical takes to pass through the machine (retention time) can be used to identify it. * **MASS SPECTROSCOPY**: A technique that can be used to find the relative molecular mass of a mystery compound (or the relative atomic mass of a mystery element). **You Can Identify Unknown Substances Using Instrumental Analysis** You could be given the results of an instrumental analysis and asked to interpret them. Here's an example. **EXAMPLE** This is the mass spectrum of an unknown alcohol. Determine the identity of the alcohol. To find the relative molecular mass of the alcohol, you look at the molecular ion peak (the M peak). This is the peak with the highest m/z value. The m/z value of the M peak is 46, so the M of the alcohol must be 46. If you calculate the molecular masses of the first few alcohols, you'll find that the one with a molecular mass of 46 is ethanol (C2H5OH). **Unfortunately, you can't get a machine to sit exams for you...** If you are ever given the results of an instrumental test to analyse, as in the example on this page, don't panic just remember that all the information you need will be right there in the question. And speaking of questions... **Q1** State one advantage of using instrumental methods for analysis.