06 Physical And Chemical Changes 2009

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CHANGES IN MATTER PHYSICAL CHANGES If paraffin wax is heated to 80 °C in a porcelain dish it will change from a yellowish white solid into a slightly yellow liquid. Now if you let it to stand for some time, the temperature will go back again to room temperature and paraffin wax will again show as a yellowish white solid with the same properties it had before the heating experiment. We can explain what happened during the experiment as a change of state: the wax’s particles that were vibrating around a fixed position (the microscopic description of a solid) gained enough energy to “release the chains” (overcome the forces) that held them in the crystal lattice and now slide past freely one another (became a liquid). As energy was dissipated to the environment and the temperature went down, the particles slowed down again to solidify or freeze to the original solid. Paraffin wax is still paraffin wax so its particles have been unchanged.

If sugar is added with some alcohol, it will “disappear” forming a solution. After some time, if we let alcohol to evaporate, sugar will re-appear as white and sweet as before. The particles in sugar according to the particle theory had been separated by alcohol mingling with them. As alcohol evaporated, the sugar particles crystallised back again.

Both examples above are physical changes. During a physical change, what is changing is the way that particles are arranged: from being closely aligned in a solid, to not so close but moving past one another in a liquid or widely separated in a gas. These changes are the changes of state. Particles of one substance can spread and “hide” mingling with other different particles as in dissolution or go back again to their partners as in crystallisation The changes of state, dissolution, diffusion, any change in the system that doesn’t change the particles themselves is a physical change. IN ANY OF THESE PREVIOUS EXAMPLES THE PARTICLES THEMSELVES SUFFER ANY CHANGE AT ALL. JUST THEIR ARRANGEMENT HAS CHANGED

2 CHEMICAL CHANGES AND CHEMICAL REACTIONS Let us repeat the first experiment but heating until paraffin vapours catch fire: light, heat, and some black smoke will appear and in the end the content of the dish completely disappears. If we wait until it cools down to room temperature, paraffin wax will not be re-formed. Once again paraffin has suffered changes but with completely different results. A more careful experiment could show that not just the black smoke was formed but other substances as water vapour and carbon dioxide were produced at the same time and that not just paraffin has disappeared but some oxygen in the air has been used up too. We have new particles formed by the same atoms as before. (Remember that atoms cannot be changed or destroyed). Chemical bonds keeping the atoms in the molecules have been broken and the new bonds that were formed lead to new different particles with properties different as the original paraffin: water, carbon dioxide, some soot)

In this kind of change, the particles have changed and new substances have been formed. ANY CHANGE IN WHICH REARRANGEMENT OF BONDS (COVALENT, IONIC METALLIC) TAKES PLACE, IS CALLED A CHEMICAL CHANGE.

In the diagram above, the first and the third drawings show the chemical change. The second drawing, the process itself in which this change happens is called the chemical reaction. If you mix some iron filings and sulphur powder, the mixture has the properties of both substances and you can separate them by different means (a magnet is the easiest way to achieve separation). But if you just heat for some seconds the mixture gets red-hot, glows and finally a dark residue is left behind. This residue does not show any of the properties of either iron or sulphur, and you will not separate sulphur from iron any more because there is no such thing as the metal iron or the yellow specks of sulphur. A new substance called iron sulphide has been formed. The covalent molecules of sulphur and the grey shiny gigantic structure of iron atoms clustered together in a sea of common electrons, have both disappeared: a black ionic substance formed by sulphur anions and iron cations has been formed.

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A CHEMICAL REACTION IS ABOUT FORMING AND BREAKING BONDS

CHEMICAL

Paraffin is made out of carbon chains with two hydrogen atoms bonded to each carbon of the chain. Can you guess in this case which bonds break as paraffin reacts with oxygen molecules (burns)? Can you guess which bonds are broken when water is formed reacting hydrogen molecules and oxygen molecules? Could you do the same for photosynthesis process? What bonds are formed in the three reactions named before? HOW CAN WE KNOW IF A CHEMICAL CHANGE HAS HAPPENED? How do we know if a system suffered a physical or a chemical change if we cannot se the particles? There are some signs generally accepted as distinctive between these two types of change: PHYSICAL CHANGES Same substances before and after change Easy to reverse Low to moderate energy exchange with environment

CHEMICAL CHANGES Different substances before and after change Not easily reversed if not impossible Generally strong energy exchange with environment

Looking at a system we can tell a chemical change has happened because there are different substances in it. A substance has a set of properties that makes it different from the rest. If new substances are formed, the system changes its properties. If a drop of bleach falls on a coloured fabric, the colour slowly disappears: a chemical reaction between bleach and the dyestuff has happened. An iron bolt left outdoors for a long time, will no longer look as a grey strong metallic object. It will appear as a crusty brownish “earthy” material. A chemical reaction has also taken place. Sometimes, as in melting wax, the properties change very evidently. But as the temperature draws back and the system returns to ambient temperature, the system returns to what it used to be: the process has been reversed, the change was just temporary. If wax is added with ether it will disappear. The properties have changed and we have no solid at all. But if ether is allowed to evaporate at 30- 40 °C wax will crystallise again showing the same properties it used to have. Physical changes are usually reversible but that is not the case for chemical changes (see if the rusted bolt will turn greyish and shiny after keeping it for a year in your bedroom!). Finally, as a chemical reaction proceeds energy exchanges with the environment take place. Take a sheet of paper and put it in a small flame for one second. It burns in air releasing a lot of energy as heat and light, because burning (combustion) is a chemical

4 reaction. Do burnt ashes look like a sheet of paper? Can you just put the ashes to cool and they turn back into the original white sheet? THE MEANING OF CHEMICAL FORMULAE It is important to know that chemists use the symbols of the elements and the formulae of the compounds instead of writing the names of the substances involved in a chemical reaction. Thus, in the previous equation iron is replaced by its symbol (Fe) and hydrochloric acid by its formula (HCl). But: what is a formula? A formula is a symbol that tells you what are the elements that form the molecule of a substance and how many of each class are bonded in the molecule. The number of atoms of a certain element in the compound is written as a subscript. The subscript “1” is always omitted (is implicit) The formula for water is H2O; it tells us that the water molecule is made of two hydrogen atoms and one oxygen atom The formula of carbon dioxide is CO2; it tells us that in a carbon dioxide molecule there are two oxygen atoms linked to one carbon atom. The formula of glucose is C6H12O6: what atoms does it contain? How many of each? But we have learned that there are substances that are not made of molecules but of ions electrically attracted together to form a solid 3-D structure like a network. Sodium chloride (common table salt) is one of these substances. Is there a formula for them? Yes, the formula in this case will tell you not the number but the ratio between positive and negative ions in the lattice (network). NaCl means that you have as many sodium cations as you have chloride anions in the structure. Al2O3 states that you will found three oxide anions for every two aluminium cations. CHEMICAL EQUATIONS The elements are represented by chemical symbols (Na, H, O, Fe) and the substances by chemical formulae (H2, NaOH, H2O, Fe2O3). Chemical reactions are represented by means of chemical equations. Let us see how a chemical reaction is written In a chemical change the REACTANTS are the substances standing before the change has happened and the PRODUCTS are the substances that remain after the reaction has occurred. • The first step to write a chemical equation is to decide which are which: find and label the reactants and the products in the reaction. Most times it is simple but many others some reactants (as oxygen in rusting) they might be not clearly shown. • Having identified the reactants and products, the next step is to write a word equation. The reactants’ names are written on the left side of the page, separated by “+” signs. The products are written on the right side also separated by “+” signs. Between both reactants and products an arrow pointing in the direction of the process, that is, from left to right will be written Suppose that you are asked to write the word equation of the reaction between iron and hydrochloric acid (aqueous hydrogen chloride) to form iron (II) chloride and hydrogen. You have to analyse which are “befores” and which are “afters”. In this case, the

5 reactants (before) are iron and hydrochloric acid and the products (after) iron (II) chloride and hydrogen. Writing both sets as previously explained: iron + hydrochloric acid

→

iron (II) chloride + hydrogen

Using the appropriate symbols the word reaction that describes the action of hydrochloric acid on iron will turn into: Fe + HCl →

FeCl2 + H2

If we inspect the symbol equation we can realise that there are more chlorine and hydrogen atoms in the products than there are in the reactants’ side of the equation! Does this mean that atoms have been created? By no means!! BALANCING EQUATIONS: LAVOISIER’S CONSERVATION OF MASS From Dalton to Bohr, the impossibility of creating or annihilating atoms during chemical reactions has been accepted and proved to hold. Hence, we must change something in the equations to make it consistent with this fact. If we write a “2” before the formula of HCl, meaning that two molecules of it are to be used in the reaction, the problem will be solved: Fe + 2 HCl →

FeCl2 + H2

Now the symbolic equation shows what is really happening and we have the same number of atoms before and after the reaction. We say that the reaction has been balanced The essential rule to remember is in the very foundations of chemical science. It is called Lavoisier’s law of the conservation of mass and elements: “The mass of a closed system and of each of the elements in it remains constant no matter the changes that may happen.” If the total mass of iron and acid that will react is, for instance, 254 g, the mass of the hydrogen and iron (II) chloride obtained will be 254 g. In case there remained some (unchanged) acid or iron, the 254 g include the unreacted material. If the reaction vessel had been an open flask, hydrogen would have escaped and the total mass should be less. This fact about gases escaping to the atmosphere or gases from the atmosphere being incorporated (because they can be part of the reactants) made chemistry unpredictable and it was not until Lavoisier’s experiments and conclusion that some rational behaviour was found in chemical changes. A very important observation was the conservation of the elements. If among the products there is a copper salt, of course that some reactant must contain copper, and vice versa; in case some element appears in the reactants’ list it should appear ion the products’. This law knocks down the search of the philosopher’s stone, that magic object that would turn vile metals into gold. Sorry Mr Potter!

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SYNTHESES AND DECOMPOSITIONS Look at the following chemical equations: S + Fe → FeS 2 H2 + O2 → 2 H2O 6 CO2 + H2O → C6H12O6

(sulphur + iron → iron (II) sulphide) (hydrogen + oxygen → water) (carbon dioxide + oxygen → glucose)

In the three of them two different reactants “link” to form one product that looks somewhat more “complicated”: this class of reactions are known as syntheses. In a synthesis two or more species, react to form a new more complex substance. Plants synthesise glucose, starch and cellulose from carbon dioxide and water. We ourselves synthesise very huge and complex molecules as the proteins starting from a score of small amino acids. From sulphur, oxygen (in air) and water chemists make the dangerous and very well known sulphuric acid. Small molecules of ethylene are linked to form a vastly known gigantic molecule: polyethylene. All these are syntheses. Now look at the following: 2 HgO → 2 Hg + O2

(mercury (II) oxide → mercury + oxygen)

2 H2O2 → 2 H2O + O2

(hydrogen peroxide → water + oxygen)

C6H12O6 → 6 CO2 + H2O

( glucose → carbon dioxide + water)

In all these examples, one substance is broken into two (maybe more) different and simpler substances. These reactions are called decompositions. The decomposition of bauxite (a mineral formed mainly by aluminium oxide) forms aluminium metal and oxygen. Bauxite is the ore we get all the aluminium from. The stepwise decomposition of glucose to carbon dioxide and water in mitochondria is the source of most of the energy in living organisms. REDOX REACTIONS When carbon reacts with oxygen to form CO2 (carbon dioxide) the reaction belongs to the class we have called syntheses. But it also belongs to a class known as redox reactions. In a restricted interpretation, A CHEMICAL SPECIES (A MOLECULE, ATOM OR ION) GETS OXIDISED WHEN IT COMBINES OR GAINS OXYGEN. SIMILARLY, IT GETS REDUCED WHEN IT LOOSES OR GIVES OXYGEN.

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Obviously, as atoms are not created or destroyed, when something is getting oxidised some other thing must be getting reduced; otherwise where should the oxygen atoms be coming from? When carbon is oxidised to carbon dioxide, oxygen atoms are giving oxygen: both oxygen atoms in the molecule are loosing their partner! According to what we have stated they are getting reduced. In the examples of the previous section, bauxite is reduced to aluminium, and mercury oxide is reduced to mercury. In the chloroplasts, carbon dioxide is reduced to glucose (looses oxygen) and in the mitochondria glucose is oxidised back to carbon dioxide.. Oxygen is a very reactive gas and attacks most metals. That is why, except for gold, silver most common metals are never found native and must be obtained reducing the minerals in which they are found. Iron is made in blast furnaces heating haematite (a natural iron oxide) with carbon monoxide. The haematite is reduced and carbon monoxide gets oxidised: Iron oxide + carbon monoxide → iron + carbon dioxide When atoms combine with oxygen they always loose control of their electrons. If the atom is a metal it will give them forming a cation; if it is a non metal it will share them but oxygen will always have the better share because it is the more electronegative element (“electron greedy” element). In this case the “electron loss” is not complete. This fact leads to a wider concept of redox reactions: an atom gets oxidised when it looses electrons (no matter against what other element) and gets reduced when it gains electrons from other elements. We will use this approach further in electricity: CORROSION AND RUSTING ARE REDOX REACTIONS Weathering (the action of atmospheric agents on objects) many times causes corrosion (eating up materials). Acid rain corrodes buildings, salty mist corrodes cars. Metals corrode forming oxides (they get oxidised). In particular iron oxide is called rust. Rusting is a major technical problem. To prevent rusting of iron we protect it avoiding contact with air: to that purpose iron pieces at the ironmonger’s are frequently covered with an oily layer. Window frames, doors etc. are painted for aesthetic reasons but most important, to prevent rusting. Iron is galvanised (covered with a thin layer of zinc) to avoid rusting; zinc is oxidised and the oxide forms a thin, tenacious, impermeable layer of zinc carbonate that will not let oxygen get to iron. COMBUSTIONS ARE REDOX REACTIONS TOO Combustion is burning. Combustion is a highly exothermic (heat forming) redox reaction in which energy is emitted as heat and light. As it burns, a fuel combines

8 with oxygen (supplied by the air in most opportunities). This redox reaction is not spontaneous (generally speaking) and must be started with heat. As it proceeds, the heat released will keep the reaction going on. So much energy is set free that the gases get so hot that they start emitting light (as redhot iron does at the blacksmith’s). The hot light-emitting gases form the flame or fire. In combustion processes we have: • A fuel, the substance from which we get energy from. Wood and fossil fuels are the most common fuels. Fossil fuels are natural gas, derivatives from petroleum (petrol, paraffine oil, gas oil etc.) and coal. • Air: air is the regular oxygen supply for a combustion • Heat: once the reaction has been started, it is self-sustained by the energy that comes from the reaction itself This three factors form what is popularly known as the FIRE TRIANGLE. If one of sides in the triangle is removed the fire will stop.

QUESTIONS AND PROBLEMS 1- Decide whether the following are physical or chemical changes: Ice melting Candle wax melting Custard being made

Iron rusting Hair drying Milk sours

Petrol burns A candle burning Perfume evaporates

Clothes being ironed Sulphur lump being crushed Wood burns

2- The drawing below shows some molecules found in air and a key to know what atom each “ball” represents. Write the formula for these molecules. Does any look familiar? Name it!

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3- Work out ball models for the following molecules: C2H2 H2O2 CF4 N2 O2 4- Write word equations for the following reactions: a- copper combines with sulphur to form copper (II) sulphide b- ethanol burns in oxygen to form water and carbon dioxide c- calcium carbonate decomposes into carbon dioxide and calcium oxide d- carbon monoxide is formed when carbon dioxide reacts with carbon 5- Balance the following equations: a- H2 + O2 → H2O b- H2O2 → H2O + O2 c- CaCO3 → CO2 + CaO d- N2 + H2 → NH3 6- Which of the reactions in exercise (5) are syntheses? Which are decompositions? 7- Reactions (a) and (b) in exercise (5) are redox reactions from the point of view we have established. Which species is being oxidised in each case? Which is being reduced? 8- According to the fire triangle: a- Why does water stop a fire? b- Why is a man wrapped up in a blanket if he catches fire? c- If you enter a room where you suspect there is a gas leak, do never put the lights on before venting out for a while. Give reasons for this safety rule. d- How can foam stop a fire? e- Water should never be used to quench cooking oil and petrol (gasoline) fires. Why?