Chemistry unit 2

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  • Created by: Megan
  • Created on: 11-04-13 21:39

Atomic and mass number

These two numbers tell you how many of each kind of particle an atom has. 

The mass number = Total number of protons and neutrons.

The atomic number = Total number of protons. 

1The atomic number tells you how many protons there are. 2. Atoms of the same element all  have the same number of  protons - so atoms of  different elements will have  different numbers of protons. 3. To get the number of neutrons, just subtract the atomic number from the mass  number. Electrons aren't counted in the mass number because their relative mass is very  small.  

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Compounds

1. Compounds are formed when atoms of two or more elements are chemically combined together. For example, carbon dioxide is a compound formed from a chemical reaction between carbon and oxygen. 

2. It's difficult to seperate the two original elements out again. 

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Isotopes

ISOTOPES ARE: DIFFERENT ATOMIC FORMS OF THE SAME ELEMENT, WHICH HAVE THE SAME NUMBER OF PROTONS BUT A DIFFERENT NUMBER OF NEUTRONS


1. The upshot is: isotopes must have the same atomic number but different mass numbers. 

2. If they had different atomic numbers, they'd be different elements altogether. 

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Ionic bonding

A SHELL WITH JUST ONE ELECTRON IS WELL KEEN TO GET RID...       All the atoms over at the left-hand side of the periodic table, have just one or two electrons in their outer shell. And they're pretty keen to get shot of them, because then they'll only have full shells left, which is how they like it. So given half a chance they do get rid, and that leaves the atom as an ion instead. Now ions aren't the kind of things that sit aroound quietly. They tend to leap at the first passing ion with an opposite charge.       

A NEARLY FULL SHELL IS WELL KEEN TO GET THAT EXTRA ELECTRON...        On the other side of the periodic table, the elements in group 6 and 7, such as oxygen and chlorine, have outer shells which are nearly full. They're obviously pretty keen to gain that extra one or two electrons to fill the shell up. When they do of course they become ions and before you know it, pop, they've latched onto the atom (ion) that gave up the electron a moment earlier. 

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Ionic compounds

1. Ionic compounds always have giant ionic lattices

2. The ions form a closely packed regular lattice arrangement. 

3. There are very strong electrostatic forces of attraction between oppositely charged ions, in all directios

4. A single crystal of sodium chloride (salt) is one giant ionic lattice, which is why salt crystals tend to be cuboid in shape. The Na+ and Cl- ions are held together in a regular lattice. 

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Ionic compounds (similar properties)

1. They all have high melting points and high boiling points due to the strong attraction between the ions. It takes a large amount of energy to overcome this attraction. When ionic compounds melt, the ions are free to move and they'll carry electric current

2. They do dissolve early in water though. The ions seperate and are all free to move in the solution, so they'll carry electric current

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Group 1 & 2 and 6 & 7

 1. Remember, atoms that have lost or gained an electron are ions.                         2. Ions have the electronic structure of a noble gas.                 3. The elements that most readily form ions are those in groups 1, 2, 6 and 7.         4.  Group 1 and 2 elements are metals and they lose electrons to form positive ions.        5. For example, group 1 elements (the alkali metals) form ionic compounds with non -         metals where the metal ion has a 1+ charge.                                                 6. Group 6 and 7 elements are non-metals. They gain electrons to form negative ions.    7. For example, group 7 elements form ionic compounds with the alkali metals where the   halide ion has a 1- charge.           8. The charge on the positive ions is the same as the group number of the element.        9. Any of the positive ions above can combine with any of the negative ions to form              an ionic compound.                    10. Only elements at opposite sides of the periodic table will form ionic compounds. 

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Formula of an ionic compound

1. Ionic compounds are made up of positively charged part and a negatively charged part. 

2. The overall charge of any compound is zero.

3. So all the negative charges in the compound must balance all the positive charges

4. You can use the charges on the individual ions present to work out the formula for the ionic compound: 

  • Sodium chloride contains Na+ (+1) and Cl- (-1) ions. 

(+1) + (-1) = 0. The charges are balanced with one of each ion, so the formula for sodium chloride = NaCl. 

  • Magnesium chloride contains Mg 2+ (+2) and Cl- (-1) ions. 

Because a chloride ion only has a 1- charge we will need two of them to balance out 2+ charge of a  magnesium ion. This gives us the formula MgCl2. 

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Covalent Bonding

1. Sometimes atoms prefer to make covalent bonds by sharing electrons with other atoms.        2. They only share electrons in their outer shells (highest energy levels).           3. This way both atoms feel that they have a full outer shell, and that makes them happy. Having a full outer shell gives them the electronic structure of a noble gas.           4. Each covalent bond provides one extra shared electron for each atom.                  5. So, a covalent bond is a shared pair of electrons.        6. Each atom involved has to make enough covalent bonds to fill up its outer shell. 

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Simple molecular substances

1. The atoms from very strong covalent bonds to form small molecules of several atoms.   2. By contrast, the forces of attraction between these molecules are very weak.        3. The result of these feeble intermolecular forces is that the melting and boiling points are very low, because the molecules are easily parted from each other. It's the intermolecular forces that get broken when simple molecular substances melt or boil - not the much stronger covalent bonds.        4. Most molecular substances are gasses or liquids at room temperature, but they can be solids.        5. Molecular substances don't conduct electricity - there are no ions so there's no electrical charge

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Giant covalent structures

1. These are similar to giant ionic structures (lattices) except that there are no charged ions.       2. All the atoms are bonded to each other by strong covalent bonds.         3. This means that they have very high melting and boiling points.        4. They don't conduct electricity - not even when molten (except from graphite).        5. The main examples are dioamond and graphite, which are both made only from carbon atoms, and silicon dioxide (silica).               

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Giant covalent structures

DIAMOND - Each carbon atom forms four covalent bonds in a very rigid giant covalent structure. This structure makes diamond the hardest natural substance, so it's used for drill tips. And it's pretty and sparkly too.

SILICON DIOXIDE - Sometimes calledsilica, this is what sand is made of. Each grain of sand is one giant structure of silicon and oxygen. 

GRAPHITE - Each carbon atom only forms 3 covalent bonds. This creates layers which are free to slide over each other, like a pack of cards - so graphite is soft and slippery. The layers are held togeether so loosely that they can be rubbed off onto paper - that's how a pencil works. This is because there are weak intermolecular forces between the layers. Graphite is the only non-metal which is a good conductor of heat and electricity. Each carbon atom has onedelocalised electron and it's these free electrons that conduct heat and electricity. 

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Metal properties

1. Metals also consist of a giant structure.        2. Metallic bonds involve the all-important free-electrons which produce all the properties of metals. These delocalised (free) electrons come from the outer shell of every metal atom in the structure.        3. These electrons are free to move through the whole structure and so metals are good conductors of heat and electricity.        4. These ekectrons also hold the atoms together in a regular structure. There are strong forces of electrostatic attraction between the positive metal ions and the negative electrons.        5. They also allow the layers of atoms to slide over each other, allowing metals to be bent and shaped

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Alloys

Pure metals often aren't quite right for certain jobs. So scientists mix two or more metals together - creating an alloy with the properties they want. 

Different elements have different sized atoms. So when another metal is mixed with a pure metal, the new metal atoms will distort the layers of metal atoms, making it more difficult for them to slide over each other. So alloys are harder

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Smart materials

1. Smart materials behave differently depending on the conditions, e.g. temperature. 

2. A good example is nitinol - a shape memory alloy. It's a metal alloy (about half nickel, half titanium) but when it's cool you can bend it and twist it like rubber. Bend it too far, though, and it stays bent. But here's the really clever bit - if you heat it above a certain temperature, it goes back to a remembered shape. 

3. It's really handy for glasses frames. If you accidently bend them, you can just pop them into a bowl of hot water and they'll jump back into shape

4. Nitinol is also used for dental braces. In the mouth it warms and tries to return to a remembered shape, and so it gently pulls the teeth with it. 

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Nanoparticles

1. Really tiny particles, 1-100 nanometres across, are called nanoparticles.        2. Nanoparticles contain roughly a few hundred atoms.        3. Nanoparticles include fullerenes. These are molecules of carbon, shaped like hollow balls or closed tubes. The carbon atoms are arranged in hexagonal rings. Different fullerenes contain different numbers of carbon atoms.         4. A nanoparticle has very different properties from the bulk chemical that it's made from.  5. Using nanoparticles is known as nanoscience. Many new uses of nanoparticles are being developed: a. They have huge surface area to volume ratio, so they could help make new industrial catalysts. b. You can use nanoparticles to make sensors to detect one type of molecule and nothing else. These highly specific sensors are already being used to test water purity. c. Nanotubes can be used to make stronger, lighter building materials. d. New cosmetics, have been made using nanoparticles. The small particles do their job but don't leave white marks on the skin. e. Nanomedicine is a hot topic. The idea is that the tiny fullerences are asorbed more easily by the body than most particles. This means they could deliver drugs right into the cells where they're needed. f. New lubricant coatings are being developed using fullerenes. These coatings reduce friction a bit like ball bearings and could be used in all sorts of places from artifical joints to gears

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Properties of plastics

Strong covalent bonds hold the atoms together in long chains. But it's the bonds between the different molecule chains that determine the properties of the plastic. 

WEAK FORCES - Individual tangled chains of polymers, held together by weak intermolecular forces, are free to slide over each other.        THERMOSOFTENING POLYMERS don't have cross-linking between chains. The forces between the chains are really easy to overcome, so it's dead easy to melt the plastic. When it cools, the polymer hardens into a new shape. You can melt these plastics and remould them as many times as you like. 

STRONG FORCES - Some plastics have stronger intermolecular forces between the polymer chains, called crosslinks, that hold the chains firmly together.        THERMOSETTING POLYMERS have crosslinks. These hold the chains together in a solid structure. The polymer doesn't soften when it's heated. Thermosetting polymers are the tough guys of the plastic world. They're strong, hard and rigid

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Polymer properties

1. The starting materials and reaction conditions will both affect the properties of a polymer. 

2. Two types of polyethene can be made using different conditions: 

  • Low density (LD) polyethene is made by heating ethene to about 200C under high pressure. It's flexible and is used for bags and bottles. 
  • High density (HD) polyethene is made at a lower temperature and pressure (with a catalyst). It's more rigid and is used for water tanks and drainpipes. 
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Relative atomic mass

1. This is just a way of saying how heavy different atoms are compared with the mass of an atom of carbon-12. So carbon-12 has Ar, of exactly 12

2. It turns out that the relative atomic mass Ar is usually just the same as the mass number of the element. 

3. In the periodic table, the elements all have two numbers. The smaller one is the atomic number (how many protons it has). But the bigger one is the mass number or relative atomic mass

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Relative formula mass

If you have a compound like MgCl2 then it has a relative formula mass, Mr, which is just all the relative atomic masses added together

For MgCl2 it would be: 

24 ---> MgCl<--- (35.5 x 2)  24 + (35.5 x 2) = 95  So the Mr for MgCl2 is simply 95

You can easily get Ar, for any element from the periodic table, but in a lot of questions they give you them anyway. And that's all it is. 

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One mole of a substance

The relative formula mass (Ar or Mr) of a substance in grams is known as one mole of that substance. 

NUMBER OF MOLES = MASS IN G / Mr 

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Calculating % of mass

Percentage mass OF AN      = Ar x No. of atoms (of that element)  x 100       ELEMENT IN A COMPOUND      Mr (of whole compound) 

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Empirical formula

Method: 

1. List all the elements in the compound (there's usually only two or three). 

2. Underneath them, write their experimental masses or percentages

3. Divide each mass or percentage by the Ar, for that particular element. 

4. Turn the numbers you get into a nice simple ratio by multiplying and/or dividing them by well-chosen numbers. 

5. Get the ratio in its simplest form, and that tells you the empirical formula of the compound. 

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Calculating masses in reactions

1. Write out the balanced equation

2. Work out Mr - just for the two bit you want 

3. Apply the rule: divide to get one, then multiply to get all  (but you have to apply this first to the substance they give you information about, and then the other one). 

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Percentage yield

The amount of product you get is known as the yield. The more reactants you start with, the higher the actual yield will be. But the percentage yield doesn't depend on the amount of reactants you started with - it's a percentage

1. The predicted yield of a reaction can be calculated from the balanced reaction equation.                2. Percentage yield is given by the formula:        PERCENTAGE YIELD = actual yield (grams) / predicted yield (grams) x 100        3. Percentage yield is always somewhere between 0 and 100%.        4. A 100% percentage yield means that you got all the product you expected to get.        5. a 0% percentage yield means that no reactants were converted into product, i.e. no product at all was made

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Yields are always less than 100%

Even though no atoms are gained or lost in reactions, in real life, you never get a 100% percentage yield. Some product or reactant always gets lost along the way - and that goes for big industrial processes as well school lab experiments. There are several reasons: 

1. The reaction is reversible:        A reversible reaction is one where the products of the reaction can themselves react to produce the orginal reactants.        This means that the reactants will never be completely converted to products because the reaction goes both ways. Some of the products are always reacting together to change back to the original reactants. This will mean a lower yield.                2. When you filter a liquid to remove solid particles, you nearly always lose a bit of liquid or a bit of solid. So, some of the product may be lost when it's seperated from the reaction mixture.        3. Things don't always go exactly to plan. Sometimes there can be other unexpected reactions happening which use up the reactants. This means there's not as much reactant to make the product you want. 

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Artificial colours

A food colouring might contain one dye or it might be a mixture of dyes. Here's how you can tell:        1. Extract the colour from a food sample by placing it in a small cup with a few drops of solvent (can be water, ethanol, salt water, etc.).        2. Put spots of the coloured solution on a pencil baseline on filter paper.        3. Roll up the sheet and put it in a beaker with some solvent - but keep the baseline above the level of the solvent.        4. The solvent seeps up the paper, taking the dyes with it. Different dyes form spots in different places.        5. Watch out though - a chromatogram with four spots means at least four dyes, not exactly four dyes. There could be five dyes, with two of them making a spot in the same place. It can't be three dyes though, because one dye can't split into two spots. 

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Gas chromatography

Gas chromatography can seperate out a mixture of compounds and help you identify the substances present.         1. A gas is used to carry substances through a column packed with a solid material.        2. The substances travel through the tube at different speeds, so they're seperated.         3. The time they take to reach the detector is called the retention time. It can be used to      help identify the substances.         4. The recorder draws a gas chromatograph. The number of peaks shows the number of      different compounds in the sample.         5. The position of the peaks shows the retention time of each substance.         6. The gas chromatography column can also be linked to a mass spectrometer. This              process is known as GC-MS and can identify the substances leaving the column              very accurately.                 7. You can work out the relative molecular mass of each of the substance from the                graph it draws. You just read off from the molecular ion peak

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The rate of reaction

Reactions can go at all sorts of different rates

1. One of the slowest is the rusting of iron.        2. A moderate speed reaction is a metal (like magnesium) reacting with acid to produce a gentle stream of bubbles.        3. A really fast reaction is an explosion, where it's all over in a fraction of a second. 

The rate of a reaction depends on four things

1. Temperature       2. Concentration       3. Catalyst        4. Surface area of solids 

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Ways to measure the rate of a reaction

Rate of reaction = amount of reactant used or amount of product formed / time

1. PRECIPITATION - a. This is when the product of the reaction is a precipitate which clouds the solution. b. Observe a mark through the solution and measure how long it takes for it to disappear. c. The quicker the mark disappears, the quicker the reaction. d. This only works for reactions where the intial solution is rather see-through. e. The result is very subjective-different people might not agree over the point where the mark disappears.       2. CHANGE IN MASS (usually gas given off) - a. Measuring the speed of a reaction that produces a gas can be carried out on a mass balance. b. As the gas is released the mass disappearing is easily measured on the balance. c. The quicker the reading on the balance drops, the faster the reaction. d. This is the most accurate of the three methods because the mass balance is very accurate. The disadvantage is gas released into room.   3. THE VOLUME OF GAS GIVEN OFF - a. This involves the use of a gas syringe to measure the volume of gas given off. b. The more gas given off during a given time interval, the faster the reaction. c. Gas syringes usually give volumes accurate to the nearest millilitre, so they're quite accurate. You have to be careful - if the reaction is too vigorous, you can easily blow the plunger out of the end of the syringe. 

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Collision theory

MORE COLLISIONS INCREASES THE RATE OF REACTION -       The effects of temperature, concentration and surface area on the rate of reaction can be explained in terms of how often the reacting particles collide successfully

1. HIGHER TEMPERATURES INCREASES COLLISION -                        When the temperature is increased the particles all move quicker. If they're moving quicker, they're going to collide more often

2. HIGHER CONCENTRATION (OR PRESSURE) INCREASES COLLISION -        If a solution is made more concentrated it means there are more particles of reactant knocking about between the water molecules which makes collisions between the important particles more likely. In a gas, increasing the pressure means the particles are more squashed up together so there will be more frequent collisions

3. LARGER SURFACE AREA INCREASES COLLISIONS -        If one of the reactants is a solid then breaking it up into smaller pieces will increase the total surface area. This means the particles around it in the solution will have more area to work on, so there'll be more frequent collisions

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Faster collisions increase the rate of reaction

Higher temperature also increases the energy of the collisions, because it makes all the particles move faster

INCREASING THE TEMPERATURE CAUSES FASTER COLLISIONS

Reactions only happen if the particles collide with enough energy

The minimum amount of energy needed by the particles to react is known as the activation energy

At a higher temperature there will be more particles colliding with enough energy to make the reaction happen.

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Catalysts speed up reactions

Many reactions can be speeded up by adding a catalyst

 A CATALYST IS A SUBSTANCE WHICH SPEEDS UP A REACTION, WITHOUT BEING CHANGED OR USED UP IN THE REACTION 

A solid catalyst works by giving the reacting particles a surface to stick to. This increases the number of successful collisions (and so speeds the reaction up). 

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Catalysts help reduce costs (industrial)

 1. Catalysts are very important for commercial reasons.         2. Catalysts increase the rate of the reaction, which saves a lot of money simply because  the plant doesn't need to operate for as long to produce the same amount of stuff.            3. Alternatively, a catalyst will allow the reaction to work at a much lower temperature.  That reduces the energy used up in the reaction (the energy cost), which is good  for sustainable development and can save a lot of money too.                                         4. There are disadvantages to using catalysts, though.         5. They can be very expensive to buy, and often need to be removed from the product  and cleaned. They never get used up in the reaction though, so once you've got them you  can use them over and over again.         6. Different reactions use different catalysts, so if you make more than one product at  your plant, you'll probably need to buy different catalysts for them.                         7. Catalysts can be poisoned by impurities, so they stop working, e.g. sulfur impurities can poison the iron catalyst used in the Haber process (used to make ammonia for fertillisers). That means you have to keep your reaction mixture very clean

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Exothermic reaction

AN EXOTHERMIC REACTION IS ONE WHICH TRANSFERS ENERGY TO THE SURROUNDINGS, USUALLY IN THE FORM OF HEAT AND USUALLY SHOWN BY A RISE IN TEMPERATURE

1. The best example of an exothermic reaction is burning fuels - also called COMBUSTION. This gives out a lot of heat - it's very exothermic.                2. Neutralisation reactions (acid + alkali) are also exothermic.        3. Many oxidation reactions are exothermic. For example, adding sodium to water produces heat, so it must be exothermic. The sodium emits heat and moves about on the surface of the water as it is oxidised.        4. Exothermic reactions have lots of everyday uses. For example, some hand warmers use the exothermic oxidation of iron in air (with a salt solution catalyst) to generate heat. Self heating cans of hot chocolate and coffee also rely on exothermic reactions between chemicals in their bases. 

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Endothermic reaction

AN ENDOTHERMIC REACTION IS ONE WHICH TAKES IN ENERGY FROM THE SURROUNDINGS, USUALLY IN THE FORM OF HEAT AND IS USUALLY SHOWN BY A FALL IN TEMPERATURE

Endothermic reactions are much less common. Thermal decompositions are a good example: 

Heat must be supplied to make calcium carbonate decompose to make quicklime. CaCO3 ---> CaO + CO2 

Endothermic reactions also have everyday uses. For example, some sports injury packs use endothermic reactions - they take in heat and the pack becomes very cold. More convenient than carrying ice around. 

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Reversible reactions

In reversible reactions, if the reaction is endothermic in one direction, it will be exothermic in the other direction. The energy absorbed by the endothermic reaction is equal to the energy released during the exothermic reaction. A good example is the thermal decomposition of hydrated copper sulfate

hydrated copper sulfate <---> anhydrous copper sulfate + water

---> If you heat blue hyrdrated copper sulfate crystals it drives the water off and leaves white anhydrous copper sulfate powder. This is endothemic.

<--- If you then add a couple of drops of water to the white powder you get the blue crystals back again. This is exothermic. 

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the pH scale

1. The pH scale is a measure of how acidic or alkaline a solution is. 

2. The strongest acid has pH 0. The strongest alkali has pH 14

3. A neutral substance has pH 7. 

AN INDICATOR IS JUST A DYE THAT CHANGES COLOUR -                The dye in the indicator changes colour depending on whether it's above or below a certain pH. Universal indicator is a combination of dyes which gives the colours. It's very useful for estimating the pH of a solution. 

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Acids and bases

    An ACID is a substance with a pH of less than 7. Acids form H^+ ions in water.                                    A BASE is a substance with a pH of greater than 7.                                          An ALKALI is a base that dissolves in water. Alkalis form OH^- ions in water.         So, H^+ ions make solutions acidic and OH^- ions make them alkaline.

 The reaction between acids and bases is called neutralisation. Make sure you learn it: 

acid + base ---> salt +water 

Neutralisation can also be seen in terms of H^+ and OH^- ions like this: 

H^+  +  OH^-  ---> H2O 

When an acid neutralises a base (or vice versa), the products are neutral, i.e. they have a pH of 7. An indicator can be used to show that a neutralisation reaction is over (universal indicator will go green). 

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Acids reacting with metals

ACID + METAL ---> SALT + HYDROGEN

 1. The more reactive the metal, the faster the reaction will go - very reactive metals (e.g.  sodium) react explosively.                       2. Copper does not react with dilute acids at all - because it's less reactive  than hydrogen.     3. The speed of reaction is indicated by the rate at which the bubbles of hydrogen are  given off.     4. The hydrogen is confirmed by the burning splint test giving the notorious squeaky  pop.             5. The name of the salt produced depends on which metal is used, and which acid is  used: 

  • HYDROCHLORIC ACID WILL ALWAYS PRODUCE CHLORIDE SALTS 
  • SULFURIC ACID WILL ALWAYS PRODUCE SULFATE SALTS 
  • NITRIC ACID PRODUCES NITRATE SALTS WHEN NEUTRALISED 
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Metal oxides and hydroxides are bases

1. Some metal oxides and metal hydroxides dissolve in water. These soluble compounds are alkalis

2. Even bases that won't dissolve in water will still react with acids. 

3. So, all metal oxides and metal hydroxides react with acids to form a salt and water

ACID + METAL OXIDE ---> SALT + WATER 

ACID + METAL HYDROXIDE ---> SALT + WATER 

(These are neutralisation reactions

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Ammonia

Ammonia dissolves in water to make an alkaline solution

When it reacts with nitric acid, you get a neutral salt - ammonium nitrate

NH3 + HNO3 ---> NH4NO3 

AMMONIA + NITRIC ACID ---> AMMONIUM NITRATE 

This is a bit different from most neutralisation reactions because there's no water produced - just the ammonium salt. 

Ammonium nitrate is an especially good fertiliser because it has nitrogen from two sources, the ammonia and the nitric acid. Kind of a double dose. Plants need nitrogen to make proteins

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Making soluble salts

1. You need to pick the right acid, plus a metal or an insoluble base (a metal oxide or metal hydroxide). E.g. if you want to make copper chloride, mix hydrochloric acid and copper oxide

2. You add the metal, metal oxide or hydroxide to the acid - the solid will dissolve in the acid as it reacts. You will know when all the acid has been neutralised because the excess solid will just sink to the bottom of the flask. 

3. Then filter out the excess metal, metal oxide or metal hydroxide to get the salt solution. To get pure, solid crystals of the salt, evaporate some of the water (to make the solution more concentrated) and then leave the rest to evaporate very slowly. This is called crystallisation

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Making soluble salts using an alkali

1. You can't use the method above with alkalis (soluble bases) like sodium, potassium or ammonium hydroxides, because you can't tell whether the reaction has finished - you can't just add an excess to the acid and filter out what's left. 

2. You have to add exactly the right amount of alkali to just neutralise the acid - you need to use an indicator to show when the the reaction's finished. Then repeat using exactly the same volumes of alkali and acid so the salt isn't contaminated with indicator. 

3. Then just evaporate off the water to crystalise the salt as normal. 

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Making insoluble salts

1. If the salt you want to make is insoluble, you can use a precipitation reaction

2. You just need to pick two solutions that contain the ions you need. E.g. to make lead chloride you need a solution which contains lead ions and one which contains chloride ions. So you can mix lead nitrate solution (most nitrates are soluble) with sodium chloride solution (all group 1 compounds are soluble). 

3. Once the salt has precipitated out (and is lying at the bottom of your flask), all you have to do is filter it from the solution, wash it and then dry it on filter paper. 

4. Precipitation reactions can be used to remove poisonous ions (e.g. lead) from drinking water. Calcium and magnesium ions can also be removed from water this way - they make water hard, which stops soap lathering properly. Another use of precipitation is in treating effluent (sewage) - again, unwanted ions can be removed. 

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Electrolysis

1. If you pass an electric current through an ionic substance that's molten or in a solution, it breaks down into the elements it's made of. This is called electrolysis

2. It requires a liquid to conduct the electricity, called the electrolyts

3. Electrolytes contain free ions - they're usually the molten or dissolved ionic substance

4. In either case it's the free ions which conduct the electricity and allow the whole thing to work. 

5. For an electrical circuit to be complete, there's got to be a flow of electrons. Electrons are taken away from ions at the positive electrode and given to other ions at the negative electrode. As ions gain or lose electrons they become atoms or molecules and are released. 

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Oxidation and reduction

1. Back in core chemistry you learnt about reduction involving the loss of oxygen. However...

2. Reduction is also a gain of electrons

3. On the other hand, oxidation is a gain of oxygen or a loss of electrons

4. So reduction and oxidation don't have to involve oxygen

5. Electrolysis always involves an oxidation and a reduction 

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Reactivity

1. Sometimes there are more than two free ions in the electrolyte. For example, if a salt is dissolved in water there will also be some H^+ and OH^- ions

2. At the negative electrode, if metal ions and H^+ ions are present, the metal ions will stay in solution if the metal is more reactive than hydrogen. This is because the more reactive an element, the keener it is to stay as ions. So, hydrogen will be produced unless the metal is less reactive than it. 

3. At the positive electrode, if OH^- and halide ions (Cl^-, Br^-, I^-) are present then molecules of chlorine, bromine or iodine will be formed. If no hallide is present, then oxygen will be formed. 

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Electroplating uses electrolysis

1. Electroplanting uses electrolysis to coat the surface of one metal with another metal, e.g. you might want to electroplate silver onto a brass cup to make it look nice. 

2. The negative electrode is the metal object you want to plate and the positive electrode is the pure metal you want it to be plated with. You also need the electrolyte to contain ions of the plating metal. (The ions that plate the metal object come from the solution, while the positive electrode keeps the solution topped up.) 

3. There are lots of different uses for electroplating: 

  • Decoration: Silver is attractive, but very expensive. It's much cheaper to plate a boring brass cup with silver, than it is to make the cup out of solid silver - but it looks just as pretty
  • Conduction: Metals like copper conduct electricity well - because of this they're often used to plate metals for electronic circuits and computers. 
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