Atoms, Compounds and Isotopes
Atoms are made up of three small particles:
- Protons (Charge + 1 and Mass 1)
- Neutrons (Charge 0 and Mass 1)
- Electrons (Charge -1 and Mass 1/2000)
There are two numbers that describe an atom: - The Mass Number tells you the total number of Neutrons and Protons in the Nucleus of a specific atom. - The Atomic Number tells you the total number of Protons in a specific atom. (And as there are the same number of Electrons as Protons in an atom, it tells you the number of Electrons).
Compounds are formed when atoms of two or more elements are Chemically Combined together
Isotopes are different atomic forms of the same element, which have the SAME number of PROTONS, but a DIFFERENT number of NEUTRONS.
This means they have different Mass Numbers but the same Atomic Numbers. If they had different Atomic Numbers they would be different elements all together.
In ionic bonding atoms lose or gain electrons to form charged particles (called ions) which are strongly attracted to each other (because of the oppostite charges they contain)
Ionic compounds have a giant ionic lattices and the ions are packed closely together in a regular lattice arrangement and there have very strong electrostatic forces of attraction between oppositely charged ions.
Ionic compunds all have simular properties. They all have high melting and boiling points due to the strong attraction bettween the ions. It takes a large amount of energy to overcome this attraction.
When ionic compund melt, the ions are free to move and they'll carry eletric current.
They also dissolve easily in water. The ions separate and our free to move in the solution, so they'll carry electric current.
Ions and Formulas
Atoms that have lost or gained electrons are ions. Ions have the electronic structure of a Noble Gas (Group 0 - Helium, Neon, Argon, Krypton, Xenon, Radon).
The elements that most readily form ions are those in Groups 1, 2, 6, and 7. Group 1 and 2 elements are metals, and they lose electrons to form positive ions. Groups 6 and 7 elements are Non-metals, and they gain electrons to form negative ions.
Ionic compounds are made up of a positive and negative part.
The overall charge of one therefore, is 0.
So all the negative charges in the compound must balance all the positive charges.
You can use the charges on individual ions 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 forumula is NaCl.
Electronic Structure of Ions
Sometimes atoms like to make Covalent Bonds by Sharing electrons with other atoms.
They only share electrons in their Outer Shells. This way both atoms feel they have a full outer shell. (This gives them the structure of a noble gas)
Each covalent bond provides one extra electron for each atom. So a covalent bond is a shared pair of electrons. Each atom has to involved has to make enough covalent bonds to fill their outer shell.
More Covalent Bonding
There are 7 important examples to learn:
- H2. Hydrogen goes around in pairs, as it only has one electron in its outer shell, and only needs one covalent bond.
- Cl2. Chlorine also goes round in pairs, as it only needs one electron to complete its outer shell.
- Methanes formula is CH4. Carbon has 4 outer electrons, which is half a full shell. So it forms 4 covalent bonds with 4 hydrogen atoms.
- Hydrogen Chloride (HCl). This is similar to H2 and Cl2, both elements need one electron to complete an outer shell, so one covalent bond is formed.
- Ammonia (NH3). Nitrogen has 5 outer electrons, and so forms 3 covalent bonds with 3 hydrogen atoms.
- Water (H2O). Oxygen has 6 outer electrons, and so forms 2 covalent bonds with 2 hydrogen atoms.
- O2. Oxygen also goes around in pairs, sharing 2 electrons with each other.
Covalent Substances: Two Kinds
Simple molecular substances: -The atoms form very strong covalent bonds to form small molecules of several atoms. But the bonds between the molecules are very weak. Because of this, the melting and boiling points are very low. -Most molecular substances are gases or liquids at room temperature, but they can be solids. Molecular substances don't conduct electricity, as there are no ions. So there is no electrical charge.
Giant Covalent Structures (Macromolecules): -These are similar to giant ionic structures (lattices) except that there are no charged ions. All atoms are bonded together by Strong covalent bonds. This means they have very high melting and boiling points. They do not conduct electricity, even when molten (Except for graphite) -The main examples are Diamond and Graphite, which are both made from Carbon atoms, and Silicon Dioxide.
Diamond, Silicon Dioxide(Silica) and Graphite
Each carbon atom contains four covalent bonds in a very rigid giant covalent structure. This structure makes diamond the hardest natural substance, so its used for drill tips
This is what sand is made of. Each grain of sand is one giant structure of silicon and oxygen.
Each carbon atom only forms three covalent bonds. This creates layers which are free to slide over each other, like a pack of cards. So they can be rubbed off on to paper (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 conducter of heat and electricity. Each carbon atom has a delocalised electron and these are what conduct the electricity and heat.
-metals also consist of a giant structure -metallic bonds free-electrons which produce all the properties of metals. They are delocalised electrons and come from the outer shell of every metal atom in the structure. -delocalised electrons are free to move through the structure and are good conductors of heat and electricity. -these electrons hold the atom together in a regular structure- there are strong forces of electrostatic attraction between the positive metal ions and the negative electrons. -they also allow the layers of atoms to slide over each other, meaning metals can be bent and shaped.
Pure metals aren't quite right for certain jobs. So scientists mix two or more metals together - creating an alloy with 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.
Smart materials: Smart materials behave differently depending on conditions. (eg. temperature). A good example is nitinol - a shape memory alloy. Its a metal alloy but when ts cool you can bend it like rubber. And when you heat it, it goes back to the remembered shape.
Its really handy for glass frames. If you accidently bend them, you can just pop them into a bowl of hot water and they'll jump back into the shape. Nitinol is also used for dental braces. In the mouth it warms and tries to go back to its remembered shape, and so it gently pulles the teeth with it.
Nanoparticles: Really tiny particles, 1-100 nanometres across, are called nanoparticles. Nanoparticles contain roughly a few hundred atoms. Nanoparticles include fullerenes. These are molecules of carbon, shaped like hollow balls or closed tubes. They are arranged in hexagonal rings. A nanaoparticle has very different properties from the 'bulk' chemical that its made from.
Using nanoparticles is called Nanoscience. Many new uses for them are being developed:
- They have a very large surface area to volume ratio, so they could help make new industrial catalysts - They can be used to make stronger, lighter materials - You can use them to make lubricant coatings for artificial joints and gears. - Nanotubes can conduct electricity, so they can be used in tiny electric circuits.
Strong covalent bonds hold the atoms together in long chains.
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 its really easy to melt the plastic. When it cools, the polymer hardens into a new shape. You can melt these platics and remould them as many times as you want.
Strong forces: Some plastics have stronger intermolecular forces between the polymer chains, called cross links, 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 its heated. Thermosetting polymers are strong, hard and rigid.
The two types of polythene can be made using different condidtions:
- Low density polythene is made by heating ethen to about 200 degrees under high pressure. Its flexible and is used for bags and bottles
- High density polythene is made at a lower temperature and pressure (With a catalyst). Its more rigid and is used for water tanks and drain pipes.
Relative Formula Mass
The Relative Atomic Mass (Ar) is just like the Mass Number of an element. For example: Heliums Mass Number is 4, so its Relative Atomic Mass is 4 as well.
The Relative Formula Mass (Mr) is just the combined total of all Ar's in a compound. For example:
- MgCl2. Ar of Mg (Magnesium) = 24. Ar of Cl (Chloride) = 35.5. 24 + (35.5 x 2) = 95. 95 is the Mr of MgCl2
One Mole of a substance is equal to its Mr or Ar in grams. For example:
- One mole of Carbon is 12g, as the Ar of Carbon is 12.
- One mole of Nitrogen is 28g, as it travels in pairs (N2), and its Ar is 14.
Calculate the % mass of an element: This just means to calculate what percentage of a compound a certain element takes up. You can use this calculation: - % Mass of an element = (Ar x No. of atoms (of that element)) DIVIDED BY (Mr of whole compound) AND THEN times the result by 100.
An example of this: - Find the % mass of sodium in Sodium Carbonate (Na2CO3) Ar of Sodium = 23. (2 atoms) Ar of Carbon = 12. Ar of Oxygen = 16 (3 atoms) Mr = (23 x 2) + 12 + (16 x 3) = 106. 106/(23 x 2) x 100 = 43.4%
This is just how you can find out the number atoms for each element in a compound. To do this, first list all the elements in the compound and how much of them there are (in grams). Then, divide each of the masses of each element, by a single mole of the element. Then, divide all the values you get after the calculations, by the smallest value you get. Here is an example:
In a substance, there is:
- 0.60g of Carbon. - 0.15g of Hydrogen. - 0.40g of Oxygen.
Divide each ones mass by a mole of the element..
- 0.60/12 = 0.05 (Carbon). - 0.15/1 = 0.15 (Hydrogen). - 0.40/16 = 0.025 (Oxygen)
Then divide each value by the smallest value..
- 0.05/0.025 = 2 (so there are 2 atoms of carbon)
- 0.15/0.025 = 6 (so there are 6 atom of carbon)
- 0.025/0.025 = 1 (so there is 1 atom of carbon)
So the formula of the substance would be C2H6O
Calculating Masses in Reactions
The percentage yield compares actual yield and the predicted (theoretical) 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, its a percentage
To calculate percentage yield, use this equation:
Percentage yield = actual yield (grams)/predicted yield (grams) x 100
The percentage yield is always somewhere between 0 and 100%. A 100% yield means you get all the product you expected, but percentage 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 aling the way.
There are several reasons for this:
- The reaction is reversible. This means that the reactants will never be completely converted to the products, as the reaction goes both ways. Some of the products are always reacting together to change back to the orginal reactants. This will mean a lower yield.
- When you filter a liquid to remove solid particles, you nearly always lose a bit of liquid or solid. So, some of the product may be lost when its seperated from the reaction mixture, meaning a lower yield.
- Things don't always go exactly to plan. Sometimes there are unexpected reactions happening that use up the reactants. This means there isn't as much reactant to make the product you want, meaning a lower yield.
Chemical Analysis and Instrumental Methods
Rate of Reaction
Reactions can go at different rates.
- One of the slowest rates in the rust of iron
- A moderate speed reaction is metals (like magnesium) reacting with acid, which produces a gentle stream of bubbles
- A very fast reaction is an explosion, which can be over in a fraction of a second
There are different factors that affect the speed of a reaction: (Learn them all)
- Concentration (Or pressure)
- The use of a catalyst
- Surface area of solids
Measuring Rates of Reaction
To calculate the rate of reaction, use this equation:
Rate of reaction = Amount of reactant used or amount of product formed/time
There are different ways of measuring the rate of reaction:
1) Precipitation: This is when they solution clouds over. You can measure it by placing the solution over a mark and see how long it takes before you can't see it anymore. This does only work when you have a transparent solution to begin with.
2) Change in mass: Weigh the solution as the reaction is taking place, and see how much the mass changes by. The quicker the reading on the balance drops, then the faster the reaction. Most accurate of the three methods.
3) The volume of gas given off: Use a gas syringe(attached to a flask) to measure how much gas is given off in a given time interval. The more gas given off during a given time interval, then the faster the reaction.
Rate of Reaction Experiments
More Rate of Reaction Experiments
The rate of reaction depends on how often and how hard the reacting particles collide with each other. Particles have to collide in order to react, and they have to collide hard enough. Higher Temperature increases collisions: - When the temperature increases, the particles all move quicker. If they're moving quicker they will start to collide more often. Higher concentration (or pressure) increases collisions: - If a solution is made more concentrated it means there are more particles of a 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. Large 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. Catalysts also help speed up reactions: - 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 particles a surface to stick to. This increases the number of successful collisions.
Energy Transfers in Reactions
An Exothermic Reaction is a reaction that gives out heat. You can tell if a reaction is Exothermic because there will usually be a rise in temperature.
- The best example of an Exothermic Reaction would be burning fuels, or Combustion
An Endothermic Reaction is a reaction that takes in heat. You can tell if a reaction is Endothermic because there will usually be a rise in temperature.
- Endothermic Reactions are much less common, but an example of one would be Thermal Decompositions.
Reversible Reactions can be both Exothermic and Endothermic.
- An example of a reaction like this is the Thermal Decomposition of hydrated copper sulfate:
Hydrated copper sulfate = (reversible reaction symbol) anhydrous copper sulfate + water
If you heat blue hydrate copper sulfate crystals, it evaporates the water from it (you can collect this by using a cooling system to condense the water vapor) and leaves white anhydrous copper sulfate crystals. Then when you pour the water back onto the white crystals, they turn back into the blue hydrated copper sulfate crystals.
Acids and Alkalis
The pH (Power of Hydrogen - Not needed now, but a fun fact ;-]) scale is a measure of how Acidic or Alkaline a solution is. The strongest Acid has a pH of 0. The strongest Alkaline has a pH of 14.
A Neutral (Neither Acid nor Alkaline) substance has a pH of 7 (e.g. pure water).
To test the pH of a solution, you use an Indicator. An Indicator is just a dye that changes colour. The dye in the indicator changes colour depending on whether is above or below a certain pH. A Universal Indicator is a combination of dyes which gives the colours Red (Acid) - Purple (Alkaline).
Acids and Bases neutralise each other. An Acid is a substance with a pH of less than 7. Acids for 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.
The reaction between Acids and Bases is called Neutralisation: Acid + Base = Salt + Water
It can also be seen in terms of H+ and OH- ions: H+ + OH- = H20 (Water!!!)
A Universal Indicator goes green to show a Neutral substance.
State symbols: What physical state is this substance in? (s) = Solid. (l) = Liquid. (g) = Gas. (aq) = Aqueous (Dissolved in water)
Acids Reacting with Metals
Metals react with Acids to give Salts: Acid + Metal = Salt + Hydrogen
- The more reactive a metal, the faster the reaction will go
- Copper does not react with dilute acids at all, as it is less reactive than Hydrogen
- The speed of reaction is indicated by the rate at which bubbles of Hydrogen are given off
- The Hydrogen is confirmed by the burning splint test, giving the Squeaky pop.
- The name of the salt depends on the metal and acid used
Hydrochloric Acid will always produce Chloride salts: - 2HCl + Mg = MgCl2 + H2 (Magnesium Chloride) - 6HCl + 2Al = 2AlCl3 + 3H2 (Aluminium Chloride)
Sulfuric Acid will always produce Sulfate salts: - H2SO4 + Mg = MgSO4 + H2 (Magnesium Sulfate) - 3H2SO4 + 2Al = Al2(SO4)3 + 3H2 (Aluminium Sulfate)
Nitric Acid produces Nitrate salts when Neutralised
Oxides and Hydroxides
Metal Oxides and Metal Hydroxides are Bases.
- Some metal oxides and metal hydroxides dissolve in water. These soluble compounds are called Alkalis
- So all metal oxides and metal hydroxides will react with acids to form a salt and water:
Acid + Metal Oxide = Salt + Water
Acid + Metal Hydroxide = Salt + Water
The combination of Metal and Acid decides the salt:
Hydrochloric acid + copper oxide = copper sulfate + water Hydrochloric acid + sodium hydroxide = sodium chloride + water
Sulfuric acid + zinc oxide = zinc sulfate + water Sulfuric acid + calcium hydroxide = calcium sulfate + water
Nitric acid + magnesium oxide = magnesium nitrate + water Nitric acid + potassium hydroxide = potassium nitrate + water
Ammonia can be neutralised with HN03 to make fertiliser.
Ammonia dissolves in water to make an Alkaline Solution. When it reacts with Nitric Acid, you get a neutral salt - Ammonium nitrate
NH3(aq) + HNO3(aq) = NH4NO3(aq)
Ammonia + Nitric acid = Ammonium nitrate
There is No water produced!
Electrolysis means Splitting up with Electricity:
- If you pass an electric current through an ionic substance thats molten or ina solution, it breaks down into the elements its made of
- It requires a liquid to conduct the electricity, called the electrolyte
- Electrolytes contain free ions, they're usually molten or dissolved ionic substances
- In either case its free ions which conduct electricity and allow the whole thing to work
- 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 (Anode) and given to other ions at the negative electrode (Cathode). As ions gain or lose electrons they become atoms or molecules and are released.
Electrolysis reactions involve Oxidation and Reduction
- Oxidation is the loss of electrons OIL
- Reduction is the gain of electrons RIG
Negative ions are attracted to the Positive electrode, and Positive ions are attracted to the Negative electrode.
An example of this is the electrolysis of Molten Sodium Chloride:
When you pass a current through Molten Sodium Chloride
- The Sodium Ions (Na+) are attracted to the negative electrode
- The Chlorine Ions (Cl-) are attracted to the positive electrode
- The Electrons that the Chlorine Ions have gained to become Ions, travel through the positive electrode, up to the power pack, back down the negative electrode and reaches the Sodium ions. This turns the Sodium and Chlorine Ions into Atoms.
A different scenario is the Electrolysis of a salt in a solution.
When a salt is dissolved in water, there will also be H+ and OH- Ions
For the example, we'll use Sodium Chloride again.
- Chlorine will be attracted to the positive electrode. The electrons it has gained to become an ion will run up the electrode and down to the negative electrode.
- The Sodium ions however, will this time will stay in the solution because they are more reactive than hydrogen.
- The H+ Ions are attracted to the negative electrode. Here they combine with the electrons from the Chlorine Ions to form Hydrogen Molecules
- The OH- Ions bond with the Sodium Ions to form Sodium Hydroxide
So the end products are Hydrogen Gas, Sodium Hydroxide and Chlorine.