Chemistry GCSE (1)

Element - made of only one type of atom.

Compound - made of two or more types of atoms joined together.

Atom - smallest particle of an element (on it's own).

Molecule - 2 or more atoms joined together (can be the same or different).

H20 = Molecule and Compound.

Cl2 = Molecule and Element.

Au = Atom and Element.

H2PO4 = Molecule and Compound.

To balance an equation, adjust the number of atoms or molecules until there are the same numbers of each type of atom on both sides of the equation. 

copper + oxygen → copper oxide

The substances involved are copper atoms (Cu), oxygen molecules in the air (O₂) and molecules of copper oxide (CuO). Unbalanced equation:

Cu + O₂ → CuO

two atoms of copper react with two atoms of oxygen to form two molecules of copper oxide

So the balanced equation is:

 2Cu + O₂ → 2CuO

All the elements are arranged in the periodic table.

The columns are knows as groups and the rows are known as periods.

All the elements in the same group have the similar properties.

Group One - The Alkali Metals.

They are the same as other metals because: 1.) They conduct electricity

 2.) They conduct heat.

They are different to other metals because: 1.) They are soft and can be cut.  2.) They are lighter than other metals. 3.) They melt easily.

We store alkali metals under oil because they are all very reactive and react very fast with cold water, so they are stored under oil away from air and water.

When the alkali metals react with water, they produce hydrogen gas. They also turn the water into an alkaline solution.

Nucleus: Protons and Neutrons.

Inner shell: Maximum of 2 electrons.

Outer shells: Maximum of 8 electrons.

Particle, Mass and Charge.

Proton:      1             +1

Neutron:    1              0

Electron:    ~             -1

Mass number = number above.

Atomic number = number below.

Bonding = Atoms join together to gain a full outer shell.

Covalent bonding = Atoms share electrons.

Steps: 

1.) Work out how many electrons are in the outer shell (Group n = n number of electrons in the outer shell).

2.) Work out how many electrons each atom needs to gain to get a full outer shell, and how the atoms could do this.

Properties of simple covalent compounds:

Simple covalent compounds have low melting and boiling points. It does not take much energy to overcome the forces of attraction between molecules.

In some cases there are only weak forces between molecules. We call these intermolecular forces. 

A solid uses energy when it melts to allow the molecules to slide over each other. A liquid uses energy to boil to allow the molecules to escape from the surface of the liquid. Substances made from simple molecules have weak intermolecular forces. This means that their melting and boiling points are lower, so it doesn't take much energy to overcome the forces attracting the molecules to each other. 

Simple molecules like those in methane don't conduct electricity because they don't have an overall electrical charge.

Diamond: 

Each carbon atom forms four strong covalent bonds - this gives it a very high melting point.

The carbon atoms form a rigid, giant covalent structure - this is why diamonds are hard.

Diamond does not conduct electricity because the electrons are not de localised - every electron is involved in the bonding - this means there are no electrons to carry the charge. 

Graphite:

Atoms in graphite are linked to other atoms by strong covalent bonds - this gives it a very high melting point.

Each carbon atom bonds to three others - this forms layers which are free to slide over each other - this is why graphite is soft and slippery.

There are weak bonds between the layers.

• Give and take electrons to gain a full outer shell.
• Bonding is always between metal and non-metal atoms.

Atoms that lose or gain electrons are known as ions.

No electrons = no protons = neutral atoms.

Example: 

Li = 3 electrons (-ve)   3 protons (+ve)

F = 9 electrons (-ve)     9 protons (+ve)

Li gives f one electron.

Li has 3 +ve protons and 2 -ve electrons   = positive charge.

F has  9+ve protons and 10 -ve electrons  = negative charge.

Nano science is the study of particles that have the size of 1 to 100nm.

A nanometre (nm) is 1,000,000,000 (1 billion) times smaller than a metre.

In an exam you need to write VERY SMALL.

Advantages.

•  lots of uses
• Increase surface area

Disadvantages.

• Don't know the long term effects
• Very expensive

e.g. Space elevator: Made with carbon nano tubes. Only 1cm of the wire needed to do this has been made. Nano particles = particles which are smaller than 100nm and have a high surface area to volume. 

Properties of ionic compounds: 

Bonding: Strong electrostatic (opposite charges attract) attraction between opposite charged ions extend in all directions - 3D lattice <- how ions are arranged.

Strong bonds > a lot of energy to overcome the bonds > high melting/boiling points.

E.g.

MgI2    Mg > 12     I > 53 (group 7)

I1+   Mg2+    I1+

Mg loses two electrons - one to one of the I atoms, but as it has an extra electrons, it gives another electron to a different I atom.

Metal ions LOSE electrons -> Positive charge.              Non metal ions GAIN electrons -> negative charge.

Solid Ionic compounds DON'T conduct electricity -> Ions can't move.

Ionic compounds which are MOLTEN or DISSOLVED conduct electricity because the ions can move. 

How to speed up a reaction:

• Increase the temperature
• Increase the concentration (amount of dissolved substance in a solution)
• Use a catalyst (speeds up a reaction without being used up)
• Increase the surface area (reaction involving a solid)

Measuring the rate of reaction:

You can measure the rate of reaction by measuring:

• How quickly a product is made
• How quickly a reactant is used up

The easiest product/reactant to measure is a gas. You can measure the volume of gas produced - gas syringe OR you can measure the mass lost. 

Collision Theory:

For a chemical reation to happen the reactants have to collide. To increase the rate of reaction the particles need to collide more and/or with more energy.

Temperature: Increasing the temperature gives the particles more energy so they: move more, collide more and collide with more energy.

For a reaction to happen, the particles need to collide with a minimum energy. This is known as ACTIVATION ENERGY.

Concentration:  If a solution is more concentrated there are more particles in the same volume. So particles are more likely to collide.

Surface area: If you increase the surface area of a solid there are more particles exposed, so it reacts quicker.

In a lump of iron, oxygen can't get to most of the atoms.

If the iron is in small bits, the oxygen molecules can collide with many more iron atoms. The iron now has a much bigger surface area. 

Catalysts can speed up reactions without the need to use so much energy.

Sometimes in a chemical reaction, the reactants make substances that we don't want, as well as those we do. The correct catalyst can speed up the reaction that makes the product we need.

Periodic Table - shows the relative atomic mass (number below).

To calculate the relative formula mass you add together all the relative atomic masses in a molecule. 

E.g. HCL

•  1 (relative atomic mass of H) + 35.5 (relative atomic mass of Cl)= 36.5

Calculation of the % by mass of an element in a compound.

• 1.) Work out the relative formula mass of the compound (add together the atomic mass)

e.g. CaC03 = 40 + 12 + (16x3) = 100.

• 2.) What is the % by mass of oxygen in CaC03

% by mass = relative atomic mass of the element / relative formula mass of the compound.     

= (16X3) / 100 X 100 = 48%

Empirical formula is the simplest formula of a compound.

e.g. C5H10 is the molecular formula

       CH2     is the empirical formula

Calculation.

What is the empirical formula of a compound which contains 0.12g of Mg and 0.8g of Br?

Write out each one                                 Mg         Br

Write how much of each underneath      0.12      0.8

Divide by the atomic mass                      24         80

Ratio of Mg:Br to make it a whole number,divide through by the smallest number.

0.005/ 0.005 = 1        0.01/0.005 = 2            =MgBr2

E.g. 2AL + Fe203  >   2Fe + AL203 

What mass of iron is produced from 8g of Fe203?

1.) Formula mass -     Fe203 (56 + 56 + (3X16) = 160g)

                                  2Fe (56 + 56 = 112g)

2.) Work out how much Fe 1g of Fe203 gives.

160/160    >    112/160

1g  >  112/160g

8g = 8 X 1   >   8 X 112/160

8g > 5.6g

Yield - The amount of a product made during a chemical reaction.

• In a chemical reaction you never make 100% yield. Not all of the reactants end up as product.

How much do we really make?

Atoms are not gained or lost in a chemical reaction. But when we carry out a chemical reaction we don't always obtain the mass of a product we expect.

This could be because:

• Some of the product may escape of get left behind in a mixture
• The reaction may be different from the one we expected
• The reaction is reversible and may not go to completion.

Percentage Yield =

The amount of product obtained / maximum possible amount X 100

Atom economy - This measures the amount of starting materials that end up as useful products.

% atom economy = mass of useful products / mass of reactants X 100.

e.g. boots process of making ibuprofen:

Mass of useful product = 206 g.

Mass of reactants = 514.5g.

% Atom economy = 206g / 514.5g X 100 = 40%.

We can measure the amount of starting materials that end up as useful products. This is the atom economy. Using reactions with a high atom economy is important for sustainable development. It can help manufacturers make chemicals more cheaply. 

Reversible Reaction.

A + B <> C + D ( reaction that goes both ways = reversible).

Hazard - High pressure = could explode

Environmental:

• Visual pollution
• Uses a lot of energy - release of air pollution
• Increase in traffic

Economic - Provides jobs for the local area

Endothermic and Exothermic reactions

Exothermic Reactions - Exo = out/exit     Give out heat, temperature rises

Endothermic Reactions - Endo = in/entrance    Takes in heat, temperature falls

Pressure: You want to increase the yield of NH3. If you increase the pressure you will increase the amount of collisions between particlles. IF you increase the pressure you will increase the amount of collisions between N2 and H2 so the rate of forward reaction will increase -> Yield of NH3 will increase. NH3 doesnt need to collide with anything to make H2 and N2 so the rate of reverse reaction won't change.

Temperature:If you increase the temperature the endothermic reaction prefers it so it's rate of reaction increases. The higher the temperature, the lower the rate of reaction. You don't do the reaction at a very low temperature as the overall rate of reaction is too slow. The particles will be moving very slowly. 

Question: The temperature used in industry needs to be high enough for the reaction to take place quickly. Explain, in terms of particles, why the rate of reaction increases when the temperature is increased.

If you increase the temperature, the endothermic reaction prefers it to its rate of reaction increases. 

Electro = Electricity. Lysis = Break apart.

Electrolysis is breaking apart compounds into elements. It is done with ionic compounds. Ionic compound is when atoms give and take electrons to gain a full outer shell. (atoms change into ions).

e.g. CuCl2     Cu gives 2 electrons away -> Cu+2

                      Each Cl gains 1 electron to make Cl -1

Ionic compounds only conduct electricity when the ions can move - which is when they are melted or dissolved. You can use electricity to split up ionic compounds into elements. We call this electrolysis. First we must melt the compound or dissolve it in water. When we do this the ions are free to move about in the liquid or solution. When we pass electricity through an ionic substance

• the positive ions move to the negative electrode
• the negative ions move to the positive electrode