Edexcel iGCSE Chemistry

Experiment for Diffusion?

1) HCl placed at one end of the test tube, ammonia solution at the other.
2) Where they meet, a ring of Ammonium Chloride appears.
3) The diagram shows that the ammonia is closer to the ring, showing that ammonia must have travelled faster, showing it's a lighter gas.

Experiment for Dilution?

1) Put a coloured substance in a solvent e.g food colouring in water.
2) Over the time, the colour levels out but is weaker than the original solution- Dilution
3) More heat = more kinetic energy = happens faster, rate of dilution quicker.

What is filtration?

Making a barrier which one component of a mixture can pass through but the other is caught by.

What is distillation?

When one substance is evaporated off while the other remains.

What is fractional distillation?

A mixture is evaporated and rises up a tube. Different substances have different boiling points and so will condense at different temperatures. As the mixture travels up the tube, the temperature decreases, substances are collected at different places. Separates a mixture into its different parts.

What is crystallisation?

A solution is warmed, allowing the solvent to evaporate; the solution is now left to cool and will form crystals.

What is chromotography?

Chromotography paper is placed in a substance, the different components of the substance will travel at different speeds due to the size of the particles.

How can information from chromatograms be used to identify the composition of a mixture?

Chromotography paper is placed in a solvent- the different compounds will travel at different speeds due to the size of their particles.

What is the atomic number?

Number of protons (same as the number of electrons)

What is the atomic mass number?

Number of neutrons & protons together

What are isotopes?

Atoms of an element with the same number of protons but a different number of neutrons

What is the RAM (relative atomic mass)?

The mass of one atom of an element in relation to Carbon

How do you work out the relative formula mass (Mr) from the relative atomic mass (RAM)?

(Number of that element in a molecule x its RAM) + (Number of other element in a molecule x its RAM)

Triangle for carrying out equations using Ar & Mr

Moles (mol) = Mass (g) / RFM (Mr)

How do you work out moles for a gas?

Moles = Mass (g) / 24,000 cm3 (24dm3)

Difference between a liquid (l) and an aqueous solution?

An aqueous solution is a solid dissolved in a liquid.

How can you obtain the formula of a simple compound experimentally?

1) Weigh your compound of two elements.
2) Remove an element through a reaction. Weigh again. This gives you the individual mass of both elements in the compound.
3) Work out the formulae by dividing the weight by its Ar.

eg 22g with an Ar of 11 = 22/11 = 2

How to work out the empirical formula?

1) Find the masses of each element in a compound by taking each element out and weighing.
2) Turn these masses into percentages of the compound.
3) Divide each % by the element's Ar & you will have the number of atoms of it in a molecule.
4) Put these values in the formula, giving you the *molecular formula*
5) To get the *empirical formula*, divide the values by their smallest factor.

eg C6.7H20 = Molecular
CH₃ = Empirical

Calculate reacting masses using experimental data and chemical equations?

If we react 2 moles of A(Ar=24) and 5 moles of B(Ar=25) we get 2x24 + 5x25 = 173g

Calculate percentage yield?

% yield = actual yield/theoretical yield.
1) See how many moles of the reactant it takes to give the number of moles of the product eg Fe₂O₃ = 2Fe (one molecule of Iron Oxide gives two molecules or Iron)
2) Thus the mass of Fe will be my yield
3) If I am told the weight of Fe₂O₃ is 100g, I work out its moles (Mol = Mass/Mr) = Mol = 100/160 = 0.625mol
4) Because the molar ratio between Fe₂O₃ & 2Fe is 1:2, I multiply 0.625 by 2 = 1.25 mol
5) I then need to work out the theoretical mass of Fe using mass = mol x Mr = 1.25 x 56 = 70g
6) If I in fact got 82g, I divide 70 by 82 and put it over 100 = 85.4%

How do you work out the molar concentration?

Concentration = Moles/Volume

Why do electrons transfer from one atom to another?

Electrons are transferred from one atom to another in an attempt to fill or empty their outermost shell to become stable.

Describe the formation of ions

An atom has no charge. If it loses or gains an electron it becomes an ion and then has a charge. Losing an electron will give it a charge of +1, gaining an electron will give it a charge of -1. If two ions meet with opposite charges, they are attracted to eachother and form an ionic compound.

What is oxidation?

Oxidation is when an atom loses an electron - OILRIG

It is also when there is a gain of oxygen

What is reduction?

Reduction is when an atom gains an electron - OILRIG

It is also when oxygen is lost.

Name the 7 common cations.

K⁺, Na⁺, Ca²⁺, Li⁺, H⁺, Mg²⁺, Al³⁺

Group 1 & 2 elements, Hydrogen & Aluminium

Name the 8 common anions

Cl⁻, Br⁻, I⁻, F⁻, OH⁻, NO3⁻, CO₃²⁻, SO₄²⁻

The halogens, Sodium Hydroxide & OHNOCOSO

What is ionic bonding?

It happens between 2 ions which are attracted to one another due to their opposite attractions. Thus the ions have electrostatic attraction. The attraction makes an ionic bond.

What are the melting & boiling points of ionic bonds?

Due to the strong force of electrostatic attraction between oppositely charged ions, the boiling & melting points are high as it takes a lot of heat energy to break these bonds.

What is the relationship between ionic charge & melting & boiling points

The more charge ions have, the *stronger the attraction between ions* will be, making the forces of electrostatic attraction more difficult to break, increasing the melting & boiling points.

What is an ionic crystal?

A three-dimensional lattice of atoms in the shape of a cube. The ions alternate positive & negative and there are strong forces of electrostatic bonds holding it all together.

What are the positions of ions in an ionic crystal?

They are in a lattice structure with negative touching positive & vice versa.

What is a covalent bond?

A bond formed between atoms by sharing a pair of electrons (one from each atom)

Why are electrons shared in covalent bonding?

Because the electrons are attracted to the positive charge in the other atoms' nuclei in the bonds.

Why do simple molecular structures have such low melting points?

A simple molecule only has a few atoms and thus is not strong and will melt & boil very easily

How strong are covalent bonds in giant covalent structures?

Bonds between molecules are covalent bonds which are very strong and require a lot of heat energy to break

How are atoms positioned in graphite?

Graphite is a *giant covalent structure* but carbon atoms are each only bonded to 3 other atoms.

How are atoms positioned in diamond?

Diamonds are giant covalent structures and each carbon atom is bonded with 4 other atoms, making it stronger than graphite

How does the structure of graphite give it its features?

In graphite, the carbon atoms form in layers. This means the layers can slide over one another very easily, making graphite very slippery so it can act as an effective lubricant.

How does the structure of diamonds give it its features?

Because it has many bonds in it, as carbon atoms are bonded with 4 others in diamond, it is extremely hard and can cut through almost anything.

Describe the structure of metals.

In a metal, atoms come together to form a lattice & the electrons become detached from their atoms, becoming delocalised. This is why metal conducts electricity. This makes the remaining ions positive.

Why are metals such good conductors of electricity?

In metals, there is a *sea of delocalised electrons*. These free electrons carry elecricity and so charge can easily pass through them. The charge can then pass straight through the metal, making them good conductors.

Why are metals so malleable?

In metals, the atoms are formed in layers on top of one another. In pure metals (because all the atoms are the same size), these layers can slide over one another- making them easy to bend if force is applied.

What is an electric current?

It can be either a flow of electrons or a flow of ions. It can be both because both have a charge.

Do covalent compounds conduct electricity? Why?

No- because there are no electrons free to move. This means that there can be no transfer of electricity and so covalent compounds don't conduct electricty.

Do ionic compounds conduct electricity?

Only when molten or in a solution. When molten, the positive & negative ions separate meaning that there are ions free to flow. They can thus conduct electricity.

What is an electrolyte?

It is a liquid that conducts electricity

How could you tell whether a liquid was an electrolyte or not?

Take a bulb with two wires. Put the two wires in the electrolyte when not touching eachother. If the bulb lights up, it is an electrolyte as electricity has been conducted in the liquid.

How do new atoms form in electrolysis?

The positive ions gather at one end and the negative ions gather at the other. The ions then lose charge when the electricity is turned off, turning them into atoms.

What is an inert electrode?

An electrode that doesn't react with any other substances.

What is the positive electrode?

Anode

What is the negative electrode?

The cathode

What goes to the negative electrode?

All metals & Hydrogen go to the cathode

What goes to the positive electrode?

All non-metals except Hydrogen go to the anode.

In the electrolysis of sulphuric acid, what goes to the electrodes?

Oxygen to the anode; hydrogen to the cathode

In the electrolysis of copper sulphate, what goes to the electrodes?

Oxygen to the anode, copper to the cathode

In the electrolysis of brine, what goes to the electrodes?

Chlorine to the anode, hydrogen to the cathode

How do you work out charge?

Charge = Current X Time (Q = It)

How do you work out the number of moles of electrons?

It / 96,500 (Charge X Time / 96,500)

In the electrolysis of sodium chloride, what goes to the electrodes?

Hydrogen at the cathode, chlorine at the anode

When does Hydrogen become a product in electrolysis?

If not already in the electrolyte, the Hydrogen in the water/solution will be a product if the metal in the solution is more reactive than Hydrogen because the metal will bond with Oxygen instead

How do you write ionic half-equations for reactions at the electrodes during electrolysis?

Take 2O²- ; The 2 at the beginning is because Oxygen appears as a molecule (O₂). The 2 at the end is because Oxygen has 2 electrons on its outer shell and thus has a charge of -2. To make just Oxygen, you have to lose electrons. Because in a molecule of Oxygen there are 4 electrons on the outer shells combined, (2 x 2 = 4), you must lose 4 electrons to make just O₂.

2O²⁻ = O₂ + 4e⁻

The product is the molecule the element usually appears as = O₂

What is a faraday?

The amount of charge (coulombs) in one mole of electrons. = 96,500

How do you work out scale factor in molar amounts calculations?

Scale Factor = Moles of product / Moles of Electrons

How do you work out the moles of a product using scale factor?

Moles of Electrons x Scale Factor

What is a group?

Groups are columns on the periodic table going from left to right. The group number is the number of electrons an element has on its outer shell.

What is a period?

Periods are rows on the periodic table going vertically. The period number is the number of shells an element has.

How do we classify metals?

Metals all conduct. They form metal oxides which are alkaline.

How do we classify non-metals?

Non-metals don't conduct. They form non-metal-oxides which are acidic.

What do elements in the same group have in common?

They react and bond similarly as they have the same number of electrons on their outer shell.

Why are noble gases so unreactive?

They are inert because they already have a full outer shell of electrons so they don't need to lose or gain electrons.

Name the three main Group 1 elements?

Lithium, Sodium & Potassium

Which of the Group 1 elements are most reactive?

Potassium, they get more reactive as you go down the group.

How does Lithium react with water?

It floats & fizzes, giving off Hydrogen. It disappears leaving a solution of Lithium Hydroxide.

How does Sodium react with water?

It fizzes and is melted (because enough heat is generated) into a silver ball which floats along the surface leaving a white trail. It disappears leaving a solution of Sodium Hydroxide.

How does Potassium react with water?

It fizzes more violently & gives off a lilac flame.

Explain the trend of reactivity in Group 1 metals?

They get more reactive as you go down the group as the atoms with more shells lose their electron more easily, because the distance between the outermost electron and the atom is larger and thus the force of attraction between them is less.

What is the physical state of Chlorine at room temperature?

Green gas

What is the physical state of Bromine at room temperature?

Brown liquid

What is the physical state of Iodine at room temperature?

Purple-black solid

What is the physical state of Fluorine at room temperature?

Yellow gas

What happens to the halogens as you go down the group?

They decrease in reactivity down the group. Melting & boiling points get higher, liquids turn to solids and the colours get darker. (Astatine = Black solid)

What is the difference between Hydrogen Chloride gas and Hydrochloric Acid?

HCl is Hydrogen Chloride dissolved in Water. The two ions become detached leaving H⁺ (acidic) & Cl⁻. Hydrogen ions make it acidic.

What is dissociation?

When an atom splits up into its ions, like when Hydrogen Chloride dissolves in Water to make HCl.

Why doesn't Hydrogen Chloride turn into an acid when dissolved into methylbenzene?

Methylbenzene is a non-polar molecule whereas Water & Hydrogen Chloride are polar molecules. Polars only dissolve in polars.

Explain the trend of reactivity in Group 7 elements?

Halogens' reactivity decreases as you go down the group. This is because when there are less shells, the positively charged protons can attract the one electron they need more easily (they only need one electron to have a full outer shell). If there are more shells, the distance between the protons & the electrons is greater and so the protons cannot attract the electrons as easily and so they are less reactive elements.

How does displacement happen with halogens?

A more reactive halogen (further up the group) will displace a less reactive halogen that is bonded as a salt. This only happens if the salt is dissolved in water or a gas. This is a reduction reaction.

What is air made up of?

78% Nitrogen, 21% Oxygen & 1% of others such as Argon & Carbon Dioxide

How can we investigate the percentage by volume of Oxygen in air?

1) Measure the initial volume of air.
2) React Copper, Iron or Phosphorus in the air/
3) Re-measure the volume of air. The volume that has been lost is the Oxygen that was lost in the reaction.

How do you prepare Oxygen in a lab?

1) Put Hydrogen Peroxide in a conical flask along with a catalyst of Manganese Oxide (Mn₃O₄)
2) Hydrogen Peroxide then turns into Water & Oxygen
3) Collect the Oxygen using the downwards displacement method.

2H₂O₂ → 2H₂O + O₂

How do Carbon & Sulphur burn in air?

They burn, giving out heat and light, and bond with Oxygen to form non-metal-oxides, which are acidic.

How does Magnesium burn in air?

It will bond with Oxygen and form a metal-oxide, which is basic/alkaline.

How do you prepare Carbon Dioxide in a lab?

The reaction of a Metal Carbonate & an acid will always give off Carbon Dioxide as a product.
1) Use Marble chips (Calcium Carbonate- most readily available Metal Carbonate) to react with dilute Hydrochloric Acid.
2) Obtain Carbon Dioxide using the upward displacement of air.

How can you get Carbon Dioxide using thermal decomposition?

When metal carbonates are heated, they decompose into Carbon Dioxide & the metal oxide.

CuCO₃ → CuO + CO₂

Describe the properties of Carbon Dioxide?

1) It is denser than air.
2) It is a very soluble gas and dissolves in water easily, especially when the pressure is high.

Why is Carbon Dioxide used in carbonating drinks & fire extinguishers?

Fire extinguishers because it is denser than air and the CO₂ will thus make a barrier between the air and the fire, stopping the burning as fire can't burn without oxygen.

Carbonating drinks because it is dissolved into drinks at high pressure, making the bubbles in the fizzy drinks.

How may Carbon Dioxide contribute to climate change?

Carbon Dioxide traps the Sun's heat & keeps Earth's temperatures moderate, called the Greenhouse Effect.

Too much Carbon Dioxide (emitted by factories and cars) will trap too much heat and transfer too much into the atmosphere, gradually heating it up the average temperature. This is called global warming.

What do we get when we react an acid with a metal?

A salt (using the metal and the non-metal from the acid) and some hydrogen.

Mg + 2HCl → MgCl₂ + H₂

Describe the combustion of Hydrogen.

Hydrogen combusts when it reacts with Oxygen. Water is created, along with a lot of energy.

2H₂ + O₂ → 2H₂O

How can we test for water?

1) Use anhydrous Copper Sulphate and react it with the given liquid.
2) If the liquid is water, the Copper Sulphate will be hydrated to become hydrous.
3) The Copper Sulphate will turn from white to blue if the liquid is water.