Chemistry GCSE: Atomic Structure


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  • Created on: 17-06-10 18:16

atomic stricture

All substances are made of atoms.

The atoms themselves contain a nucleus (containing protons and neutrons) surrounded by electrons.

An atom is the smallest particle that can be recognised as being an element.

The periodic table shows a list of elements.

The elements are substances that only contain one kind of atom.

Atoms of elements have a chemical symbol. The first letter is always a capital letter. The second letter (if it has one) is always a lower case. Be careful that your handwritten lower case “l” does not look like a capital “L”.

The periodic table collects elements with similar properties in vertical columns called groups.

Atoms join together (bond) by gaining, losing or sharing electrons to form compounds.

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atomic stricture

The chemical formula of a substance shows the number of each kind of atom that are joined together. For an ionic compound, the formula shows the ratio of each kind of atom.

In any chemical reaction, the total mass of the starting materials (reactants or reagents) is always the same as the total mass of the materials at the end (products). Sometimes you might be confused in this because some substance may be added or taken away that are not included in your weighings.

Chemical equations (symbol ones) must always be balanced. That is they must have the same number of each kind of atom on both sides.

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Rocks and building

Limestone is a building material. It has the chemical name calcium carbonate.

Limestone can also be used to make quicklime (calcium oxide), slaked lime (calcium hydroxide), cement and glass.

Most metal carbonates decompose on heating (thermal decomposition) to give the metal oxide and carbon dioxide gas. This is done to limestone in a lime kiln to form quicklime.

Sodium carbonate and potassium carbonate are two carbonates that cannot be decomposed using a Bunsen burner flame.

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Rocks and metals

Metals are obtained from ores which are dug out of the Earth.

Apart from for the very unreactive metals, chemical reactions are needed to extract the metal. Many of these reactions require a great deal of energy, either to run the reaction itself or to provide the raw materials.

A metal that is mixed (ie not a compound) with other elements is called an alloy.

Alloys usually have different properties from their parent metals and alloys with specific properties can be designed. In particular, aluminium, titanium, iron, copper and gold are useful and so are their alloys.

Recycling metals is a good way to conserve the Earth’s resources of these metals.

Recycling also saves energy resources because it is much less energy-demanding to re-use a metal than have to extract fresh metal from its ore.

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Crude oil

Hydrocarbons are molecules that contain only hydrogen and carbon atoms.

Crude oil is almost entirely made up of a mixture of hydrocarbons.

Most of the hydrocarbons in crude oil are saturated hydrocarbons (hydrocarbons with only single bonds).

The family of saturated hydrocarbons is called the alkanes.

All alkanes have the same general chemical formula (CnH2n+2).

Crude oil is broken down into groups of molecules with similar boiling points by fractional distillation.

Many of the fractions of crude oil are used as fuels.

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crude oil

Burning fuels causes pollution of the atmosphere.

Carbon dioxide (a product of burning) is a “greenhouse gas” and contributes to global warming.

Sulfur dioxide is formed because there are sulphur impurities in crude oil that are not removed at the fractional distillation stage. When the sulphur burns, it forms sulphur dioxide which as well as being an irritant also contributes to acid rain.

Particulate matter is also released from burning fossil fuels.

Chemists are trying to produce less polluting fuels (such as ULS petrol).

Pollution would also be reduced if less fuel were use

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C1A Questions

Do you know three things an atom can do with its electrons to form a bond?

What can you say about the total mass of the products compared to the total mass of the starting materials?

Can you name the products when a metal carbonate is strongly heated?

Can you write a word equation for the thermal decomposition of strontium carbonate?

Can you write a balanced chemical equation for the thermal decomposition of calcium carbonate?

What is cement?

What is mortar?

How is lime mortar different?

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Rates of reaction

The rate of a reaction is measured as either the amount of reactant used up in a given time or the amount of product formed in a given time.

In order for a reaction to occur, the particles must collide.

If more collisions can be made to take place in a given time, the rate of the reaction will increase.

Some collisions don’t lead to a reaction taking place. Only collisions with sufficient energy can lead to a successful reaction. So, if we can increase the success rate of the collisions, this will also lead to an increase in the rate of reaction.

The minimum energy that must be possessed by the particles for the collision to be successful is called the activation energy.

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Rates of reaction

Increases in concentration, pressure of gases and surface area of solids all lead to more collisions taking place per second.

Increase in temperature leads to more collisions per second but also collisions that have, on average, a greater energy so the success rate is higher too.

Catalysts lower the activation energy by providing an alternative route and so the success rate is higher.

Catalysts are usually specific to a particular reaction. They are very important in a number of indusral processes.

Concentrations are usually measured in units of mol dm-3 (moles per litre).

Equal volumes of gases at the same temperature and pressure contain the same number of molecules (and moles) of gas.

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Energy and reactions

When chemical reactions occur there is an energy exchange with the surroundings.

If heat is given out to the surroundings, it is said to be an exothermic reaction, if heat is absorbed from the surroundings it is said to be an endothermic change. Beware, if a reaction “gets hot” it may seem like it is absorbing heat from outside but really it is the chemicals reacting, giving their heat energy to the water or container etc and then the heat spreads out to the surroundings.

It is an exothermic change. If a reaction can go in both forward and backward directions it is called a reversible reaction.If the forward direction is exothermic, the reverse direction must be equally endothermic. If the forward reation is endothermic, the reverse must be equally exothermic.

If the system is closed (ie substances can’t escape or be added), a reversible reaction can come to equilibrium.

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Energy and reactions

At equilibrium, the forward and reverse reactions keep going but at the same speed so that there is no overall change.

The equilibrium position can be adjusted by changing conditions of temperature and pressure. This can be used to our advantage in industry.

An increase in temperature favours the endothermic direction.

An increase in pressure favours the direction with a decrease in the number of moles of gas.

The conditions for reversible reactions in industry also take into account the needs to have a good rate and a minimal cost. A compromise is sometimes needed.

Energy use (and waste) must be minimised for the sake of maximum profit and environmental concern.

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Electrolysis is the passage of electricity to cause a chemical reaction.

Only ionic substances can undergo electrolysis.

Ionic substances must be either molten (liquid) or dissolved in aqueous solution) for electrolysis to take place.

Positive ions (cations) are attracted to the negative electrode (cathode) where they gain electron(s) to form the neutral atoms.

Negative ions (anions) are attracted to the positive electrode (anode) where they lose electron(s) to form the neutral atoms.

The details of the electrode reactions can depend on what material the electrode is made of, the concentration of the ions and other factors.

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Gain of electron(s) is called reduction, loss of electron(s) is called oxidation.

Remember Oxidation Is Loss of electrons, Reduction Is Gain of electrons (OILRIG)

Positive Anode Negative Cathode (PANiC)

Anion is an ion that goes to the anode

Three important industrial electrolysis reactions are:

Electrolysis of sodium chloride solution to give hydrogen, chlorine and sodium hydroxide.

Electrolysis of aluminium oxide to give aluminium (and oxygen)

Purification of copper by making the impure copper the anode and collecting pure coper at the cathode.

Half equations (using e-) can be used to describe electrode reactions.

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Acids, alkalis and salts

Insoluble salts can be made by precipitation (two aqueous solutions react to give an aqueous solution and a precipitate)

Soluble salts can be made by reacting a metal, a metal oxide, a metal hydroxide or a metal carbonate with an acid. The final solution can be slowly evaporated to leave crytals of the salt.

The pH scale measures the acidity or alkalinity of a solution.

A low pH means that there is a high concentration of H+(aq) ions (hydrated protons) and so the solution is strongly acidic.

A high pH value means that the solution has a high concentration of OH-(aq) ions (hydrated hydroxide ions) and so is strongly alkaline.

Neutralisation is the reaction between H+(aq) and OH-(aq) to form water.

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