Chemistry GCSE- Additional science-Specification and Revision Notes

KEY AREAS FOR CHEMISTRY ADDITIONAL SCIENCE GCSE REVISION

1. How do sub-atomic particles help us to understand the structure of substances? 2. How do structures influence the properties and uses of substances? 3. How much can we make and how much do we need to use? 4. How can we control the rates of chemical reactions? 5. Do chemical reactions always release energy? 6. How can we use ions in solutions? 

1. How do sub-atomic particles help us to understand the structure of substances? 

This is about simple particle theory including atomic structure and bonding.  

The arrangement of electrons in atoms can be used to explain what happens when elements react and how atoms join together to form different types of substances. 

TIP: Visit these links and learn by heart . Do the tests as well.

http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/bonding/

http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/atomic_structure/

Also you need to know:

• to represent the electronic structure of the first twenty elements of the periodic table in the following forms: 

   

Sodium, Na

   and 2,8,1 

• to represent the electronic structure of the ions in sodium chloride, magnesium oxide and calcium chloride in the following forms: 

for sodium ion (Na +)  and     [2,8] and  in ionic form as above

• to represent the covalent bonds in molecules such as water, ammonia, hydrogen, hydrogen chloride, chlorine, methane and oxygen and in giant structures such as diamond and silicon dioxide in the following forms: 

• When atoms form chemical bonds by transferring electrons, they form ions.  Atoms that lose electrons become positively charged ions.  Atoms that gain electrons become negatively charged ions.  Ions have the electronic structure of a noble gas (Group 0). 

• The elements in Group 1 of the periodic table, the alkali metals, 

have similar chemical properties.  They all react with non-metal 

elements to form ionic compounds in which the metal ion has a 

single positive charge. 

• The elements in Group 7 of the periodic table, the halogens, have 

similar chemical properties.  They react with the alkali metals to 

form ionic compounds in which the halide ions have a single 

negative charge. 

• An ionic compound is a giant structure of ions.  Ionic compounds 

are held together by strong forces of attraction between 

oppositely charged ions.  These forces act in all directions in the 

lattice and this is called ionic bonding.  

• When atoms share pairs of electrons, they form covalent bonds. 

These bonds between atoms are strong.  Some covalently bonded 

substances consist of simple molecules such as H2 

,Cl2

, O2

, HCl, 

H2O and CH4. 

Others have giant covalent structures 

(macromolecules), such as diamond and silicon dioxide. 

HT  ™ Metals consist of giant structures of atoms arranged in a regular 

pattern.  The electrons in the highest occupied energy levels (outer 

shell) of metal atoms are delocalised and so free to move through 

the whole structure.  This corresponds to a structure of positive 

ions with electrons between the ions holding them together by 

strong electrostatic attractions.

2. How do structures influence the properties and uses of substances? 

Substances that have simple molecular, giant ionic and giant covalent structures 

have very different properties. 

 Ionic, covalent and metallic bonds are strong.  

The forces between molecules are weaker, eg in carbon dioxide and iodine.  

Nanomaterials have new properties because of their very small size. 

TIP: Visit this link and learn by heart . Do the tests as well.

http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/bonding/structure_propertiesrev7.shtml

You need to know:             

                  

• to relate the properties of substances to their uses 

• to suggest the type of structure of a substance given its properties 

• to evaluate developments and applications of new materials, eg 

nanomaterials, smart materials. 

You need to know:  

• Substances that consist of simple molecules are gases, liquids or solids that have relatively low melting points and boiling points.   

HT  ™ Substances that consist of simple molecules have only weak forces between the molecules (intermolecular forces).  It is these intermolecular forces that are overcome, not the covalent bonds, when the substance melts or boils. 

• Substances that consist of simple molecules do not conduct electricity because the molecules do not have an overall electric charge. 

• Ionic compounds have regular structures (giant ionic lattices) in which there are strong electrostatic forces in all directions between oppositely charged ions.  These compounds have high melting points and high boiling points. 

• When melted or dissolved in water, ionic compounds conduct electricity because the ions are free to move and carry the current. 

• Atoms that share electrons can also form giant structures or macromolecules.  Diamond and graphite (forms of carbon) and silicon dioxide (silica) are examples of giant covalent structures (lattices) of atoms.  All the atoms in these structures are linked to other atoms by strong covalent bonds and so they have very high melting points. 

• In diamond, each carbon atom forms four covalent bonds with other carbon atoms in a giant covalent structure, so diamond is very hard. 

• In graphite, each carbon atom bonds to three others, forming layers.  The layers are free to slide over each other and so graphite is soft and slippery.   

HT  ™ In graphite, one electron from each carbon atom is delocalised.  These delocalised electrons allow graphite to conduct heat and electricity. 

HT  ™ Metals conduct heat and electricity because of the delocalised electrons in their structures.  

• The layers of atoms in metals are able to slide over each other and so metals can be bent and shaped. 

• Nanoscience refers to structures that are 1-100nm in size, of the order of a few hundred atoms.  Nanoparticles show different properties to the same materials in bulk and have a high surface area to volume ratio, which may lead to the development of new computers, new catalysts, new coatings, highly selective sensors and stronger and lighter construction materials.

3. How much can we make and how much do we need to use? 

The relative masses of atoms can be used to calculate how much to react and how 
much we can produce, because no atoms are gained or lost in chemical reactions.  In 
industrial processes, atom economy is important for sustainable development. 

You need to know:

to calculate chemical quantities involving formula mass (Mr) and percentages of elements in compounds 

HT ™ to calculate chemical quantities involving empirical formulae, reacting masses and percentage yield 

HT ™ to calculate the atom economy for industrial processes and be able to evaluate sustainable development issues related to this 

TIP: VISIT THIIS LINK, READ AND REVISE. PRACTISE USING PEN AND PAPER.
economy.http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pre_2011/chemcalc/chemcalc_higherrev1.shtml



• Atoms can be represented as shown: 

 Mass number       23 
            Na 
 Atomic Number   11 

The relative masses of protons, neutrons and electrons are: 

Name of particle                    Mass 

Proton                                          1 

Neutron                                        1 

       Electron                             Very small 

• The total number of protons and neutrons in an atom is called its mass number. 
• Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element.    

HT  ™ The relative atomic mass of an element (Ar) compares the mass of atoms of the element with the 12 C isotope.  It is an average value for the isotopes of the element. 

• The relative formula mass (Mr ) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula. 

• The relative formula mass of a substance, in grams, is known as one mole of that substance. 

• The percentage of an element in a compound can be calculated from the relative mass of the element in the formula and the relative formula mass of the compound. 

HT  ™ The masses of reactants and products can be calculated from balanced symbol equations. 

• Even though no atoms are gained or lost in a chemical reaction, it is not always possible to obtain the calculated amount of a product because: 

− the reaction may not go to completion because it is reversible 
− some of the product may be lost when it is separated from the reaction mixture 
− some of the reactants may react in ways different to the expected reaction. 

 HT ™ The amount of a product obtained is known as the yield.  When compared with the maximum theoretical amount as a percentage, it is called the percentage yield. 

• The atom economy (atom utilisation) is a measure of the amount of starting materials that end up as useful products.  It is important for sustainable development and for economical reasons to use reactions with high atom economy. 

• In some chemical reactions, the products of the reaction can react to produce the original reactants.  Such reactions are called reversible reactions and are represented: 

                                                                    A + B   
                                                                                  → (show arrows both ways)   C + D 

For example: 

ammonium chloride  → (show arrows both ways)   ammonia + hydrogen chloride       
                                         

HT ™ When a reversible reaction occurs in a closed system, equilibrium is reached when the reactions occur at exactly the same rate in each direction. 

HT  ™ The relative amounts of all the reacting substances at equilibrium depend on the conditions of the reaction.

• Although reversible reactions may not go to completion, they can still be used efficiently in continuous industrial processes, such as the Haber process that is used to manufacture ammonia. 

• The raw materials for the Haber process are nitrogen and hydrogen.  Nitrogen is obtained from the air and hydrogen may be obtained from natural gas or other sources. 

• The purified gases are passed over a catalyst of iron at a high temperature (about 450 °C) and a high pressure (about 200 atmospheres).  

Some of the hydrogen and nitrogen reacts to form ammonia.  The reaction is reversible so ammonia breaks down again into nitrogen and hydrogen: 

nitrogen + hydrogen  ammonia 

• On cooling, the ammonia liquefies and is removed.  The remaining hydrogen and nitrogen is re-cycled. 

HT  ™ The reaction conditions are chosen to produce a reasonable yield of ammonia quickly.   

4. How can we control the rates of chemical reactions? 

Being able to speed up or slow down chemical reactions is important in everyday life 

and in industry.  Changes in temperature, concentration of solutions, surface area of 

solids and the presence of catalysts all affect the rates of reactions. 

TIP: Visit this link and learn by heart . Do the tests as well.

http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/reaction/ratesrev3.shtml

You need to know:

• to interpret graphs showing the amount of product formed (or reactant used up) with time, in terms of the rate of the reaction 

• to explain and evaluate the development, advantages and disadvantages of using catalysts in industrial processes. 

When chemical reactions occur, energy is transferred to or from the surroundings. 

• An exothermic reaction is one that transfers energy, often as heat, to the surroundings.  Examples of exothermic reactions include combustion, many oxidation reactions and neutralisation. 

• An endothermic reaction is one that takes in energy, often as heat, from the surroundings.  Endothermic reactions include thermal decompositions. 

• If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction.  The same amount of energy is transferred in each case.  For example:

hydrated copper  sulfate(blue) endothermic 
                                                         exothermic  
                                                                               anhydrous copper sulfate (white) +  water 

The reverse reaction can be used as a test for water. 

HT  ™ When a reversible reaction occurs in a closed system, equilibrium is reached when the reactions occur at exactly the same rate in each direction. 

HT  ™ The relative amounts of all the reacting substances at equilibrium depend on the conditions of the reaction. 

HT  ™ If the temperature is raised, the yield from the endothermic reaction increases and the yield from the exothermic reaction decreases. 

HT  ™ If the temperature is lowered, the yield from the endothermic reaction decreases and the yield from the exothermic reaction increases. 

HT  ™In gaseous reactions, an increase in pressure will favour the reaction that produces the least number of molecules as shown by the symbol equation for that reaction. 

HT  ™ These factors, together with reaction rates, are important when determining the optimum conditions in industrial processes, including the Haber process. 

• It is important for sustainable development as well as economic reasons to minimise energy requirements and energy wasted in industrial processes.  Non-vigorous conditions mean less energy is used and less is released into the environment.

5. Do chemical reactions always release energy? 

TIP: Visit this link and learn by heart . Do the tests as well.

http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/exothermic/

Chemical reactions involve energy transfers.  Many chemical reactions involve the 
release of energy.  For other chemical reactions to occur, energy must be supplied.  In 
industrial processes, energy requirements and emissions need to be considered both 
for economic reasons and for sustainable development. 

You need to know:
                   
• to describe the effects of changing the conditions of temperature and pressure on a given reaction or process 

• to evaluate the conditions used in industrial processes in terms of energy requirements. 
       
• When chemical reactions occur, energy is transferred to or from the surroundings. 

• An exothermic reaction is one that transfers energy, often as heat, to the surroundings.  Examples of exothermic reactions include combustion, many oxidation reactions and neutralisation. 

• An endothermic reaction is one that takes in energy, often as heat, from the surroundings.  Endothermic reactions include thermal decompositions. 

• If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction.  The same amount of energy is transferred in each case.  

For example: 

hydrated 
copper  sulfate (blue) endothermic 
                                                    exothermic 
                                                         anhydrous copper sulfate (white) 

The reverse reaction can be used as a test for water. 

HT  ™ When a reversible reaction occurs in a closed system, equilibrium is reached when the reactions occur at exactly the same rate in each direction. 


HT  ™ The relative amounts of all the reacting substances at equilibrium depend on the conditions of the reaction. 

HT  ™ If the temperature is raised, the yield from the endothermic reaction increases and the yield from the exothermic reaction decreases. 

HT  ™ If the temperature is lowered, the yield from the endothermic reaction decreases and the yield from the exothermic reaction increases. 

HT  ™ In gaseous reactions, an increase in pressure will favour the reaction that produces the least number of molecules as shown by the symbol equation for that reaction. 

HT  ™ These factors, together with reaction rates, are important when determining the optimum conditions in industrial processes, including the Haber process. 

• It is important for sustainable development as well as economic reasons to minimise energy requirements and energy wasted in industrial processes.  

Non-vigorous conditions mean less energy is used and less is released into the environment.

6.  How can we use ions in solutions? 

Ionic compounds have many uses and can provide other substances.  Electrolysis is used to produce alkalis and elements such as chlorine and hydrogen.  Oxidation reduction reactions do not just involve oxygen.  Soluble salts can be made from  acids and insoluble salts can be made from solutions of ions. 

You need to know:

• to predict the products of electrolysing solutions of ions 

• to suggest methods to make a named salt 

• to explain and evaluate processes that use the principles described 

in this unit 

HT  ™ to complete and balance supplied half equations for the reactions occurring at the electrodes during electrolysis. 

You need to know and include:

• The state symbols in equations are (s), (l), (g) and (aq). 

• When an ionic substance is melted or dissolved in water, the ions are free to move about within the liquid or solution. 

• Passing an electric current through ionic substances that are molten or in solution breaks them down into elements.  This process is called electrolysis. 

• During electrolysis, positively charged ions move to the negative electrode, and negatively charged ions move to the positive electrode.