Structures and Bonding 1
- atoms are made of protons, neutrons and electrons.
- protons and electrons have equal and opposite electric charges. Protons are positively charged, and electons are negatively charged.
- atoms are arranged in the periodic table in order of their atomic number
- neutrons have no electic charge. They are neutral.
- the electrons in an atom are arranged in energy levels or shells
- atoms with the same number of electrons in their outer shell belong in the same group of the periodic table.
- the number of electrons in the outer shell of an atom determines the way that the atom behaves in chemical reactions.
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Structures and Bonding 2
- elements react to form compounds by gaining or losing electrons or by sharing electrons.
- the electrons in Group 1 react with the elements in Group 7 because Group 1 elements can lose an electron to gain a full outer shell. This electrons can be given to an atom grom Group 7, which then also gains a full outer shell.
- ionic compounds are held together by strong forces between the oppositely charged ions.
- other elements that can form ionic compounds include those in Group 2 and 6.
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Structures and Bonding 3
- covalent bonds are formed when atoms share electrons.
- many substances containing covalent bonds consist of molecules, but some have giant covalent structures.
- the atoms (or ions) in metals are arranged in regular layers.
- the positive ions in metals are held together by electrons from the outer shell of each metal atom. These delocalised electrons are free to move throughout the metal lattice.
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Structures and Properties 1
- it takes a lot of eneergy to break the bonds which hold a giant ionic lattice together. so ionic compounds have very high melting points- they are all solids at room temperature.
- ionic compounds will conduct electricity when we melt them or dissolve them in water because their ions can then move freely.
- substances made up of simple molecules have low melting points and boiling points.
- the forces between simple molecules are small. These weak intermolecular forces explain their low melting points and boiling points.
- simple molecules have no overall charge, so they cannot carry electrical charge. Therefore substances containing simple molecules do not conduct electricity.
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Structures and Properties 2
- some covalently bonded substances contain giant structures.
- these substances have high melting points and boiling points.
- the giant structure of graphite contains layers of aoms that can slide over each other which make graphite slippery. The atoms in diamond have a different structure and cannot slide like this- so diamond is a very hard substance.
- graphite can conduct electricity because of the delocalised electrons along its layers.
- we can bend and shape metals because the layers of atoms (or ions) in a metal can slide over each other.
- delocalised electrons in metals allow them to conduct heat and electricity well.
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How much? 1
- the relative mass of protons and neutrons is 1.
- the mass number of an atom tells you the total number of protons and neutrons in its nucleus.
- isotopes are atoms of the same element with different numbers of neutrons.
- we compare the masses of atoms by measuring them relative to atoms of carbon-12.
- we work out the relataive formula mass of a compound from the relative masses of the elements in it.
- one mole of any substance always contains the same number of particles.
- the relative atomic masses of the elements in a compound can be used to work out its percentage composition.
- we can calculate emphirical formulae given the masses or percentage composition of elements present.
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How much? 2
- chemical equations tell us the number of moles of substances in the chemical reaction.
- we can use chemical equations to calculate the masses of reactants and products in a chemical reaction from the masses of one mole of each of the substances involved in the reaction.
- the yield of a chemical reaction describes how much productis made.
- the percentage yield of a chemical reaction tells us how much product is made compared with the maximum amount that would be made (100%).
- factors affecting the yield of a chemical reaction include product being left behind in the apparatus and difficulty separating the products from the reaction mixture.
- it is important to maximise atom economy to conserve resources and reduce pollution.
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How Much? 3
- In a reverse reaction the products of the reaction can react to make the original reactants.
- in a closed system the rate of the forward and backwards (reverse) reactions are equal at equilibrium.
- changing the reaction conditions can change the amounts of products and reactants in a reaction mixture.
- ammonia is an important chemical for making ither chemicals, including fertilisers.
- ammonia is made from nitrogen and hydrogen in the Haber process.
- we carry out the Haber process under conditions which are chosen to give a seasonable yield of ammlonia as quickly as possible.
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Rates of Reactions 1
- knowing and controlling the rate of chemical reactions is important in living cels, in the laboratory and in industry.
- we can measure the rate of a chemical reaction by following the rate at which reactants are used up. Alternatively, we can measure the rate at which products are made.
- the minimum amount of energy that particles must have in order to react is called the activation energy.
- the rate of a chemical reaction increases if the surface area of any solid reactant is increased.
- reactions happen more quickly as the temperature increases.
- a 10 degrees C increase in temperature at room temperature roughly doubles the rate of a reaction.
- the rate of a chemical reaction increases with temperature because the particles collide more often and they have more energy.
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Rates of Reactions 2
- increasing the concentration of reactants increases the frequency of collisions between particles, increasing the rate of reaction.
- increasing the pressure of reacting gases results in particles collide more often, increasing the rate of reaction.
- a catalyst speeds up the rate of a chemical reaction.
- a catalyst is not used up during a chemical reaction.
- catalysts are generally quite expensive because they are made of precious metals.
- catalysts lower the activation energy needed for a chemical reaction to occur.
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Energy and Reactions 1
- energy may be transferred to or from the reacting substances in a chemical reaction.
- a reaction where energy is transferred from the reacting substances into its surroundings is called an exothermic reaction.
- a reaction where energy is transferred to the reacting substances from its surroundings is called an endothermic reactions.
- in reverse reactions, one reaction is exothermic and the other is endothermic.
- in any reversible reaction, the amount of energy released when the reaction goes in one direction is exactly equal to the energy absorbed when the reaction goes in the opposite direction.
- we can change the amount of products formed at equilibrium by changing the temperature at which we carry out a reverse reaction.
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Energy and Reactions 2
- the Haber process uses a pressure of around 200 to 350 atmospheres to increase the amount of ammonia produced.
- although higher pressures would produce more ammonia, they would make the chemical plant too expensive to build.
- a temperature of about 450 degrees C is used for the reaction. Although lower temperatures would increase the amount of ammonia at equilibrium, the ammonia would be produced too slowly.
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- electrolysis involves splitting up a substance using electricity.
- ionic substances can be electrolysed when they are molten or in solution.
- in electrolysis positive ions move to the negative electrode (cathode) and negative ions move to the positive electrode (anode).
- in electrolysis, the ions move towards the oppositely charged electrodes.
- at the electrodes, negative ions are oxidised while positive ions are reduced.
- reactions where reduction and oxidation happen are called redox reactions.
- when electrolysis happens in water, the less reactive element is usually produced at an electrode.
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- when we electrolyse brine we get three products- chlorine gas, hydrogen gas and sodium hydroxide solution.
- chlorine is used to kill microbes in drinking water and swimming pools, and to make hydrochloric acid, disinfectants, bleach and plastics.
- hydrogen is used to make margarine and hydrochloric acid.
- sodium hydroxide is used to mke bleach, paper and soap.
- copper extracted from its ore contains impurities such as gold and silver.
- copper is purified by electrolysis to remove these impurities.
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Acids, Alkalis and Salts 1
- acids are substances which produce H+ ions when we add them to water.
- bases are substances that will neutralise acids.
- an alkali is a souble base. Alkalis produce OH- ions when we add them to water.
- we use the pH scale to show how acidic or alkaline a solution is.
- when we react an acid with a base a neutralisation reaction occurs.
- the reaction between an acid and a base produces a salt and water.
- salts can also be made by reacting a metal with an acid. This reaction prduces hydrogen gas as well as a salt.
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Acids, Alkalis and Salts 2
- an indicator is needed when we produce a salt by reacting an alkali with an acid to make a soluble salt.
- insoluble salts can be made by reacting two solutions to produce a precipitate.
- precipitation is an important way of removing some substances from wastewater.
Mole: The relative formula mass of a substance in grams.
Precipitate: A solid material produced from a solution.
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