- Elements react together to form compounds by gaining or losing electrons (ionic) or sharing electrons (covalent).
- The elements in Group 1 react with the elements of Group 7. As they react, atoms of Group 1 can each lose one electron to gain the stable electronic structure of a Noble Gas (full outer shell). This electron can be given to an atom from Group 7, which then also achieves a full outer shell.
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- Ionic compounds are held together by strong forces of attraction between the oppositely charged ions. This is called ionic bonding.
- Ionic bonds between charged particles result in an arrangement of ions that we call a giant structure or a giant ionic lattice.
- Besides the elements in Groups 1 and 7, other elements that can form ionic compounds include those of Groups 2 and 6.
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- The charges on the ions in an ionic compound always cancel each other out. E.g: Ca2+ and Cl- become CaCl2.
- The formula of an ionic compound shows the ratio of ions present in the compound.
- Sometimes we need brackets to show the ratio of ions in a compound, e.g. magnesium hydroxide - Mg(OH)2
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- Covalent bonds are formed when atoms share pairs of electrons.
- Many substances containing covalent bonds consist of simple molecules, but some have giant covalent structures.
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- The atoms in metals are closely packed together and arranged in regular layers.
- We can think of metalllic bonding as positively charged metal ions which are held together by electrons from the outermost shell of each metal atom. These delocalised electrons are free to move throughout the giant metallic lattice.
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- It takes a lot of energy to break the many strong ionic bonds which hold a giant ionic lattice together. So ionic compounds have high melting points. They are all solids at room temperature.
- Ionic compounds will conduct electricity when we melt them or dissolve them in water. That's because their ions can then move more freely around and can carry charge through the liquid.
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- Substances made up of simple molecules have low melting points and boiling points.
- The forces between simple molecules are weak. These weak intermolecular forces explain why substances made of simple molecules have low melting points and boiling points.
- Simple molecules have no overall charge, so they cannot carry electrical charge. Therefore substances made of simple molecules do not conduct electricity.
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- Some covalently bonded substances have giant structures. These substances have high melting points and boiling points.
- Graphite contains giant layers of covalently bonded carbon atoms. However, there are no covalent bonds between the layers. This means they can slide over each other, making graphite soft and slippery. The atoms in diamond have a different structure and cannot slide like this, so diamond is a very hard structure.
- Graphite can conduct electricity because of the delocalised electrons along its layers.
- As well as diamond and graphite, carbon also exists as fuellerenes which can form large cage-like structures based on hexagonal rings of carbon atoms.
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- We can bend and shape metals because the layers of atoms (or ions) in a giant metallic structure can slide over each other.
- Delocalised electrons in metals enable electricity and heat to pass through the metal easily.
- If a shape memory alloy is deformed, it can return to its original shape on heating.
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- Monomers affect the properties of the polymers that they produce.
- Changing reaction conditions can also change the properties of the polymer that is produced.
- Thermosoftening polymers will soften or melt easily when heated.
- Thermosetting polymers will not soften but will eventually char if heated very strongly.
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- Nanoscience is the study of small particles that are between 1 and 100 nanometres in size.
- Nanoparticles behave differently from the materials they are made from on a large scale.
- New developments in nanoscience are very exciting but will need more research into possible issues that might arise from their increased use.
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- The relative mass of protons and neutrons is 1.
- The atomic number of an atom is its number of protons (which equals its number of electrons).
- The mass number of an atom is the total number of protons and neutrons in its nucleus.
- Isotopes are atoms of the same element with different numbers of neutrons.
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- We compare the masses of atoms by measuring them relative to atoms of carbon-12.
- We work out the relative formula mass of a compound by adding up the relative atomic masses of the elements in it, in the ratio shown by its formula.
- One mole of any substance is its relative formula mass, in grams.
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- The relative atomic masses of the elements in a compound and its formula can be used to work out its percentage.
- We can calculate empirical formulae given the masses or percentage composition of elements present.
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- Balanced symbol equations tell us the number of moles of substances involved in a chemical reaction.
- We can use balanced symbol equations to calculate the masses of reactants and products in a chemical reaction.
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- The yield of a chemical reaction describes how much product is made.
- The percentage yield of a chemical reaction tells us how much product is made compared with the maximum amount that could 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's important to maximise yield and minimise energy wasted to conserve the Earth's limited resources and reduce pollution.
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% YIELD = amount of product produced/maximum amount of product possible
- In a reversible reaction, the products of the reaction can react to make the original reactants.
- We can show a reversible with the double arrow sign.
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- Additives may be added to food in order to improve its appearance, taste and how long it will keep (its shelf life).
- Food scientists can analyse foods to identify additives e.g. by using paper chromatography.
- Modern instrumental techniques provide fast, accurate and sensitive ways of analysing chemical substances.
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- Compounds in a mixture can be separated using gas chromatography.
- Once separated, compounds can be identified using a mass spectrometer, by their different retention times.
- The mass spectrometer can be used to find the relative molecular mass of a compound from its molecular ion peak.
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- We can find out the rate of a chemical reaction by following the amount of reactants used up over time.
- Alternatively, we can find out the rate of reaction by following the amount of products made over time.
- The slope of the line at any given time on the graphs drawn from such experiments tells us the rate of reaction at that time. The steeper the slope, the faster the reaction.
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- Particles must collide with a certain amount of energy before they can react.
- 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 is increased. This increases the frequency of collisions between reacting particles.
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- Reactions happen more quickly as the temperature increases.
- Increasing the temperature increases the rate of reaction becuase the particles are colliding more frequently and more energetically. More of the collisions result in a reaction because a higher proportion of particles have energy greater than the activation energy.
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- Increasing the concentration of reactants in solutions increases the frequency of collisions between particles, and so increases the rate of reaction.
- Increasing the pressure of reacting gases also increases the frequency of collisions and so increases the rate of reaction.
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- A catalyst speeds up the rate of a chemical reaction.
- A catalyst is not used up during a chemical reaction.
- Different catalysts are needed for different reactions.
- Catalysts provide another surface to react on.
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- Catalysts are used whenever possible in industry to increase the rate of reaction and reduce energy costs.
- Traditional catalysts are often transition metals or their compounds, which can be toxic and harm the environment if they escape.
- Modern catalysts are being developed in industry which result in less waste and are safer for the environment, e.g. enzymes.
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- Energy may be transferred to or from the reacting substances in a chemical reaction.
- A reaction in which energy is transferred from the reacting substances to their surroundings is called an exothermic reaction.
- A reaction in which energy is transferred to the reacting substances from their surroundings is called an endothermic reaction.
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- In reversible 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.
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- Exothermic changes can be used in hard warmers and self-heating cans. Crystallisation of a supersaturated solution is used in reusable warmers. However, disposable, one-off warmers can give off heat for longer.
- Endothermic changes can be used in instant cold pcks for sports injuries.
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- Acids are substances which produce H+ ions when we add them to water.
- Bases are substances that will neutralise acids.
- An alkali is a soluble hydroxide. Alkalis produce OH- ions when we add them to water.
- We can use the pH scale to show how acidic or alkaline a solution is.
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- 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 suitable metal that's more reactive than hydrogen with an acid. This reaction produces hydrogen gas as well as salt. A sample of the salt made can then be crystallised out of solution by evaporating off water.
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- An indicator is needed when a soluble salt is prepared by reacting an alkali with an acid.
- Insoluble salts can be made by reacting two salts to produce a precipitate.
- Precipitation is an important way of removing some metal ions from industrial wastewater.
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- Electrolysis breaks down a substance using electricity.
- Ionic compounds can only be electrolysed when they are molten or in solution. That's because their ions are then free to move to the electrodes.
- In electrolysis, positive ions move to the negative cathode while the negative ions move to the positive anode.
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- In electrolysis, the ions move towards the oppositely charged electrodes.
- At the electrodes, negative ions are oxidised while positive ions are reduced.
- When electrolysis happens in water, the less reactive element, between hydrogen and a metal, is usually produced at the negative electrode. At the positive electrode, we often get oxygen gas given off from the discharged hydroxide ions.
Oxidised = A reaction where oxygen is added to a substance (OR when electrons are lost from a substance).
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- Aluminium oxide is electrolysed in the manufacture of aluminium metal.
- The aluminium oxide is mixed with molten cryolite to lower its melting point.
- Aluminium forms at the negative electrode an oxygen at the positive electrode.
- The positive carbon electrodes are replaced regularly as they gradually burn away.
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- When we electrolyse brine we get three products - chlorine gas, hydrogen gas and sodium hydroxide solution (an alkali).
- Chlorine is used to make bleach, which kills bacteria, and to make plastics.
- Hydrogen is used to make margarine.
- Sodium hydroxide is used to make bleach, paparer and soap.
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- We can electroplate objects to improve their appearance, protect their surface and to use smaller amounts of precious metals.
- The object to be electroplatedis made the negative electrode in an electrolysis cell. The plating metal is made the positive electrode. The electrolyte contains ions of the plating metal.
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