- The periodic table of elements developed as chemists tried to classify the elements. It arranges them in a pattern in which similar elements are grouped together.
- One of the first suggestions was made by John Dalton who arranged elemnts in order of their masses.
- Next, Newtons' table put elements in order of atomic mass, but failed to take account of elements that were unknown at the time, meaning several elements were odd and didn't fit the pattern.
- Mendeleev's periodic table left gaps for the unknown elements that he predicted, and so provided the basis for the modern periodic table.
- The atomic (proton) number of an element determines its position in the periodic table.
- The number of electrons in the outermost shell (highest energy level) of an atom determines what Group its in and its chemical properties.
- We can explain trends in reactivity as we go down the Groups in terms of:
- the distance between the outermost electrons and the nucleus
- the number of occupied inner shells (energy levels) in the atoms.
- The elements in Group 1 of the periodic table are called the alkali metals.
- Their melting points and boiling points decrease going down the group.
- The reactivity of the alkali metals increases going down the group.
- The metals all react with water to produce hydrogen and an alkaline solution containing the metal hydroxide.
- They form 1+ ions in reactions to make ionic compounds. These are generally white and dissolve in water, giving colourless solutions.
- Compared with the alkali metals, transition elements have much higher melting points and densities. They are also stronger and harder, but are much less reactive.
- The transition elements do not react vigorous with oxygen or water.
- A transition element can form ions with different charges, in compounds that are often coloured.
- Transition elements and their compounds are important industrial catalysts.
- The halogens (Group 7) all form ions with a single negative charge in their ionic compounds with metals.
- The halogens form covalent compounds by sharing electrons with other non-metals.
- A more reactive halogen can displace a less reactive halogen from a solution of one of its salts.
- The reactivity of the halogens decreases going down the group.
- Hard water contains dissolved compounds such as calcium and magnesium salts.
- The calcium and/or magnesium ions in hard water react with soap producing a precipitate called scum.
- One type of hard water can produce a solid scale when it is heated, reducing the efficiency of heating systems and kettles.
- Hard water is better than soft water for devloping and maintaining teeth and bones. It may also help to prevent heart diseases.
- Soft water does not contain ions that produce scum or scale.
- Hard water can be softened by removing the ions that produce scum and scale.
- Temporary hardness is removed from water by heating/boiling it. Permanent hardness is not changed by heating.
- The hydrogencarbonate ions in temporary hard water decompose on heating. The carbonate ions formed react with Ca2+ (aq) and Mg2+ (aq) ions, making precipitates.
- Either type of hard water can be softened by adding washing soda or by using an ion-exchange resin of sodium ions to remove calcium and magnesium ions.
- Water for drinking should contain only low levels of dissolved substances and microbes.
- We can make water pure by distilling it, but this requires large amounts of energy which makes it expensive.
Water is made fit to drink by passing it through several stages:
- Settlement tank - sand and soil settle out.
- Aluminium sulfate and lime are added to the water - Smll particles of dirt clump together so they sink to the bottom of the water.
- Fine sand filter - Removes any remaining particles of mud or grit, so the water is clear.
- Chlorine - A small amount is added to kill any harmful bacteria.
- pH - Gets checked and corrected until it's neutral.
- Chlorine is added to water to sterilise it by killing microbes.
- Fluoride helps to improve dental health.
- Some argue against the fluoridtion of public water supplies. For example, they think that people should have the right to choose if they want to take extra fluoride or not.
- When fuels and food react with oxygen, energy is released in an exothermic reaction.
Energy released = mass of water x specific heat x rise in temp.
heated capacity of water
- A simple calorimeter can be used to compared the energy released by different foods or fuels ina aschool chemistry lab.
- We can calculate the energy change for reactions in solution by measuring the temperature change and using the equation: Q = m c (change)T (this is given in the exam)
- Neautralisation and displacement reactions are both examples of reactions that we can use this technique for.
- We can show the relative difference in energy of reactants and products on energy level diagrams.
- Catalysts lower the activation energy so a greater proportion of reactant particles have enough energy to react.
- Bond breaking is endothermic because bonds need energy to break.
- Bond making is exothermic because when new bonds are formed, energy is released.
- In chemical reactions, energy must be supplied to break the bonds between atoms in the reactants.
- When new bonds are formed between atoms in a chemical reaction, energy is released.
- In an exothermic reaction, the energy released when new bonds are formed is greater than the energy absorbed when bonds are broken.
- In an endothermic reaction, the energy released when the new bonds are formed is less than the energy absrobed when bonds are broken.
- We can calculate the overall energy change in a chemical reaction using bond energies.
- Much of the world relies on fossil fuels. However, they are non-renewable and they cause pollution. Alternative fuels need to be found soon.
- Hydrogen is one alternative. It can be burned in combustion engines or used in fuel cells to power vehicles. It doesn't release CO2 but does use a lot of energy to produce.
- Most Group 1 and Group 2 metals can be identified in their compounds using flame tests.
Lithium - Crimson
Sodium - Yellow
Potassium - Lilac
Calcium - Red
Barium - Green
- Sodium hydroxide solution can be used to identify different metal ions, depending on the precipitate that is formed.
Copper (||) ions - Blue
Iron (||) ions - Green
Iron (|||) ions - Redish Brown
- We identify carbonates by adding dilute acid, which produces carbon dioxide gas. The gas turns limewater cloudy.
- We identify halides by adding nitric acid, then silver nitrate solution. This produces a precipitate of silver halide.
- Chloride = White, Bromide = Cream, Iodide = pale yellow
- We identify sulfates by adding hydrochloric acid, then barium chloride solution. This produces a white precipitate of barium sulfate.
- Titration is used to measure accurately how much acid and alkali react completely.
- The point at which an acid-alkali reaction is complete is called the end point of the reaction.
- We use an indicator to show the end point of the reaction between an acid and an alkali.
To calulate the concentration on a solution, given the mass of solute in a certain volume:
- Calculate the mass (in grams) of solute in 1cm(cubed) of solution.
- Calculate the mass (in grams) of solute in 1000cm(cubed) of solution.
- Convert the mass (in grams) to moles.
To calculate the mass of solute in a certain volume of solution of known concentration:
- Calculate the mass in grams of the solute there is in 1dm(cubed) [1000cm(cubed)] of solution.
- Calculate the mass (in grams) of solute there is in 1cm(cubed) of the solution.
- Calculate the mass (in grams) of solute there is in the given volume of the solution.
- Scientists working in environmental monitoring, medicine and forensic science all need to analyse substances.
- The results of their analysis are often matched against existing databases to identify substances (or suspects in the case of forensics).
- In a reversible reaction the products of the reaction can react to re-form the original reactants.
- In a closed system the rate of the forwrd reaction and reverse reactions are equal at equilibrium.
- Changing the reaction conditions can change the amounts of products and reactants in a reaction mixture at equilibrium.
- Pressure can affect reverisble reactions involving gases at equilibrium. Increasing the pressure favours the reaction with the least number of molecules of gas formed. Decreasing the pressure favours the reaction with the greater number of molecules of gas formed.
- We can change the amount of products formed at equilibrium by changing the temperture at which we carry out the reversible reaction.
- Increasing the temperature favours the endothermic reaction. Decreasing the temperature favours the exothermic reaction.
- Ammonia is an important chemical for making other products, 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 reasonable yield of ammonia as quickly as possible.
- Any unreacted nitrogen and hydrogen are recycled in the Haber process.
- The Haber process uses a pressure of around 200 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 and run, as well as being too dangerous.
- A temperature of around 450 degrees C is used for the reaction. Although lower temperatures would increase the yield of ammonia, it would be produced too slowly.
- Iron catalysts are used to speed up this reaction.
- The homogulous serious of alcohols contain the -OH functional group.
- The homogulous serious of carboxylic acids contain the -COOH functional group.
- The homogulous serious of esters contain the -COO functional group.
- Alcohols are used as solvents and fuels, and ethanol is the main alcohol in alcoholic drinks.
- Alcohols burn in air, forming carbon dioxide and water.
- With sodium metal, alcohols react to form a solution, and hydrogen gas is given off.
- Ethanol can be oxidised to ethanoic acid, either by chemical oxidising agents or by the action of microbes. Ethanoic acid is the main acid in vinegar.
- Solutions of carboxylic acids have a pH value less than 7. Carbonates gently fizz in their acidic solutions, releasing carbon dioxide gas.
- Aqueous solutions of weak acids have a higher pH value than solutions of strong acids with the same concentration.
- Esters are made by reacting a carboxylic acid and an alcohol together with an cid catalyst.
- Esters are volatile compounds used in flavourings and perfumes.
- Alcohols, carboxylic acids and esters have many uses which benefit society.
- Some of these substances, such as ethanol and solvents, can be abused.
- In future, the use of biofuels, such as ethanol and esters, could help society as crude oil supplies run out.
- Future uses of biofuels might conflict with the need to feed the world.