Chemistry a Flashcards
Why did atomic number not exist in the 1800s ?
They had no idea about atomic structure or of protons and electrons
Until recently, what were the two ways of categorising elements ?
- Their physical and chemical properties
2. Their relative atomic mass
How were the elements first ordered ?
Relative atomic mass
In what year did new land form his element arrangement ?
1864
What did Newland notice ?
That every 8th element had similar properties - Newlands Octaves
Why was Newland’s element arrangement criticised ?
- His groups contained elements that didn’t share properties
- He mixed up metals and non metals
- Didn’t leave gaps for undiscovered elements
In what year did Mendeleev create his own periodic table ?
1869
What did Mendeleev do ?
He put the elements in order of atomic mass
What did Mendeleev find when grouping his elements ?
He had to leave gaps in order to keep elements with the same properties in the same column
Why was the fact he left gaps so influential ?
He predicted the properties of undiscovered elements
Why did many scientists think that the periodic table wasn’t important ?
There wasn’t much evidence to support the periodic table
What evidence was soon found that suggested Mendeleev’s periodic table was correct ?
New elements were found and they fitted into the gaps he left
What happened in the 19th century which meant that nearly all scientists accepted that the periodic table was useful ?
Scientists discovered protons, neutrons and electrons
What does the periodic table allow us to do ?
Predict element’s chemical properties
What do elements in the same group have ? (Excluding transition metals)
The same number of electrons in their outer shell (highest energy level)
What does the group number represent ?
The amount of electrons on the outer shell
How are electrons held in place ?
The positive charge of the nucleus attracts the negative electrons.
The further away from the nucleus, the…
The less attraction the electron has
What is shielding ?
When the attraction of the nucleus is reduced because the ‘inner electrons’ get in the way
What is energy level ?
The amount of shells of electrons an atom has
Why do group 1 metals become more reactive as you go down the periodic table ?
The combination of an increased distance and increased shielding means that an electron in a higher energy level is lost more easily.
As you go down group 1, what happens to the boiling and melting points ?
They decrease
Why can some group 1 metals float ?
They have a lower density than water
What are the group 1 metals also known as and why ?
The alkali metals - when they mix with water they form a metal hydroxide which dissolves to form an alkali solution
What are the first three group 1 elements ?
Lithium
Sodium
Potassium
What do group 1 metals have on their outer shell ?
1 electron
What compounds do the alkali metals form ?
Ionic compounds because they lose the electron (ionic bonding)
What happens when you add alkali metals to water ?
They float on the surface and fizz. The metal displaces the hydrogen in the water, forming hydrogen gas and metal hydroxide
What happens to the metal hydroxide in the water ?
It dissolves in the water, forming an alkali solution. It would turn universal indicator purple
Why do group 7 non-metals become less reactive as you go further down the periodic table ?
Increased distance and shielding means the higher the energy level the less likely it is to gain an election. This is because there is less attraction to draw the electron in
As you go further down group 7, what happens to the melting and boiling point ?
It increases. The group 7 and group 1 elements follow opposite patterns
What is another name for the group 7 elements ?
Halogens
How do halogens form halides ?
They gain an electron from a non-metal to form an atom with a complete outer shell (ionic bonding)
What can more reactive halogens do ?
Displace less reactive halogens
What properties do transition metals have ?
- Good conductors
- Dense, strong and shiny
- Much less reactive than group 1 metals
- Higher melting points than other metals
Why do some transition metals form different colour compounds, even when they are the same element ?
Transition metals often have more than one ion
What is an example of an element that forms different coloured compounds ?
- Iron - Fe2+ usually produces green compounds
* Iron - Fe3+ usually produces red/brown compounds
How do the colours in gemstones and pottery glaze occur ?
They are transition metals. Transition metals produce colourful compounds.
What benefits can transition metals have on reactions ?
Transition metals and their compounds all make good catalysts
What is iron used as a catalyst in ?
Haber process - used to make ammonia
What is manganese oxide used as a catalyst for ?
Decomposition of hydrogen peroxide
Where is a nickel catalyst used ?
Hydrogenation- turning oils into fats
When you mix soft water with soap what do you get ?
Lather
When you mix hard water with soap what do you get ?
Scum
What happens when you heat hard water ?
Scale is formed
What is scale ?
Calcium carbonate - limescale/limestone
Why is scale build up bad ?
It can reduce the efficiency of heating systems and they may need replacing. It can eventually block pipes
Why do kettles take longer to boil water with hard water ?
The hard water forms scale, the scale acts as a thermal insulator. If the scale is around the heating element then it will take longer for the energy to reach the water
Which ions cause hard water ?
Calcium - Ca2+
Magnesium - Mg2+
How do hard water ions get into water ?
When rainwater falls on some rocks, compounds like magnesium sulphate and calcium sulphate dissolve in the water
How are hard water ions good for you ?
- Calcium ions are good for your teeth and bones
- Studies have suggested people who drink hard water have a reduced chance of heart disease because of the extra minerals
What causes temporary hard water ?
Hydrogencarbonate ion
What causes permanent hard water ?
Dissolved calcium and magnesium sulphate
How do you make temporary hard water soft ?
Boil it. When heated the calcium hydrogencarbonate decomposes to form calcium carbonate which is insoluble. The solid is the limescale on your kettle.
Why can you not boil permanent hard water to make soft water ?
Heating the ions won’t remove them
What method makes both forms of hard water soft ?
By adding sodium carbonate (washing soda)
How does sodium carbonate remove hard water ?
It reacts with the calcium and hydrogen to make an insoluble precipitate. The ions are no longer dissolved in the water, so it is soft. The insoluble precipitate can be removed
What is an ‘ion exchange column’ ?
It is a resin in which water can be passed through. It contains sodium ions which swap places with the calcium or magnesium ions that cause hard water
How do you compare the hardness of water samples ?
Titration
How are titration reactions used to compare water hardness ?
Fill a burette with soap and a flask with the same amount. Add 1cm^3 of soap and shake well. Repeat these steps until a good lasting lather forms. Measure the amount of soap needed to form the lather. Repeat all of the above but this time boil the water prior to adding soap to remove temporary hardness
What is a good lasting lather ?
When bubbles cover the surface for at least 30 seconds
Why does water need to be good quality ?
It must be free of poisonous salts and harmful microbes. Microbes can cause diseases such as cholera
Where does most of our drinking water come from ?
Reservoirs, water flows into it from rivers and groundwater sources
What happens to water in a water treatment works ?
- Passed through a mesh screen to remove big bits like twigs (screening)
- Chemicals are added to bind solids together and make them sink to the bottom
- The water is filtered through gravel beds to remove all solids
- Chlorination kills any harmful microbes
Some people are not satisfied with their tap water, what do they do with it ?
They buy filters that contain carbon and silver to remove unwanted substances. Carbon removes the chlorine taste and the silver removes germs
Why is distilled water not used as tap water ?
It is to expensive to distill water
Why is fluorine added to tap water ?
To reduce tooth decay
Why is chlorine added to water ?
To prevent disease
What do some scientists think adding chlorine is bad ?
The chlorine can react with some natural substances in the body to produce toxic by-products. These are thought to increase the risk of cancer
What can fluorine cause in the long term ?
Cancer and bone problems
What is a reversible reaction ?
When the products of the reaction can themselves react to produce the original reactants
What is equilibrium ?
The amount of reactants and products will reach a certain balance and stay there. Reactions are taking place in both directions but they balance out
What condition is needed for equilibrium ?
It needs to be a closed system
What two factors affect the ‘position of equilibrium’ ?
Temperature and pressure
Why would scientists want to move the ‘position of equilibrium’ ?
They can have more product and less reactants - increasing yield
True or false - reversible reactions are either endothermic or exothermic both ways
False - reversible reactions are exothermic in one direction and endothermic in the other
What happens if you increase the temperature of a reversible reaction ?
The endothermic side of the reaction will benefit and increase because it will use up the extra energy
What happens if you increase the pressure in a reversible reaction ?
The reaction which produces less volume will benefit
What is the haber process ?
An industrial process which produces ammonia
What is the formula for making ammonia (2NH3) ?
N2 (g) + 3H2 (g) 2NH3 (g)
Reversible reaction
Where does the nitrogen and hydrogen, needed for the haver process come from ?
Nitrogen - from air, which is 78% nitrogen
Hydrogen - from natural gas or fossil fuels
Why does only some of the nitrogen and hydrogen react to form ammonia ?
It is a reversible reaction. Some of the ammonia will break down into nitrogen and hydrogen. The reaction reaches equilibrium
What conditions are needed for the haber process ?
Pressure - 200 atmospheres
Temperature - 450 degrees Celsius
Catalyst - Iron
How is the pressure chosen in the haber process ?
Higher pressure benefits the forward reaction (making ammonia) because there are 4 molecules of reactants which form 2 molecules of products
Why is a pressure higher than 200 atmospheres not chosen ?
Event though it would benefit the reaction, the plant would be too expensive to build to contain that pressure
How does a high temperature affect the haber process ?
- It is exothermic so would move the equilibrium the wrong way. The lower the temperature the greater the yield
- it increases the rate of reaction. The higher the temperature the faster the reaction
How is the temperature (450 degrees c) chosen ?
It is a comprise between maximum yield and speed of reaction. It is better to wait 20 seconds for a 10% yield than 60 seconds for 20% yield
How is the ammonia collected ?
It is formed as a gas but is cooled in the condenser. It liquifies and is then removed
What happens to the hydrogen and nitrogen that didn’t form ammonia ?
They are recycled by pumping back to where the reaction starts so nothing is wasted
What does the iron catalyst do in the haber process ?
Makes the reaction faster, it doesn’t effect the equilibrium position only gets to the position quicker
What would have to be done without the iron catalyst ?
The temperature would have to rise even further, reducing the yield even more
What is the general formula of alcohol ?
What is the functional group of alcohols ?
- C(n)H(2n+1)OH
* -OH
What is produced when alcohols burn in air ?
Carbon dioxide and water
What happens when the first three alcohols are mixed with water ?
They dissolve to form neutral solutions
What is formed when alcohols are reacted with sodium ?
- Hydrogen
* Alkoxides - ethanol produces sodium ethoxide
Why is ethanol the main alcohol used in drinks and not methanol ?
Methanol causes blindness when drunk, although ethanol is still toxic to the liver and brain
Why are the first three alcohols such good solvents ?
They can dissolve substances that water can’t
What are the first three alcohols ?
Methanol
Ethanol
Propanol
What is methylated spirit ?
Ethanol mixed with chemicals, used to clean paintbrushes and as a fuel
Why is ethanol a good fuel ?
It burns cleanly and it’s not smelly
How can ethanol be used in cars ?
It can be mixed with petrol, the more ethanol the less pollution produced
Why is ethanol a good alternative to petrol ?
- Renewable
* Countries who have no oil deposits can grow sugar cane instead
What is the functional group of carboxylic acids ?
-COOH
What are the first three carboxylic acids ?
Methanoic acid
Ethanoic acid
Propanoic acid
What is formed when you react carboxylic acids with sodium carbonate ?
Carbon dioxide
Salts - sodium ethanoate
What happens when carboxylic acids are added to water ?
They dissolve to form acidic solutions
Why are dissolved carboxylic acids considered weak acids ?
When they dissolve, they only ionise partially - releasing hydrogen ions. This means they have a higher pH (less acidic)
What happens when you leave wine open ?
The ethanol inside it is oxidised, forming ethanoic acid and water. This is when wine goes off
How is vinegar made ?
By dissolving ethanoic acid in water
How are carboxylic acids used ?
- Longer chains are used to make soaps and detergents
* In the preparation of esters
Why aren’t carboxylic acids used as solvents ?
They are good solvents however they make the solution acidic
What is the functional group for esters ?
-COO-
How are esters made ?
By mixing an alcohol and carboxylic acid. Water is made as well
How do you work out the name of the ester ?
- The alcohol forms the first part + ‘yl’
- The acid forms the second part + ‘oate’
Eg. Ethanol + Propanoic acid –> ethyl propanoate
Why are esters used in perfumes ?
- They smell nice
* They are volatile so they evaporate quickly
Why are esters mixed with alcohols and not water ?
They don’t mix with water
Why are esters sometimes dangerous ?
- Inhaling the fumes can irritate mucus membranes
- They’re fumes are heavier than air and are flammable
- Some esters are toxic
Why are esters better solvents than others ?
They don’t release nearly as many toxic fumes as others. They have replaced other solvents in paints
What is the difference between concentration and strength of acids ?
- concentration is how watered down something is
* strength is how well it has ionised in water
When is methyl orange used ?
- strong acid
* weak alkali
When is phenolphthalein used ?
- weak acid
* strong alkali
