Chemistry Unit 3 Flashcards
What 2 things does tap water have to be free of?
Poisonous salts (phosphates, nitrates)
Harmful microbes
What is the problem with adding chlorine to tap water?
Can react with other substances in the water to produce toxic by-products
Describe the method of using titration to compare the hardness of water samples
Fill a burette with 50cm^3 soap solution
Add 50cm^3 of the first water sample to a flask
Use the burette to add 1cm^3 to the flask
Put a bung in the flask and shake it for 10 seconds
Repeat the last 2 steps until a long lasting lather is formed
Record how much soap was needed to form a lather
Repeat the experiment for the different water samples
What is the problem with adding chemicals to tap water in general?
We don’t get to choose whether we want the chemicals in our tap water
What are the 2 problems with scale?
The scale can block up pipes which might need to be replaced
Scale is a thermal insulator so scale in the kettle causes the kettle to take longer to boil reducing its efficiency
What causes water to be hard?
Rain falling on some types of rocks like limestone, chalk and gypsum can dissolve compounds like magnesium sulphate and calcium sulphate
Describe how water from reservoirs is treated so it can be tap water
Passes through a mesh screen to remove big bits like twigs
Chemicals are added to make solids and microbes stick together and fall to the bottom
The water is filtered through gravel beds to remove all solids
Water is chlorinated to kill any harmful microbes left
What are the 2 advantages of hard water?
Ca2+ ions are good for healthy teeth and bones
People who live in hard water areas seem to be at less risk of heart disease
What is the problem with adding fluoride to water?
Can cause cancer and bone problems in high doses
What 2 things are added to tap water?
Fluoride and chlorine
Why is chlorine added to tap water?
To prevent disease from harmful microbes
How do we obtain completely pure water?
Distillation
Describe how adding washing soda removes hardness of water
The added carbonate ions react with the calcium and magnesium ions to form an in soluble precipitate of calcium carbonate and magnesium carbonate
The calcium and magnesium ions are no longer dissolved so they can’t make it hard
Ca2+ + (CO^3)2- –> CaCO^3
What causes hard water to produce scale?
Hard water produces scale when it is heated
Describe how ion exchange columns remove hardness of water
The column has many sodium ions in a resin
When the hard water runs through it, the calcium and magnesium ions are exchanged for sodium ions
Na^2Resin + Ca2+ –> CaResin + 2Na+
What causes hard water to produce scum?
When hard water is mixed with soap it takes longer to form a lather because the calcium and magnesium ions in the hard water react with the soap to form scum which is insoluble
What are 2 types of hardness and what are they caused by?
Temporary caused by hydrogen carbonate ion (HCO^3-)
Permanent is caused by mainly calcium sulphate
What is hard water?
Water that contains lots of calcium 2+ and magnesium 2+ ions
What is the problem with scum being produced instead of a leather?
More soap is needed to make a lather so more money is spent on soap
Describe how only temporary hardness is removed and what results from this
I will use calcium as an example
When heated, calcium hydrogen carbonate decomposes to form calcium carbonate (scale), water and carbon dioxide
What are the 2 things that hard water produces and what are their chemical compositions?
Scum (calcium and magnesium ions reacting with soap)
Scale (mostly calcium carbonate)
What are the 2 ways of removing both temporary and permanent hardness?
Adding washing soda (sodium carbonate)
Running it through ion exchange columns
What will the results be for the hard water titration experiment?
The hard water samples will have a much higher amount of soap to form a lather
Why is fluoride added to tap water?
Helps reduce tooth decay
Why don’t we put distilled water into taps?
Too expensive
What are the 3 issues with burning fuels for energy?
Has various environmental effects like global warming
It will be expensive to change to other energy sources that are better
Crude oil is a finite resources and will run out eventually
What are the 3 disadvantages of hydrogen gas fuel?
You need a special, expensive engine
You need to use energy from another source to make hydrogen gas from water
It’s very explosive so it is hard to store
Describe how the calculation of calorimetry is done
The mass of the fuel burned = the mass of spirit burner before - mass of burner after
Temperature change of water = temperature after - temperature before
The mass of water is 50g
It takes 4.2 joules to heat up 1g of water by 1C (specific heat capacity)
The energy transferred by the amount of fuel burned = mass of water (50g) * specific heat capacity of water (4.2) * temperature change of water
What is calorimetry?
The method used to calculate how much energy is stored in a given mass of a fuel
What is a fuel cell?
An electrical cell that’s supplied with a fuel and oxygen and uses energy from the reaction between them to generate electricity
Calculate the overall energy change for the reaction H^2 + Cl^2 –> 2HCl
Assume that:
H-H = 1kJ/mol
Cl-Cl = 2kJ/mol
H-Cl = 2kJ/mol
H^2 + Cl^2 –> 2HCl
H-H + Cl-Cl –> 2H-Cl
1kJ/mol + 2kJ/mol - 2(2kJ/mol) = -1kJ/mol
This means that -1kJ/mol is taken up by the reaction
This would be stated as 1kJ/mol has been released by the reaction
What causes a reaction to be endothermic and why?
A reaction is endothermic if more bonds are broken than formed because energy has to be supplied to break bonds
What is activation energy?
The amount of energy needed to break the bonds of reacting molecules (to trigger a reaction)
Describe how the practical part of calorimetry is done
Put 50g of water inside the copper can and record its temperature using a thermometer
Weigh the spirit burner with the fuel inside it and its lid on top of it then put the spirit burner underneath the copper can
Turn on the spirit burner and take off its lid
Put the lid over the spirit burner when the temperature of the water reaches 50C and turn it off
Weigh the spirit burner with the lid on top of it again
Describe how energy transfer of a reaction can be measured and explain why everything is done
For the equipment:
You put a polystyrene cup inside a beaker and fill all of the gaps between the 2 with cotton wool (to insulate the cup and reduce heat loss)
You also put a lid on the cup to also reduce heat kiss
For the method:
You take the temperature (thermometer) of the 2 reactants (liquid or solution) and if they are the same, you record the temperature and put the right amounts of the reactants for a complete reaction in the cup and close the lid
You open the lid when the reaction is complete and take the temperature to see how much energy has been transferred or absorbed
What effect would a catalyst have on an energy level diagram?
The activation energy would be lower
What would an energy level diagram look like for an endothermic reaction?
The curve will go upwards and then down slightly meaning that the activation energy is shown and the energy of the products is higher than the energy of the reactants
What is the advantage of fuel cells?
It doesn’t need recharging because only fuel and oxygen needs to be supplied
What is bond energy?
The amount of energy needed to break the bonds of a specific molecule which is the same amount of energy released when that bond is formed
What are the 2 advantages of using fuel cells in cars and replacing engines?
Could help countries to become less dependent on crude oil
No environmentally harmful waste products
What is an exothermic reaction?
A reaction that gives out energy to its surroundings usually in the form of heat and is shown as a rise in temperature
What is the advantage of using hydrogen gas fuel?
The only product is water so it is very environmentally friendly
What would the energy level diagram look like for an exothermic reaction?
The curve would go up first to represent the activation energy and then go down further than the start height to represent the products having less energy than the reactants
What causes a reaction to be exothermic and why?
A reaction is exothermic if more bonds are formed than broken because energy is released when bonds are formed
What is an endothermic reaction?
One that takes in energy (usually heat) and is shown by a fall in temperature
Describe how the equipment is set up for calorimetry
A copper can is suspended above a spirit burner
This equipment is within a draught excluder
How do catalysts work?
They lower the activation energy by providing an alternative pathway for the reaction
What is an energy level diagram?
A graph that shows if a reaction is exothermic or endothermic and the activation energy
How does hydrogen gas fuel work?
If hydrogen gas is burned, that provides the activation energy for hydrogen to react with oxygen in the air to form water
This is an exothermic reaction meaning energy is given out so hydrogen gas can be used as a fuel
Describe how testing for sulphates works
Add dilute hydrochloric acid and then barium chloride solution
A white precipitate of barium sulphate means the original compound was a sulphate
List the 6 ions that can be identified by precipitation with NaOH and give the colour of their precipitation
Calcium (Ca2+) - White
Copper (Cu2+) - Blue
Iron (Fe2+) - Green
Iron (Fe3+) - Brown
Aluminium (Al3+) - White at first but redissolves to turn colourless
Magnesium (Mg2+) - White
Describe how testing for halides works
You add dilute nut ric acid and then silver nitrate solution and a colour Ed precipitate will form
Describe how flame tests are done
Clean a wire loop by dipping it into hydrochloric acid and rinsing it with distilled water
Dip the wire loop into a sample of the compound and put it in the clear blue part of the Bunsen flame
List the 3 halides that can be tested and give the colour of their precipitate
Chloride (Cl-) - White
Bromide (Br-) - Cream
Iodide (I-) - Yellow
Describe how carbonate testing is done
You can test to see if a gas is carbon dioxide by bubbling it through line water. It is carbon dioxide if the limewater turns cloudy
You can use this to test for carbonates because carbonates react with dilute acids to form carbon dioxide
List the 5 ions that can be flame tested and give the colour of the flame for each
Lithium (Li+) - Crimson
Sodium (Na+) - Yellow
Potassium (K+) - Lilac
Calcium (Ca2+) - Red
Barium (Ba2+) - Green
What are the 2 chemical tests for negative ions?
Carbonate testing
Testing for halides and sulphates
Describe how precipitation with NaOH testing is done
Many metal hydroxides are insoluble and precipitate out of solution and have a characteristic colour
Add a few drops of sodium hydroxide solution to the solution of the mystery compound
If you get a coloured insoluble hydroxide, you can tell what the positive ion is
What are the 2 chemical tests for positive ions and which one would you try first?
Flame tests (try this first)
Precipitation with NaOH
How do you draw alcohols?
H H
| |
H - C - C - O - H
| |
H H
For ethanol etc.What other 2 things can alcohols be used for?
Solvent
Fuels
What is the functional group of alcohols?
-OH
How do you draw esters?
H O
| //
H - C - C H H
| \ | |
H O - C - C - H
| |
H H
For ethyl ethanoate
The top left part is the carboxylic acid part and the bottom right is the alcohol
The top line has been shifted to the right
What are the 3 main chemical properties for most alcohols?
They burn in air to produce carbon dioxide and water
Dissolve completely in water to form neutral solutions
They react with sodium to give hydrogen and sodium ethoxide
What is the functional group of esters?
-COO-
What is the functional group of carboxylic acids?
-COOH
What is the naming system for carboxylic acids?
Replace the “e” with “oic acid”
What is the alcohol used in drinks and what is the problem with it?
Ethanol is used in drinks and it damages the liver and brain
In what 2 contexts are alcohols used as solvents?
Ethanol is used as the solvent for perfumes and aftershave lotions
Methylated spirit is ethanol with other chemicals mixed with it and it’s used to clean paint brushes
What is a solvent?
A chemical that can dissolve substances that water can’t dissolve
What is the general formula of an alcohol?
C and then subscript n
H and then subscript 2n + 1
Then OH
How are carboxylic acids made?
An example is butanol + oxygen –> butanoic acid + water
What would the chemical formula for propanol be?
C^3 H^7 OH
What is the problem with acidic solutions of carboxylic acids?
They are weak acids
What is the main use of esters and why?
Used for flavourings and perfumes because they smell nice and are also volatile meaning that the evaporated molecules can be detected easily by smell receptors
How else can you produce alcohols?
An example would be ethene + water –> ethanol
In what 2 ways are alcohols used as fuels?
They are burned in spirit burners
They can be mixed with petrol for use in a car
What is the naming system for esters?
The alcohol forms the first part and the carboxylic acid forms the second part
An example is: for methane and propanoic acid, the ester is methyl propanoate
What are the 4 advantages of using ethanol as a fuel?
No smell
Clean burning so no pollution
Some developing countries can ferment sugar from sugar canes to get ethanol
Sugar cane is a renewable source
What are the 3 main ways that carboxylic acids react?
Ethanoic (example) acid + metal carbonate –> metal ethanoate + carbon dioxide + water
Dissolves in water to ionise, release H+ ions and make the solution acidic
Carboxylic acid + alcohol –> ester + water
How do you draw carboxylic acids?
H H O
| | //
H - C - C - C
| | \
H H OH
For propanoic acid
What is the naming system for alcohols?
Replace the final “e” with “ol”
What are the 5 uses of carboxylic acids?
Ethanoic acid can be dissolved in water to make vinegar
Citric acid is another carboxylic acid which is present in fruits
Soaps and detergents are made from carboxylic acids and long chain carbon atoms
Used to make eaters
Can be used as a solvent but it makes the solution acidic
What is the general formula for a carboxylic acid?
C and then subscript n
H and then subscript 2n+1
Then COOH
What is the other main use of esters?
Solvents for paint, ink, glue etc.
What are the 4 disadvantages of esters?
Very flammable and this makes them dangerous
Don’t mix well with water and not very soluble
Some esters irritate mucous membranes in the nose and mouth
Some are toxic
How are esters made?
Alcohol and carboxylic acids react in the presence of an acid catalyst (usually sulphuric) to form the ester and water
What do titrations tell us?
How much acid is needed to neutralise an alkali or vice versa
What is the concentration of a substance?
The amount of substance in a given volume
How do you calculate concentration in grams per dm^3?
Multiply the concentration in mol/dm^3 by the relative formula mass to get the concentration in g/dm^3
What is concentration measured in?
Moles per dm^3
Or
Grams per dm^3
What is the formula triangle that tells us the concentration in moles per dm^3?
Number of moles is over concentration in mol/dm^3 and volume in dm^3
What colour is phenolphthalein in acid and alkali?
It is colourless in acid and pink in alkali
What is one mole of a substance?
The relative atomic mass in grams
Describe how to do a titration
You put some alkali in a flask with some phenolphthalein indicator
Put the acid into a burette and add it to the flask a bit at a time, swirling the flask after every addition
When the indicator changes from pink to colourless, stop adding acid and record how much acid it took to neutralise the alkali
How does exothermic / endothermic relate to reversible reactions?
One way of the reaction is exothermic and the other is endothermic
What can a catalyst be used for in a reversible reaction?
Adding a catalyst means the reaction reaches equilibrium quicker
What are the 2 reasons that the catalyst is important?
Makes the reaction faster so equilibrium is reached much more quickly
Without the catalyst, the temperature would have to be raised further to get a quick enough reaction which would negatively affect the equilibrium position
What is the Haber process?
The reversible reaction that produces ammonia gas from nitrogen gas and hydrogen gas
What is the equilibrium position?
The ratio of the amounts of reactants and products when equilibrium is reached
What is a reversible reaction?
A reaction where the products of the reaction can react to produce the original reactants
Why is the temperature of the Haber process 450C?
The forward reaction is exothermic so a lower temperature will move the equilibrium position in favour of the forward reaction
However, the temperature must still be quite high so that the rate of reaction is still high
A compromise is made and this is why the temperature is 450C
What is the pressure, temperature and catalyst for the Haber process?
Pressure: 200 atmospheres
Temperature: 450 degrees C
Catalyst: iron
What has to be true for equilibrium to occur?
The reaction takes place in a closed system
What is a closed system?
When the area where the reaction is taking place doesn’t allow any reactants or products escape
What is equilibrium of a reversible reaction?
When the amount of reactants and products reaches a certain balance and stay there
Describe the Haber process
H^2 and N^2 are mixed in a 3:1 ratio
The mixed gas flows into the reaction vessel where the 450C temperature and 200 atmospheres pressure is. It travels past trays of iron catalyst
Then the gasses go into the condenser where the ammonia is condensed and the unused hydrogen and nitrogen are recycled back to the start
You are left with liquid ammonia
What is the chemical reaction for the Haber process?
N^2 + 3H^2 <–> 2NH^3
How does pressure affect the equilibrium position and why?
Most reaction have a greater volume on one side
If you raise the pressure, it will encourage the reaction that produces less volume and vice versa
How does temperature affect the equilibrium position and why?
If you raise the temperature, the endothermic reaction will increase to use up the extra heat
If you lower the temperature, the exothermic reaction will increase to give out more heat
What is volume?
The amount of molecules in a substance
How is the nitrogen and hydrogen for the Haber process obtained?
Nitrogen is taken from the air
Hydrogen usually comes from natural gas or crude oil
What 2 factors affect the equilibrium position?
Temperature and pressure
How is the pressure set to move the equilibrium position in favour of the forward reaction and why?
The pressure is very high because there are only 2 molecules for the product and 4 molecules for the reactants
How does adding a catalyst affect the equilibrium position and why?
It doesn’t affect the position because it speeds up both reactions by the same amount
What are the 4 main chemical properties of transition metals?
Good conductors of heat and electricity
Dense, strong and shiny
Much less reactive than alkali metals
Generally higher melting points than alkali metals
What are the 2 other basic properties of the alkali metals and why?
Very reactive (1 outer electron)
Low density
Describe how more reactive halogens displace less reactive ones
The more reactive halogen is fed as a gas into a solution of a salt of the less reactive halogen
2 solutions are formed: the less reactive halogen (diatomic still) and the same salt solution but with the more reactive halogen
What is common about the way that the halogens naturally are?
They are all diatomic (exist as pairs of atoms)
How are alkali metals stored and transferred?
Stored in oil and handled with tongs etc.
What are alkali metals?
Group 1 metals (one outer electron)
What is a halide?
A halogen ion (1- charge)
What are the 3 other properties of transition metals?
Usually have more than one ion (like Fe^2+ and Fe^3+)
Compounds of them are very colourful due to the transition metal ion
Transition metals and their compounds are very good catalysts
What are the 2 things that change as you go down group 7?
Less reactive
Higher melting and boiling points
Why do the halogens get less reactive as you go down?
Harder to gain other electron because outer shell is further from nucleus
Describe what happens in terms of chemistry when an alkali metal is put into water
The alkali metals and water react to form a metal hydroxide and hydrogen gas (H^2)
The metal hydroxide dissolves in water to give alkaline solution
For example, 2Na + 2H^2O –> 2NaOH(aq) + H^2
What can the more halogens do?
Displace less reactive ones
What are halogens?
Group 7 (7 outer electrons)
Describe the features of fluorine, chlorine, bromine and iodine
Fluorine is a very reactive, poisonous yellow gas
Chlorine is a fairly reactive, poisonous dense green gas
Bromine is a dense, poisonous, red-brown volatile liquid
Iodine is a dark grey crystalline solid or a purple vapour
Describe what can be seen when an alkali metal reacts with water
The alkali metal floats around the surface, fizzing
The hydrogen gas that is given off is ignited by the heat of the metal to produce tiny explosions (pops)
What are the 2 things that change as you go down group 1?
They become more reactive
They have lower melting and boiling points
Why do alkali metals get more reactive as you go down?
The outer electron is further from the nucleus so it is more easily lost
Describe what is produced when alkali metals react with non-metals by ionic bonding
White compounds that dissolve in water to form colourless solutions
What were the 3 reasons that people started supporting the periodic table?
Newly discovered elements fitted into the gaps he left
It was a useful tool for predicting properties of elements
Atomic structure was discovered and this matched up with his periodic table
In the early 1800s, what were the only 2 ways to categories elements and why weren’t anymore known?
Their chemical and physical properties
Their relative atomic mass
Atomic number wasn’t discovered because there was no knowledge of atomic structure (protons and electrons)
What were the 3 ways that Newlands’ work was criticized?
His groups contained elements that didn’t have similar properties like carbon and titanium (titanium is a transition metal)
He mixed up metals and non-metals
He didn’t leave any gaps for undiscovered elements
What are the 2 ways that the modern periodic table is different to Mendeleev’s?
It is ordered by atomic number instead of mass number
Each period has one more shell of electrons
Why didn’t many scientists think the periodic table was important?
There wasn’t much evidence to say that the elements really did for together in that way
Describe how the modern periodic table is organised
The group number represents the number of electrons in the outer shell and the period number represents the number of shells of electrons
The mass number of an element is the number of protons and neutrons and the atomic number is the number of protons
The middle block is the transition metals which don’t belong to a group
The staircase-shaped line divides the metals on the left from the non-metals
Who was the first person to invent a periodic table?
Newlands
Describe Newlands’ periodic table
Newlands had invented a law of octaves which said that every eighth element (in order of atomic mass) had similar properties
He listed all of the currently known elements in rows of 7 so that his law of octaves was followed
Describe Mendeleev’s periodic table
He put the elements in order of atomic mass like Newlands but he left gaps in order to keep elements with similar properties in the same vertical columns
He left big gaps above the transition metals (first 2 rows) to follow his rule
When new elements were discovered and put in the table according to their mass number, they fitted in terms of properties
Who invented the next periodic table?
Mendeleev
