Chemistry Flashcards
1
Q
What are the first three alkanes and alkenes?
No of carbon atoms
A
Alkanes:
Methane
Ethane
Propane
Alkenes:
Ethene
Propene
Butene
2
Q
What is a functional group?
A
It is what is responsible for the alcohol’s properties.
3
Q
What is the functional group of an alcohol?
A
-OH
E.g.
Ethanol - C2H5OH or CH3C2OH
H H
| |
H - C - C - OH
| |
H H
4
Q
What are the uses of alcohol?
A
Drinks and spirits Nail polish remover Anti septic Cleaners Stain removers
5
Q
How acidic are alcohols?
A
Alcohols react with magnesium and sodium to form neutral solution is when dissolved in water.
Alcohol is a slightly acidic because they can release a hydrogen ion.
6
Q
What is the product of methanol and oxygen and propanol and oxygen?
A
Methanol + oxygen -> (methanoic acid) carbon dioxide + water
Propanol + oxygen -> (Propanoic acid) carbon dioxide + water
7
Q
What happens when an alcohol reacts with a metal?
A
Hydrogen gas is produced
8
Q
What can (weak) ethanoic acid be used for?
A
Vinegar
9
Q
What is the product of a reaction between a (carboxylic) acid and a metal e.g. ethanoic acid and sodium?
A
Acid + metal -> salt + hydrogen
Ethanoic acid + sodium -> sodium ethanoate + hydrogen
10
Q
What is the product of a reaction between a (carboxylic) acid and a metal carbonate e.g. ethanoic acid and sodium carbonate?
A
Acid + metal carbonate -> carbon dioxide + salt + water
Ethanoic acid + sodium carbonate -> carbon dioxide + sodium ethanoate + water
11
Q
What is the functional group of a carboxylic acid?
A
-COOH
E.g.
Ethanoic acid CH3COOH
H
|
H - C - C = O
| |
H OH
12
Q
What are the first 4 carboxylic acids? (Based on their corresponding alcohol & no of carbons)
A
Methanoic acid
Ethanoic acid
Propanoic acid
Butanoic acid
13
Q
What (organic) substance is produced when an alcohol reacts with a carboxylic acid e.g. Ethanol and ethanoic acid?
A
Alcohol + carboxylic acid -> ester + water
Ethanol + ethanoic acid -> ethyl ethanoate + water
14
Q
What is the functional group of an ester?
A
-COO-
If the hydrogen atom in the carboxylic (-COOH) group is replaced by a hydrocarbon group, the compound is an ester.
E.g.
Ethyl ethanoate CH3COOCH2CH3
H
|
H - C - C = O H H
| \ | |
H \ O - C - C - H
| |
H H
15
Q
What type of reaction takes place when a carboxylic acid is produced from alcohol?
A
Oxidation
16
Q
What is an ester?
A
There are another type of organic group of compounds.
They are prepared by reacting a carboxylic acid an alcohol.
They are often used for the fruity smells.
17
Q
How is an ester named?
A
By the name of the alcohol then the name of the carboxylic acid
E.g. If propanol and methanoic acid are used, it would be named:
Propyl methanoate
18
Q
What’s produced in the reaction of ethanol and hexanoic acid?
A
Ethyl hexanoate and water
Alcohol + carboxylic acid -> ester + water
19
Q
How is a carboxylic acid formed?
A
By the oxidation of an alcohol e.g. Ethanoic acid is produced from a reaction between ethanol and oxygen.
20
Q
Ethanol reacts with sodium to produce a gas. What gas is produced?
A
Hydrogen
Any alcohol that reacts with sodium forms hydrogen
21
Q
What’s a homologous series?
A
This series of molecules that have a general formula.
E.g. Alkenes and alkanes.
22
Q
What is electrolysis?
A
The separation of ionic compounds into elements in a solution by use of electricity / an electric current.
An electrical current is passed through the solution - positive compounds move to the cathode and negative compounds move to the anode.
23
Q
What’s an electrolyte?
A
The substance broken down in electrolysis
A liquid which can be electrolysed
24
Q
What are ions?
A
Charged particles
25
Why can electrolytes conduct electricity?
They contain charged particles which are able to move to the anode or cathode in the solution
26
Why can sodium chloride only conduct electricity when molten or in solution?
The ions are free to move and therefore can carry charge and conduct electricity.
27
What happens during the electrolysis of copper chloride?
CuCl2 (aq) -> Cu (s) + Cl (g)
The copper, having a positive charge, moves to the cathode and is reduced.
The chlorine, having a negative charge, moves to the anode and is oxidised.
28
What is the half equation for the electrolysis of copper chloride at the electrodes?
Anode:
2Cl- - 2e- = Cl2
Cathode:
Cu2+ + 2e- = Cu
29
What are redox reactions?
Reduction and oxidation
Oxidation
Is
Loss of e-
Reduction
Is
Gain of e-
30
What is brine and how are the different compounds separated?
A solution of sodium chloride in water separated by electrolysis.
31
What happens during the electrolysis of brine?
The brine solution contains sodium ions (Na+), chloride ions (Cl-), hydrogen ions (H+) and hydroxide ions (OH-).
- When electrolysed, chlorine (being a negative ion) moves to the anode where it is oxidised and loses (2) electrons which forms chlorine gas.
- This leaves sodium and water.
- Water is formed by a reversible reaction and can form H+ and OH- ions. The least reactive (positive) ion moves to the cathode which is why hydrogen is formed rather than sodium.
- The sodium and hydroxide ions then react to produce a sodium hydroxide solution.
32
What are the products of the electrolysis of brine and what are their uses?
Hydrogen: margarine and hydrochloric acid
Chlorine: bleach, disinfectants, plastics
Sodium hydroxide: (strong alkali) soap, paper, bleach, controlling pH and neutralising acids.
33
What is the test for the presence of chlorine gas?
Using damp litmus paper which turns white (from pink) if chlorine is present.
34
What are the half equations for the electrolysis of brine at the electrodes?
Anode:
2Cl- - 2e- -> Cl2
Cathode:
2H+ + 2e- -> H2
Leaving an NaOH solution
35
How is aluminium extracted?
It's more reactive than carbon so can't be can't be done by displacement - it's done by electrolysis.
2Al2O3 -> 4Al + 3O2
- Bauxite, Al2O3, is purified and melted to be electrolysed. Aluminium oxide melts at over 2000°C so is mixed with another ionic compound called cryolite so that the mixture melts at roughly 850°C.
- The cryolite stays in the cell and fresh aluminium oxide is added as aluminium and CO2 are produced.
- At the negative electrode, aluminium ions are reduced to aluminium atoms by gaining electrons. The molten aluminium is collected from the bottom of the cell.
- At the anode, oxide ions are oxidised and lose electrons to become oxygen atoms
36
What are the half equations for the electrolysis of aluminium at the electrodes?
Anode:
2O2- -> 2O2 + 4e-
OR
6O2- - 12e- -> 3O2
Cathode:
Al3+ + 3e- -> Al
OR
4Al3+ + 12e- -> 4Al
37
Within the electrolysis of aluminium, why does the anode have to regularly be replaced?
During the electrolysis of aluminium, the oxide ion (being negative) moves to the anode where oxygen is formed. As the electrodes are made of carbon, the oxygen produced reacts with the carbon to form carbon dioxide so the electrodes gradually burn away over time.
38
What's electroplating?
Using electrolysis to put a thin coating of metal on an object e.g. Coating of gold or silver, done to look more attractive, prevent corrosion increase hardness or reduce costs.
To do so, the object being plated is used as the cathode and the anode is made of the plating material. At the anode, atoms of the plating metal lose electrons to form metal ions which go into the solution. At the cathode, metal ions from the solution gain electrons to form metal atoms which are deposited on the object being plated.
39
What happens during electrolysis?
Ionic compounds are broken up as an electric current passes through the solution.
- The electrical circuit consists of 2 electrodes which make contact with the electrolyte. (Electrodes of often made of inert substances so the don't react with the products).
- The ions in the electrolyte move to the electrodes (depending on their charge) where they are discharged to produce elements.
- Positive ions are attracted to the cathode where they form metals or hydrogen, depending on the ions in the electrolyte. (They are reduced - gain e-).
- Negative ions are attracted to the anode where they lose their charge to form non-metallic elements (they are oxidised - lose e-).
40
Why would hydrogen be formed at the cathode rather than a metal?
Water contains hydrogen and hydroxide ions.
When a solution of ions are electrolysed in water, hydrogen may be produced at the cathode if the other positive ions are (metals) more reactive than hydrogen.
41
What happens when an alcohol burns in air?
It forms carbon dioxide and water
They are used as fuels
42
What is formed when sodium reacts with an alcohol?
Hydrogen gas however he reactions are less vigorous than the reaction between sodium and water
43
What happens when an alcohol is oxidised?
Alcohols can be oxidised by chemical oxidising agents e.g. Potassium dichromate to produce carboxylic acids. Some microbes in the air can also oxidise solutions of ethanol to produce ethanoic acid which turns alcoholic drinks sour.
44
What happens when carboxylic acid is dissolved in water?
It forms a pH below 7 like a typical acid.
| They also fix when added to carbonates as they react to produce carbon dioxide (& a salt and water)
45
What makes carboxylic acids different to acids?
They react with alcohols in the presence of an acid catalyst to produce an ester which is a volatile, smelly substance
46
Why are carboxylic acids weak?
In aqueous solutions, HCl ionises completely to hydrogen ions and chloride ions.
HCl -> H+ + Cl -
Acids that ionise completely are known as strong acids
When ethanoic acid dissolves in water, it doesn't ionise completely and some of the ethanoic acid molecules remain as molecules in the solution:
CH3COOH -> CH3COO- + H+
Acids that don't ionise completely are weak acids
In aqueous solutions of equal concentrations, weak acids have a higher pH and react slower
47
What are the issues of organic chemistry?
Alcohol, carboxylic acid and esters can be used for food, solvents, food etc but can lead to health problems.
Biofuels offer an alternative to fossil fuel but crops used of the production may use land needed to grow food.
48
When were the elements discovered and how were they arranged?
During the 19th century, many elements were discovered but scientists didn't know about the structure of atoms and protons so they were classed on their properties and atomic weights.
In 1863 Newlands proposed his law of octaves which so stated that similar properties are repeated every eighth element. He designed a table but failed to take into account elements which hadn't been found.
In 1869 Mendeleev produced an improved table with gaps for undiscovered elements so groups of known elements had similar properties.
When some of the missing elements were discovered, people more readily accepted.
49
How was the modern periodic table established?
Scientists found out about protons and electrons at the start of the 20th century and soon models of the arrangement of electrons in atoms were found.
Elements were then arranged in order of their atomic number and aligned in vertical groups.
The groups of elements have similar chemical properties due to how their atoms have the same number of electrons in their highest occupied energy level (outer shell)
50
What is the reactivity within the groups of the periodic table?
The reactivity depends on the total number of electrons
Going down a group, there are more occupied energy levels and larger atoms
When metals react they lose electrons so the reactivity of metals in a group increases going down the group.
When non-metals react they gain electrons so the reactivity of non-metals decreases going down a group
51
What are properties of group 1 elements?
The group 1 elements are called alkali metals.
They all metals that react with air and water.
They are often solid at room temperature with low melting and boiling points that decrease going down the group.
They have low densities so they float on water
They all have one electron in the highest occupied energy level. They lose this electron in reactions to form ionic compounds which their ions have a single positive charge
They react with group 7 halogens to form salts that are often white or colourless
Reactivity increases going down the group because the outer electron is less strongly attracted to the nucleus as the number of occupied energy level is increases and the atoms get larger
52
What are the properties of the transition elements?
They are found in the periodic table between groups two and three
They are all metals and sometimes called transition metal
Except for mercury they have higher melting and boiling points than the alkali metals
They are malleable and ductile and they are good conductors of heat and electricity
They react only slowly or not at all with oxygen and water
Most are strong and dense and are you sport as building materials off and as alloys
They form positive ions with various charges e.g. Fe 2+ and Fe 3+
Compounds of transition metals are often brightly coloured
Many transition metals and their compounds are catalysts for chemical reactions
53
What are the properties of group 7 of the periodic table?
The halogens on nonmetallic elements
They exist as small molecules made up of pairs of atoms
They have low melting and boiling points that increase going down the group
All of the halogens have seven electrons in the highest occupied shell
They all form ionic compounds with metals in which the halide ions have a charge of -1
The halogens also bond covalently with nonmetals forming molecules
Reactivity of the halogens decreases going down the group because the attraction of the outer electrons to the nucleus decreases as the number of occupied energy shells increases.
54
What is soft water?
What that lathers easily with soap
55
What is hard water?
Hard water are uses more soap to produce lather and to wash effectively.
This is because hard water contains dissolved compounds that react with soap to form an insoluble solid called scum
56
What ions are found in hard water?
Calcium and/or magnesium ions in a hard water react with soap producing a precipitate called scum.
When water is in contact with rocks some compounds dissolve. If the water contains these ions they will react with the soap.
57
What happens when hard water is heated?
When heated one type of hard water is called temporary hard water.
This softens after being boiled and produces an in soluble solid called limescale. Scale can be deposited in kettles boilers and pipes reducing the efficiency of heating systems and causing blockages.
Permanent hard water is not softened by boiling and does not produce limescale when heated.
58
What is better for you, hard water or soft?
Hard water us better than soft for developing and maintaining teeth and bones.
It may also help to prevent heart disease.
59
What does soft water contain?
Soft water may contain dissolved substances but it does not contain dissolved salts that react with soap to produce scum. Nor does it produce scale when heated.
60
What does hard water contain?
Calcium and magnesium ions that react with soap to form scum.
61
How can hard water be softened?
By removing he calcium and magnesium ions that react with soap.
1) Some types of hard water are affected by heating while others aren't.
- Temporary hard i water is softened by boiling as, when heated, the calcium and magnesium form insoluble scale which removes them from the hard water.
- Permanent hard water is not softened by boiling and doesn't produce scale.
2) A method of softening either type of water is by precipitating the ions that cause hardness, done by adding washing soda/sodium carbonate.
The sodium carbonate reacts with calcium and magnesium ions in the water to form solid calcium and magnesium carbonate that cannot react with soap.
3) Another method is to use an ion-exchange column.
This is packed with resin containing sodium or hydrogen ions.
When hard water passes through the resin, calcium and magnesium ions attach to the resin while the sodium or hydrogen ions take their place in the water.
Sodium and hydrogen ions don't react with soap.
62
Hats the difference between scum and scale?
Scum is formed when soap reacts with dissolved compounds in hard water.
When temporary hard water is heated it produces scale which covers pipes and heating elements.
63
How is temporary hard water softened by heating?
It contains hydrogencarbonate ions, HCO3-.
The hydrogencarbonate ions decompose when heated to produce carbonate ions, water and carbon dioxide.
2HCO3- -> CO3 2- + H2O + CO2
The carbonate ions react with calcium and magnesium ions in the water to produce precipitates of calcium carbonate and magnesium carbonate that are deposited as scale.
64
How is water made safe to drink?
1) Water is taken in from a source
2) It passes a large filter/screen which removes debris, branches and animals from the water.
3) Sedimentation and filtration is then used to remove solids
4) Coagulation and flocculation - (using aluminium sulphate) (removing small masses)
5) Chemicals and disinfectants such as chlorine
Water filters can improve the taste of water - they contain carbon and an ion-exchange resin that removes some soluble substances.
There's also often silver which prevents growth of bacteria.
65
How can pure water be obtained?
By distillation.
| This requires large amounts of energy to boil the water and so it would be very expensive large scale.
66
What are the issues with water treatment?
- Hard water can cause problems with heating and washing systems but has health benefits if drank and can be treated.
- Chlorine kills microbes however it is toxic and can produce other poisonous compounds so must be controlled.
- Fluoride ions in toothpastes and water supplies prevent tooth decay however some argue people should be allowed to take extra fluoride or not.
67
What type of reaction is it when fuels and food react with oxygen?
Exothermic - energy is released
68
Ho can the amount of energy released when a substance burns be measured?
By using a calorimeter - the simplest is a glass of water so that, when a substance burns and heats the water, the temperature rise depends on the amount of energy released but this isn't very accurate.
1 cal = 4.2 J
You need to calculate the amount of energy transfer to the water
69
What's the equation for the energy transferred to water?
Q = mc^T
Energy transferred (joules) = mass (g) * S.H.C of water (J/g°C) * temp change (°C)
70
How can energy transfer in solutions be calculated?
Energy is either transferred to or from the solution.
It can be done in an insulated container to prevent energy transfer to surroundings.
We measure the change using Q = mc^T
When a solid is added to water or aqueous solution, we assume the volume doesn't change and 1cm3 solution has a mass of 1g.
If we know the number of moles involved in the reaction, we can calculate energy change KJ/mol
71
What do energy level diagrams show?
Energy changes for chemical reactions.
The difference between energy levels of reactants and products is the energy change for the reaction.
For an exothermic reaction, the graph shows the product has LESS energy than the reactants
For an endothermic reaction, the graph shows the product has MORE energy than the reactants
A catalyst increases the rate of reaction by lowering the activation energy so the highest amount of energy needed is lowered.
72
What is the activation energy?
During a chemical reaction bonds in the reactants must be broken for the reaction to occur.
(Breaking bonds is endothermic as energy is taken in)
The minimum energy for the reaction to happen is called the activation energy.
When new bonds in the products are formed, energy is released so this is exothermic.
73
What energy transfer in an exothermic and endothermic reaction?
Exothermic: the energy released when new bonds are formed is greater than the energy absorbed when bonds are broken.
Endothermic: the energy released when new bonds are formed is less than the energy absorbed when bonds are broken.
74
What is bond energy?
The energy needed to break the bond between two atoms.
An equal amount of energy is released when the bond forms between two atoms and so we can use bond energies to calculate the overall energy change for a reaction (KJ/mol)
75
How are bond energies calculated?
```
C-C 347
C-H 413
H-O 464
O=O 498
C=O 805
```
The balanced equation for the reaction is needed to calculate the energy change for a reaction
Then calculate:
- the total amount of energy needed to break all the bonds in the reactants
- the total amount of energy released in making all the bonds in the products
- the difference between the two totals
E.g.
Use the bond energies to calculate the energy change for burning methane
CH4 + 2O2 -> CO2 + 2H2O
Bonds broken: (4 * 413) + (2 * 498) = 2648
Bonds formed: (2 * 805) + (4 * 464) = 3466
Difference = 3466 - 2648 = 818 KJ
Energy change = 818KJ/mol
76
Calculate the energy change for burning propane:
C3H8 + 5O2 -> 3CO2 + 4H2O
```
C-C 347
C-H 413
H-O 464
O=O 498
C=O 805
```
Bonds broken = (2 * 347) + (8 * 413) + (5 * 498) = 6488
Bonds made = (6 * 805) + (8 * 464) = 8542
Energy change = 6127 - 6488 = 2054 KJ/mol
77
What are the issues with fuel?
Fossil fuels are non renewable and produce pollution
Hydrogen can be used as it burns easily and releases a lot of energy per gram. It produces no carbon dioxide when burned, only water.
It can be used in combustion engines or in fuel cells to power vehicles
It can be produced from renewable sources however disadvantages include supply storage and safety problems
Vehicles that use fuel cells need to match the performance convenience and cost of petrol and diesel vehicles
78
What is the test for positive ions?
Some can be identified using a flame test or by using sodium hydroxide solution.
Most group 1 and 2 metals can be identified using the flame test e.g Na, Li, K etc.
79
How is sodium hydroxide used to identify positive ions?
Hydroxides of most metals that have ions with 2+ or 3+ are insoluble in water.
When sodium hydroxide is added to the solutions of these ions a precipitate of the metal hydroxide is formed.
Aluminium, calcium and magnesium ions form white precipitates. When excess sodium hydroxide solution is added the precipitate of aluminium hydroxide dissolves.
80
What are the results of the sodium hydroxide test for positive ions for aluminium, calcium, magnesium, copper, iron II and iron III?
(Colour precipitate)
Aluminium hydroxide dissolves in excess solution
Magnesium hydroxide is white
Calcium hydroxide is white
Copper hydroxide is blue
Iron II hydroxide is green
Iron III hydroxide is brown
We can show the reactions using balanced ionic equations:
Fe 3+ + 3OH- -> Fe(OH)3
81
What are the 3 tests for negative ions?
Carbonate ions
Halide ions
Sulfate ions
82
What's the test for carbonate ions (negative ions)?
Add dilute hydrochloric acid to the substances to see if it fizzes.
If it does and the gas produced turns limewater milky, the substance contains carbonate ions.
E.g.
2HCl + CaCO3 -> CaCl2 + H2O + CO2
83
What's the test for halide ions (negative ions)?
Add dilute nitric acid and then add silver nitrate solution
- chloride ions give a white precipitate
- bromide ions give a cream precipitate
- iodide ions give a yellow precipitate
E.g.
AgNO3 + NaCl -> AgCl + NaNO3
84
What's the test for sulfate ions (negative ions)?
Add dilute hydrochloric acid and then barium chloride solution.
If a white precipitate forms, sulfate ions are present.
E.g.
BaCl2 + MgSO4 -> BaSO4 + MgCl2
85
Why must you add nitric acid and not hydrochloric acid or sulfuric acid when testing with silver nitrate solution for halides?
Hydrochloric acid contains chloride ions (Cl-) and sulfuric acid contains sulfate ions (SO4 2-) which both give precipitates with silver nitrates.
86
What's a titration?
A method for measuring the volumes of two solutions that react together.
It is used to measure accurately how much acid and alkali react together completely.
To do a titration, a pipette is used to measure accurately the volume (e.g. of alkali) that is put into the conical flask. An indicator is then added to the liquid in the flask.
A burette is filled (e.g. with acid) which is then added gradually to the flask.
When the indicator changes colour the end point has been reached. The volume of acid used is found from the initial and final burette readings.
The titration should be done several times to improve the repeat ability of the results.
87
What is the equation to calculate the number of moles?
No of moles = mass in grams / relative formula mass
88
How are concentrations of titrations calculated?
Concentrations of solutions are measured in grams per decimetre cubed (g/dm3) or moles per decimetre cubed (mol/dm3).
If we know the mass or the number of moles of a substance dissolved in a given volume we can calculate its concentration.
If we know the volume of a solution and it's concentration we can calculate the mass or the number of moles of the substance in any volume of solution.
If the concentration of one of the solutions is known and the volumes that react together are known, the concentration of the other solution can be calculated.
This information can be used to find the amount of a substance in a sample.
The concentrations are calculated using balanced symbol equations and moles.
89
Titration calculation
50 cm3 of solution was made using 5.6g of potassium hydroxide, KOH.
What is it's concentration in g/dm3 and mol/dm3?
1 cm3 of solution contains (5.3/50)g
So 1dm3 of solution contains (5.6/50) * 1000g = 112g
Concentration of solution = 112g/dm3
1 mole KOH = (39 + 16 + 1)g = 56g
112g/56g = 2 mole
Concentration of solution = 2 mol/dm3
90
Titration calculation
| What is the mass of sodium hydroxide in 100cm3 of a solution with a concentration of 0.2 mol/dm3?
100cm3 contains 100 * 0.2/1000 mol = 0.02 mol
1 mol NaOH = 40g
0.02 * 40 = 8g
8g
91
A student found that 25.0 cm3 of sodium hydroxide solution with an unknown concentration reacted with exactly 20.0 cm3 of 0.50 mol/dm3 hydrochloric acid.
What was the concentration of the sodium hydroxide solution?
The equation for this reaction:
NaOH + HCl -> NaCl + H2
The concentration of the HCl is 0.50 mol/dm3, so 0.50 mol of HCl are dissolved in 1000cm3 of acid.
Therefore 20 cm3 of acid contains 20 * 0.50/1000 mol = 0.010 mol HCl
The equation for the reaction tells us that 0.010 mol of HCl will react exactly with exactly 0.010 mol of NaOH (1:1)
This means there must have been 0.010 moles of NaOH in the 25 cm3 of solution in the conical flask.
Therefore the concentration of NaOH solution =
(0. 010/25) * 1000 =
0. 40 mol/dm3
92
When is it necessary to use quantitative analysis?
When we need to know how much or the quantity of a substance in a sample.
93
Why is it necessary to have large databases for DNA analysis to be used to identify individuals?
DNA results / profile are different for every individual.
94
What is meant by equilibrium?
When the rates of forward and reverse reactions of a reversible reaction are equal
OR
When the amounts of reactants and products in a reversible reaction are constant.
95
What's a reversible reaction?
One where the products of the reaction can react to re-form the original reactants.
96
What is a closed system?
When is equilibrium achieved in a closed system?
One where no reactants or products can escape.
In a closed system, equilibrium is achieved when the rates of the forward and reverse reactions are equal.
97
How can the amounts of products or reactants be changed for a reversible reaction?
By changing the reaction conditions.
Increasing the concentration of a reactant will cause more products to be formed as the system tries to achieve equilibrium.
If a product is removed, more react mats will react to try to achieve equilibrium so more product is formed.
E.g.
ICl + Cl2 ICl3
If chlorine is added, the concentration of chlorine is increased and more ICl3 is produced.
If chlorine is removed, the concentration of chlorine decreases so more ICl is produced.
98
How does a change in pressure affect reversible reactions involving gases at equilibrium?
The position of equilibrium shifts to cancel out that change.
For reversible reactions that have different numbers of molecules of gases on one side of the equation than the other changing the pressure will affect the position of equilibrium.
E.g. If pressure is increased, the position of equilibrium will shift to try to reduce the pressure (favouring the reaction that produces fewer molecules of gas).
99
What are the effects of changing the pressure of gases at equilibrium in a reversible reaction?
If the forward reaction produces MORE molecules of gas:
- an increase in pressure decreases the amount of products formed
- a decrease in pressure increases the amount of products formed
If the forward reaction produces FEWER molecules of gas:
- an increase in pressure increases the amount of products formed
- a decrease in pressure decreases the amount of products formed
Increasing pressure favours the reaction with the smaller number of molecules of gas formed.
Decreasing the pressure favours the reaction with the larger number of molecules of gas formed.
100
What are the effects of changing the temperature of gases at equilibrium in a reversible reaction?
Reversible reactions are exothermic in one direction and endothermic in the other.
If the forward reaction is exothermic:
- increase in temp decreases the amount of products formed
- decrease in temp increases the amount of products formed
If the forward reaction is endothermic:
- increase in temp increases the amount of products formed
- decrease in temp decreases the amount of products formed
Increasing temp favours the endothermic reaction
Decreasing the temp favours he exothermic reaction
101
What's the Haber process?
| What occurs during the process?
The Haber process is used to manufacture ammonia (which is used to make fertilisers and other chemicals).
Nitrogen from the air and hydrogen, which is usually obtained from natural gas, are purified and mixed in the correct proportions.
The gases are passed over an iron catalyst at a temperature of approx 450°C and a pressure of approx 200 atmospheres
These conditions are chosen to give a fast rate of reaction and a reasonable yield of ammonia.
(It's a reversible reaction:
N2 + 3H2 -> 2NH3)
Some of the ammonia that is produced breaks down into nitrogen and hydrogen and the yield of ammonia is only approx 15%.
The gases that exit the reactor are cooled so the ammonia condenses. The liquid ammonia is separated from the up reacted gases.
The unreacted gases are recycled so they are not wasted.
102
Why is there an optimum pressure for the Haber process?
N2 + 3H2 -> 2NH3
The products have fewer molecules of gas than the reactants so the higher the pressure, the greater the yield of ammonia.
However, the higher the pressure the more energy is needed to compress the gas.
Higher pressures also need stronger reaction vessels and pipes which increases costs.
A pressure of about 200 atmospheres is often used as a compromise between the costs and the yield.
103
Why is there an optimum temperature for the Haber process?
N2 + 3H2 -> 2NH3
The forward reaction is exothermic so the lower the temp, the greater the yield of ammonia.
However the reaction rate decreases as the temp is lowered and the iron catalyst becomes ineffective so it would take a longer time to produce any ammonia.
Therefore a compromise temp of about 450°C is usually used to give a reasonable yield in a short time.
104
What are the energy levels of an exothermic reaction?
The energy released when new bonds are formed is greater than the energy absorbed when bonds are broke.
105
What are the energy levels of an endothermic reaction?
The energy released when new bonds are formed is less than the energy absorbed when bonds are broken
106
How do you calculate the number of moles?
No' of moles = mass in grams / relative formula mass Mr
107
What's the equation to calculate moles, using concentration and volume?
Moles = concentration * volume
108
How do you work out the number of moles a substance has (using concentration and volume)
Moles = concentration * volume
