4. Electrochemistry + 5. Chemical energetics Flashcards
What happens to a molten ionic compound when a current is passed through it?
When an electric current is passed through a molten ionic compound the compound decomposes or breaks down
In what forms can an ionic compound undergo electrolysis? Why?
in a molten state
in an aqueos state
Ionic compounds in the solid state cannot conduct electricity either since they have no free ions that can move and carry the charge
Why can covalent compounds not undergo electrolysis?
Covalent compounds cannot conduct electricity hence they do not undergo electrolysis
Electrode definition
Electrode is a rod of metal or graphite through which an electric current flows into or out of an electrolyte
Electrolyte definition
Electrolyte is the ionic compound in molten or dissolved solution that conducts the electricity
Anode definition
Anode is the positive electrode of an electrolysis cell
Anion definition
Anion is a negatively charged ion which is attracted to the anode
Cathode definition
Cathode is the negative electrode of an electrolysis cell
Cation definition
Cation is a positively charged ion which is attracted to the cathode
How would you set up the equipment for the electrolysis of lead (III) bromide)
Add lead (II) bromide into a beaker and heat so it will turn molten, allowing ions to be free to move and conduct an electric charge
Add two graphite rods as the electrodes and connect this to a power pack or battery
Turn on power pack or battery and allow electrolysis to take place
What happens at the anode during the electrolysis of lead (III) bromide?
Negative bromide ions move to the positive electrode (anode) and lose two electrons to form bromine molecules. There is bubbling at the anode as brown bromine gas is given off
What happens at the cathode during the electrolysis of lead (III) bromide?
Positive lead ions move to the negative electrode (cathode) and gain electrons to form a grey lead metal which deposits on the surface of the electrode
Half equation for anode
electrolysis of lead (III) bromide
2Br- —> Br2 + 2e-
Half equation for cathode
electrolysis of lead (III) bromide
Pb2+ + 2e- —> Pb
When using an aqueos solution, what needs to be taken into account?
Aqueous solutions will always have water (H2O)
H+ and OH– ions from the water are involved as well
To which electrode are OH- ions attracted?
OH– ions and non-metal ions (anions) are attracted to the positive electrode
What will be formed at the anode? (electrolysis of an aqueos solution)
Either OH– or non-metal ions will lose electrons and oxygen gas or gas of non-metal in question is released, eg. chlorine, bromine, nitrogen
What is the rule for deciding what non-metal will be formed at the anode? (electrolysis of an aqueos solution)
The product formed depends on which ion loses electrons more readily, with the more reactive ion remaining in solution
What is the reactivity series for anions?
More reactive
→ SO42-
→ NO3-
→ OH-
→ Cl-
→ Br-
→ I-
Less reactive
What will form at the cathode? (electrolysis of an aqueos solution)
H+ and metal ions attracted to the negative electrode but only one will gain electrons
Either hydrogen or metal will be produced
What is the rule for deciding what will be formed at the cathode? (electrolysis of an aqueos solution)
If the metal is above hydrogen in reactivity series, then hydrogen will be produced and bubbling will be seen at the cathode
What factor (apart from reactivity) also influences the products of the electrolysis of an aqueos solution?
Concentrated and dilute solutions of the same compound give different products
What is the rule for which anion will be produced in a concentrated solution?
For anions, the more concentrated ion will tend to be produced over a more dilute ion
In the electrolysis of a molten compound, what will be produced at the cathode and the anode?
cathode - metal
anode - non-metal
How can you determine that hydrogen was produced at the cathode?
If the gas produced at the cathode burns with a ‘pop’ when a sample is lit with a lighted splint then the gas is hydrogen
How can you determine that oxygen was produced at the anode?
If the gas produced at the anode relights a glowing splint dipped into a sample of the gas then the gas is oxygen
What colour is bromine gas?
red-brown
What colour is chlorine gas?
yellow-green
What colour is fluorine gas?
pale yellow
How would you set up the equipment for copper refining?
In the set-up, the impure metal is always the anode, in this case the impure copper
The cathode is a thin sheet of pure copper
The electrolyte used is an aqueous solution of a soluble salt of the pure metal at the anode, e.g: CuSO4
Why is electrolysis used to purify metals?
Electrolysis can be used to purify metals by separating them from their impurities
What happens to copper atoms during copper refining?
Copper atoms at the anode lose electrons, go into solution as ions and are attracted to the cathode where they gain electrons and form now purified copper atoms
What happens to the anode during copper refining?
The anode thus becomes thinner due to loss of atoms and the impurities fall to the bottom of the cell as sludge
What happens to the cathode during copper refining?
The cathode gradually becomes thicker
Where do electrons move during electrolysis?
During electrolysis the electrons move from the power supply towards the cathode
Where do positive ions move during electrolysis?
Positive ions within the electrolyte move towards the negatively charged electrode which is the cathode
Where do positive ions move during electrolysis?
Positive ions within the electrolyte move towards the negatively charged electrode which is the cathode
What happens to positive ions at the cathode?
Here they accept electrons from the cathode and either a metal or hydrogen gas is produced
Where do negative ions move during electrolysis?
Negative ions within the electrolyte move towards the positively charged electrode which is the anode
What happens to negative ions at the anode, if it is inert?
If the anode is inert (such as graphite or platinum), the ions lose electrons to the anode and form a nonmetal or oxygen gas
What happens to negative ions at the anode, if it is reactive?
If the anode is a reactive metal, then the metal atoms of the anode lose electrons and go into solution as ions, thinning the anode
What is an electrochemical cell a source of?
An electrochemical cell is a source of electrical energy
What is the simple design of an electrochemical cell?
The simplest design consists of two electrodes made from metals of different reactivity immersed in an electrolyte and connected to an external circuit
How does an electrochemical cell work, using zinc and copper as an example?
Zinc is the more reactive metal and forms ions more easily, releasing electrons as its atoms form ions
The electrons give the more reactive electrode a negative charge and they then flow around the circuit to the copper electrode
What causes a voltage to be produced by an electrochemical cell?
The difference in the ability of the electrodes to release electrons causes a voltage to be produced
What is the correlation between the reactivity of a metal and the voltage produced? (electrochemical cell)
The greater the difference in the metal’s reactivity, the greater the voltage
Electroplating definition
Electroplating is a process where the surface of one metal is coated with a layer of a different metal
What is the rule when deciding which metal is used to coat the other one? (electroplating)
The metal being used to coat is a less reactive metal than the one it is covering
What is the basic setup of equipment for electroplating?
The anode is made from the pure metal used to coat
The cathode is the object to be electroplated
The electrolyte is an aqueous solution of a soluble salt of the pure metal at the anode
What are 2 uses of electroplating?
Electroplating is done to make metals more resistant to corrosion or damage, e.g: chromium and nickel plating
It is also done to improve the appearance of metals, e.g: silver plating cutlery
Conductor definition
Conductors of electricity allow electrical charge to pass through them easily
3 examples of conductors
Conductors can be:
Solids such as metals or graphite
Liquids such as molten lead bromide or molten metals
Solutions such as sodium chloride solution
What metal is a good conductor and is used in electrical wiring? What other quality does it have which makes it suitable?
Copper is used extensively in electrical wiring as it is an excellent conductor and is malleable and easy to work with
What metal is used in overhead cables?
Aluminium is used in overhead cables which are reinforced with a steel core
Why do aluminium cables have a steel core?
The steel core provides extra strength and prevents the cable from breaking under its own weight
Why is aluminium used in overhead cables rather than copper?
Although not as good a conductor as copper, it is less dense and cheaper than copper
Insulator definition
Insulators resist the flow of electricity and do not conduct
What are 3 examples of insulators?
Most insulators are solids of plastic, rubber or ceramic
How are plastics used as insulators?
Plastics are used as insulators and are placed around electrical wiring and for some tool and machine handles
When are ceramics used as insulators?
Ceramics are used in very high voltage lines where contact between the power line and the metal of the pylon would be dangerous
What type of metals and metal compounds does the earth’s crust contain? What is the problem?
The Earth’s Crust contains metals and metal compounds such as Gold, Iron Oxide and Aluminium Oxide, but when found in the Earth, these are often mixed with other substances
What needs to be done with metals found in the earth’s crust to make them useful?
To be useful, the metals have to be extracted from their ore
What processes can be used to extract a metal from its compound?
electrolysis, using a blast furnace or by reacting with more reactive material
How are metals above carbon extracted?
Extracted by electrolysis of molten chloride or molten oxide.
How are metals below carbon extracted?
Extracted by heating with a reducing agent such as carbon or carbon monoxide in a blast furnace.
What is the difference in costs between electrolysis and reduction?
E - large amounts of electricity required, so an expensive process
R - cheap process as carbon is cheap and can be source of heat as well
- What is bauxite purified to?
Extraction of aluminium
The Bauxite is first purified to produce Aluminium Oxide Al2O3
- What is aluminium oxide dissolved in and why?(Extraction of aluminium)
Aluminium Oxide has a very high melting point so it is first dissolved in molten cryolite producing an electrolyte with a lower melting point, as well as a better conductor of electricity than molten aluminium oxide. This also reduces expense considerably
- What is the electrolyte?
Extraction of aluminium
The electrolyte is a solution of aluminium oxide in molten cryolite at a temperature of about 1000 °C.
- What keeps the electrolyte molten?
Extraction of aluminium
The cell operates at 5-6 volts and with a current of 100,000 amps. The heat generated by the huge current keeps the electrolyte molten
- How is aluminium extracted from the cell?
Extraction of aluminium
The molten aluminium is siphoned off from time to time and fresh aluminium oxide is added to the cell.
What is the half-equation at the cathode?
Extraction of aluminium
The Aluminium melts and collects at the bottom of the cell and is then tapped off:
Al3+ + 3e- → Al
What is the half-equation at the anode?
Extraction of aluminium
2O2- – 4e– → O2
What happens at the anode which leads to it being reduced in size?
(Extraction of aluminium)
Some of the Oxygen Produced at the positive electrode then reacts with the Graphite (Carbon) electrode to produce Carbon Dioxide Gas:
C (s) + O2 (g) → CO2 (g)
*This causes the carbon anodes to burn away, so they must be replaced regularly.
What is brine?
Brine is a concentrated solution of aqueous sodium chloride
What are the products when brine is electrolysed?
When electrolysed it produces chlorine, hydrogen and sodium hydroxide
What ions does brine contain?
The electrolyte is concentrated sodium chloride which contains the following ions: Na+, H+, Cl– and OH–
What happens at the cathode during the electrolysis of brine?
The H+ ions are discharged at the cathode as hydrogen gas
What happens at the anode during the electrolysis of brine?
The Cl– ions are discharged at the anode as chlorine gas
What remains behind after the electrolysis of brine?
The Na+ and OH– ions remain behind and form the NaOH solution
Exothermic definition
A reaction in which energy is given out to surroundings (temperature of environment increases)
What are 3 examples of exothermic reactions?
Combustion of fuels
Reaction of acids and metals
Neutralisation reactions
Endothermic definition
A reaction in which energy is taken in from surroundings (temperature of environment decreases)
The energy change is positive
What are 3 examples of endothermic reactions?
Thermal decomposition of carbonates
Electrolysis
First stages of photosynthesis
What determines whether a reaction is endo/exothermic?
energy
Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break bonds and the energy released when the new bonds are formed
How can you determine if a reaction is endothermic by the energy?
If more energy is absorbed than is released, this reaction is endothermic
More energy is required to break the bonds than that gained from making the new bonds
Is the change in energy positive or negative if the reaction is endothermic? Why?
The change in energy is positive since the products have more energy than the reactants
- positive ΔH value
What symbol is used to show the change in heat energy?
The symbol ΔH (delta H)
What is H the symbol for, and what does this mean?
H is the symbol for enthalpy, which is a measure of the total heat of reaction of a chemical reaction
How can you determine if a reaction is exothermic by the energy?
If more energy is released than is absorbed, then the reaction is exothermic
More energy is released when new bonds are formed than energy required to break the bonds in the reactants
Is the change in energy positive or negative if the reaction is exothermic? Why?
The change in energy is negative since the products have less energy than the reactants
Therefore an exothermic reaction has a negative ΔH value
What are energy level diagrams?
These are graphical representations of the heat changes in chemical reactions
What is shown on the y-axis?
energy level diagram
The enthalpy of the reactants and products is displayed on the y-axis
What is shown on the x-axis?
energy level diagram
The reaction pathway is shown on the x-axis
What do the arrows show?
energy level diagram
Arrows on the diagrams indicate whether the reaction is exothermic (downwards pointing) or endothermic (upwards pointing)
In an exothermic reaction, do the reactants or products have more energy? Why?
During an exothermic reaction, energy is given out
This means that the energy of the products will be lower than the energy of the reactants, so the change in enthalpy (ΔH) is negative
How is an exothermic reaction represented on an energy level diagram?
This is represented on the energy-level diagram with a downwards arrow (from the reactants to the products) as the energy of the products is lower than the reactants
In an endothermic reaction, do the reactants or products have more energy? Why?
During an endothermic reaction, energy is absorbed
This means that the energy of the products will be higher than the energy of the reactants, so the change in enthalpy (ΔH) is positive
How is an endothermic reaction represented on an energy level diagram?
This is represented on the energy-level diagram with an upwards arrow (from the reactants to the products) as the energy of the products is higher than the reactants
What does each chemical bond have which is specific to it?
Each chemical bond has a specific bond energy associated with it
What is bond energy?
This is the amount of energy required to break the bond or the amount of energy given out when the bond is formed
What can bond energy be used to calculate?
This energy can be used to calculate how much heat would be released or absorbed in a reaction
What is necessary to calculate the heat released or absorbed during a reaction?
To do this it is necessary to know the bonds present in both the reactants and products
How can you calculate the heat released or absorbed during a reaction?
Add together all the bond energies for all the bonds in the reactants – this is the ‘energy in’
Add together the bond energies for all the bonds in the products – this is the ‘energy out’
Calculate the energy change: Energy change = energy in – energy out
Fuel definition
A fuel is a substance which releases energy when burned
What is produced upon combustion when a fuel is a hydrocarbon?
When the fuel is a hydrocarbon then water and carbon dioxide are produced in combustion reactions
What shows the efficiency of a fuel?
The efficiency of a fuel refers to how much energy is released per unit amount
How can we measure the efficiency of fuels?
We can measure the efficiency of fuels by calorimetry
How can we calculate the heat of combustion?
A known mass of the fuel is combusted and used to heat up a known mass of water to calculate its heat of combustion
What is the method for an experiment with a calorimeter?
Using a measuring cylinder, place 100 cm3 of water into a copper can
Measure and record the initial temperature of the water
Fill the spirit burner with test substance and measure and record its mass
Place the burner under the copper can and light the wick
Stir the water constantly with the thermometer and continue heating until the spirit burner burns out
Measure and record the highest temperature of the water
What calculations are needed? (experiment with a calorimeter)
Temperature change of water = final temperature – initial temperature
Number of moles burned = change in mass ÷ molecular mass
Amount of energy = change in temperature x mass of water x specific heat capacity
Amount of energy per mole (J mol-1) = total amount of energy ÷ moles burned
How is hydrogen used as a fuel?
Hydrogen is used in rocket engines and in fuel cells to power some cars
What are advantages of hydrogen as a fuel?
It releases more energy per kilogram than any other fuel (except for nuclear fuels)
It does not pollute as it only produces water on combustion, no other product is formed
What are disadvantages of hydrogen as a fuel?
Expensive to produce and requires energy for the production process
Difficult and dangerous to store and move around (usually stored as liquid hydrogen in highly pressurised containers)
How is Uranium-235 used in power stations?
Uranium-235 undergoes decay and gives off heat energy which nuclear power stations harness
The heat it produces is used to heat water to steam, which in turn is used to power turbines to generate electricity
What are advantages of nuclear fuel?
Nuclear fuel energy is clean as it does not produce pollutants such as CO2 or oxides of nitrogen or sulfur
What are disadvantages of nuclear fuel?
But nuclear power plants are expensive to build and maintain as well as being potentially dangerous in the event of an accident as radioactive materials may be released
What is a fuel cell?
A fuel cell is an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode
In which industry are fuel cells becoming more common?
These cells are becoming more common in the automotive industry to replace petrol or diesel engines
What is pumped through the electrodes in a fuel cell?
H2 and O2 are pumped through two porous electrodes where the half-reactions occur
What is the reaction at the anode? (fuel cell)
2H2 → 4H+ + 4e-
What is the reaction at the cathode? (fuel cell)
4H+ + O2 + 4e- → 2H2O
What is the overall reaction in a fuel cell?
2H2 + O2 → 2H2O
How does a fuel cell work?
The electrons move around the external circuit from the cathode to the anode
This movement of electrons is used to drive an electric motor
What are advantages of fuel cell?
They do not produce any pollution
They produce more energy per kilogram than either petrol or diesel
No power is lost on transmission as there are far fewer moving parts than in an internal combustion engine
What are disadvantages of fuel cell?
Materials used in producing fuel cells are expensive
High-pressure tanks are needed to store the oxygen and hydrogen in sufficient amounts
Fuel cells are affected by low temperatures, becoming less efficient
Hydrogen is expensive to produce and store
Electrolyte
A liquid containing free moving ions that is broken down by electricity during the process of electrolysis.
Electrolysis
The breakdown of a substance containing ions by electricity.
The positive electrode
Anode
The negative electrode
Cathode
What sort of compounds are usually electrolysed?
Ionic compounds.
Covalent compounds can only do it if they react in the water to form ions.
Equation at the anode
n- -> n + e-
Equation at the cathode
n+ + e- -> n
Will the more or less reactive element be formed at the cathode?
Less reactive
What is the ‘order of discharge’ at the anode?
Halide ion > Hydroxide > All other negatively charged ions.
What gas is often given off at the anode?
Oxygen
Name of aluminium ore
Bauxite
What substance is mixed with aluminium during electrolysis and why?
Molten cryolite
It reduces the melting point
What happens at the cathode during the electrolysis of aluminium?
Al3+ ions are reduced to form Al atoms.
This forms aluminium at the bottom of the tank and is then siphoned off.
What happens at the anodes during the electrolysis of aluminium?
O2- ions turn into oxygen atoms.
These bond in pairs to form oxygen gas.
The oxygen reacts with the carbon anodes, forming Carbon Dioxide.
Due to the this the anodes must be replaced regularly.
What three products are formed in the electrolysis of brine and where?
> Chlorine gas- anode
Hydrogen gas- cathode
Sodium Hydroxide solution
How is brine electrolysed in industry?
Using a diaphragm cell, where the two electrodes are separated by a porous membrane.
Why is sodium hydroxide produced in the electrolysis of brine?
During the electrolysis of brine, Na+, Cl-, H+ and OH- are present. Chlorine and Hydrogen gas are emitted during the electrolysis process, which leaves only sodium and hydroxide.
2 uses of chlorine.
> When reacted with sodium hydroxide it can be used as bleach.
Plastics such as PVC
Use for hydrogen
> Makes margarine when combined with veritable oils.
Use for sodium hydroxide
> Important alkali in industry
Bleach
Soap
Paper
Electroplating
The process of depositing a thin layer of metal onto an object during electrolysis.
Why do we electroplate objects?
> To protect the metal beneath from corrosion.
To make the object look more attractive.
To increase the hardness or resistance to scratching.
To save money.
In electroplating, what substances make up the anode and cathode?
> Cathode- Object to be electroplated
> Anode - Metal to plate the Object.
Exothermic
A reaction that gives out energy to the surroundings.
Endothermic
A reaction that takes in energy from the surroundings.
Sign for enthalpy change
△H
Are the products of exothermic energy calculations positive or negative?
Negative
Uses of exothermic reactions
> Hand warmers
> Self-heating cans
Use of endothermic reactions
> Cold packs for sports injuries.
Test for water
Anhydrous copper sulfate (white) + Water = Hydrated copper sulfate (blue)
If a reaction releases energy when it goes in one direction…
… It will absorb exactly the same amount of energy when it goes in the opposite direction.
energy released =
mass of object heated x specific heat capacity x change in temperature
What is the unit of energy?
Joules (although calories are still used sometimes)
Calorimeter
A machine used to measure the amount of energy released from an object.
They can be made inaccurate by energy losses.
How do catalysts increase the rate of reaction?
By providing an alternative pathway for the products which has a lower activation energy.
This means that a greater proportion go reactant particles have enough energy to react.
Breaking bonds…
aBsorbs energy (endothermic)
foRming bonds…
Releases energy (exothermic)
How can you measure enthalpy change?
By using calorimetry
What does calorimetry allow you to measure?
The amount of energy transferred in a chemical reaction
What reactions does calorimetry measure?
Dissolving
Displacement
Neutralisation
How do you measure the energy transferred in these reactions?
Take the temperature of the reagents (making sure they are the same)
Mix them
Measure the temperature of the solution at the end of the reaction
What can you put the reagents into when carrying out the experiment?
A polystyrene cup (insulator)
Why should the polystyrene cup be put in a beaker of cotton?
Gives more insulation as lots of energy can be lost to the surrounds
Also a lid can be put on the cup to reduce energy lost by evaporation
How can you measure combustion using calorimetry?
Burn a fuel and use the flame to heat up some water - this method uses a metal container, usually copper, as it conducts heat well.
What is the method for working out the enthalpy change in combustion using calorimetry
Put 50g of water in the copper container and record the temperature
Weigh the spirit burner and lid
Put the spirit burner underneath the container and light the wick
Heat the water stirring constantly until the temperature reaches about 50 degrees
Put out the flame and measure the final temperature
Calculate the enthalpy change from the measurements
What does the combustion experiment involve?
Heating water by burning a liquid fuel
What do you need to measure to get how much energy is supplied by each gram of fuel?
How much fuel you’ve burned
- Temperature change of the water
What is specific heat capacity?
The amount of energy needed to raise the temperature of 1 gram of water by 1 degrees in water
What is the specific heat capacity for water?
4.2 J/g/degrees - so it takes 4.2 joules to raise the temperature of 1g of water by 1 degrees
Work out the mass of methylated spirit burnt using the following figures:
Mass of spirit burner + lid before heating = 68.75g
Mass of spirit burner + lid after heating = 67.85g
68.75 - 67.85 = 0.90g
Work out the temperature rise of 50g of water due to heating with the following figures:
Temp of water in copper before heating = 21.5 degrees
Temp of water in copper after heating = 52.5 degrees
52.5 - 21.5 = 31.0 degrees
Work out the total energy produced in this experiment
(knowing that 0.90g of fuel produces enough energy to heat up 50g of water by 31 degrees and it takes 4.2 joules of energy to heat up 1g of water by 1 degrees)
4.2 x 50 x 31 = 6510 joules
So if 0.9 of methylated produces 6510 joules of energy, how much does 1g of methylated produced?
6510/0.9 = 7233J or 7.233 KJ
How do you work out the energy per mole of a substance?
Calculate the amount of energy transferred
Work out how many moles of fuel produced the heat (moles=mass/mr)
Work out the heat produced by 1 mole
Work out the energy per mole of methylated spirit
Total energy produced = 6.510 kj
Mr = 44.6
Mass of meths burnt = 0.90g
Moles = 0.90/44.6 = 0.020 moles
-6.510/0.020 = -325.5 kj/mol
Why does the total energy produced in the methylated spirit turn into a negative number?
Combustion is a exothermic reaction
What happens during a chemical reaction?
Old bonds are broken and new bonds are formed
What process describes breaking bonds?
Endothermic - energy must be supplied to break existing bonds
What process describes the formation of bonds?
Exothermic - energy is releases when new bonds are formed
In terms of energy what happens in an exothermic reaction?
The energy released in bond formation is greater than the energy used in breaking old bonds
What is an exothermic reaction?
One which gives out energy to the surroundings usually in the form of heat shown by a rise in temperature
In terms of energy what happens in an endothermic reaction?
The energy required to break bonds is greater than the energy released when new bonds are formed
What is an endothermic reaction?
One which takes in energy from the surroundings usually in the form of heat shown by a fall in temperature
What is the overall change in energy in a reaction called?
The enthalpy change (symbol = ΔH)
What are the units of ΔH?
KJ/mol - its the amount of energy in kilojoules per mole of reactant
What value can the enthalpy change be?
Either positive or negative
If the reaction is exothermic, what is the value of the enthalpy change?
Negative because the reaction is giving out energy
If the reaction is endothermic, what is the value of the enthalpy change?
Positive because the reaction takes in energy
If the reaction is endothermic, what is the value of the enthalpy change?
Positive because the reaction takes in energy
What do energy level diagrams show?
How the energy levels of the reactants change when they react to form the products (shows if its exothermic or endothermic)
What do exothermic reactions look like on an energy level diagram?
The products are at a lower energy than the reactants
What does the difference in height bwteen the reactants and products represent?
The energy given out in the reaction (per mole) - the enthalpy change is -ve
What does the initial rise in the line represent?
The energy needed to break the old bonds (activation energy)
What do endothermic reactions looks like on energy level diagrams?
The products are at a higher energy than the reactants, so thr enthalpy change is +ve
What is activation energy?
The minimum energy needed by reacting particles for the reaction to occur
What does a catalyst do in reactions?
Makes them faster by providing an alternative reaction pathway with a lower activation energy
What curve represents the activation energy on this graph?
The lower curve as it shows the less initial energy needed for the reaction to begin
What remains the same in the reaction with a catalyst?
The overall energy change - catalysts are not used up during reactions
How can you calculate the enthalpy change for a reaction?
Look at the bonds that are made and broken
What is bond energy?
The amount of energy in a bond
What does each type of chemical bond have?
A particular bond energy associated with it
What bonds do you add up?
The energy of the bonds that are broken
What bonds do you subtract?
The energy of the bonds that are made
Using the bond energies for the following reaction H2 + Cl2 —> 2HC
H-H: +436 kj/mol
Cl-Cl: +242 kj/mol
H-Cl: +431 kj/mol
Work out the energy needed for breaking the bonds
436 + 242 = 678 kj
Using the bond energies for the following reaction H2 + Cl2 —> 2HC
H-H: +436 kj/mol
Cl-Cl: +242 kj/mol
H-Cl: +431 kj/mol
Work out the energy needed to form the bonds
2 x 431 = 862kj
What formula can you use to calculate the differences in the energy calculated in the formation of HCl?
Enthalpy change = Total energy absorbed to break bonds - total energy released in making bonds
What is the enthalpy change in the formation of HCl (breaking=678 and forming=862)?
678 - 862 = -184 kj/mol
What does the enthalpy change of the formation of HCl show?
The value is negative so it in an exothermic reaction
What is electrolysis?
breakdown of an ionic compound (when molten or in aqueous solution) by passage of electricity
What is an electrolyte?
Aqueous solution of ionic substance or molten ionic salt
Why does an electrolyte conduct electricity?
Conducts electricity due to the presence of mobile ions
What are electrodes?
Rods which help current enter the electrolyte
What are the products formed from the electrolysis of Molten lead(II) bromide?
At cathode: lead
At anode: bromine
What are the products formed from the electrolysis of Concentrated hydrochloric acid?
At cathode: Hydrogen
At anode: chlorine
What are the products formed from the electrolysis of Concentrated aqueous sodium chloride?
At cathode: hydrogen
At anode: chlorine
What is formed at the cathode?
Metal/Hydrogen
What is formed at the anode?
Non-metal/Oxygen (OH-)
What is electroplating?
Coating one metallic object with another metal using electrolysis
What are the materials needed for electroplating? (3)
Anode made of metal you want to electroplate object with
Ions of same metal as anode in solution
Object to be plated at cathode
What are the uses of electroplating? (3)
Improving metal appearance
Galvanise
Prevent corrosion
Why is aluminum used for electricity cables? (4)
Light
Non-corrosive
Good conductor
Cheaper than copper
Why is copper used in electrical wiring? (2)
Good conductor
Ductile
Why are plastics & ceramics used as insulators?
Don’t conduct electricity
Conduct heat poorly
How is copper purified?
By using an impure copper anode and a pure copper cathode
Using inert electrodes
What are the materials needed for the purification of copper with copper electrodes?
Anode: Impure copper
Cathode: Pure copper
Copper sulfate solution
What happens during electrolysis using copper electrodes?
the metal atoms at the anode lose electrons to form positive ions
What are the materials needed for the purification of copper with inert electrodes?
Inert anode: Graphite
Inert cathode: Graphite
Copper sulfate solution
What happens during electrolysis using inert electrodes?
Negative ions in the electrolyte lose electrons to anode
What happens at the anode?
Reduction of positive ions
What is the main ore of aluminum?
Bauxite
What are the electrodes used in the extracting aluminum made of?
Carbon
What is the electrolyte in the cell?
molten aluminium oxide dissolved in molten cryolite
Why is aluminum oxide dissolved in cryolite?
Decrease melting point, improve conductivity
Describe the process of extracting aluminium
1) Bauxite treated w/ sodium hydroxide to form sodium aluminate and heated to form aluminum oxide
2) Aluminum oxide dissolved in cryolite
3) Electrolysis using carbon electrodes
4) At cathode aluminum ions gain electrons, reduced to aluminum
5) At anode, oxide ions are oxidised to oxygen
6) Oxygen reacts w/ carbon
What are the products in extracting aluminum?
At cathode: aluminum
At anode: oxygen
What is brine?
Concentrated aqueous solution of sodium chloride
Identify the ions present in brine (4)
Na+ , Cl-, H+ and OH-
Where is brine electrolyzed?
In a diaphragm cell
What are the products electrolysis of brine? (3)
At cathode: Hydrogen
At anode: Chlorine
Sodium hydroxide forms in solution
What are the uses of chlorine? (2)
treating drinking water
- making bleach
What are the uses of hydrogen? (2)
making ammonia
- making margarine
What is the use of sodium hydroxide?
soap
Explain ionic movement in electrolysis
In a molten ionic compound, ions able to move
When current applied, cations move towards cathode - anions move towards anode
Cations reach cathode and gains electrons (reduction)
Anions reach anode and lose electrons (oxidation)
Where does reduction occur?
Cathode
Where does reduction occur?
Cathode
What is copper refining?
When copper is purified by electrolysis
In copper refining, what electrodes are used?
Anode: Impure copper
Cathode: Pure copper
During electrolysis using inert electrodes, what happens to the negative ions in the electrolyte?
They lose their electrons to the anode
In copper refining using inert electrodes, why does the eletrolyte lose its blue colour?
Copper ions in solution are turning into copper atoms at the cathode
In copper refining using 2 copper electrodes, what happens to the anode?
As the anode is not inert, it loses electrons and copper ions go into solution - anode gets smaller
In copper refining using 2 copper electrodes, why does the electrolyte remain the same deep blue colour?
Copper ions removed from solution at cathode are replaced in solution by copper ions formed at the anode - going in to solution
What is the overall equation for the electrolysis of aluminium oxide using electrolysis?
2Al2O3 => 4Al + 3O2
What are exothermic reactions?
Chemical reactions in which heat is given out
What is bond forming an example of?
Exothermic reaction
What are the energy changes that occur during an exothermic reaction?
Energy released is more than the energy absorbed
Bond forming __ energy
releases
What happens to heat during an exothermic reaction?
Heat is given out and transferred from the chemical to surrounding
What happens to the temperature of the mixture during an exothermic reaction?
Temperature of mixture will rise until the highest temperature is reached, then fall to room temp.
What are the energy transformations that occur during an exothermic reaction?
Transfer chemical to heat energy
What are three common examples of exothermic reactions?
Combustion of fuels
Rusting iron
Metal corroding
Neutralisation between acid and alkali
Respiration
What are endothermic reactions?
Chemical reactions in which heat is absorbed/taken in
What is bond breaking an example of?
Endothermic reaction
What are the energy changes that occur during an endothermic reaction?
Energy absorbed is more than the energy released
Bond breaking __ energy
requires
What happens to heat during an endothermic reaction?
Heat is absorbed and transferred from surroundings to reactants
What happens to heat during an endothermic reaction?
Heat is absorbed and transferred from surroundings to reactants
What happens to the temperature of the mixture during an endothermic reaction?
Temperature of mixture falls until lowest temp is reached, then rise until room temp
What are the energy transformations that occur during an exothermic reaction?
Transfer heat to chemical energy
What are common examples of endothermic reactions? (2)
Photosynthesis
Thermal decomposition
What is a fuel?
substance that can be burned to release energy
What happens when fuel is burned/ the combustion of fuel? (2)
Energy is stored inside the fuel, burning the fuel releases it
Combustion reactions turns fuel and oxygen into water and carbon dioxide (and releases energy)
Why is hydrogen used as a fuel?
Its environmentally friendly
What happens in a hydrogen fuel cell?(2)
In hydrogen fuel cell, hydrogen reacts with oxygen without burning
Energy released is used to generate electricity (which is used to drive an electric motor)
What are three advantages of fuel cells?
Lightweight
High efficiency
Only water is made – no additional pollutants
How are radioisotopes used a source of energy?
These isotopes are used to produce nuclear fuels
What are 2 common radioisotopes used as a source of energy?
Urainium235
Plutonium239
What are the products that would be released if petrol and natural gas (methane) were used in energy production?
Carbon dioxide and water
What is the equation for the reaction that takes part in a fuel cell?
2H2 + O2 –> 2H2O
What is an exothermic reaction?
Reaction in which energy is given out to surroundings
Give examples of exothermic reactions
Combustion of fuels
Reaction of acids and metals
Neutralisation
What is an endothermic reaction?
Reaction in which energy is taken in from surroundings
Give examples of endothermic reactions
Photosynthesis
Thermal decomposition of carbonates
Electrolysis
What is endothermic?
bond breaking positive change in temperature value is positive
What is exothermic?
Bond forming negative change in temperature value is negative
How can you calculate the energy of a reaction
Add together all bond energies for all the bonds in reactants- energy in
Add together all bond energies for all the bonds in products- energy out
Calculate the energy change by energy in- energy out
What is the equation for energy change?
Energy taken in - Energy given out
What is a fuel?
A substance which releases energy when burned
What does a fuel release?
Carbon dioxide and water
How can we measure the efficiency of fuels?
By a calorimeter
Where is hydrogen used?
In rocket engines and in fuel cells to power some car
What are the advantages of fuel cells?
Releases more energy/kilogram
It does not pollute
What are the disadvantages of fuel cells?
Expensive to produce and requires energy for production process
Difficult to store and move around
Affected by low temps
How does Uranium-235 release energy?
Undergoes decay and gives off heat energy
What are the advantages of using Uranium-235?
It is clean as it does not release polluting gases
What are the disadvantages of using Uranium-235?
Nuclear power plants are expensive and radioactive materials may be released by accident
How does a hydrogen fuel cell work?
Donates electrons at one electrode and oxygen gains electrons at the other electrode
What happens at the anode in a fuel cell?
2H2 –> 4H+ +4e-
What happens at the cathode in a fuel cell?
4H+ +02 +4e- –> 2H2O
What is the overall reaction in a fuel cell?
2H2 + O2 –> 2H2O
define electrolysis
the decomposition of an
ionic compound, when molten or in
aqueous solution, by the passage of an electric current
which is the positive electrode?
anion
which is the negative elctrode?
cathode
What is the the electrolyte ?
the molten or aqueous substance that undergoes electrolysis
Why are metal objects are electroplated?
to improve their appearance and resistance to corrosion
State the use of hydrogen–oxygen fuel cell
uses hydrogen and oxygen to produce electricity with water as the only chemical product
How are metals electroplated
Describe the disadvantages of
using hydrogen–oxygen fuel cells in comparison with gasoline /petrol engines in vehicles
Describe the advantages of
using hydrogen–oxygen fuel cells in comparison with gasoline /petrol engines in vehicles
what is an exothermic reaction?
reaction that transfers
thermal energy to the surroundings leading to an increase in the temperature of the surroundings
What is an endothermic reaction?
reaction that takes in
thermal energy from the surroundings leading to a decrease in the temperature of the surroundings
What is the transfer of thermal energy during a reaction called?
the enthalpy change ∆H
is ∆H negative or positive in exothermic reactions
negative
is ∆H negative or positive in endothermic reactions
positive
define activation energy
minimum energy that colliding particles must have to react
what is the symbol for activation energy
Ea
is bond breaking an endothermic or exothermic reaction?
endothermic
is bond making an edothermic or exothermic reaction?
exothermic
what is an example of an exothermic reaction?
combustion
neutralisation
what is an example of an edothermic reaction?
hand warmers
self heating cans
How can we calculate the enthalpy change of a reaction using bond energies
Add together all the bond energies for all the bonds in the reactants – this is the
‘energy in’.
Add together the bond energies for all the bonds in the products – this is the
‘energy out’.
Calculate the energy change: energy in – energy out
What does it mean if If the energy out > energy in?
the energy change will be negative showing an exothermic reaction
What does it mean if the energy out<energy in?
will be positive showing
an endothermic reaction.
What can Reaction profiles be used to show?
an be used to show the relative energes of
reactants and products, the activation energy and the overall
energy change of a reaction.
