Chapter 6 and 7 Flashcards
What is an energy level diagram?
A diagram to show the energy released when the bonds are broken in the reactant and remade in the products.
It shows the overall change in energy for that reaction
The overall energy change for a reaction is the difference between the energy need to break bonds and the energy need to form bonds
What are exothermic reactions?
Reactions that give out heat to their surroundings (you will see an increase in temperature)
Thermal energy is transferred from the chemicals to the surroundings
What are endothermic reactions?
Reactions that take in heat from their surroundings (you will see a decrease in temperature)
Thermal energy is transferred from the surroundings to the chemicals
What is electrolysis?
The use of electricity to split ionic compounds into their ions.
What is an electrolyte?
An aqueous or molton solution of ions
What is oxidation?
The gain of oxygen or loss of electrons
What is reduction?
The loss of oxygen or gain of electrons
Electrolysis requires what to conduct electricity?
Charged particles that are free to move
Why is cryolite added to the electrolysis process of Aluminium oxide?
To lower the temperature at which aluminium oxide melts
Why is the electrolysis of aluminium oxide so expensive?
It requires a large amount of electricity to produce the temperatures needed to melt the aluminium oxide
What is the +ve electrode called?
Anode – it attracts -ve ions (anions)
What is the –ve electrode called?
Cathode – it attracts +ve ions (cations)
What happens if two +ve ions are present in the electrolyte?
In aqueous solutions, the least reactive +ve ions will go to the cathode
Which -ve ion will be attracted to the anode in aqueous electrolytes?
In aqueous solutions oxygen (-ve ion) will be attracted to the anode unless a halogen ion is present
What happens at the anode?
The -ve ions lose electrons to turn back into atoms. We can show this using half ionic equations
What happens at the cathode?
The +ve ions gain electrons to turn back into atoms. We can show this using half ionic equations
What is the half equation for the formation of oxygen at the anode?
2O- ————–> O2 + 2e-
What is the half equation for the formation of aluminium at the cathode?
Al3+ + 3e - ——————> Al
What is mean bond energy?
The energy needed to break a particular chemical bond
What is activation Energy?
The minimum energy needed to start a reaction
How is activation energy shown on an energy level diagram?
Activation energy is shown as an arrow from the reaction line to the top of the curve
How would you show a change in activation energy when a catalyst is used?
What is the test for chlorine?
Bleaches wet litmus paper
At the electrode, Mg2+ ion is converted back into an atom. Has it been oxidised or reduced? Explain your answer.
Mg2+ has been reduced as it has gained two electrons to become an atom of Mg.
Mg2+ + 2e- ————-> Mg
What does OIL RIG mean in terms of oxidation and reduction?
Oxidation
is
loss
(of electrons)
Reduction
is
gain
(of electrons)
Give two examples of an exothermic reaction.
Combustion, neutralisation, some oxidation reactions such as rusting, respiration, heat packs
Give two examples of an endothermic reaction.
Thermal decomposition, photosynthesis, cold packs
How is a current formed in an electrochemical cell?
(higher only)
In a fuel cell, the electrodes are two different metals. The potential difference (voltage) between the metals cause electrons to flow
Electrons flow from the most to least reactive metal producing an electrical current
What is a battery?
(higher only)
A battery is formed from two or more cells
Why is the reactivity series important in electrochemical cells?
(higher only)
The position of the metals in the reactivity determines the voltage. The greater the difference in the position of the two metals used as electrodes the higher the voltage
Explain why a current won’t flow if the electrodes are made of the same metal?
(higher only)
For a current to flow there must be a difference in the metals position in the reactivity series. This can only happen if the metals are different.
What is voltage?
(higher only)
Voltage is a measure of the potential difference in electrical energy between two parts of a circuit i.e. the different metal electrodes in the electrochemical cell.
The bigger the difference in energy, the bigger the voltage.
In Chemistry, we are using the position of the metal in the reactivity series to reflect its electrical energy.
Note: Current is the rate at which charge is flowing
What is a happening in an electrochemical cell?
(higher only)
Chemical energy is being converted into electrical energy.
Non-rechargeable batteries have a limited amount of chemicals that are eventually used up.
How is a fuel cell different from a non- rechargeable battery?
(higher only)
Non-rechargeable batteries have a limited supply of chemicals that are used up
Fuel cells have a continuous supply of chemicals most often oxygen (from the air) and a fuel (hydrogen)
What is the law of conservation of energy?
Energy can not be created or destroyed it can only be transferred from one form to another
What two factors can change the voltage of an electrochemical cell?
(higher only)
Choice of metals used for the electrodes
Choice of the electrolyte (type or concentration)
How would you calculate bond energy?
(higher only)
To calculate bond energy
Add together 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.
Why are bond energies important?
(higher only)
In chemical reactions energy is transferred as bonds in the reactants are broken and new bonds form in the products
Bond breaking is endothermic and bond forming is exothermic. We can work out the overall energy change of a reaction by looking at the bond energies
The overall energy change for a reaction is the difference between the energy need to break bonds and the energy need to form bonds
