Energy changes Flashcards
What is electrolysis?
The splitting up of a substance using electricity
Why does the ionic substance have to be molten or in solution?
So that the ions are free to move to the electrodes
What is the electrolyte?
The molten ionic substance or solution of the ions
What are the positive and negative electrodes called?
Cathode - negative electrode
Anode - positive electrode
What are electrodes made from?
Graphite, aluminium or carbon
What happens during electrolysis?
Positively charged ions are attracted to the negative electrode and the negatively charged ions are attracted to the positive electrode. When the ions reach the electrodes they can lose their charge and become elements
What’s different about the electrolysis of aqueous solutions?
They contain hydrogen ions (H+) and hydroxide ions (OH-) as well as the ions of the dissolved substance. These ions are formed from the water that the compound is dissolved in
What happens if the metal ion is more reactive than hydrogen in the electrolysis of an aqueous solution?
The H+ ions go to the negative electrode to form hydrogen gas
What happens if the metal is less reactive than hydrogen in the electrolysis of an aqueous solution?
The metal ions go to the negative electrode and form the metal
What happens at the positive electrode in the electrolysis of an aqueous solution?
Oxygen is produced unless there is a halide ion present, which means the halogen is produced instead
What happens in the electrolysis of brine?
The ions in the solution are H+ ions, Na+ ions, Cl- ions and OH- ions
The hydrogen ions go to the negative electrode, the chloride ions go to the positive electrode and this leaves a solution of sodium hydroxide
What happens in an exothermic reaction?
Heat is transferred to the surroundings; this could be air or water and then the surroundings have more heat energy so the temperature increases
What are some examples of exothermic reactions?
Combustion
Respiration
Neutralisation
Many oxidation reactions
Hand warmers
What happens in an endothermic reaction?
Heat energy is taken away from the surroundings and stored as chemical energy, this means that the surroundings have less heat energy and so the temperature decreases
What are some examples of endothermic reactions?
Thermal decompositions
Some dissolving processes
Photosynthesis
Sports injury cooling packs
What is the activation energy?
The minimum amount of energy required for particles to react
How can you tell if a reaction is endothermic?
If more energy is required to break the bonds than is released from making the bonds, the reaction is endothermic
How can you tell if a reaction is exothermic?
If more energy is released from making the bonds than required to break the bonds, the reaction is exothermic
When is cryolite used?
In the electrolysis of aluminium oxide in order to lower its melting point and save energy
How is a cell made?
By connecting two different metals which are in contact with an electrolyte
What does the voltage produced by a cell depend on?
The type of electrode and electrolyte used; the greater in reactivity between the two metals, the greater the voltage
What happens in a non-rechargeable battery?
One of the reactants is used up, causing the reaction to stop and the battery no longer works
What are non-rechargeable batteries also called?
Alkaline batteries
What happens in a rechargeable battery?
A reversible reaction is used which can go forwards and backwards when an external electricity current is applied
Why does a fuel cell never stop working?
Because the reactants are continually replaced
How is energy released in a fuel cell?
In the form of electrical current and heat
What happens at the negative electrode in a fuel cell and what is the equation?
The hydrogen molecules lose electrons to form H+ ions and become oxidised
H2 → 2H+ + 2e-
How is electrical current created around the fuel cell?
The electrons move around the circuit to the positive electrode, creating an electrical current
What happens at the positive electrode in a fuel cell?
The hydrogen ions and electrons react with the oxygen to form water and the oxygen atoms are reduced
4H+ + O2 + 4e- → 2H2O
What is the overall reaction in a fuel cell?
2H2 + O2 → 2H2O
What are the advantages of fuel cells?
Don’t need to be charged overnight
Continuous supply of electricity
No loss of efficiency over time
Only waste product is water so it’s good for the environment
Longer life than rechargeable cells or batteries that add to landfill
No toxic chemicals to dispose of at the end of the cell’s life
What are the disadvantages of fuel cells?
Expensive and unreliable technology in motor vehicles
Hydrogen is difficult to store as it’s highly flammable
Hydrogen is produced from methane (a non-renewable resource) and the process releases carbon dioxide
Rechargeable cells and batteries are much cheaper
Hydrogen is produced by electrolysis which uses fossil fuels and carbon dioxide
