Topic 5 - Energy Changes (No Pracs) Flashcards
What is an exothermic reaction
A reaction which transfers energy to the surroundings (shown by a rise in temperature)
What are some examples of exothermic reactions
Combustion, neutralisation, many oxidation reactions
What are some everyday uses of exothermic reaction
- hand warmers
- self heating cans of hot chocolate and coffee
What’s an endothermic reaction
A reaction which takes in energy from the surroundings (shown by a fall in temperature)
What’s an example of an endothermic reaction
Thermal decomposition
Are endothermic or exothermic reactions more common
Exothermic
What are some everyday uses of endothermic reactions
Sports injury packs
Is bond breaking endothermic or exothermic
Endothermic - energy must be supplied
Is bond formation endothermic or exothermic
Exothermic - energy is released
What is the overall energy change of a reaction in terms of bond energies
The sum of the energies needed to break bonds in the reactants minus the energy released when the new bonds are formed
What’s an electrochemical cell
A basic system made up of two different electrodes in contact with an electrolyte
How does an electrochemical cell work
1) the two electrodes can conduct electricity
2) the electrolyte contains ions that react with the electrodes
3) the chemical reactions between the electrodes and the electrolyte set up a charge difference between the electrodes
4) if the electrodes are then connected by a wire then charge flows and electricity is produces
How do you increase the voltage of an electrochemical cell
1) change the type of electrodes
2) change the electrolyte
3) connect more cells
How does changing the electrodes in an electrochemical cells increase the voltage
Different metals with react differently with the same electrolyte - the bigger the difference in reactivity of the electrodes the larger the voltage
How does changing the electrolyte in a electrochemical cell change the voltage
Different ions in a solution will react differently with the metal electrodes used
What type of reactions does non rechargeable batteries contain
Irreversible
How can a reaction in a rechargeable cell be reversed
By connecting it to an external electric current
What’s a fuel cell
Is an electrical cell that’s supplied with a fuel and oxygen and uses energy from the reaction between them to produce electrical energy efficiently
Whats an example of a fuel cell
Hydrogen - oxygen fuel cell
How do fuel cells work
When the fuel enters the cell it becomes oxidised and sets up a potential difference within the cell
How do hydrogen oxygen fuel cells work
1) hydrogen goes into the anode compartment and oxygen goes into the cathode compartment
2) hydrogen is oxidised
3) oxygen gains electrons from the cathode and reacts with the H+ ions from the electrolyte to make water
4) the electrons flow through an external circut from the anode to to the cathode- this is the electric current
What’s the overall reaction in a hydrogen - oxygen fuel cells
2H2 + O2 > 2H20
What are the downsides of conventional fuels
- finite supply
- very polluting
What are the advantages of full cell vehicles
- don’t produce as many pollutants
- less expensive than batteries
- need to be recharged less than batteries
