energy_changes Flashcards
(18 cards)
Conservation of energy principle
Energy is conserved in chemical reactions. The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place.
Exothermic reaction
A reaction where energy is transferred to the surroundings so that the surroundings temperature increases - combustion
Endothermic reaction
A reaction where energy is taken in from the surroundings so the surroundings temperature decreases - thermal decomposition
Activation energy
Minimum amount of energy that particles need to react.
Reaction profile
Reaction profile is a graph which shows the relative energies of reactants and product
Bond energies in chemical reactions
Energy is supplied to break bonds and energy is released when bonds are made; exothermic - energy released from forming bonds is greater than that needed to break the bonds; endothermic - energy needed to break bonds is greater than energy released making them.
Enthalpy change equation
Energy of reaction = sum of bonds broken - sum of bonds made.
Cell
A cell is composed of two electrodes dipped in an electrolyte solution. It produces electricity from a chemical reaction.
Battery
A battery consists of two or more cells connected in series.
Voltage from a cell
Identities of metals used as electrodes and the identity and concentration of an electrolyte.
Advantages of cells and batteries
(+) more or less cheap
Disadvantages of cells and batteries
(-) harmful chemicals.
Rechargeable cells
Chemical reactions are reversed when an external current is supplied.
Non-rechargeable cells
Reactants are used up
Fuel cell
Fuel cells are supplied by fuel and oxygen to oxidise the fuel to generate electricity.
Overall reaction in a hydrogen fuel cell
Cathode: 2 H2 → 4 H+ + 4 e−; Anode: O2 + 4 H+ + 4 e−→ 2 H2O; Overall: 2 H2 + O2 → 2 H2O.
Advantages of hydrogen fuel cells
No pollutants
Disadvantages of hydrogen fuel cells
Flammable
