Energy changes Flashcards
What is the conservation of energy principle?
Energy is conserved in chemical reaction. The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place
What is an exothermic reaction? Give examples
A reaction where energy is transferred to the surroundings so that the surroundings temperature increase - combustion, oxidation reaction and neutralisation (acid+ alkali) reactions. Negative sign of energy change.
What is activation energy?
Minimum amount of energy that particles need to react
What is a reaction profile?
Reaction profile is a graph which shows the relative energies of reactants and product, as well as activation energy of the reaction.
What occurs in a chemical reaction in terms of bond energies? Describe exothermic and endothermic reactions in terms of bond breaking/forming.
Energy is supplied to break bonds is realised 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
What is the equation to find enthalpy change in terms of bond energies?
Energy of reaction= sum of bonds broken - sum of bonds made
What is a cell?
A cell is composed of two electrodes dipped in an electrolyte solution. It produces electricity from a chemical reaction.
What is a battery?
A battery consists of two or more cells connected in series.
What determines the volage obtained from a cell?
Identities of metals used as electrodes and the identity and concentration of an electrolyte.
State the advantage and disadvantages of using cells and batteries.
(+) more or less cheap, some are rechargeable, a convenient source of electrical energy
(-) harmful chemicals
Describe rechargeable and non-rechargeable cell
Rechargeable - chemical reactions are reversed and external current is supplied
Non-rechargeable - reactants are used up, cannot be recharged
What is a fuel cell? What is the overall reaction in a hydrogen fuel cell? What are the half equations?
Fuel cells are supplied by fuel and oxygen to oxidise the fuel to generate electricity.
What is the overall reaction in hydrogen fuel cell? What are the half equations?
Cathode: 2 H2 –> 4H+4 e-
Anode: O2+4 H+ + 4e- –> 2 H2O
Overall: –> 2 H2 + O2 –> 2 H2O
What are the advantages and disadvantages of hydrogen fuel cells?
Advantages: no pollutants, no recharging
Disadvantages: flammable, H2 difficult to store, fossil fuel production, toxic chemicals, expensive production of H2 by electrolysis
