Energy Changes Flashcards
What is the 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.
What is an exothermic reaction? Give examples (3) and state enthalpy
A reaction where energy is transferred to the surroundings so that the surroundings temperature increases
Combustion, oxidation reactions and neutralisation (acid + alkali) reactions. Negative enthalpy
What is an endothermic reaction? Give examples and state enthalpy
A reaction where energy is taken in from the surroundings so the surroundings temperature decreases
Thermal decomposition, reaction of citric acid and sodium hydrogencarbonate. Positive enthalpy
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 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
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 voltage obtained from a cell?
Identities of metals used as electrodes and the identity and concentration of an electrolyte
State the advantages (3) and disadvantages (1) of using cells and batteries.
(+) cheap, some are rechargeable, a convenient source of electrical energy
(-) harmful chemicals
Describe rechargeable and non-rechargeable cells
Rechargeable – chemical reactions are reversed when an external current is supplied
Non-rechargeable – reactants are used up, cannot be recharged
What is a fuel cell?
Fuel cells are supplied by fuel and oxygen to oxidise the fuel to generate electricity.
What is the overall reaction in a hydrogen fuel cell? What are the half equations?
Anode: 4H+ → 2H² + 4e‐
Cathode: 2H²O → 4H+ + 4e‐ +2O²
Overall: 2 H² + O² → 2 H²O
What are the advantages(2) and disadvantages(5) 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
