Unit 5 - 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
A reaction where energy is transferred to the surroundings so that the
surroundings temperature increases – combustion, oxidation reactions and
neutralisation (acid + alkali) reactions. Negative sign of energy change.
What is an endothermic reaction? Give examples
A reaction where energy is taken in from the surroundings so the surroundings
temperature decreases – thermal decomposition, reaction of citric acid and
sodium hydrogencarbonate. 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 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 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 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?
Cathode: 2 H2 → 4 H+ + 4 e−
Anode: O2 + 4 H+ + 4 e−→ 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
