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.
Higher Tier Only
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?
Higher Tier Only
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? What is the
overall reaction in a hydrogen
fuel cell? What are the half
equations?
Reactions
Higher tier only
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?
Reactions
Higher tier only
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
