Topic 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.
Advantage: more or less cheap, some are rechargeable, a convenient source of electrical energy
Disadvantage: 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
