Energy changes Flashcards
What is the conversion of energy principle?
Energy is conserved in chemical reactions. The amount of energy 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 surrounding so that the surrounding temperature increases.
Combustion, oxidation reactions and neutralisation reactions.
Negative sign of energy change.
What is an endothermic reaction? Give examples
A reaction where energy is taken from the surrounding so the surrounding temperature decreases.
Thermal decomposition, reaction of citric acid and sodium hydrogen carbonate.
Positive sign of energy change.
What is activation energy?
The minimum amount of energy that particles need to collide/react.
What is a reaction profile?
Reaction profile is a graph which shows the relative energies of reactants and products, 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 energy needed to break the bonds.
Endothermic - energy needed to break bonds is greater than energy released to make bonds.
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 2 electrodes dipped in an electrolyte solution. It produces electricity from a chemical reaction.
What is a battery?
A battery consists of 2 or more cells connected in series.
What determines the voltage obtained from a cell?
The greater the difference in reactivity of metals and the concentration of electrolyte.
State the advantages and disadvantages of using cells and batteries
Advantages:
Cheap, rechargeable, a convenient source of electrical energy.
Disadvantages:
Harmful chemicals.
Describe rechargeable and non-rechargeable cells
Rechargeable - chemical reactions are reversed when an external current is supplied.
Non-rechargeable - reactant 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 a potential difference and electricity.
What is the overall reaction in a hydrogen fuel cell? What are the half equations?
Anode: 2H₂ → 4H⁺ + 4e⁻
Cathode: O₂ + 4H⁺ + 4e⁻ → 2H₂O
Overall: 2H₂ + O₂ → 2H₂O
What are the advantages and disadvantages of hydrogen fuel cells?
Advantages:
No pollutants, no recharging as long as fuel is supplied, more energy per kg than petrol or diesal.
Disadvantages:
Flammable, H₂ difficult to store, sometimes produced by non-renewable resources, affected by low temperatures, expensive production of H₂ by electrolysis.
