C5 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. Positive 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 inn terms of bond energies?
Energy of reaction = sum of bonds broken - sum of bonds made
What is a battery?
A battery consists of two or more cells connected in series.
What is a cell?
A cell is composed of two electrodes dipped in an electrolyte solution. It produces electricity from a chemical reaction.
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?
Fuel cells are supplies by fuel and oxygen to oxidise the fuel to generate electricity.
2 H2 (g) + O2 (g) → 2 H2 O (l)
In summary, the two half-reactions in a hydrogen fuel cell are the oxidation of hydrogen gas at the anode and the reduction of oxygen gas at the cathode.
What is the overall reaction in a hydrogen fuel cell? What are the half equations?
Cathode: 2 H2 → 4H+ + 4 e-
Anode: O2 + 4 H+ + 4 e- →2 H20
Overall: 2 H2 + O2 → 2 H20
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
Endothermic reaction:
An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases. In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds.
Activation energy:
The minimum amount of energy for particles to collide with in order for a successful reaction to occur.
Exothermic reaction:
An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases. In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds.
Overall energy change of the reaction:
The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed.
Reaction profile:
Reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction.
