F325 - Energy Flashcards
What is the enthalpy change of solution?
The energy released when one mole of an ionic solid dissolves in sufficient water to form a solution of infinite dilution.
What is lattice enthalpy?
Enthalpy change when one mole of an ionic compound is formed from it’s gaseous ions.
What is the enthalpy of hydration?
The energy released when one mole of gaseous ions dissolves in sufficient water to form a solution of infinite dilution.
What is the enthalpy change of atomisation?
Enthalpy change when one mole of gaseous atoms are formed from the element in its standard state.
What is the first electron affinity?
Enthalpy change when one electron is added to each of one mole of gaseous atoms to form one mole of gaseous 1- ions.
What is the second electron affinity?
Enthalpy change when one electron is added to each of one mole of gaseous 1- ions to form one mole of gaseous 2- ions.
What is the first ionisation energy?
Energy needed to remove one electron from each atom in one mole of gaseous atoms of an element.
What is meant by the term successive ionisation energies?
Energies needed to remove each successive electron from each atom in one mole of gaseous positively charged ions.
What is the enthalpy of formation?
Enthalpy change when one mole of a compound is made from its constituent elements.
What properties affect the magnitude of lattice enthalpy?
Size of ions (ionic radius) and charge on ions (charge density).
What happens to the lattice enthalpy as the size of the ion increases?
Becomes less exothermic (less negative)
What happens to the lattice enthalpy as the charge on the ion increases?
Becomes more exothermic (more negative)
What is entropy?
The degree of disorder of a system.
Which state if matter would have the lowest entropy?
Solids (as their particles are arranged in near rows and are ordered)
Which state of matter would have he highest entropy?
Gases (as the interactions between molecules are negligible)
What happens to the magnitude of entropy when a solid dissolves? Why?
Increases as the lattice dissolves (breaks down) to be in solution.
What happens to the magnitude of entropy for a reaction where a larger number of moles of gas are produced?
Increases (more disordered)
What happens to the magnitude of entropy for a reaction where a larger number of moles of gas are reacted?
Decreases (less disordered)
How is the entropy change for a reaction calculated?
Change in entropy = entropy of products - entropy of reactants
What does the tendency for a reaction to take place depend on?
- T, temperature
- S, entropy change in the system
- H, enthalpy change with the surroundings
What is meant by the term free energy change of a reaction?
The maximum useful work that can be done by a chemical reaction.
What is the Gibbs free energy equation?
{delta}G = {delta}H - T{delta}S
What is needed for a reaction to be spontaneous?
Free energy is negative (more than 20kJ/mol)
How can an endothermic reaction occur spontaneously?
- enthalpy value is small
- entropy is positive (increase in disorder)
- temperature is high enough so that the overall energy is negative
What is meant by the term redox?
When reduction and oxidation take place simultaneously.
What is meant by the term oxidation number?
A number assigned to the atoms of each element in an ion or compound determined from the number of electrons gained or lost.
What is a half-equation?
An equation that describes what is happening in one half-cell. Or an equation which describes only the reduction or only the oxidation reaction.
What is an oxidising agent?
A species that takes one or more electrons away from another species in a reaction?
What is a reducing agent?
A species that donates one or more electrons to another species in a reaction.
What is added to balance the oxidation numbers in a half-equation?
Electrons
What is added to balance charge in a half-equation?
Hydrogen ions
What is added to balance hydrogen and oxygen in a half-equation?
Water
What needs to be balanced when constructing an overall redox equation?
Electrons
What is the standard electrode potential?
The electrode potential of a half-cell when measured with a standard hydrogen electrode as the other half-cell. All conditions must be standard.
What is taking place in the half-cell if the standard electrode potential is negative?
The half-cell donates electrons to the standard hydrogen electrode. Greater tendency to proceed in the reverse direction.
What takes place in a half-cell when the standard electrode potential is positive?
The half-cell receives electrons from the standard hydrogen electrode. Greater tendency to proceed in forward direction.
What is used as the electrode in a half-cell when measuring the electrode potential is ions of the same element in different oxidation states?
Platinum
What are the standard conditions required for measuring the standard electrode potentials?
- 1 atm of pressure/100 kPa/ 1 bar
- 298 K
- 1 mol/dm^3 solutions
What are the limitations associated with predicting whether or not a reaction will occur using standard electrode potentials?
- actual conditions are not likely to be standard
- if concentration is not standard then it can be measured against the SHE
- if concentration is greater on LHS E will be more positive
- if concentration is greater on RHS E will be less positive
- does not predict rate of reaction which might be too slow
What is a difference between a storage cell and a fuel cell?
Fuel cells use energy from the reaction of a fuel with oxygen and storage cells use redox reactions.
Where do the electrons flow in a cell?
External circuit
Where do the ions flow in a cell?
Across the salt bridge/membrane
What is the only byproduct in a hydrogen-oxygen fuel cell?
Water
What do the anode and cathode do with electrons respectively in a fuel cell?
Anode - supplies
Cathode - receives
What are the limitations of hydrogen-oxygen fuel cells?
- limited lifetime
- expensive to produce (electrodes and membrane)
- production of toxic products in manufacturing
- storage of hydrogen
- refuelling
- engineering fuel cell vehicles
- transport of hydrogen to filling stations
- people adapting to change
How can the storage of hydrogen problem be overcome?
- store hydrogen as a liquid (still costly and expensive)
- adsorb onto a surface (eg nanotubes)
- absorb by metal alloys
What are the advantages of hydrogen-oxygen fuel cells?
- less pollution and carbon dioxide
- greater efficiency
- hydrogen economy is better (obtained from electrolysis of water)
