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135 Cards in this Set
- Front
- Back
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Lattice Enthalpy What are ionic bonds? |
Electrostatic forces of attraction between oppositely charged ions. |
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Lattice Enthalpy Define lattice enthalpy |
The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions. |
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Lattice Enthalpy Is Lattice enthalpy endothermic or exothermic? |
Lattice enthalpy is an exothermic change. It always has a negative sign because energy is given out when ionic bonds are formed from gaseous ions. |
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Lattice Enthalpy What does Lattice enthalpy indicate? |
The strength of an ionic lattice - it is a measure of the ionic bond strength. |
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Lattice Enthalpy Large negative value for LE =? |
Strong electrostatic forces of attraction between the oppositely charged ions in the lattice. |
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Lattice Enthalpy Do covalent structures have a lattice enthalpy? |
No. There are no ions in the structure. |
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Lattice Enthalpy How are lattice enthalpies measured? |
It is impossible to measure directly because it is impossible to form 1 mole of an ionic lattice from gaseous ions. Instead you use Hess' law by constructing Born-Haber cycles. |
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Lattice Enthalpy Define standard enthalpy change of formation. |
The enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states under standard conditions. |
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Lattice Enthalpy Define standard enthalpy change of atomisation. |
The enthalpy change that takes place when one mole of gaseous atoms forms from the element in its standard state. |
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Lattice Enthalpy Define First ionisation energy |
The enthalpy change accompanying the removal of one electron from each atom in one mole of gaseous atoms to form one mole of gaseous +1 ions. |
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Lattice Enthalpy Define second ionisation energy |
The enthalpy change accompanying the removal of one electron from each ion in one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions. |
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Lattice Enthalpy Define First electron affinity |
The enthalpy change accompanying the addition of one electron to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions |
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Lattice Enthalpy Define second electron affinity |
The enthalpy change accompanying the addition of one electron to each ion in one mole of gaseous 1- ions to form one mole of gaseous 2- ions. |
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Lattice Enthalpy Write the equation for the enthalpy change of formation of potassium Chloride |
K (s) + 1/2 Cl2 (g) ----> KCl (s) |
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Lattice Enthalpy Write the equation for the standard enthalpy change of atomisation of a) K (g) b) Cl |
K (s) ---> K (g) 1/2 Cl2 (g) ----> Cl (g) |
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Lattice Enthalpy Draw the equation for the first ionisation energy of K. |
K (g) ---> K+ (g) + e- |
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Lattice Enthalpy Draw the equation for the second ionisation energy of Ca + . |
Ca + (g) ----> e- + Ca 2+ (g) |
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Lattice Enthalpy Draw the equation for the first electron affinity of Cl. |
Cl (g) + e- ----> Cl - (g) |
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Lattice Enthalpy Draw the equation for the second electron affinity of O- |
O- (g) + e- ---> O2- (g) |
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Lattice Enthalpy Endothermic or exothermic? Why? a) Formation b) Atomisation c) ionisation d) affinity |
a) usually exothermic for ionic compounds b) always endothermic - breaking bonds. c) endothermic because the electron being lost has to overcome attraction from the nucleus in order to leave the atom. d) Exothermic because the electron is attracted into the outer shell of an atom by the nucleus |
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Lattice Enthalpy
Draw a full Born Haber cycle for NaCl (s) . |
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Lattice Enthalpy Delta Hf = ? |
The sum of all other enthalpy changes |
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Lattice Enthalpy Define standard enthalpy change of solution |
^Hs* - the enthalpy change that takes place when one mole of a compound is completely dissolved in water under standard conditions |
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Lattice Enthalpy Is the process of dissolving exothermic or endothermic? |
either. |
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Lattice Enthalpy Draw the equation for the standard enthalpy change of solution for KCl. |
KCl (s) + aq ---> K + (aq) + Cl - (aq) |
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Lattice Enthalpy What happens when an ionic solid dissolves in water? |
- The ionic lattice breaks down into gaseous ions - the ions are hydrated |
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Lattice Enthalpy What does breaking the ionic solid involve? and what does this require? |
Breaking down the crystal lattice and separating the ions. Overcoming the attractive forces between the oppositely charged ions requires energy. |
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Lattice Enthalpy So what is the enthalpy changed involved in breaking ionic lattice? |
= to the minus sign of the lattice enthalpy. |
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Lattice Enthalpy If the lattice enthalpy for KCl is -711 kJmol-1, what is the enthalpy change for the breakdown of the ionic lattice? |
+ 711 kJmol-1 |
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Lattice Enthalpy What happens in the second stage of dissolving? |
Hydration of the gaseous ions - the gaseous ions bond with the water molecules. The positively charged ions are attracted slightly to the slightly negative oxygen atoms in water, and the negatively charge ions are attracted to the slightly positive hydrogen atoms in water. |
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Lattice Enthalpy Define the standard enthalpy change of hydration |
The enthalpy change that takes place when one mole of isolated gaseous ions is dissolved in water forming one mole of aqueous ions under standard conditions. |
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Lattice Enthalpy Is hydration exothermic or endothermic? |
Exothermic. |
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Lattice Enthalpy
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K+ (g) + aq ---> K + (aq) |
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Lattice Enthalpy How can you calculate the lattice enthalpy of an ionic solid? |
Using 1) the enthalpy changes of hydration of the constituent gaseous ions 2) the enthalpy change of solution of the ionic solid |
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Lattice Enthalpy
To calculate the lattice enthalpy of an ionic solid, what Born-Haber cycle thingys do you draw? |
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Lattice Enthalpy
Calculate the lattice enthalpy: |
LE + Enthalpy change of solution = hydrations.
? + 26 = -363 - 322 ?= -363-322-26 = =711kJmol-1 |
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Lattice Enthalpy What does a large exothermic value for a lattice enthalpy mean? |
There is a large electrostatic force of attraction between the oppositely charged ions and that the ionic bonds are strong. |
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Lattice Enthalpy What are the two factors that affect lattice enthalpy? |
Ionic size and ionic charge. |
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Lattice Enthalpy Describe the effect of ionic size on lattice enthalpy |
Small ions can pack together closely in a lattice and attract eachother strongly. Large ions are further apart in their lattice and the forces of attraction between them are weaker. SO: as ionic radius increases: - Attraction between the ions decreases - Lattice enthalpy becomes less negative - (less exothermic) |
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Lattice Enthalpy Describe the effect of ionic charge on lattice enthalpy. |
The compounds with the most negative lattice enthalpies (most exothermic) are those which have small, highly charged ions. When two highly charged ions of opposite charge are present in the lattice they attract each other strongly. A very exothermic reaction will take place when a lattice is formed from two highly charged ions. |
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Lattice Enthalpy Describe how, and explain why, the lattice enthalpy of MgO differs from BaO |
The lattice enthalpy of MgO is more exothermic. This is because Mg 2+ ions have a smaller ionic radius than Ba 2+ ions, so the ions in MgO are closer together than in BaO. There is stronger attraction in MgO than BaO . |
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Lattice Enthalpy Give one reason why MgO is a good material to use for making the lining of furnaces. |
MgO has a highly negative LE which means that there is a strong attraction between its ions. compounds with highly exothermic lattice enthalpies have high melting points and so will be stable inside the working furnace at high temperatures. |
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Lattice Enthalpy Define standard enthalpy change of hydration. |
The enthalpy change that occurs when one mole of isolated gaseous ions dissolve in water forming one mole of aqueous ions under standard conditions. |
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Lattice Enthalpy What two factors affect the size of hydration enthalpy? |
-Ionic size -Ionic charge |
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Lattice Enthalpy Describe how Ionic size affects hydration enthalpy |
As the ionic radius become smaller, the value of the enthalpy change of hydration becomes more negative. Hydration depends on the ability of an ion to attract and bond with water molecules. Small ions exert more attraction on water molecules and more energy is released. |
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Lattice Enthalpy Describe how ionic charge affects hydration enthalpy |
As the charge on an ion increases it has a greater attraction for water molecules, and hence the hydration enthalpy is more negative. |
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Lattice Enthalpy What happens to ionic size across period 3? |
Ionic size decreases |
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Enthalpy and Entropy Define Entropy |
S, the quantitative measure of the degree of disorder in a system |
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Enthalpy and Entropy What sign is Entropy ? |
Always positive because all substance possess some degree of disorder because particles are always in constant motion. |
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Enthalpy and Entropy Which is preferred, order or disorder? Give an example of how this happens in nature. |
Disorder. Gas molecules spread out over time to fill a space, increasing their entropy over time. |
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Enthalpy and Entropy What four things can entropy help us explain? |
- A gas spreading through a room -Heat from a fire spreading through a room -Ice melting in a warm room -Salt dissolving in water |
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Enthalpy and Entropy At 0K, perfect crystals have |
Zero entropy |
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Enthalpy and Entropy What happens to the entropy of a pure substance with increasing temperature |
entropy increases.
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Enthalpy and Entropy What happens to entropy as substances change from solid -> liquid -> gas |
Entropy increases in the state that gives more randomness |
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Enthalpy and Entropy What happens to entropy when a solid lattice dissolves? |
It increases. |
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Enthalpy and Entropy Draw the equation for the dissolving of CuSO4 |
CuSO4*5H2O (s) ---> Cu2+ (aq) + SO42- (aq) + 5H2O(l) |
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Enthalpy and Entropy Define standard entropy. |
S* - the entropy content of one mole of the substance under standard conditions. Units - J K-1 mol -1 |
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Enthalpy and Entropy How do you calculate the entropy change of a reaction? |
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Enthalpy and Entropy a) If a change makes a system more random, Delta S is.... b) If a change makes a system less random, Delta S is... |
a) +ve b) -ve |
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Enthalpy and Entropy
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Enthalpy and Entropy What do spontaneous processes lead to? |
Lower energy and increased stability. |
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Enthalpy and Entropy Describe how exothermic reactions occur spontaneously at room temp. |
The enthalpy content of the chemical system decreases during the reaction and excess energy is released to the surroundings - which increases stability. |
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Enthalpy and Entropy How can endothermic reactions occur spontaneously at room temp? |
The enthalpy content of a chemical system increases during the reaction with energy being taken in from the surroundings. This must also increase stability. - the reason this can occur is because of entropy.
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Enthalpy and Entropy When is a process spontaneous? |
If a chemical system becomes more stable and its overall energy decreases. |
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Enthalpy and Entropy What affects the overall energy decrease in a spontaneous reaction? |
Enthalpy and Entropy |
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Enthalpy and Entropy What does the entropy contribution to the overall energy depend on? |
Temperature. |
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Enthalpy and Entropy Energy derived from entropy = |
TΔ S T in Kelvin |
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Enthalpy and Entropy As temperature increases, what happens to the energy derived from entropy? |
It becomes more significant. |
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Enthalpy and Entropy What are the three factors that determine whether a process is spontaneous? |
- T, Kelvin - Entropy change ^S - Enthalpy change, ^H with the surroundings |
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Enthalpy and Entropy How is the relationship between these three factors expressed? |
Free energy change, ΔG: Δ G = Δ H - TΔ S. |
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Enthalpy and Entropy Define the free energy change/Gibbs free energy? |
Δ G, the balance between enthalpy, entropy and temperature for a process: Δ G = Δ H - T Δ S A process can take place spontaneously when Δ G < 0. |
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Enthalpy and Entropy What is a spontaneous process? |
A process that proceeds on its own and results in a more stable chemical system and a decrease in overall energy. |
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Enthalpy and Entropy Spontaneous processes happen when... |
delta G is negative (<0) |
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Enthalpy and Entropy Most exothermic reactions are... |
spontaneous, even if entropy decreases and the system becomes more ordered because enthalpy contributes more to delta G than entropy. |
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Enthalpy and Entropy
Fill in the following table |
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Enthalpy and Entropy What is required for an endothermic reaction to take place spontaneously ? |
Delta S MUST BE +VE The temperature must be high enough so that T^S > ^H. |
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Enthalpy and Entropy
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Enthalpy and Entropy What happens to entropy when there is an increase in number of gaseous molecules (like when a gas is evolved (produced) Give an example |
Entropy increases Mg (s) + 2HCl (aq) ---> MgCl2 (aq) + H2 (g) |
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Electrode Potentials and Fuel Cells What does oxidation and reduction involve? |
the transfer of electrons |
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Electrode Potentials and Fuel Cells What are the oxidation numbers of the following: a) uncombined element b) combined oxygen c) combined hydrogen d) simple ion e) combined fluorine |
a) 0 b) 2- c) 1+ d) ionic charge e) -1 |
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Electrode Potentials and Fuel Cells Define a) oxidation b) reduction c) redox |
a) the loss of electrons or an increase in oxidation number b) the gain of electrons of a decrease in oxidation number c) a reaction in which oxidation and reduction both take place |
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Electrode Potentials and Fuel Cells What is a) an oxidising agent b) a reducing agent |
a) the substance that is reduced - it takes electrons from the substance that is oxidised b) the substance that is oxidised - it accepts electrons from the substance that is reduced |
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Electrode Potentials and Fuel Cells
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Poooooooooo |
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Electrode Potentials and Fuel Cells What is the function of an electrochemical cell? |
To control the electron transfer in redox reactions to produce electrical energy. It is the basis of all cells and batteries. |
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Electrode Potentials and Fuel Cells What does a half cell consist of? |
An element in two oxidation states - the simplest half cell has a metal placed in an aqueous solution of its ions. |
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Electrode Potentials and Fuel Cells Describe a copper half cell. |
It consists of a solution containing Cu2+ (aq) ions and a strip/rod of copper metal. (0 oxidation state) An equilibrium exists at the surface of the copper between these oxidation states of copper: Cu2+ (aq) + 2e- <---> Cu (s) Forward reaction = reduction = electron gain Reverse reaction = oxidation = electron loss |
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Electrode Potentials and Fuel Cells How do you write the equilibrium? |
With the electrons on the left hand side. |
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Electrode Potentials and Fuel Cells What is the electrode potential of a half cell? |
Indicates its tendency to lose or gain electrons in the equilibrium. |
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Electrode Potentials and Fuel Cells How are electrochemical cells made? |
By connecting two half cells with different electrode potentials: one half cell releases electrons, the other gains electrons. The difference in electrode potentials is measured with a voltmeter. |
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Electrode Potentials and Fuel Cells What are the three types of cells? |
1) Cells from metal/metal ion half cells 2) Non metal/non-metal ion half cells 3) Metal ion/metal ion half cells |
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Electrode Potentials and Fuel Cells Draw a copper-zinc electrochemical cell. |
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Electrode Potentials and Fuel Cells Describe how the two half cells are joined and the function of the: a) wire b) salt bridge c) voltmeter |
The two half cells are joined using a wire and a salt bridge. a) The wire connects the two metals, allowing electrons to be transferred between the two half cells. b) The salt bridge connects the two solutions, allowing ions to be transferred between the half cells. c) The voltmeter has a high resistance that is used to minimise the current that flows. |
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Electrode Potentials and Fuel Cells How can you make a simple salt-bridge? |
Soaking a strip of filter paper in an aqueous solution of an ionic compound that doesn't react with either of the half cell solutions. Usually KNO3 and NH4NO3 are used. |
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Electrode Potentials and Fuel Cells Write the redox equilibrium for the copper-zinc electrochemical cell. |
Zn 2+ (aq) + 2e- <--> Zn (s) Cu 2+ (aq) + 2e- <--> Cu (s) the Zn/Zn2+ equilibrium releases electrons more readily than the Cu equilibrium. so, the Zn equilibrium releases electrons into the wire, making zinc the negative electrode. Electrons flow along the wire to the Cu electrode of the Cu half cell. The Zn loses electrons so moves to the left: Zn 2+ (aq) + 2e- <--> Zn (s) <---------------------------------------- The Cu gains electrons and moves to the right: Cu 2+ (aq) + 2e- <--> Cu (s) -----------------------------------------> |
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Electrode Potentials and Fuel Cells What is the problem with using half cells from a non-metal and aqueous ions when constructing a cell? |
There is no electrode to connect the wire to, so electrons cannot be transferred.
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Electrode Potentials and Fuel Cells How is this problem overcome? |
A platinum electrode is placed in the solution so that it is in contact with the non-metal and aqueous ions. The platinum is inert and doesn't react at all - its purpose is to transfer electrons into and out the half cell via a connecting wire. The surface of the platinum electrode is coated with platinum black, a spongy coating in which electrons can be transferred between the non-metal and its ions. |
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Electrode Potentials and Fuel Cells Describe how a hydrogen half cell is set up |
2H + (aq) + 2e- <--> H2 (g) HCl is used as a source of H+ ions (aq) H2 gas - 100 kPa (1atm) Inert platinum electrode to allow electrons to pass into or out of the half cell via a connecting wire. |
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Electrode Potentials and Fuel Cells What does a metal ion/metal ion half cell contain? |
Ions of the same element in different oxidation states. |
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Electrode Potentials and Fuel Cells Describe a Fe3+/Fe2+ half cell |
Fe3+ (aq) + e- <--> Fe2+ (aq) contains: -a solution with Fe 2+ and Fe 3+ ions with the same concentrations -an inert platinum electrode to allow electrons to pass into or out of the half cell via the connecting wire |
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Electrode Potentials and Fuel Cells What is a standard hydrogen half cell used as? |
the reference standard for the measurement of cell e.m.f |
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Electrode Potentials and Fuel Cells What is e.m.f? |
Electromotive force - the voltage produced by a cell when no current flows. |
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Electrode Potentials and Fuel Cells How do you compare the tendency of different half cells to release or accept electrons? |
By measuring the emf when the half cells are combined separately with a Hydrogen half cell (you use standard half cells and standard conditions) |
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Electrode Potentials and Fuel Cells Define standard electrode potential of a half cell |
E* - the emf of a half cell compared with a standard hydrogen half cell, measured at 298K with solution concentrations of 1mol dm-3 and a gas pressure of 100kPa (1 atm) |
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Electrode Potentials and Fuel Cells What is the standard electrode potential of a hydrogen half cell? |
0V. |
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Electrode Potentials and Fuel Cells How do you measure a standard electrode potential? Draw a diagram of Zn2+/Zn half cells. |
A standard electrode potential is measured by connecting a standard half cell to a standard hydrogen half cell. Reading on the voltmeter = standard electrode potential. pic |
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Electrode Potentials and Fuel Cells Define standard cell potential of a cell |
The emf between the two half cells making up the cell under standard conditions |
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Electrode Potentials and Fuel Cells What is the cell reaction? |
The overall chemical reaction taking place in the cell |
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Electrode Potentials and Fuel Cells What else can standard electrode potentials be used for? |
Predicting the feasibility of a redox reaction. |
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Electrode Potentials and Fuel Cells pic |
pic |
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Electrode Potentials and Fuel Cells When comparing two redox equilibria, it is conventional to... |
show electrons on the left hand side |
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Electrode Potentials and Fuel Cells What is the electrochemical series? |
A list of redox equilibria arranged with the most negative E* value at the top. Electrode Potentials and Fuel Cells |
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Electrode Potentials and Fuel Cells Describe the preferred direction of reaction with increasing E* |
Increasing E* = wants to go to the right. |
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Electrode Potentials and Fuel Cells How can concentration affect electrode potential? What problem does this cause for using electrode potentials for predictions of feasibility? |
If you increase the concentration of the ion on the left hand side, the equilibrium will oppose the change by moving to the right, electrons are removed from equilibrium so the electrode potential becomes more positive. thus, a change in electrode potential resulting from conc changes = predictions made on the standard value may not be valid. |
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Electrode Potentials and Fuel Cells What else could mean that a reaction might not take place? |
- Reaction rate may be slow because of high Ea - Conditions for the reaction may be different from standard conditions, changing the electrode potential value - Standard electrode potentials apply to aqueous equilibria and many reactions take place under very different conditions |
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Electrode Potentials and Fuel Cells What are the rules for electrode potentials? |
- The larger the difference between E* values, the more likely it is that a reaction with occur - If the difference between E* values is less than 0.4 V, then a reaction is unlikely to take place |
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Electrode Potentials and Fuel Cells a) draw the two equilibria half equations that occur in a Zinc and copper cell ( Zn = E* = -0.76 ) (Cu = E* = 0.34) b) add the half equations to give the overall cell reaction equation c) Calculate the emf. |
<------------------------------------ Zn 2+ (aq) + 2e- <--> Zn (s) NEGATIVE TERMINAL Cu 2+ (aq) + 2e- <--> Cu (s) POSITIVE TERMINAL ------------------------------------> E*cell = E* positive terminal - E* negative terminal 0.34 - (-0.76) = 1.10V |
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Electrode Potentials and Fuel Cells What three types of cells are there? Describe |
- Non rechargeable cells - provide electrical energy until the chemicals have reacted to such an extent that the voltage falls. The cell is then 'flat' and is discarded. - Rechargeable cells - The chemicals in the cell react, providing electrical energy. The cell reaction can be reversed during recharging - the chemicals in the cell are regenerated and the cell can be used again. Common examples include: - Nickel and cadmium batteries, used in rechargeable batteries - Lithium-ion and lithium polymer batteries, used in laptops - Fuel cells- the cell reaction uses external supplies of a fuel and an oxidant, which are consumed and needs to be provided continuously. The cell with continue to provide electrical energy so long as there is a supply of fuel and oxidant. |
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Electrode Potentials and Fuel Cells Describe a hydrogen-oxygen fuel cell |
A fuel cell uses energy from the reaction of a fuel with oxygen to create a voltage. -The reactants flow in and products flow out while the electrolyte remains in the cell - Fuel cells can operate virtually continuously so long as the fuel and oxygen continue to flow into the cell. Fuel cells do not have to be recharged. |
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Electrode Potentials and Fuel Cells Draw a hydrogen- oxygen fuel cell |
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Electrode Potentials and Fuel Cells Draw the redox equilibria for the hydrogen oxygen fuel cell, overall and calculate E*cell. H20 = -0.83 O2 = 0.40 |
pic |
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Electrode Potentials and Fuel Cells What are FCVs? |
Fuel cell vehicles - they use hydrogen gas or hydrogen rich fuels (methanol, natural gas and petrol) - these are mixed with water and converted into hydrogen gas by an on board 'reformer', which operates at 250-300 degrees to generate the H2 gas. |
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Electrode Potentials and Fuel Cells Write the equation for the mixture of hydrogen rich fuel methanol with water. |
CH3OH + H2O --> 3H2 + CO2 the hydrogen can then be fed into fuel cells |
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Electrode Potentials and Fuel Cells How else have fuel cells been developed with methanol? |
Methanol is used as the fuel rather than hydrogen. |
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Electrode Potentials and Fuel Cells Advantages of a methanol fuel cell compared with use of hydrogen gas? |
- liquid fuel is easier to store than H2 (g) - methanol can be generated from biomass |
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Electrode Potentials and Fuel Cells What are the advantages of fuel cell vehicles? |
1) Less pollution and less CO2 - combustion of HC fuels produces CO2 which contributes to the greenhouse effect. Incomplete combustion = CO which must be removed by catalytic converters. Hydrogen fuel cells produce only small amounts of CO2 and pollutants. 2) Greater efficiency - a petrol engine is less than 20% efficient in converting chemical energy by the combustion of petrol. Much of the chemical energy is wasted as heat. Hydrogen fuel cell vehicles are 40-60% efficient in converting the fuels energy. This means that fuel consumption drops by more than half compared with a petrol or diesel car. |
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Electrode Potentials and Fuel Cells What are the problems with storing H2 (g) ? |
It is a gas with a very low boiling point. |
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Electrode Potentials and Fuel Cells What are the three ways hydrogen can be stored? |
1) as a liquid under pressure - even under pressure a very low temp is required and liquid hydrogen would need to be stored in a giant thermos flask to prevent it from boiling 2) adsorbed onto the surface of a solid material 3) absorbed with some solid materials |
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Electrode Potentials and Fuel Cells What are 5 limitations of hydrogen fuel cells? |
1) Large-scale storage and transportation is difficult. 2) Storing a pressurised liquid is hard 3) Adsorbers or absorbers have a limited lifetime 4) Current fuel cells have a limited life time and require regular replacement and disposal = costly 5) fuel cells use toxic chemicals in their production |
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Electrode Potentials and Fuel Cells What needs to be resolved before the use of hydrogen as a fuel? |
- accepted politically + by general public - Sort out problems with handling and maintenance of hydrogen systems - Since hydrogen is an energy carrier rather than an energy source it must first be manufactured by electrolysis of water or by reacting methane with steam. Problem? = more energy involved in making hydrogen than is saved by its use. solution? use renewable forms of energy to produce the H2? solar? wind? ? |
