5.2 Energy Flashcards
Lattice enthalpies give an indication of the strength of ionic bonding
How would the lattice enthalpies of magnesium chloride and calcium chloride differ?
Explain your answer
(3 marks)
-lattice enthalpy of MgCl2 is more exothermic than CaCl2
-because magnesium ion (Mg2+ ) is smaller than calcium ions (Ca2+) or Mg2+ has a greater charge density
-therefore the attraction between Mg2+ and Cl- is greater (than between Ca2+ and Cl- )
Define the term lattice enthalpy
(2 marks)
-the enthalpy change that accompanies the FORMATION of ONE MOLE of an ionic lattice from its GASEOUS IONS under standard conditions
The first ionisation energy of sodium is more endothermic than that of potassium
Explain why
(2 marks)
-sodium has a smaller atomic radius OR it has fewer shells
-therefore nuclear attraction increases
The enthalpy changes of solution of the magnesium halides show a trend from MgF2 to MgI2
Explain why it is difficult to predict whether the enthalpy change of solution becomes more exothermic or less exothermic down the group from MgF2 to MgI2.
(4 marks)
-halide ion gets larger down the group
-lattice enthalpy is less exothermic down the group/ halide ion has less attraction for Mg2+
-hydration enthalpy is less exothermic down group/ halide ion has less attraction for H2O
-difficult to predict whether lattice enthalpy or hydration enthalpy has bigger effect
Lattice enthalpy is used as a measure of…
-strength of ionic bonding in a giant ionic lattice
-more exothermic lattice enthalpy values mean stronger ionic bonds (stronger electrostatic attraction)
What is the definition of first ionisation energy?
-the energy change that accompanies the removal of 1 mole of electrons from one mole of gaseous atoms
-this is an endothermic process
What is the definition of standard enthalpy change of formation?
-the enthalpy change that accompanies the formation of one mole of a compound from its elements
What is standard enthalpy change of atomisation?
-one mole of gaseous atoms is formed from its elements in its standard state
-it is always endothermic because bonds have to be broken
What is first and second electron affinity?
First electron affinity- one mole of of gaseous 1- ions are formed from gaseous atoms. This is an exothermic process because the electron is attracted into the outer shell of an atom by the nucleus.
Second electron affinity-one mole of gaseous 2- ions are formed from gaseous 1- ions. This is an endothermic process because the electron is repelled by the 1- ion.
What do arrows pointing up represent in a Born-Haber cycle?
-to represent endothermic changes
What do arrows pointing down represent in a Born-Haber cycle?
-exothermic changes
What do we use to work out unknown enthalpy changes using Born-Haber cycles?
What is the definition of enthalpy change of solution?
-the enthalpy change that takes place when ONE MOLE OF SOLUTE is COMPLETELY DISSOLVED in water
-under standard conditions
What is the definition of enthalpy change of hydration?
-the enthalpy change that takes place when DISSOLVING ONE MOLE of GASEOUS IONS in WATER
What is the equation we use when calculating enthalpies changes of solution and hydration ?
Lattice enthalpy + enthalpy of solution= enthalpy of hydration
What is entropy?
-S
-a measure of the dispersal of energy in a system it is greater the more disordered a system
-
What is standard entropy?
-the entropy content of one mole of the substance under standard conditions
What is standard entropy change of reaction?
-the entropy change that accompanies a reaction in molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states
What is the equation for standard entropy change ?
What is an oxidising agent?
-the species that is reduced in a reaction and causes another species to be oxidised
What is a reducing agent?
-the species that is oxidised in a reaction and causes another species to be reduced
How do you use half equations to construct full redox equations?
-identify the oxidation and reduction equations half equations
-balance the electrons (by multiplying the entirety of the half equations)
-add the two half equations together and cancel the electrons as well as any ions with the same charges that appear on both sides and any water molecules that appear on both sides
What are the steps to using oxidation numbers to construct redox equations?
-construct an unbalanced equation using the information given
-identify oxidation number changes
-balance the oxidation number changes so that total increase in oxidation number = total decrease in oxidation number
-check to see if anything else needs to be balanced (add H2O when equations needs extra O or H, add H+ when reaction is carried out in acidic conditions, add OH- when reaction is carried out in alkaline conditions
Describe the redox titrations between Fe2+ and MnO4-
-Maganate (VII) MnO4- is a common oxidising agent usually obtained from potassium permanganate (VII) KMnO4. It has a deep purple colour but becomes colourless when it is reduced from +7 to +2 oxidation states (this usually occurs in the presence of H+ ions- so acidic solutions)
-in the reaction between the iron ions and maganate ions te iron ions are oxidised while the maganate ions are reduced
-the iron solution is measured into a conical flask using a pipettes and pipette filler
-the END POINT is when the solution in the conical flask has a permanent pale pink colour. As all the Fe2+ ions have reacted and the MnO4- can no longer be reduced to the colourless Mn2+
Why are titrations between Fe2+ and MnO4- carried out in the presence of sulfuric acid not hydrochloric acid?
-as hydrochloric acid reacts with MnO4-
Describe redox titrations between I2 and S2O3^2-
-the reaction of iodine to iodide ions occurs in the presence of thiosulfate ions S2O3^2-
-we can use aqueous iodine ions and aqueous thiosulfate to determine the concentration of an unknown reducible species. Often this involves an initial reaction between the unknown oxidising agent and iodide ions, which has iodine as a product
-the iodine then goes on to react with thiosulfate ions, from a solution with a known concentration being added from a burette
-in the presence of starch a blueblack colour will remain present as long as their is iodine.Once this has all reacted with the thiosulfate ions this will disappear marking the end point of the reaction
Define the term standard electrode potential difference
-Emf of a cell/ voltage/ potential difference
-comprising half cell combined with standard hydrogen electrode
-conc =1moldm^-3 , pressure (of H2)= 1 atm/ 100kPa, temp= 298K
What does a half cell made of metals and the metals aqueous ions consist of?
-a half cell comprises of an element in two oxidation states
-a metal half cell will consist of a metal (e.g. a strip of that solid metal) placed in an aqueous solution of its ions
-the forward reaction involves the gain of electrons (reduction)
-the reverse reaction involves the loss of electrons (oxidation)
How is a hydrogen half cell set up?
What does an half cell made of ions of the same element in different oxidation states consist of?
-consists of ions of the same element in different oxidation states e.g. Fe2+ (aq) and Fe3+ (aq) ions
-these ions need to be in contact with a platinum electrode as their is no solid piece of metal that could act as an electrode- this inert platinum electrode allows electrons to pass into or out of the half cell via a connecting wire
How can the standard electrode potential of a half cell be measured?
We can determine the standard electrode potential of a half cell by connecting it to a hydrogen half cell. The tendency for
different half cells to accept or release electrons is measured as an electromotive force (e.m.f.), or voltage, measured in volts, V.
The hydrogen half cell has an e.m.f. value of 0 V, so it can be used as a reference to measure other half cells against.
How are cells made from half cells?
-by connecting together two half cells with different electrode potentials- one half cell releases electrons, the other half cell gains electrons
-the two half cells are joined using a wire-this allows the electrons carrying charge to flow through it and a salt bridge- this connects the two solutions and allows ions carrying charge to be transferred between the half cells they are usually made from a piece of filter paper soaked in an aqueous solution of an ionic substance
What is the equation to calculate the standard cell potential?
What does a positive standard electrode potential mean?
-positive potentials mean the substances are more easily reduced and will gain electrons
What does a negative standard cell potential mean?
-negative potentials mean the substances are more easily oxidised and will lose electrons and become more stable
How can standard electrode potentials be used to predict the feasibility of a reaction?
-they can be used to predict if a stated reaction is likely to proceed under standard conditions
1-write out their half equations as reductions, use the data from the electrochemical series to predict which cell is being oxidised (the one with the most negative electrode potential)
2-take the oxidised equation and reverse it
3-balance the amount of electrons and combine the 2 equations
4-compare this equation to the reaction stated in the question to see if they match
5-confirm that it is feasible by calculating the cell potential (a feasible reaction will have a positive value)
What are the limitations to using standard cell potentials to predict the feasibility of a reaction?
-non standard conditions can cause the electrode potential to change
-if the concentration of a product/reactant is changed the equilibrium will shift to counteract this, this can increase or decrease the cell potential
-the reaction may have a high activation energy and the reactants may not reach this
What are the benefits and risks of electrochemical cells?
-Lithium-based cells are rechargeable
-however there are risks of toxicity and a rapid discharge of current which can lead to fires
How do fuel cells work?
-they use the energy from the reaction between a fuel (either hydrogen or a hydrogen rich fuel) and oxygen to create a voltage
.the reactants flow in and products flow out while the electrolyte remains in the cell
.fuels can operate continuously so long as fuel and oxygen continue to flow into the cell. Fuels cells do not need to be recharged
-A fuel cell is an electrochemical cell in which a fuel donates electrons at one electrode (anode) and oxygen gains electrons at the other electrode (cathode)
-water is the only product
What are the advantages and disadvantages of using fuels cells for energy rather than fossil fuels?
List solids, liquid and gases in the order of greatest entropy
Entropy of solids> entropy of liquids > entropy of gases
How does the number of gas molecules affect the entropy?
-if the number of gas molecules changes during a reaction this changes the entropy
-an increase in the number of gas molecules causes an increase in entropy
-a decrease in the number of gas molecules causes a decrease in entropy
What is the equation for Gibbs free energy?
-gibbs free energy is measured in kJmol^-1
-this equation takes the form y=mx + c so can be represented graphically as a straight line graph of ΔG (y-axis) against T (x-axis)
-ΔG represents the change in Gibbs free energy, ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy.
For a reaction to occur spontaneously ΔG must be ________
Negative
How can you find the minimum temperature at which a reaction is spontaneous?
Large increases in entropy will cause ________ in ΔG because the term - TΔS will become _____
Large negative values for ΔH (e.g. highly exothermic reactions) will result in more _________ values for ΔG
Decreases
Larger
Negative
What are the limitations of predictions made by ΔG about feasibility?
-although a reaction may be thermodynamically feasible, it still may not be able to occur due to kinetic factors as the kinetic factors may lead it to occur at such a slow rate, so that in reality it does not appear to occur at all
-the reaction may require a large activation energy that is not provided by the reaction conditions, therefore it will not occur spontaneously
What is the symbol for greater than?
>
the crocodile eats the thing that is greater ,”she face the bigger one and gobble it down”
5>1
What is the symbol for less than?
<
2<5
Crocodiles turn their back on what is less
(4 marks)
The enthalpy and entropy of a reaction both have a negative sign.
Discuss how the feasibility of this reaction will change as the temperature increases
(2 mark)
Draw out the Born-Haber cycle for NaCl(s)
Label the two routes
What two factors affect the exothermic value of a lattice enthalpy and enthalpy change of hydration?
-ionic charge
-IONIC radius
