5.2 Energy

Question 1

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)

Answer 1

-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- )

Question 2

Define the term lattice enthalpy
(2 marks)

Answer 2

-the enthalpy change that accompanies the FORMATION of ONE MOLE of an ionic compound from its GASEOUS IONS under standard conditions

Question 3

The first ionisation energy of sodium is more endothermic than that of potassium
Explain why
(2 marks)

Answer 3

-sodium has a smaller atomic radius OR it has fewer shells

-therefore nuclear attraction increases

Question 4

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)

Answer 4

-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

Question 5

Lattice enthalpy is used as a measure of…

Answer 5

-strength of ionic bonding in a giant ionic lattice
-more exothermic lattice enthalpy values mean stronger ionic bonds (stronger electrostatic attraction)

Question 6

What is the definition of first ionisation energy?

Answer 6

-the energy change that accompanies the removal of 1 mole of electrons from one mole of gaseous atoms

-this is an endothermic process

Question 7

What is the definition of standard enthalpy change of formation?

Answer 7

-the enthalpy change that accompanies the formation of one mole of a compound from its elements

Question 8

What is standard enthalpy change of atomisation?

Answer 8

-one mole of gaseous atoms is formed from its elements in its standard state
-it is always endothermic because bonds have to be broken

Question 9

What is first and second electron affinity?

Answer 9

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.

Question 10

What do arrows pointing up represent in a Born-Haber cycle?

Answer 10

-to represent endothermic changes

Question 11

What do arrows pointing down represent in a Born-Haber cycle?

Answer 11

-exothermic changes

Question 12

What do we use to work out unknown enthalpy changes using Born-Haber cycles?

Answer 12

Question 13

Explain what is meant by the term enthalpy change of hydration

Answer 13

Question 14

Explain what is meant by the term enthalpy change of solution?
(1mark)

Answer 14

Enthalpy change for 1 mole of a solute dissolving in water

Question 15

What is the equation we use when calculating enthalpies changes of solution and hydration ?

Answer 15

Lattice enthalpy + enthalpy of solution= enthalpy of hydration

Question 16

What is entropy?

Answer 16

-S
-a measure of the dispersal of energy in a system it is greater the more disordered a system
-

Question 17

What is standard entropy?

Answer 17

-the entropy content of one mole of the substance under standard conditions

Question 18

What is standard entropy change of reaction?

Answer 18

-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

Question 19

What is the equation for standard entropy change ?

Answer 19

Question 20

What is an oxidising agent?

Answer 20

-the species that is reduced in a reaction and causes another species to be oxidised

Question 21

What is a reducing agent?

Answer 21

-the species that is oxidised in a reaction and causes another species to be reduced

Question 22

How do you use half equations to construct full redox equations?

Answer 22

-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

Question 23

What are the steps to using oxidation numbers to construct redox equations?

Answer 23

-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

Question 24

Describe the redox titrations between Fe2+ and MnO4-

Answer 24

-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 the 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+

Question 25

Why are titrations between Fe2+ and MnO4- carried out in the presence of sulfuric acid not hydrochloric acid?

Answer 25

-as hydrochloric acid reacts with MnO4-

Question 26

Describe redox titrations between I2 and S2O3^2-

Answer 26

-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

Question 27

Define the term standard electrode potential difference

Answer 27

-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

Question 28

What does a half cell made of metals and the metals aqueous ions consist of?

Answer 28

-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)

Question 29

How is a hydrogen half cell set up?

Answer 29

Question 30

What does an half cell made of ions of the same element in different oxidation states consist of?

Answer 30

-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

Question 31

How can the standard electrode potential of a half cell be measured?

Answer 31

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.

Question 32

How are cells made from half cells?

Answer 32

-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

Question 33

What is the equation to calculate the standard cell potential?

Answer 33

Question 34

What does a positive standard electrode potential mean?

Answer 34

-positive potentials mean the substances are more easily reduced and will gain electrons

Question 35

What does a negative standard cell potential mean?

Answer 35

-negative potentials mean the substances are more easily oxidised and will lose electrons and become more stable

Question 36

How can standard electrode potentials be used to predict the feasibility of a reaction?

Answer 36

-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)

Question 37

What are the limitations to using standard cell potentials to predict the feasibility of a reaction?

Answer 37

-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

Question 38

What are the benefits and risks of electrochemical cells?

Answer 38

-Lithium-based cells are rechargeable -however there are risks of toxicity and a rapid discharge of current which can lead to fires

Question 39

How do fuel cells work?

Answer 39

-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

Question 40

What are the advantages and disadvantages of using fuels cells for energy rather than fossil fuels?

Answer 40

Question 42

List solids, liquid and gases in the order of greatest entropy

Answer 42

Entropy of solids

Question 43

How does the number of gas molecules affect the entropy?

Answer 43

-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

Question 45

What is the equation for Gibbs free energy?

Answer 45

-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.

Question 46

For a reaction to occur spontaneously ΔG must be ________

Answer 46

Negative

Question 47

How can you find the minimum temperature at which a reaction is spontaneous?

Answer 47

Question 48

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

Answer 48

Decreases Larger Negative

Question 49

What are the limitations of predictions made by ΔG about feasibility?

Answer 49

-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

Question 50

What is the symbol for greater than?

Answer 50

> the crocodile eats the thing that is greater ,”she face the bigger one and gobble it down” 5>1

Question 51

What is the symbol for less than?

Answer 51

< 2<5 Crocodiles turn their back on what is less

Question 52

(4 marks)

Answer 52

Question 53

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)

Answer 53

Question 54

Draw out the Born-Haber cycle for NaCl(s) Label the two routes

Answer 54

Question 55

What two factors affect the exothermic value of a lattice enthalpy and enthalpy change of hydration?

Answer 55

-ionic charge -IONIC radius

Question 57

What are the states of fluorine, chlorine, bromine and iodine at standard conditions?

Answer 57

Fluorine-gas Chlorine-gas Bromine-liquid Iodine-solid

Question 58

Answer 58

A Condensation= gas-> liquid Entropy- decreases More bonds are made- exotérmica

Question 59

Draw a table that shows the effect of enthalpy and entropy changes on ΔG

Answer 59

Question 60

Answer 60