Chemistry Key Definitions And Things To Remember

Question 1

Define The Standard Enthalpy Of Formation.

Answer 1

The enthalpy change when one mole of a compound is formed from its elements under standard conditions, all reactants and products in their standard states.

Question 2

Define The Standard Enthalpy Of Combustion.

Answer 2

The enthalpy change when one mole of a compound is completely burned in oxygen under standard conditions, all reactants and products in their standard states.

Question 3

Define The Standard Enthalpy Of Atomisation.

Answer 3

The standard enthalpy change when one mole of gaseous atoms is formed from an element in its standard state.

Question 4

Define Mean Bond Enthalpy (Aka bond dissociation enthalpy).

Answer 4

The enthalpy change when one mole of gaseous molecules each break a covalent bond to form to free radicals (atoms), averaged over a range of compounds.

Question 5

Define 1st Ionisation Enthalpy.

Answer 5

The standard enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to give one mole of gaseous ions with a single positive charge.

Question 6

Define 2nd Ionisation Enthalpy.

Answer 6

The standard enthalpy change when one mole of electrons is removed from one mole of gaseous 1+ ions to give one mole of gaseous ions each with a 2+ charge.

Question 7

Define 1st Electron Affinity.

Answer 7

The standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions each with a single negative charge under standard conditions.

Question 8

Define 2nd Electron Affinity.

Answer 8

The standard enthalpy change when one mole of electrons is added to a mole of gaseous ions each with a single negative charge, to form a mole of ions each with a two negative charge.

Question 9

Define Lattice Formation Enthalpy.

Answer 9

The standard enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions.

Question 10

Define Lattice Dissociation Enthalpy.

Answer 10

The standard enthalpy change when one mole of a solid ionic compound dissociates into its gaseous ions.

Question 11

Define Standard Enthalpy Of Solution.

Answer 11

The standard enthalpy change when one mole of a solute dissolves in enough solvent to form a solution in which the ions are far enough apart to not interact with each other.

Question 12

Define The Standard Enthalpy Of Hydration.

Answer 12

The standard enthalpy change when one mole of gaseous ions is converted to one mole of aqueous ions.

Question 13

Define Hess’s Law.

Answer 13

The enthalpy change for a chemical reaction is the same, regardless of the route taken from reactants to products.

Question 14

State the meaning of the term periodicity.

Answer 14

Repeating patterns/trends of physical or chemical properties across a period.

Question 15

If something can act as either an acid or base, it is:

Answer 15

Amphoteric

Question 16

Give the percentage uncertainty equation.

Answer 16

% Uncertainty = (uncertainty/value measured) x100

Question 17

What is the half neutralisation point of a pH curve?

Answer 17

The point when enough base has been added to neutralise exactly half of the acid (or vice versa). On a pH curve: half way between 0 and equivalence point on x-axis. [HA]=[A-] so equal moles of salt and acid so Ka=[H+] therefore pKa=pH.

Question 18

What are the bond angles and shape for a molecule with 2 bonding pairs?

Answer 18

Linear, 180°

Question 19

What are the bond angles and shape of a molecule with 3 bonding pairs?

Answer 19

Trigonal Planar, 120°

Question 20

What are the bond angles and shape of a molecule with 4 bonding pairs?

Answer 20

Tetrahedral, 109.5°

Question 21

What are the bond angles and shape of a molecule with 5 bonding pairs?

Answer 21

Trigonal Bipyramidal, 120° and 90°

Question 22

What are the bond angles and shape of a molecule with 6 bonding pairs?

Answer 22

Octahedral, 90°

Question 23

What are the bond angles and shape of a molecule with 2 bonding pairs and 1 lone pair?

Answer 23

V-Shaped, 117.5° (120-2.5 as one lone pair)

Question 24

What are the bond angles and shape of a molecule with 3 bonding pairs and 1 lone pair?

Answer 24

Trigonal Pyramidal, 107° (109.5-2.5 as one lone pair)

Question 25

What are the bond angles and shape of a molecule with 2 bonding pairs and 2 lone pairs?

Answer 25

V-Shaped, 104.5° (109.5-2(2.5) as 2.5 for each lone pair)

Question 26

What are the bond angles and shape of a molecule with 4 bonding pairs and 1 lone pair.

Answer 26

Seesaw Molecule, 119°, 89° (More than 5 e- pairs so different rules, angles from original structure have 1° subtracted from them)

Question 27

What are the bond angles and shape of a molecule with 3 bonding pairs and 2 lone pairs? (Two answers)

Answer 27

T-Shaped, 89° (90-1, despite having two lone pairs. You just need to know this one, don't try and figure it out with logic) Or Trigonal Planar, 120°, lone pairs are in an axial position (on opposite sides) so their stronger repulsion cancels each other out leaving, a trigonal planar shape as if they didn't exist in the first place (still draw them though)

Question 28

What are the bond angles and shape of a molecule with 5 bonding pairs and 1 lone pair?

Answer 28

Square Pyramidal, 89° (more than 5 pairs so different rules. 90-1 as original structure is octahedral but one lone pair instead of a bonding pair)

Question 29

What are the bond angles and shape of a molecule with 4 bonding pairs and 2 lone pairs?

Answer 29

Square Planar, 90°, Lone e- pairs in axial position (Opposite each other) so their stronger repulsions cancel out leaving essentially an octahedral, 90°, but without the top and bottom bonding pairs.

Question 30

Give an equation to represent the ionisation of a molecule 'X' in Electrospray Ionisation.

Answer 30

X(g) + H+ ---> XH+(g) (Remember, when looking at the mass spectrum, the Mr will be the Mr of 'X' +1 as it had gained a proton, so its actual relative molecular mass one subtracted from the value in the spectrum)

Question 31

Give an equation to represent the ionisation of a molecule 'X' in Electron Impact Ionisation.

Answer 31

X(g) ---> X+(g) + e- | Electrons collide with electrons in molecule, ionising them

Question 32

Heterogeneous Catalyst Example, Haber Process. Name catalyst and show reversible equation (making ammonia)

Answer 32

Catalysed by Solid Fe | 3H2+N2 (reversible arrow) 2NH3

Question 33

Heterogeneous Catalyst Example Contact Process Name catalyst and show reaction equation (making sulfuric acid) and catalyst reformation. Give a suggestion for the use of the product

Answer 33

Vanadium (V) Oxide SO2(g) + V2O5(s) ---> SO3(g) + V2O4(s) V2O4(s) + 1/2O2(g) ---> V2O5(s) Used to make sulfuric acid (group 3) SO3 + H2O ---> H2SO4

Question 34

Heterogeneous Catalyst Example | Manufacturing of methanol

Answer 34

Solid Cr2O3 Catalyst CH4(g) + H2O(g) --> CO(g) + 3H2(g) CO(g) + 2H2(g) --> CH3OH(g)

Question 35

How does a solid Heterogeneous Catalyst work for gas/liquid reactants

Answer 35

- Reaction occurs on the active sight of the surface of the catalyst - Reactants adsorb into surface - Bonds weaken - Reaction occurs - Products desorb from the surface

Question 36

How to make a solid Heterogeneous Catalyst more efficient

Answer 36

Increase its surface area and spread it over an inert support medium

Question 37

How do solid Heterogeneous Catalysts get poisoned? And how is it prevented?

Answer 37

-Impurities block the active sights (Stronger adsorb and don't desorb) -Purifying the reactants prevents poisoning

Question 38

For Homogeneous Catalyst Reaction, Peroxudisulfate ions to oxidise iodide ions to iodine, give the half equations uncatalysed and explain why this his high activation energy

Answer 38

``` S2O8^2- + 2e- --> 2SO4^2- 2I- ---> I2 + 2e- So overall S2O8^2- + 2I- ---> 2SO4^2- + I2 Which has two -ve ions reacting and therefore they repel, so high Ea ```

Question 39

For Homogeneous Catalyst Reaction, Peroxudisulfate ions to oxidise iodide ions to iodine, give the half equations catalysed and name the catalyst. Explain why this has a much lower Ea

Answer 39

``` Catalysed by Fe2+ ions in 2 step process (Or Fe3+ and swap steps 1 and 2) Step 1: 2Fe2+ ---> 2Fe3+ + 2e- S2O8^2- + 2e- ---> 2SO4^2- Step 2 (Catalyst Re-Formed) 2I- ---> I2 + 2e- 2e- + 2Fe3+ ---> 2Fe2+ Has +ve and -ve ions reacting so they attract so low Ea ```

Question 40

Give the half equations for the reaction you need to know for Autocatalysis when uncatalysed and explain the low Ea (Oxidation of ethanedioc acid by manganate (VII) ions)

Answer 40

5e- + 8H+ MnO4- ---> Mn2+ + 4H2O C2O4^2- ---> 2CO2 + 2e- High Ea as two -ve ions repel

Question 41

Give the half equations for the reaction you need to know for Autocatalysis when catalysed by its own product, and mention what the product is and why it decreases Ea

Answer 41

``` Catalyst: Mn2+ Step 1: 5e- + 8H+ + MnO4- ---> Mn2+ + H2O Mn2+ ---> Mn3+ + e- Step 2: C2O4^2- ---> 2CO2 + 2e- 2e- + 2Mn3+ ---> 2Mn2+ ``` -ve ions and +ve ions attract so low Ea

Question 42

Explain concentration against time graph for Autocatalysis reaction

Answer 42

- Slow rate at start - As no catalyst initially - So high Ea as two -ve ions react - More Mn2+ formed so faster rate - Due to increasingly catalyst reaction - Rate decreases again and levels off - As reactants get used up

Question 43

Give the colour change for MnO4- ---> Mn2+

Answer 43

Purple to colourless

Question 44

To form Vanadium (II), (III) and (IV) ions from Vanadium (V), what is required, and what is the colour of Vanadium (V)

Answer 44

Zinc (Zn) to be oxidised to Zn2+ | Colour: Yellow

Question 45

Colour of Vanadium ions, from (V) to (II)

Answer 45

``` Ya. Yellow. (V) Bastard. Blue. (IV) Got Green. (III) Punched. Purple. (II) Going down (like the bastard) form V ```

Question 46

What happens to Cobalt (II) hexa-aqua ions in Alkaline conditions? E.g H2O2, NH3, or O2

Answer 46

Oxidised to Cobalt (III)

Question 47

Half equations for Cobalt (II) Hexa-aqua ions (acts as acid) reacting with H2O2 (base so dissociation)

Answer 47

H2O2 + 2e- ---> 2OH- | [Co(OH)6]4- ---> [Co(OH)6]3- + e-

Question 48

Equations for the reaction of Cobalt (II) hexa- aqua ions with NaOH (like the same reaction with Cu instead) and give the colour change

Answer 48

[Co(H2O)6]2+(aq) + 2OH- ---> [Co(H2O)4(OH)2](s) + 2H2O | Pink solution to blue ppt

Question 49

Reaction of [Co(H2O)4(OH)2](s) with NH3 and state colour change

Answer 49

Reaction of [Co(H2O)4(OH)2](s) + 6NH3 ---> [Co(NH3)6]2+ (aq) + 4H2O + 2OH- Blue precipitate to pale brown solution

Question 50

Give the half equations for the reaction of [Co(NH3)6]2+ (aq) with O2 and H2O

Answer 50

O2 + 2H2O + 4e- ---> 4OH- | [Co(NH3)6]2+ (aq) ---> [Co(NH3)6]3+ (aq) + e-

Question 51

Give the colour change for Fe2+ to Fe3+

Answer 51

Green to brown

Question 52

Give the colour change for Cr2O7 2- to Cr3+

Answer 52

Orange to green

Question 53

How are Transition Metal Ions Coloured?

Answer 53

- Transition metals are coloured due to partially filled d-sub shell - In pure transition metal, d-orbitals are of equal energy AKA 'ground state' - In transition metal compounds, due to other atoms present, d-orbitals have slightly different energies - So e- can be excited from one d-orbital to another - When one e- moves from lower energy d-orbital to a higher one, energy is needed to make the transition. - This energy is taken from white light and a specific wavelength of light is absorbed which is missing from the light that gets reflected/transmitted to give the compound it's colour.

Question 54

What are the steps of using Colorimetry for finding the concentration of a solution

Answer 54

1. Multiple samples if known different concs of solution, with appropriate ligand added to intensify the colour 2. Test each known sample for transmittance/absorbance 3. Plot Conc Vs transmittance graphs 4. Test solution of unknown concentration for transmittance 5. Use graph to find corresponding concentration (Transmittance -> 1.0 = All light passed through, 0.5=50% absorbed, 0=All light absorbed)

Question 55

What happens when chlorine is added to water, what type of reaction is it and why is the product useful?

Answer 55

Cl2 + H2O HClO + HCl Disproportionation (Oxidised and Reduced) HClO kills bacteria

Question 56

How is bleach made? (NaClO)

Answer 56

Cl2 + 2NaOH ---> NaClO + NaCl + H2O

Question 57

What happens when Chlorine reacts with water in sunlight?

Answer 57

2Cl2 + 2H2O ---> 4HCl + O2

Question 58

What does CRAM stand for and how do we use it to distinguish which has the more exothermic lattice formation enthalpy, MgO or Na2O

Answer 58

Charge, Radius, Attraction, More exothermic or endothermic Mg2+ has greater charge than Na+, and a smaller atomic radius so a greater attraction to O2- so MgO has a more exothermic lattice formation enthalpy

Question 59

When conditions change for an electrode, what may happen? (e.g altered temp, Conc, pressure)

Answer 59

Equilibrium position will shift More or less e- produced Larger or smaller E° Affects calc

Question 60

What is a salt bridge and what is its purpose?

Answer 60

Salt bridge is a solution and connects both beakers. It's ions are free to move and balance the charges in either electrode. It must be UNREACTIVE with half cells. (KNO3 usually used)

Question 61

If two metal ions have SAME state, how are they indicated in cell representation?

Answer 61

With a comma

Question 62

When given a table of electrode potentials what is the main rule?

Answer 62

For most positive E°, SOWR strongest oxidising agent on left Weakest reducing agent on right

Question 63

When are platinum electrodes used?

Answer 63

When no solid metal in reaction, so metal ions with two diff charges in same solution E.g FeSO4 and Fe2(SO4)3 is a solution of Fe2+ and Fe3+ It is unreactive but still conductive

Question 64

What is the half equation for a standard hydrogen electrode? And what are the conditions? Does the other electrode need the conditions?

Answer 64

2H+ + 2e- (reversible arrow) H2 Temp: 298k Conc: 1moldm-3 of H+ sol (e.g 1moldm-3 of HCl or 0.5moldm-3 of H2SO4) Pressure: 100KPa Connected electrode must also have these same conditions

Question 65

How can you test for Ammonium Ions

Answer 65

Add NaOH NH4+ + -OH ---> NH3(g) + H2O Red litmus paper turns blue

Question 66

What is an example of a reachable battery?

Answer 66

Lithium Ion battery

Question 67

What occurs at the negative electrode of a lithium Ion battery?

Answer 67

Graphite (carbon) with a layer of Li(s) on it dissociates | Li(s) --> Li+ + e-

Question 68

What happens on the positive electrode of a lithium ion battery

Answer 68

Li+ ions and CoO2 from electrolyte (both have same state) form lithium cobalt oxide Li(CoO2)(s) on a platinum electrode CoO2 + Li+ + e- ---> Li(CoO2)

Question 69

For an alkaline membrane hydrogen oxygen fuel cell, give the half equation for the anode (negative electrode) and the cathode (positive electrode).

Answer 69

H2(g) + 2OH-(aq) reversible arrow 2H2O(l) + 2e- | 0.5O2(g) + H2(g) + 2e- reversible arrow 2OH- (aq)

Question 70

For an acidic membrane hydrogen oxygen fuel cell, give the half equation for the anode (negative electrode) and the cathode (positive electrode).

Answer 70

H2(g) Reversible Arrow 2H+(aq) + 2e- | 0.5O2(g) + H2(g) + 2e- reversible arrow H2O(l)