3. Mocks revision

Question 1

Intermolecular forces

Answer 1

attractive forces between molecules
1| Van der Waals
2| Dipole-dipole
3| Hydrogen bonds

Question 2

Van der Waals, what does its strength depend on

Answer 2

the result of random movement of electrons leading to the formation of temporary dipole and therefore induced dipoles in molecules – strength depends on molecular mass, bond is effective only over a short range

Question 3

Dipole-dipole

Answer 3

the result of electrostatic attraction between molecules with permanent dipoles (polar molecules)– significantly stronger than Van der Waals – ion-dipole forces are attractive forces between an ion and a polar molecule – e.g. London dispersion forces

Question 4

Hydrogen bonds

Answer 4

occur in molecules that contain H bonded to a highly electronegative atom, the strongest of all intermolecular forces

Question 5

List and draw resonance hybrids

Answer 5

1| Carbonate ion
2| Nitrate ion
3| Sulfur dioxide
4| Ozone
5| Benzene
6| Ethanoate ion

Question 6

Delocalization of electron

Answer 6

occurs whenever alternate double and single carbon atoms are present – two or more Lewis structures are used to represent the structure – a formal charge

Question 7

Sigma and pi bonds difference

Answer 7

sigma bonds are formed when two atomic orbitals (ss, sp, or pp) of different atoms overlap directly and pi bonds are formed when two p orbitals of different atoms overlap sideways

Question 8

How do free radicals break down the ozone layer?

Answer 8

stratospheric ozone is in dynamic equilibrium with oxygen (continually being formed and decomposed) – CFCs/nitrogen bonds get broken by UV light and the chlorine free radicals break down ozone faster than it should be broken down: Cl * (g) + O3 (g) → ClO* (g) + O2 (g) and ClO* (g) + O* (g) → Cl * (g) + O2 (g)

Question 9

Sp3 hybridization

Answer 9

there are four electrons (two in s and 2 in p subshell), one electron gets promoted to the p subshell so that all orbitals have one electron and then the two subshells equalize to be on the same E level – this is called the hybridized state – same thing for sp2 (three e-) and sp (2 e-) hybridization

Question 10

Enthalpy change (ΔH)

Answer 10

heat transferred from a closed system to the surroundings during a chemical reaction, it is negative in exothermic and positive in endothermic reactions

Question 11

What formula is used calculate heat

Answer 11

Q=mcΔT

Question 12

Bond enthalpy, how is it calculated, what is the limitation of using them?

Answer 12

E change when one mol of a covalent bond in the gaseous state is formed from its gaseous atoms – bond enthalpy = reactants-products – all reactants and products have to be in a gaseous state and average bond enthalpies are used as theoretical

Question 13

Hess’ law

Answer 13

the heat of a whole reaction is equivalent to the sum of its steps (adding equations)

Question 14

What is a byproduct of incomplete combustion of organic compounds, and how is the choice of fuel used determined?

Answer 14

CO and soot, by calculating its specific energy using the formula: E released from the fuel/mass of fuel consumed

Question 15

Compare advantages and disadvantages of different fossil fuels

Answer 15

coal: large quantities available, safe, long lifespan
causes global warming, acid rain, coal waste
crude oil: easily transported and convenient
causes global warming, acid rain, pollution
natural gas: easily transported, clean, yields most E
causes global warming, risk of explosions due to leaks

Question 16

Biofuels
biological carbon fixation
hydrogen fuel cell

Answer 16

renewable source, it is direct combustion of plant material
the production of organic compounds from carbon dioxide (photosynthesis)
converts chemical E directly to electrical E, doesn’t need an external source of energy for charging

Question 17

Standard enthalpy change, how is it calculated

Answer 17

the enthalpy change when molar quantities of reactants in their normal states react to form products in their normal state under standard conditions of temperature and pressure (25°C and 1 atm), it is zero for free elements
Δ Hrxn = ΣnΔHof, products - ΣnΔHof, reactants, where n is the number of moles

Question 18

Standard enthalpy change of formation, example

Answer 18

the enthalpy change when one mole of a compound is formed from its elements in their standard states
Na (s) + ½ Cl (g) -> NaCl (s)

Question 19

Standard enthalpy change of atomization, example

Answer 19

the enthalpy change when one mole of atoms in the gaseous state is formed from the element in its standard state under standard conditions
½ Cl (g) -> Cl (g) / Na (s) -> Na (g)

Question 20

First electron affinity, example

Answer 20

the enthalpy change when one mole of gaseous atoms each gain one electron
Cl (g) -> Cl- (g)