SL

Question 1

Reasons why experimental enthalpy value may be different

Answer 1

• Not all of the heat is transferred to the water
• Loss of heat from the water to the surroundings
  mass and specific heat capacity of the thermometer, beaker, stirrer are required
• The combustion of ethanol is incomplete because there is limited oxygen is available (the literature value assumes that it is complete combustion) experiment not conducted under standard conditions (the literature value is - it assumes that water is a liquid and not a gas)
• the alcohol is not pure and incorrect reading of the mass of alcohol (some is lost due to evaporation)

Question 2

What influences bond enthalpy

Answer 2

bond length, number of bonds, bond polarity

Question 3

Reason for this difference when calculating the heat of combustion from bond energies

Answer 3

When we calculate reactions using bond energies the substances are in the gaseous state, with no intermolecular forces involved. The experimental values for the enthalpy of combustion involves CO2 and water in their Standard states, liquids

Bond energies are averaged over a wide variety of related compounds so they represent an approximation of the true value

Question 4

Factors that affect lattice enthalpy

Answer 4

Ionic radius - The smaller the ionic radius of the ion, the stronger the electrostatic attraction the oppositely charged ions and the higher the melting point / lattice enthalpy

lonic Charge - The greater the charge on the ion, the stronger the electrostatic attraction between oppositely charged ions and the higher the melting point and lattice enthalpy.

Question 5

Why does Incomplete Combustion of Alkanes occur?

Answer 5

Occurs if oxygen is limited or when the compound has a high percentage of carbon content (these compounds produce more soot when oxygen is limited)
This can be used to distinguish different organic compounds

Question 6

Incomplete Combustion of Alcohols Properties

Answer 6

Complete combustion produces a hot blue flame and incomplete combustion produces a yellow flame
The combustion of organic compounds that contain elements other than carbon, hydrogen and oxygen produces a range of other products

Question 7

Combustion of Metals

Answer 7

Group 1 metals have shiny surfaces. which tarnishes very quickly (reacts with oxygen) when exposed to moist air
Since Group 1 metals are very reactive, they are stored in oil
The s-block metals form ionic oxides that are basic - PH > 7
Transition metals have variable oxidation states and so form many different oxides
When copper is heated in a flame it is tarnished (surface appears black) due to the formation of copper(il) oxide:

Question 8

Combustion of Non-metals

Answer 8

Non-metals (p-block elements) form covalent oxides which are acidic
Some non-metals can show a range of oxidation states and forms different oxides when they undergo combustion
Sulfur combusts in air to produce sulfur dioxide

Question 9

The combustion of carbon (coal)

Answer 9

Produces carbon dioxide:
C(s) + O2 (g) → CO2(g)
The heat released is used to convert water into steam which drives a turbine, which in turn, spins a generator to create (the overall result is the conversion of chemical energy into electricity)

Question 10

Exception to combustion of non-metals being exothermic

Answer 10

The combustion reaction:
N2 (g) + O2 (g) → 2 NO2 (g)
* is endothermic because it only occurs at high temperatures because it is unfavourable at standard conditions (in part due to the strength of the triple bond in nitrogen)
* the release of nitrogen oxides from combustion engines into the atmosphere lead to acid rain and photochemical smog (react with sunlight to produce a brown haze)

Question 11

How were fossil fuels formed

Answer 11

Fossil fuels were formed by the reduction of buried organisms (plant and animals) under pressure that contain carbon, hydrogen, nitrogen, sulfur and oxygen. in anaerobic conditions

Question 12

What are fossil fuels used for?

Answer 12

a) to do work (gasoline combusts and the gas expand and drive a piston in a car engine)
b) generate heat (gas boiler that is used to heat water in your home)
c) generate steam to drive electricity generation (coal-fired power plant)

Question 13

How was coal formed?

Answer 13

Coal was formed by large quantities of dead plant matter that were buried and isolated from oxygen, it can form coal over million of years. the action of heat and pressure changed prehistoric forests (plant material) into peat, then lignite, then to bituminous soft coal, and finally anthracite (hard coal).

Question 14

How does the specific energy increase in the process of coal?

Answer 14

At each stage the formation, the carbon content increases
and the specific energy of the coal also increases (peat has the lowest)
Anthracite is the best form of coal because it contains more than 92% of carbon by mass and has a specific energy greater than 35000 kJ/kg

Question 15

What is crude oil?

Answer 15

Mixture of straight-chain, branched, cyclic and, aromatic hydrocarbons
Is used as a fuel for transport and electricity generation
Used to produce polymers, pharmaceuticals, dyes, and solvents

Question 16

How was crude oil produced?

Answer 16

Produced over millions of years from the remains of marine
animals and plants
These creatures died and accumulated as sediment at the bottom of the ocean before becoming trapped under layers of rock

Question 17

How is crude oil refined?

Answer 17

Sulfur impurities are removed
It is then separated into compounds (fraction) with Similar boiling points
Short-chain hydrocarbons (5 - 10 C atoms are used as motor vehicle fuels) are very valuable

Question 18

What is natural gas?

Answer 18

Primarily methane, it also has varying amounts of ethane, propane, and butane as well as hydrogen sulfide.

Question 19

How is natural gas formed?

Answer 19

Formed in the Same way as crude oil
It can be formed from the decomposition of crude oil and coal deposits

Question 20

Uses of natural gas

Answer 20

It is the cleanest of the fossil fuels because it can be easily treated to remove impurities and combustion is complete → no carbon monoxide
Where it is available it flows through pipes (little energy is required to transport the gas from the ground to the consumer)
Setting up this infrastructure is very expensive
Therefore in some countries liquified gas (butane or propane) is used instead for domestic cooking and heating

Question 21

Carbon Content and Incomplete Combustion of Alkanes

Answer 21

The higher the carbon content of the alkane, the greater the tendency for incomplete combustion

Question 22

Pattern between alkane chains and carbons

Answer 22

If we look at the first 8 straight-chain alkanes, we see that the percentage content increases as the chain gets larger
This table suggests that incomplete combustion in the alkanes increases with the length
of the carbon chain

Question 23

Carbon Content and Specific energy of Alkanes

Answer 23

the higher the percentage carbon content (and the lower the percentage of hydrogen) the
lower the specific energy)
The higher alkanes (which produce less energy) found in petroleum and carbon in coal produce less energy for the same level of carbon dioxide pollution

Question 24

What is the greenhouse effect?

Answer 24

The greenhouse effect occurs when greenhouse gases in a planet’s atmosphere insulate the planet from losing heat to space, raising its surface temperature

Question 25

Effects of global warming

Answer 25

a) changes in agriculture (crop yields)
b) changes in ecosystems due to desertification (is a type of gradual land degradation of fertile land into arid desert due to a combination of natural processes and human activities) and loss of cold-water fish habitats

Question 26

Examples of renewable energy sources

Answer 26

photovoltaic cells (convert sunlight into electricity)
biofuels wind biomass
ocean (tidal and wave)
geothermal hydroelectric power

Question 27

What are biofuels?

Answer 27

Plants use solar energy to convert CO2 and H20 into glucose and
oxygen
The conversion of fully oxidized carbon in the air into carbon in organic matter is known as biological fixation
Some of the organic matter can be processed into biofuels (alternative to fossil fuels and are renewable - they can be grown in a short period of time)

Question 28

Photosynthesis as a biofuel

Answer 28

Chlorophyll in plants are able to absorb the energy of visible sunlight which is then used to convert water and carbon dioxide into carbohydrates (C6H1206 - glucose)
The reaction of photosynthesis can be summarized by the following reaction (MEMORIZE):
6 CO2(g) + 6 H2O(l) → C6H12O6(s) + 6 O2(g)
Some of the carbon in glucose is incorporated into other organic matter (such as oils) within plant cells –> this organic material is made into biofuels

Question 29

Ethanol as a biofuel

Answer 29

Ethanol is produced by fermentation of starch or sugar-rich crops (corn, sugar, cane, wheat)
C6H1206(s) → 2 C2H5OH(l) + 2 CO2(g)
The ethanol obtained by fermentation can be burned (combusted) to produce energy directly

Question 30

Advantages of ethanol

Answer 30

1) Lower greenhouse emissions and it is renewable
* produces lower emissions of CO and nitrogen oxides
* carbon-neutral: having a balance between emitting carbon and absorbing carbon from the atmosphere
2) Reduced crude oil use
This means the the amount of oil use (and so the carbon footprint) per person will decrease where ethanol is available

Question 31

Disadvantages of ethanol

Answer 31

1) Lower specific energy and potential damage to engines
2) High energy cost of distillation
3) Food versus fuel debate:
When corn (also called maize) is used as a source of ethanol (in the US), critics claim that the price of corn on the world markets is forced up

Question 32

Relationship between temperature and entropy

Answer 32

decrease in temperature and decreasing entropy
increase in temperature and increasing entropy

Question 33

What are spontaneous reactions?

Answer 33

Spontaneous processes are those that increase the total entropy of the universe

Question 34

K value rules

Answer 34

At 0 K, all exothermic reactions have a negative Gibbs energy, AG, and are therefore spontaneous
At 0 K, all endothermic reactions have a positive Gibbs energy, AG, and are therefore non-spontaneous

Question 35

Relationship between Q and K

Answer 35

*If Q<K, the concentration of the reactants is greater than that at equilibrium
so the forward reaction is favoured until the equilibrium is reached (therefore the reactants decrease and products increase until the rates are equal)

If Q> K, the concentration of the products is greater than that at equilibrium, so the reverse reaction is favoured until the equilibrium is reached (therefore the products decrease and reactant increase until the rates are equal)