Reactivity 1.3

Question 1

What is combustion?

Answer 1

the chemical reaction of a substance with oxygen to produce CO2 and water

Question 2

What are the two important reactants for a combustion reactant?

Answer 2

fuel (a combustible substance)
oxygen gas

Question 3

Are combustion reactions exo/endothermic?

Answer 3

highly exothermic
molecules produced are energetically stabler than reactant fuel and oxygen gas

Question 4

Is an exothermic reaction positive or negative?

Answer 4

negative, energy is released

Question 5

Is an endothermic reaction positive or negative?

Answer 5

positive, energy is absorbed

Question 6

Which substances undergo combustion?

Answer 6

organic compounds
many non-metals (form non-metal oxides, no fixed ratios)
nearly all metals

Question 7

In which state are organic compounds flammable?

Answer 7

they are flammable regardless of the state

Question 8

What happens when organic compounds are combusted with a lack of oxygen?

Answer 8

incomplete combustion, may form carbon monoxide or soot

Question 9

What causes the lack of oxygen in incomplete combustion?

Answer 9

combustion reaction occurring too quickly, depleting quantity of available oxygen around the burning fuel
or limited O2 concentration

Question 10

What factors may lead to incomplete combustion?

Answer 10

• limited O2 concentrations
• low combustion temp.
• large fuel particles
• presence of water in solid fuels

Question 11

What are observations during incomplete combustion? How does this differ from complete combustion?

Answer 11

releasing smoke
producing an orange or yellow flame

when enough oxygen is present, flame is much hotter and bluer, and there is no smoke formed

Question 12

What defines the products in incomplete combustion?

Answer 12

the less oxygen you have, the lower ratio of carbon-oxygen atoms

CO2 –> CO –> C

Question 13

What are fossil fuels?

Answer 13

mixtures of many compounds that formed through the decomposition of prehistoric organisms over time, deep within the Earth’s crust

Question 14

Are fossil fuels pure?

Answer 14

no - comprise of a range of compounds, not just one

Question 14

In which 3 main categories are fossil fuels divided into?

Answer 14

coal, crude oil, and natural gas

Question 15

What is coal’s composition and state at room temp?

Answer 15

Mainly carbon - traces of H, O, N, S and metals
solid

Question 16

What is crude oil’s composition and state at room temp?

Answer 16

Mixture of medium/long chain hydrocarbons - traces of S, N, O and metals
liquid

Question 17

What is natural gas’s composition and state at room temp?

Answer 17

Mostly methane - traces of small hydrocarbons, H2O, CO2
gas

Question 18

What are advantages of fossil fuels?

Answer 18

• highly exothermic when combusted
• relatively easy to transport, convenient
• abundant in many countries
• relatively inexpensive to obtain compared to set-up costs
• boost economy, jobs, income, taxes
• support industries such as agriculture, manufacturing and power generation

Question 19

What are disadvantages of fossil fuels?

Answer 19

• mining, transport, and processing is harmful to local and global environments
• threat of oil spills, pipeline leaks, cole mine explosions
• burning produces substances harmful for health and environment
• global warming
• respiratory problems
• acidification of lakes and soil
• non-renewable

Question 20

What is the greenhouse effect?

Answer 20

a phenomenon that occurs when certain gases such as CO2, methane or NOx absorb infrared radiation. this IR radiation has been emitted from the earths surface after being heated by the sun.

greenhouse gases then emit the IR radiation in all directions, so more energy is trapped within the atmosphere, leading to higher temperatures

Question 21

Which types of fuel release least/more CO2?

Answer 21

most - coal
middle - crude oil
least - natural gas

Question 22

How can one calculate the mol of CO2 produced per MJ of energy?

Answer 22

mol CO2/kJ of energy released per mol * 1000

Question 23

Which types of fuels produce the least/most volume of CO2?

Answer 23

most - coal
middle - crude oil
least - natural gas

Question 24

How to calculate the amount of CO2 produced in combustion of different fuels?

Answer 24

number of moles of CO2 x 22.7

Question 25

How can you calculate energy per unit mass?

Answer 25

MJ/kg

Question 26

Which fuels have the highest/lowest energy per unit mass?

Answer 26

most - natural gas middle - crude oil least - coal

Question 27

What are consequences of incomplete combustion?

Answer 27

- less energy released per mol (compared to complete combustion) - harmful by-products (eg CO and C) - unreacted fuel can be a fire/explosion hazard

Question 28

Which fuel has the greatest tendency to undergo incomplete combustion and why?

Answer 28

coal - presence of impurities such as N, S and metals that also react with oxygen

Question 29

Which fuel has the lowest tendency to undergo incomplete combustion - natural gas or crude oil- and why?

Answer 29

natural gas - methane is a shorter chain hydrocarbon, requires less oxygen than longer chain hydrocarbons like crude oil

Question 30

What is a non-renewable energy source?

Answer 30

an energy resource which is used up faster than it can be formed and will eventually run out

Question 31

What is a renewable energy resource?

Answer 31

a resource that can be replenished at the same rate that it is used up.

Question 32

What are biofuels?

Answer 32

fuels derived from biological sources, made from combustible organic molecules

Question 33

What is the logic behind a biofuel?

Answer 33

- energy from the sun is harnessed by plants/organisms - chlorophyll generates electrons to power photosynthesis - chlorophyll-containing pigments then produce glucose - some glucose molecules are used immediately, others are converted to other forms - we can use these other molecules and convert them to biofuels

Question 34

What is the MAIN idea behind the production of biofuels?

Answer 34

the breakdown or decomposition of larger organic molecules into combustible short-chain hydrocarbons

Question 35

What are advantages of biofuels?

Answer 35

- plants use CO2, offsetting CO2 emissions of burning - short-chain hydrocarbons: cleaner, more often complete combustion - refining/synthesis is less harmful to the environment - relatively inexpensive compared to other renewables - we can preserve fossil fuel stores for future use in other applications - can be used directly after synthesis – no mining or processing at a refinery as with fossil fuels

Question 36

What are disadvantages of biofuel production?

Answer 36

- uses a lot of plant, requiring a lot of land and monoculture - still releases CO2 - growth of crops requires the use of fossil fuels

Question 37

What happens in a fuel cell that is different from conventional techniques?

Answer 37

electrical energy is derived directly from a chemical reaction conventional methods use heat from combustion that undergoes many transfers and transformations to become electrical energy

Question 38

What does a fuel cell consist of?

Answer 38

a spontaneous exothermic chemical reaction that is divided in 2 regions

Question 39

What occurs in a fuel cell?

Answer 39

- two half reactions occur separately in 2 different locations, separated by a membrane and connected by a wire allowing flow of electrons - electrons formed on one side, flow to the other where they are used - movement of electrons through the wire creates an electrical current - this current can be used to directly charge a load/operate machinery