reactivity 1.3 Flashcards
3 things combustion (burning) reactions need
fuel, oxygen AND a source of ignition/trigger
why are combustion reactions exothermic?
because they occur with the generation of heat and light (in the form of flame)
all metals can _______, but not all metals can ________
oxidize, combust
some metals will only combust if they have what?
high surface area (i.e. they are finely divided as filings or a powder)
does copper combust? why or why not?
no because no flame is formed when it is directly heated in air
what do s-block metals form as they undergo combustion?
basic (pH wise) ionic oxides
what is the standard example of more reactive metals that combust in air?
magnesium, which burns with a bright white flame
two metals that combust but do not form typical oxides (metal & what they form)
sodium forms sodium peroxide Na₂O₂
iron forms iron(II, III) oxide, Fe₃O₄
what do p-block non-metals form when they undergo combustion?
acidic covalent oxides
why are many organic compounds used as fuels?
because they release relatively large amounts of energy when combusted
why do organic compounds not usually undergo spontaneous combustion?
their combustion reactions have a high activation energy
organic compounds commonly used as fuels x2
hydrocarbons - esp alkanes AND alcohols
define complete combustion?
when fuels (i.e. hydrocarbons/alcohols) are burnt in excess oxygen. all C and H will be oxidized (products CO2 and H2O)
qualities of lower alcohols x2
burn with an almost invisible flame AND make good fuels
how can ethanol be produced sustainably?
via fermentation of sugars
why do cars that run on ethanol require bigger fuel tanks/to fill up more often?
ethanol’s energy density is lower than gasoline
two benefits of blending ethanol with gasoline/diesel
increases energy density AND makes it safer in case of a fire (easier to see flames)
in a bunsen burner how is complete combustion characterized? x2
by a blue AND non-luminous flame
incomplete combustions products x2
water AND carbon monoxide or carbon
in a bunsen burner how is complete combustion characterized? x1
yellow flame
why is carbon monoxide dangerous? x5
colourless, odorless, toxic, poisionous AND binds to haemoglobin in the blood (so it limits its capacity to transport oxygen)
with a very reduced supply of oxygen, in what form will carbon be produced?
soot
how can the production of soot can be used to distinguish between different organic compounds?
compounds with a higher % of carbon content tend to undergo incomplete combustion and produce more soot in the process
fossil fuels contain _____
hydrocarbons
coal advantages x6
Relatively cheap
Abundant
Long lifespan (compared to other fossil fuels)
Can be converted into liquid fuels and gases
Relatively safe
Products from the combustion of coal have other uses, e.g. ash can be used to make roads
oil advantages x4
Easy to store and transport in pipelines and tankers
Impurities can be easily removed
Releases a lot of energy per kg( Known as high energy / enthalpy density)
Easily processed (Fractional distillation and cracking are used to produce other useful chemicals, e.g. shorter-chain alkanes and alkenes)
natural gas advantages x4
Cheapest of the fossil fuels
Easy to store and transport in pipelines and pressurised containers
Large amounts of energy per unit mass (Known as high specific energy)
Relatively clean (Complete combustion of natural gas happens with a blue flame producing minimal to no harmful compounds)
(Does not contribute to acid rain)
coal disadvantages x5
Combustion produces large amounts of pollution (Carbon dioxide, Sulfur dioxide, Particulates)
Associated with Global warming, Acid rain, Global dimming / visual pollution
Difficult to transport
Issues around mining (Destruction of habitats, Noise pollution, Health of miners, Safety issues)
Potentially radioactive
oil disadvantages x5
Combustion produces large amounts of pollution (Carbon dioxide, Sulfur dioxide, Carbon monoxide)
Associated with Global warming, Acid rain, Global dimming / visual pollution, Photochemical smog
Uneven worldwide distribution
Oil spills affecting habitats
Safety issues around drilling
natural gas disadvantages x5
Combustion produces large amounts of pollution (Carbon dioxide)
Associated with Global warming
Expensive and time-intensive to produce
Expensive to store
Safety issues around storage in pressurised containers
order of specific energy of different common fuels x8 (low to high)
wood, coal, ethanol, biodisel, crude oil, diesel, gasoline, natural gas
4 things that happen as the hydrocarbon chain increases
increased carbon content (more C/CO/CO2 produced)
stronger LDF = less volatile hydrocarbon
releases less energy per unit mass of fuel
more likely that incomplete combustion will take place
why does volume of CO2 fluctuate within a year?
photosynthesis removes CO2 from the atmosphere
define greenhouse gas
a gas that absorbs radiation emitted from the Earth’s surface, trapping it in the atmosphere so that it is not lost to space
4 examples of greenhouse gases
CO2, methane, nitrous oxides, water vapour
sources of atmospheric carbon dioxide
Combustion of wood and fossil fuels
Respiration of plants and animals
Thermal decomposition of carbonate rocks
The effect of acids on carbonates
steps of the greenhouse effect x4
- shortwave radiation from the sun strikes the Earth’s surface it is absorbed and re-emitted from the surface of the Earth as infrared radiation
- The infrared radiation passes through the atmosphere where some thermal energy passes straight through and is emitted into space
- But some infrared radiation is absorbed by greenhouse gases and re-emitted in all directions
- This reduces the thermal energy lost into space and traps it within the Earth’s atmosphere, keeping the Earth warm
3 main biofuels
bioethanol, biodiesel, biogas
what are organic compounds (used for biofuels) produced from?
biological carbon fixation
how is bioethanol produced?
via photosynthesis (fermentation of glucose)
what is biodiesel made from?
renewable vegetable oils (Natural triglyceride oils are converted to esters of methanol, which makes them less viscous)
how does transesterification work?
the triglyceride is converted into the less viscous fatty acid methyl ester (FAME) using methanol (acid protonates carbonyl group) (alkali deprotonates alcohol/methanol)
what is more commonly used to catalyse the reaction in transesterification?
an alkaline catalyst such as NaOH / KOHAcids and alkalis
is transesterification reversible?
yes
what is biogas?
a renewable fuel that is released when organic matter, such as food or animal waste, is broken down by microorganisms in the absence of oxygen
what does biogas contain?
mainly methane and CO2 BUT can also have small amounts of hydrogen sulphide & other chemicals
advantages of biofuel x4
Carbon neutral, renewable and sustainable if crops / trees are replanted
Reduce greenhouse emissions / pollution
Biodiesel and biogas can reduce the amount of waste going to landfill sites as the waste can be used to produce them
Biofuel production could provide money for less developed countries as they have the space to grow the crops required
define fuel cell
an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode
disadvantages of biofuel x4
High costs (Conversion of engines and machinery to run on biofuels instead of petrol / diesel AND Harvesting and transportation)
Many developed countries don’t have the space to be able to produce enough plants to make biofuels because the land is needed for food production (This can lead to deforestation in an effort to meet the demand)
Removes nutrients from the ground / uses large amounts of fertilizers and pesticides
Biofuels typically have lower specific energy than fossil fuels
how do fuel cells work?
As the fuel enters the cell it becomes oxidised which sets up a potential difference or voltage within the cell
different _____ and ______ can be used to set up different types of fuel cells
electrolytes AND fuels
what is an important fuel cell
hydrogen-oxygen
a hydrogen fuel cell consists of …. x4
a reaction chamber with separate inlets for hydrogen and oxygen gas
an outlet for the product - water
an electrolyte of aqueous sodium hydroxide
a semi-permeable membrane that separates the hydrogen and oxygen gases
advantages of hydrogen fuel cells x4
Water is the only reaction product, so fuel cells present obvious environmental advantages over other types of cells
The reaction is the same as hydrogen combusting in oxygen, but since the reaction takes place at room temperature without combustion, all the bond energy is converted into electrical energy instead of heat and light
There are no harmful oxides of nitrogen produced, which are usually formed in high-temperature combustion reactions where air is present
Fuel cells have been used on spacecraft, where the product can be used as drinking water for astronauts
disadvantages of hydrogen fuel cells x5
Hydrogen is a highly flammable gas and the production and storage of hydrogen carries safety hazards
Very thick walled cylinders and pipes are needed to store hydrogen which has economic impacts
The production of hydrogen is a by-product of the crude oil industry, which means it relies on a non-renewable, finite resource
Until a cheap way is found to make hydrogen, its widespread use in fuel cells will be limited
Hydrogen has high energy density, that is, the amount of energy contained in 1g of the fuel is high compared to other fuels, but because it is a gas, its energy density per unit volume is low which means larger containers are needed compared to liquid fuels
one difference between fuel cells and other cells
the fuel cell operates continuously as long as there is a supply of hydrogen and oxygen; the energy is not stored in the cell.
how is a methanol fuel cell different from a hydrogen fuel cell?
the methanol fuel cell uses methanol as the course of hydrogen ions and produces carbon dioxide and water while releasing energy (the fuel or source of hydrogen ions, H+, is methanol rather than hydrogen)
a methanol fuel cell contains … x4
a reaction chamber with separate inlets for methanol and oxygen
outlets for the carbon dioxide and water products
an electrolyte, typically a proton exchange membrane
a semi-permeable membrane that separates the hydrogen and oxygen gases
how to find the overall reaction of a hydrogen fuel cell based on two half equations?
The overall reaction is found by combining the two half equations and cancelling the common terms
how to find the overall reaction of a methanol fuel cell based on two half equations? x3 steps
Multiplying the O2 equation by 1.5
Combining the two half equations
Cancelling the common terms
advantages of methanol fuel cells over hydrogen fuel cells x5
Methanol is easier to store and transport than hydrogen
The fuel cell does not require high pressure or temperature
The membrane has a longer lifespan as it is operating in an aqueous environment
Methanol has a greater energy density (energy per unit volume) than hydrogen
Methanol can be produced from renewable resources through fermentation (This means that methanol is cleaner than hydrogen because its production has less impact on the environment in terms of greenhouse gases.)
disadvantages of methanol fuel cells over hydrogen fuel cells x6
Methanol is very toxic and highly flammable
Methanol is most commonly made from non-renewable fossil fuels
The fuel cell produces a lower voltage / lower power per unit mass of the cell
The fuel cell has a low efficiency because methanol can pass through the available membrane materials
The reaction at the anode requires a more highly efficient catalyst that contains expensive precious metals, usually ruthenium and palladium
The fuel cell produces the greenhouse gas, carbon dioxide
