8 Flashcards
Formation of the early atmosphere and oceans
The Earth’s surface was originally molten for many millions of years. It was so hot that any atmosphere just dispersed into space. Eventually things cooled down a bit and a thin crust formed, but volcanoes kept erupting.
There was intense volcanic activity for the first billion years after the Earth was formed, and the volcanoes gave out lots of gas. Scientists think that these gases went on to form the early atmosphere and the oceans. There are lots of different theories, but the most popular theory suggests that the early atmosphere was probably mostly carbon dioxide (CO), with little or no oxygen (O2). This is quite like the atmospheres of Mars and Venus today. Volcanic activity probably also released nitrogen, which built up in the atmosphere over time, as well as water vapour, and small amounts of methane (CH) and ammonia (NH).
As the Earth cooled, the water vapour in the atmosphere condensed, forming the oceans.
Decreasing the amount of carbon dioxide
Although the early atmosphere was mostly carbon dioxide, it didn’t stay that way for long. Most of the carbon dioxide was gradually removed from the atmosphere. This happened in 2 ways.
* Absorption by the oceans
* Absorption by plants and algae
Absorption by the oceans
The oceans are a natural store of carbon dioxide. When the oceans formed, a lot of the carbon dioxide from the atmosphere dissolved into them. This dissolved carbon dioxide then went through a series of reactions to form carbonate precipitates that formed sediments on the seabed. When marine animals evolved, their shells and skeletons contained carbonates from the oceans. When they died, they formed sedimentary rocks such as limestone, locking the carbon dioxide away.
Absorption by plants and algae
Green plants and algae evolved over most of the Earth. Algae evolved first- about 2.7 billion years ago. Then over the next billion years or so, primitive green plants also evolved. They absorbed some of the carbon dioxide in the atmosphere and used it for a process called photosynthesis.
photosynthesis
As well as absorbing the carbon dioxide in the atmosphere, green plants and algae produced oxygen by photosynthesis - this is when plants use light to convert carbon dioxide and water into:
Increasing the amount of oxygen
As well as absorbing the carbon dioxide in the atmosphere, green plants and algae produced oxygen by photosynthesis.
As the oxygen level built up in the atmosphere over time, organisms that couldn’t tolerate it were killed off. The increase in oxygen allowed more complex life (like animals), that needed more oxygen, to evolve. The oxygen also created the ozone layer O3 which blocked harmful rays from the Sun and enabled even more complex organisms to evolve.
Eventually, as the levels of O2 increased and CO2 decreased, the atmosphere reached a composition similar to what it is today, with virtually no CO2 left.
Test for oxygen
You can test for the presence of oxygen in the lab. To test for oxygen, put a glowing splint inside a test tube containing the gas. If oxygen is present, it will relight the glowing splint
What is greenhouse gases?
Greenhouse gases, such as carbon dioxide, methane and water vapour, are present in small amounts in the Earth’s atmosphere. They act like an insulating layer, keeping the Earth warm.
What is the greenhouse effect?
All particles absorb certain frequencies of radiation. The sun emits short wavelength electromagnetic radiation which passes through the Earth’s atmosphere, as it isn’t absorbed by greenhouse gases. The short wavelength radiation reaches the Earth’s surface, is absorbed, and then re-emitted as long wavelength, infrared (IR) radiation. This radiation is absorbed by greenhouse gases in the atmosphere. The greenhouse gases then re-radiate it in all directions - including back towards Earth. The IR radiation is thermal radiation, so it warms the surface of the Earth. This is the greenhouse effect.
Human activity and greenhouse gases
It’s thought that human activities have caused a rise in greenhouse gas concentrations in the atmosphere. For example, the level of carbon dioxide is increasing.
Global Warming
The level of carbon dioxide in the atmosphere has increased because we are adding more CO2 to the atmosphere and less is being removed from it. We are also adding to the amount of other greenhouse gases in the atmosphere, such as methane.
Increased levels of greenhouse gases in the atmosphere enhance the greenhouse effect as more IR radiation is absorbed and radiated back towards Earth, which causes the Earth to get warmer - this is global warming.
Increasing energy consumption
Over the last 150 years or so, the world’s human population has shot up, and we’ve become more industrialised. Both factors mean that we’ve increased our energy consumption and are burning more up and more fossil fuels.
Examples
* An increasing global population means that more energy is needed for lighting, heating, cooking, transport and so on.
* People’s lifestyles are changing too. This means that the average energy demand I per person is also increasing (since people have more electrical gadgets and more people have cars or travel on planes, etc.)
Burning more fossil fuels means that carbon, that was ‘locked up’ in the fuels, has been released into the atmosphere in the form of CO2.
Deforestation
More people also means more land is needed to build houses and grow food. We’ve been chopping down forests (known as deforestation) to create this extra space. This is a problem as plants absorb carbon dioxide by photosynthesis. So fewer plants means less carbon dioxide is being removed from the atmosphere.
Methane and farming
The greenhouse gas methane is also causing problems. Like carbon dioxide, the concentration of methane has also risen a lot in recent years due to increased human activity. For example, in livestock farming, cows produce large amounts of methane. Paddy fields, in which rice is grown, produce a fair bit too. So, the larger the population gets, the more we need to farm to produce food, and the more methane is produced.
Though it’s currently only present in tiny amounts in our atmosphere, the increasing concentration of methane is an issue as it’s a highly effective greenhouse gas,
Carbon dioxide and global warming
Historically, temperature change at the Earth’s surface is correlated to the level of carbon dioxide in the atmosphere. Recently, the average temperature at the Earth’s surface has been increasing as the level of carbon dioxide has increased.
Even though the Earth’s temperature varies naturally, most scientists agree that the extra greenhouse gases from human activity (mainly through burning fossil fuels) are causing an increase in temperature. This temperature increase is known as global warming and is a type of climate change. Global warming could even lead to further climate change - which may have lots of effects with negative consequences.
Examples of negative consequences of global warming
- An increase in global temperature could lead to polar ice caps and glaciers melting, causing a rise in sea levels, increased flooding in coastal areas and coastal erosion.
- Changes in rainfall patterns (the amount, timing, and distribution) may cause some regions to get too much or too little water. This, along with changes in temperature, may affect the ability of certain regions to produce food.
- The frequency and severity of storms may also increase.
- Changes in temperature and the amount of water available in a habitat may affect wild species, leading to differences in their distribution.
Challenges in obtaining climate change data.
The current average global temperature and carbon dioxide level can be worked out pretty accurately, as they’re based on measurements taken all over the world. Historical data is less accurate - less data was taken over fewer locations and the methods used to collect the data were less accurate. If you go back far enough, there are no records of the global temperature and carbon dioxide level at all.
But there are ways to estimate past data. For example, you can analyse fossils, tree rings or gas bubbles trapped in ice sheets to estimate past levels of atmospheric carbon dioxide. The problem with using these kinds of measurements is that they’re much less precise than current measurements made using instrumental sampling. Another issue with measuring carbon dioxide level is the location of sampling. Different areas may have varying carbon dioxide levels and may be unrepresentative of the global level.
Mitigating the effects of climate change
To slow down or mitigate climate change, we need to cut down on the amount of greenhouse gases we’re releasing into the atmosphere. To reduce carbon dioxide emissions, we can try to limit our use of fossil fuels. This could be doing things on a personal level, like walking or cycling instead of driving or turning your central heating down.
On a larger scale, the UK government has formed plans to encourage the public and industry to become more energy efficient, to create financial incentives to reduce CO2 emissions, to use more renewable energy and to increase research into new energy sources.
Technology could also be used to reduce some of the effects of climate change. For example, flood defences can be used which could keep homes safe from flooding. However, the long-term effects of climate change are hard to predict, and new technology may have knock-on environmental implications.
What is crude oil?
Crude oil is a complex mixture of many different hydrocarbons (Compounds which contain only hydrogen and carbon). Its formed at high temperatures and pressures from the remains of animals and plants that died millions of years ago. Because it takes so long for crude oil to form it’s said to be a finite resource once it’s used up we can’t replace it.
Crude oil is our main source of hydrocarbons and is used as a raw material to create lots of useful substances for the petrochemical industry. Most of the hydrocarbons in crude oil are alkanes hydrocarbons with the general formula CnH2n+2). The carbon atoms in these molecules are arranged in chains or rings.
Fractional distillation process
In fractional distillation, the crude oil is pumped into a piece of equipment known as a fractionating column. This fractionating column has a temperature gradient running through it - it’s hottest at the bottom and coldest at the top.
The crude oil is heated so that most of it evaporates (turns into a gas). It’s then piped in at the bottom of the column, where the liquid part (bitumen) is drained off. The gas rises up the column and gradually cools. Different compounds in the mixture have different boiling points, so they condense (turn back into a liquid) at different temperatures. This means they condense at different levels in the fractionating column.
Hydrocarbons that have a similar number of carbon atoms have similar boiling points, so they condense at similar levels in the column.
E.g - Hydrocarbons with lots of carbon atoms have high boiling points, so they condense near the bottom of the column.
E.g - Hydrocarbons with a small number of carbon atoms have low boiling points, so they condense near the top of the column.
The various fractions are constantly tapped off from the column at the different levels where they condense. Each fraction contains a mixture of hydrocarbons with similar boiling points.
Homologous series
Hydrocarbons which share similar chemical properties can be grouped together in homologous series. A homologous series is a family of molecules which have the same general formula and share similar chemical properties. The molecular formulae of neighbouring compounds in a homologous series differ by a CH2 unit.
The physical properties of compounds in a homologous series vary between the different molecules. For example, there is a gradual increase in boiling point as the molecules get bigger. Alkanes and alkenes (hydrocarbons with at least one double bond between carbon atoms) are two different homologous series of hydrocarbons.
Boiling points
The intermolecular forces of attraction break a lot more easily in small molecules than they do in bigger molecules. This is because the forces are much stronger between big molecules than they are between small molecules.
A large molecule contains many points along its length where it can be attracted to another molecule. So, even if it can overcome these forces at a few points along its length, it’s still got lots of other places where the force is still strong enough to hold it in place.
Because of this, it takes a lot more energy to break the intermolecular forces between large molecules than between small molecules. That’s why large molecules have higher boiling points than small molecules do.
- Longer the hydrocarbon
- More points of contact
- more intermolecular forces to break
- more energy needed to overcome intermolecular forces
- higher boiling point
Complete combustion of hydrocarbons
If you burn hydrocarbons, the carbon and hydrogen react with oxygen from the air to form carbon dioxide and water releasing energy. This makes hydrocarbons great fuels. When there’s plenty of oxygen, the only products are carbon dioxide and water - this is called complete combustion. This is the equation for the complete combustion of a hydrocarbon:
Incomplete combustion
In plenty of oxygen, hydrocarbons combust completely to produce only watee and carbon dioxide. However, when you burn them when there is insufficient oxygen in the air, they undergo incomplete combustion. This can happen in some appliances, e.g. boilers that use carbon compounds as fuels.
The products of incomplete combustion contain less oxygen than the products of complete combustion. Carbon dioxide and water are still produced, but carbon (in the form of soot) and the toxic gas carbon monoxide (CO) can also be produced.
Sulfur dioxide and acid rain
When fossil fuels are burned, they release mostly carbon dioxide gas (a major cause of global warming - see page 267). They also release other harmful gases - in particular sulfur dioxide and various nitrogen oxides. The sulfur dioxide (SO₂) comes from sulfur impurities present in fossil fuels.
When sulfur dioxide mixes with the water in the clouds, it reacts with water to form dilute sulfuric acid. This then falls as acid rain which causes lakes to become acidic and many plants and animals die as a result. Acid rain also kills trees, damages limestone buildings and ruins some stone statues (see Figure 3). It can also make metals corrode. Links between acid rain and human health problems have also been suggested.
What is cracking?
Short-chain hydrocarbons are flammable so make good fuels and are in high demand. Long-chain hydrocarbons form thick gloopy liquids which aren’t all that useful, so a lot of the longer saturated hydrocarbons (alkanes) produced from fractional distillation are turned into smaller, more useful ones by a process called cracking. Cracking produces both alkenes (which are unsaturated) and alkanes (which are saturated).
Some of the products of cracking are useful as fuels, like petrol for cars and kerosene for jet fuel. Cracking also produces alkene molecules such as ethene, which are needed for making polymers (mostly plastics).
Alkane is usually longer than the alkene
How cracking works
Cracking is a thermal decomposition reaction breaking molecules down into at least two new ones by heating them. In cracking, vaporised hydrocarbons are passed over a powdered catalyst.
A lot of energy is needed to break the strong covalent bonds in the long chain hydrocarbons, so cracking is carried out at temperatures of 400 °C -700 °C.
A pressure of 70 atm and an aluminium oxide catalyst are also used. The long-chain molecules split apart or ‘crack’ on the surface of the specks of catalyst.
cracking alkanes in the lab
The apparatus shown in Figure 4 can be used to crack alkanes in the lab. During this reaction, the alkane is heated until it is vaporised. It then breaks down when it comes into contact with the catalyst, producing a mixture of short-chain alkanes and alkenes.
Interpreting data on cracking
The examiner might give you a table like the one below to show supply and demand for different crude oil fractions. A fraction will be suitable for cracking if it contains long-chain hydrocarbons and its demand is less than the percentage of crude oil that it makes up.
