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Molecular orbitals and hybridisation BeCl
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OCR June 2015 F321 Q4.
What is the difference between the xylem and the phloem?
Phloem and xylem are two different types of transport tissue in plants.
Xylem is the tissue that transports water and solutes from the roots to the leaves. Water is drawn up from the roots by the process of transpiration, water loss from the leaves. There are adaptations to reduce excessive water loss. Xylem vessels are made of dead cells with thick, strengthened cellulose cell walls and a hollow lumen.
Phloem transports products of photosynthesis away from the leaves, for example to the roots or storage tissues. This process is called translocation. Phloem consists of columns of living cells.
What is the difference between aerobic and anaerobic respiration?
Nutrient molecules are broken down during the process of respiration, to release energy. Respiration can either be aerobic, or anaerobic. By the way, breathing is not the same as respiration: breathing is ventilation, whereas respiration involves a series of chemical reactions.
Oxygen is needed for aerobic respiration. Glucose is fully broken down in the presence of oxygen: glucose + oxygen → carbon dioxide + water. C
6
H
12
O
6
+ 6O
2
→ 6CO
2
+ 6H
2
O. A large part of aerobic respiration takes place inside mitochondria, the power stations of the cell. It is a process that happens all the time in animals and plants. It releases a large amount of energy.
Oxygen is not needed for anaerobic respiration. This process does not release as much energy as aerobic respiration (about 19 times less!) as the reactions do not break down glucose completely. For example, in animals, muscles use anaerobic respiration during hard exercise, where glucose is converted to lactic acid: glucose → lactic acid, C
6
H
12
O
6
→ 2C
3
H
6
O
3
. Lactic acid accumulates in the muscles during vigorous exercise, and the lactic acid has to be converted (oxidised) to water and carbon dioxide later. This oxygen debt (oxygen is required to break down the lactic acid; known as EPOC, excess post-exercise oxygen consumption) has to be repaid when the exercise stops. This is why you have to keep breathing deeply for a while after finishing your exercise.
Anaerobic respiration is used by plants and microorganisms, which convert glucose to ethanol and carbon dioxide: glucose → ethanol + carbon dioxide, C
6
H
12
O
6
→ 2C
2
H
5
OH + 2CO
2
. Ethanol is the alcohol that we drink in alcoholic drinks (brewing industry). Bubbles of carbon dioxide make bread rise (bread-making).
What are geotropism and phototropism?
Geotropism and phototropism are two types of tropisms, responses to a stimulus. They help the plant to grow towards light and water, both of which plants need for photosynthesis. Plant hormones like auxins are involved. Auxins are made in the tips of stems and roots and control the direction of growth in response to stimuli like light and gravity.
Tropism can be negative or positive. Negative means away from the stimulus. The root tip displays negative phototropism as it grows away from the light and therefore has a lower change of drying out. Stem tips show negative geotropism as they grow away from 'gravity' (i.e. upwards) which means they have a better chance of finding light.
Positive tropism means towards the stimulus. Stem tips show positive phototropism because they grow towards light so that they find light for photosynthesis. Root tips, on the other hand, display positive geotropism (grow towards gravity), enhancing the chance of encountering water.
What is the difference between mitosis and meiosis?
Both meiosis and mitosis are ways in which cells reproduce. The difference is that in mitosis two identical cells are produced, both having the same number of chromosomes as the parent cell. In meiosis, however, four daughter cells are produced in two successive sets of steps, and these daughter cells contain only half the number of chromosomes as the parent cell. Meiosis is used to produce gametes for sexual reproduction.
How do you find the formula of an ionic compound, for example in an acid base reaction?
The first thing to do is check the group of each component in the Periodic Table; e.g. Mg is in group II, and Cl is in group 7. Group I elements form +1 ions, group II element +2 ions, group III elements +3 ions, whereas group 6 elements form -2 ions and group 7 elements -1 ions. The reason for this is that when forming these ions noble gas configuration is achieved, and noble gas configuration is very stable. So the magnesium ion that is formed is Mg
2+
: Mg atom has an electronic configuration of [Ne] 3s
2
(1s
2
2s
2
2p
6
3s
2
), but the magnesium ion has lost its two outer electrons and has an electronic configuration of 1s
2
2s
2
2p
6
, the electronic configuration of the noble gas neon.
If you have to find the salt that is made up of, for example, magnesium and chloride, you would look at the group number of each of the elements and find the charges as described above: Mg
2+
and Cl
-
. You then need to balance the charges to make a neutral compound. At the moment you have got 2 positive charges on the magnesium ion, and just one negative charge on the chloride ion. Therefore you need two chloride ions (i.e. two negative charges) to balance the 2 positive charges on magnesium. Magnesium chloride is therefore MgCl
2
. The little numbers tell you how many of the atoms there are in the compound. Once you have established what it is (in our example the '2' on the chloride ion: MgCl
2
), you don't change it again. If you're trying to balance an equation (e.g. an acid base reaction), only play around with the big numbers in front of the compounds, e.g. 2 MgCl
2
.
How do you balance chemical equations?
When you try to balance an equation, remember to only change the numbers in front of compounds or atoms. Do not change any of the little subscript numbers. E.g. only change the '2' in front of 2 MgCl
2
but not the little 2 behind Cl. This is very important as changing the little subscript numbers would result in a different compound, and chances are your new compound doesn't exist. Let's look at an example. A question might ask you to predict the products and balance the equation for an acid base reaction: MgO + HCl → ??. The first step is to work out what the products are. The acid is HCl and the base MgO. So your products will be a salt formed from the metal ion on the base and the part of the acid that is not 'H', as well as water. In order to find out what the salt is, check the group number (see my other entry 'How do you find the formula of an ionic compound, for example in an acid base reaction?' for help) and balance the charges; you will find that the salt is MgCl
2
. So the first, unbalanced version of your equation is now: MgO + HCl → MgCl
2
+ H
2
O. Now check if there are the same number of each atom on both sides. You can write down the number of atoms under your equation to help you see this. There is 1 Mg atom on both sides. There is 1 O atom on both sides. There is 1 H on the left and 2 H on the right, and there is 1 Cl on the left and 2 Cl on the right. It’s easy to see now that you need 2 HCl on the left to balance the equation: MgO + 2 HCl → MgCl
2
+ H
2
O.
Let's look at a more complicated example. A question may ask you to write a balanced equation for the reaction between aluminium and oxygen to form aluminium oxide, Al
2
O
3
. Remember that when it says 'to form' or 'reacts to' this means that whatever forms something is the reactant, and what is formed is the product. Looking at the Periodic Table tells you that your reactants are Al and O
2
(remember that oxygen is diatomic). So writing this as a first, unbalanced equation gives you: Al + O
2
→ Al
2
O
3
. Now we have 1 Al on the left but 2 on the right, and 2 O on the left but 3 on the right. Let's start with oxygen: find the lowest common multiple of 2 and 3 which is 6. Does this help us? This gives Al + 3O
2
→ 2 Al
2
O
3
. Now the O's are balanced, but the Al's are still not balanced. We've now got 4 Al on the right, but only one on the left. We can now put a 4 in front of the Al of the left, and the equation is all balanced: 4Al + 3O
2
→ 2Al
2
O
3
. Sometimes it can be a bit fiddly to get the equation balanced. If you are stuck, you can always try and start with another atom. For example, if we had first looked at Al in the last example, we would have had to adjust our number again (you would have put a 2 in front of the Al on the left in your first step).
How do you decide how a metal is extracted from its ore?
Ores are rocks that contain sufficient metal or metal compounds to make it worthwhile extracting them. The method that is used to extract a metal from its ore depends on where it is situated in the reactivity series (so how reactive it is means how stable the ore is). An example of the reactivity series can be found
here
.
Metals at the top are very reactive, whereas the metals at the bottom are very unreactive and are found as their metals, as uncombined elements. Any metal above carbon in the reactivity is extracted by electrolysis, where electricity is passed through the molten ore.
Metals in the middle, below carbon, are extracted by heating them with carbon (charcoal, coke). The metals at the bottom do not have to be extracted as they are found as their metals in the earth's crust, e.g. silver, gold, platinum.
What are redox reactions?
Redox reactions are chemical reactions that involve both oxidation and reduction. Oxidation is when oxygen gets added to a substance in a reaction; it is also defined as the loss of electrons in a reaction. Reduction is when oxygen is removed in a reaction, or when electrons are gained.
Remember OIL RIG - Oxidation Is Loss of electrons; Reduction Is Gain of electrons.
An example of a redox reaction is rusting: iron + oxygen + water → hydrated iron(III) oxide, where iron is oxidised and oxygen is reduced.
What are the typical reactions between acids and bases, and how do you know what products are formed?
In an acid base reaction, the products are always salt and water. Examples of acids are hydrochloric acid (HCl), nitric acid (HNO
3
) and sulfuric acid (H
2
SO
4
). Metal hydroxides, metal oxides and metal carbonates are all bases. If the base is a carbonate, you get an extra product which is carbon dioxide.
These are the general reactions you will encounter:
Acid + metal hydroxide → salt + water
Acid + metal oxide → salt + water
Acids + metal carbonate → salt + water + carbon dioxide.
How do you know what the salt is that is formed in the reaction? Identify which reactant is the acid and which one is the base. A salt is always the metal from the base plus the part of the acid that is not H. Hydrochloric acid, HCl, always forms chloride salts (adding 'Cl' to the metal). Sulfuric acid, H
2
SO
4
, always forms sulphate salts (adding 'SO
4
' to the metal). Nitric acid, HNO
3
, always forms nitrate salts (adding 'NO
3
' to the metal). For example, hydrochloric acid + sodium hydroxide → water + sodium chloride. Potassium hydroxide + nitric acid → potassium nitrate + water.
Remember that when you have to find the chemical formula of the salt that your charges have to balance and you have to form a neutral compound. Chloride has a -1 charge (Cl
-
); nitrate is a polyatomic ion (consists of more than one atom) and has an overall charge of -1 (NO
3
-
); sulfate is a polyatomic ion and has a -2 charge (SO
4
2-
). Check what the charge of the metal ion is by looking at the group number. Group I metals form +1 ions, group II metals +2 ions, and group III metals +3 ions. Some salts are easy to make, e.g. sodium chloride is NaCl because sodium, being in group I, forms +1 ions, and chloride is -1. Therefore, +1 + (-1) = 0, so NaCl is neutral.
Magnesium sulfate is also easy as the magnesium ion is +2 (group II), and sulfate is -2. So Magnesium sulfate is MgSO
4
.
Magnesium nitrate is a bit more difficult as the magnesium ion is +2, and nitrate is -1. You now have to balance the positive (+2) and the negative (-1) charges to make them zero. What you have to do then is take two of the negatively charged ions to balance the charges. Your salt is therefore Mg(NO
3
)
2
. Remember that because nitrate is polyatomic and the charge of -1 applies to the whole ion, you have to put the nitrate ion in brackets (the same would be true if you had to use a multiple of a sulfate ion).
How do I name organic compounds?
You need be able to recognise and name alkanes, alkenes and alcohols.. When you are given an organic molecule, count up the number of carbons that are in a single straight line first. If there is only one carbon, your compound with start with 'meth-'. For two carbons, the compound will start with 'eth-', for 3 carbons it will start with 'prop-', and for 4 carbons it will start with 'but-'. This is what I will call stem later on.
Now check what the compound looks like.
Are there any double bonds? If yes, the compound is an alkene and ends in '-ene'. So if, for example, you have got 2 carbons joined by a double bond, you would put 'eth-' and '-ene' together to make 'ethene'. For 4 carbons, it is 'but-' and '-ene' so 'butene'. Note that 'methene' does not exist as you need a double bond between two carbon atoms for an alkene.
If there are no double bonds, is there an -OH anywhere in the compound? If yes, the compound is an alcohol. For alcohols you need to add '-an-' between the stem (unless there is a double bond in the structure) and the ending '-ol'. So if you have got one carbon, you put 'meth-' and '-an-' and '-ol' = 'methanol'. For 3 carbons, it's 'prop-' and '-an-' and '-ol' = 'propanol'.
If there are neither double bonds nor any -OH groups and there are only single bonds between the carbons, the compound is an alkane. Alkanes end in '-ane'. So one carbon makes it 'meth-' and '-ane' = 'methane'; 2 carbons 'eth-' and '-ane' = 'ethane', etc.
What is the difference between mass and weight?
The terms 'mass' and 'weight' are often used interchangeably in everyday language. However, in physics they mean two different things.
Mass is a measure of how much matter an object has, measured in kilograms. Weight on the other hand, is a force as it is a measure of how strongly gravity pulls on that matter. As a force it is measured in Newtons.
Forces are calculated using the equation
F = ma
(force = mass x acceleration). In the case of weight, acceleration is the gravitational field strength which is approximately 10 N kg
-1
on Earth. The equation for weight can be rewritten as
W = mg
(weight = mass x gravitational field strength). From this equation you can see that your weight depends on both the gravitational field strength and the mass. You can't change the mass of an object without altering the object, but you can take the object to an environment where the gravitational field strength is different.
So, if you go somewhere where the pull of gravity is stronger or weaker, your weight will change. For example, on the moon the pull of gravity is weaker, so your weight would be lower, but your mass would stay the same because you are still made up of the same amount of matter.
Is there a simple way to rearrange simple equations in physics?
Many equations in physics are of the type A = B x C, for example the equation for force (
F
) from mass (
m
) and acceleration (
a
):
F = ma
. If you don't like rearranging equations, have a look at the image below. You can convert equations like these into triangles, where you just cover up the quantity you want to calculate (with your thumb) and then enter the numbers into the remaining quantities. Take the equation
F = m a
(force = mass x acceleration).
Enter the equation in a triangle like the one above. You need to make sure that the terms that are multiplied (in our example
m
and
a
) are in the lower half of the triangle, where there is an 'x' (for multiplied by) between the two cells. This leaves
F
for the top of the triangle. If you want to calculate
F
, you put your thumb over
F
in the triangle. The triangle tells you that you need to multiply the numbers for
m
and
a
.
If you want to calculate
m
, you put your thumb over
m
in the triangle. You are then left with a fraction:
F/a
. This means that you have to divide the number for
F
by the number for
a
.
If you want to calculate
a
, you put your thumb over
a
in the triangle. You are then left with a fraction:
F/m
. This means that you have to divide the number for
F
by the number for
m
.
You can do this for any equation where you have to either divide one number by another, or multiply one number by another.
Le'’s try this with an equation where we have to divide one number by another, e.g.
v = d/t
(speed = distance/time). When you look at your equation visually it already tells you that
d
is at the top of the triangle, as you are dividing it by
t
. So
t
has to be at the bottom. This only leaves
v
and we only have one cell left in the triangle – in the lower half.
What happens to the particles in a material when the material is heated?
Particles in different states of matter contain different amounts of energy. Particles in solids cannot move from place to place, so the solid has a fixed shape. The particles are also close in space and there is not space to move into, so solids cannot be compressed or squashed.
In a liquid the particles can move around each other so liquids flow and can take the shape of the container they're in. Liquids cannot be squashed or compressed because the particles are close in space and there isn’t much space for them to move into.
The particles in a gas can move quickly and in all directions, so gases flow and completely fill their container. The particles have space to move into as they are far apart, so gases can be compressed or squashed.
In terms of energy, for any given material, the particles in solids have the lowest amount of energy, liquids medium and gases the highest amount of energy. This shows as more vibration and more moving around and faster, the more energy the particles have.
The effect of heating is that the particles gain energy and vibrate more (or if they are free to move around, to move around more). This is all part of the kinetic theory of matter. So when you heat a material, if you supply enough energy you turn a solid into a liquid (melting, at the melting point) or a liquid into a gas (evaporation). The opposite happens when you cool a material. The particles now have lower energy, so when you cool a gas enough you can turn it into a liquid (condensing), and a liquid into a solid (solidification).
How fast do electromagnetic waves of different frequency travel in space?
In space (in a vacuum) all electromagnetic waves travel at the same speed: the speed of light, which is 299 792 458 m/s or approximately 300 000 000 m/s = 3 x 108 m / s.
What are current and voltage (potential difference)?
Current is related to the flow of charge: it is defined as charge passing a point in a time interval. The equation for this is
I = Q/t
(current = charge/time). In metals current is due to a flow of electrons. For a current to flow you need a continuous circuit. Current is measured in Amperes (A) using an Ammeter, which is connected in series.
Confusingly, there is something called the conventional current and the actual direction of flow of electrons. Conventional current flows from the positive to negative pole of a source. The actual direction of flow, however, is in the opposite direction, from negative to positive because electrons are negatively charged and therefore attracted to the positive pole of the source.
Voltage is now more commonly called potential difference (p.d.) and describes what drives the current between two points in a circuit. It is measured in Volts (V). Without a potential difference there is no flow of current. It is a little bit like water flowing down the hill: there is a (gravitational) potential difference to allow the water to flow down the hill. Current will flow if there is a potential difference between two points. Potential difference is the energy applied to the charge; the equation for this is
V = E / Q
(potential difference = energy / charge).
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