This is all the content required for Chemistry paper 2. Each topic/lesson subheading has a link to either a cognito or Free Science lessons video. This doc states if the topic is triple content. Also includes required practicals.
Chemistry paper 2 video notes
THE RATE AND EXTENT OF CHEMICAL CHANGE
Mean rate of reaction:
Mean rate of reaction = amount of product / reactant formed / time taken
Using Tangents to Determine Rate:
Effect of concentration on rate:
- Chemical reactions can only take place when the reacting particles collide with each
other. The collisions must have sufficient energy. The rate of a chemical reaction is
determined by the frequency (number) of successful collisions per second.
- Activation energy: the minimum amount of energy that particles must have to react.
- Reactions are rapid initially because we have a large number of reactant molecules.
This means that we have a large number of collisions per second. Overtime reactions
slow down because the number of reactant molecules is running out. That means we
have a smaller number of collisions per second. Finally the reaction stops because
all of the reactant molecules have run out. This means that the number of collisions
per second is now 0.
- Increasing the concentration of reactants in solution, the pressure of reacting gases,
and the surface area of solid reactants increases the frequency of collisions and so
increases the rate of reaction. The rate is proportional to the concentration
Effect of surface area on rate:
- Smaller sized blocks of solid reactant have a greater surface area to volume ratio
than larger blocks. This means that they have more particles on the surface so there
are more collisions per second. This increases the rate of reaction.
Effect of temperature on rate:
- Particles in a reaction, some particles have more energy than others. Imagine that
two particles with only a small amount of energy collide. Because they have low
energy, they cannot overcome the activation energy barrier. In that case, they would
not collide successfully and no reaction would occur.
- Two particles with a large amount of energy collide and now they have enough
energy to overcome the activation energy barrier and collide successfully. A reaction
would take place.
- Increasing the temp increases the rate of reactions. It increases the energy because
the particles now move faster. This increases the frequency of collisions.
Catalysts:
- Catalysts increase the rate of chemical reactions without being changed or used up
during the reaction. They allow us to carry out reactions quickly without needing to
increase the temperature. This saves money. Because catalysts are not used up
during the reaction, we can reuse them again.
- The rate of reaction depends on the number of particles that have enough energy to
cross the activation energy barrier and collide successfully.
- Catalysts increase the rate by providing a different pathway for the reaction that has
a lower activation energy. They decrease the activation energy which increases the
proportion of particles with energy to react.
- In an energy profile diagram, the activation energy would be lower as catalysts speed
up the reaction by lowering the activation energy.
, - When a catalyst is present, the particles require less energy to cross the activation
energy barrier. This means that more particles can successfully collide per second.
That increases the rate of the reaction.
Reversible reactions:
- In some chemical reactions, the products of the reaction can react to produce the
original reactants. These are called reversible reactions.
- The direction of reversible reactions can be changed by changing the conditions. For
example, for forward reaction use hot conditions and for reverse use cool.
- An example of this is the Haber process: hydrogen + nitrogen → ammonia.
- If a reversible reaction is exothermic in one direction, it is endothermic in the opposite
direction. The same amount of energy is transferred in each case, just for one the
energy will be lost and for the other the same amount will be gained.
- When a reversible reaction occurs in a closed system, equilibrium is reached when
the reactions occur at exactly the same rate in each direction.
Concentration and reversible reactions:
- Equilibrium means in a closed system, the rate of the forward and backward
reactions are equal.
- Le Chatelier’s principle: If a system is at equilibrium and a change is made to the
conditions, then the system responds to counteract the change.
- 2NO2 → N2O4
- This is a reversible reaction. If we increase the concentration of NO2, at equilibrium
the system will counteract any changes that we make. If we change the
concentration of the reactants or products then the system is no longer at equilibrium.
So the forward and reverse reactions are not taking place at the same rate. The
concentrations of all the substances will change until equilibrium is reached.
- Because we have increased the concentration of NO2, the position of equilibrium
shifts towards the products (to the right) so more N2O4 will be formed until
equilibrium is reached again.
- If we decrease the concentration of N2O4 the system is no longer at equilibrium.
More NO2 will react to form N2O4 until equilibrium is reached again. If we increase
the concentration of N2O4, the equilibrium shifts to the left so more NO2 will be
formed until equilibrium is reached again.
Temperature and reversible reactions:
- If temperature is increased the equilibrium shifts in the direction of the endothermic
reaction. For example, if the forward reaction is endothermic and temperature is
increased, equilibrium shifts to the right to produce more product.
- If the temperature is decreased, equilibrium moves in the direction of the exothermic
reaction.
- In endothermic reactions, an increase in temperature causes an increase in yield but
a decrease in temperature causes a decrease in yield.
- In exothermic reactions, an increase in temperature causes a decrease in yield but a
decrease in temperature causes an increase in yield.
- By cooling we are removing heat and the equilibrium shifts right and the color gets
lighter.
- By heating we are adding heat and the equilibrium shifts left and the color gets
darker
Pressure and reversible reactions:
, - In gaseous reactions, an increase in pressure will favour the reaction that produces
the least number of molecules as shown by the symbol equation for the reaction.
- If pressure is increased, equilibrium shifts to the side of the equation with
fewer moles of gas. In this reaction: N2 + 3H2 → 2NH3 is reversible and
makes ammonia. Nitrogen has 1 molecule and hydrogen as 3 molecules so
we have 4 molecules in total on the left. On the right we have 2 molecules
of ammonia. If we increase the pressure, equilibrium shifts to the right as
there are fewer moles of gas on the right, making more product.
- If we reduce the pressure, then the position of the equilibrium shifts to the side with
the larger number of molecules. If both sides in a reversible reaction have the same
number of molecules, changing pressure has no effect on the position of equilibrium.
- For the larger volumes of gas (more moles), an increase in pressure decreases the
yield but a decrease in pressure causes an increase in yield.
- However for smaller volumes of gas (less moles), an increase in pressure produces a
greater yield but a decrease in pressure means less yield.
- High pressures are more dangerous because of the greater risk of explosion. Energy
costs would also be high.
ORGANIC CHEMISTRY
Crude oil and hydrocarbons:
- Crude oil is a finite resource found in rocks. Crude oil is the remains of an ancient
biomass consisting mainly of plankton that was buried in mud.
- Crude oil is a mixture of molecules called hydrocarbons. Hydrocarbons are
molecules made up of hydrogen and carbon atoms only.
- Hydrocarbons like methane and ethane are called alkanes. They have the general
formula CnH2n+2.
- The first four members of the alkanes are methane, ethane, propane and butane.
(monkeys eat peanut butter).
Properties of hydrocarbons:
- As the size of the hydrocarbon molecule increases, the molecules get more viscous.
- Short chain hydrocarbons are extremely flammable.
- Short chains have low boiling points.
- Shorter chains are more volatile
Combustion of hydrocarbons:
- Hydrocarbon fuels release energy when combusted (burned). During combustion the
carbon and hydrogen atoms in the fuel react with oxygen so the carbon and
hydrogen are oxidised. If the oxygen is unlimited, this reaction produces carbon
dioxide and water. This is complete combustion.
- Hydrocarbon + oxygen → carbon dioxide + water.
Fractional distillation of crude oil:
- Crude oil is a mixture of hydrocarbons. Each hydrocarbon has a different boiling
point. Longer chain hydrocarbons have a higher boiling point than shorter chains.
- In fractional distillation, crude oil is separated into fractions. Fractions contain
hydrocarbons with a similar number of carbon atoms.
- Firstly crude oil is heated to a very high temperature in the fractionating column. This
causes the crude oil to boil and evaporate. The vapours now rise up the column and
condense when they reach their boiling point. The column is hotter at the bottom and
cooler at the top. The many hydrocarbons in crude oil can be separated into
fractions. Very long chain hydrocarbons have very high boiling points so these are
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