Chemical Kinetics
→ Basics:
- STP (Standard Temperature & Pressure)- 1 atm at 27.315 degrees Kelvin/ 0 degrees Celsius
- Chemical Reactions- represented by a word equation, which uses the names of
the reactants and products, or by a chemical equation, which uses chemical formulas.
1. Evolution (release) of gas- ie: Zn + H2SO4 → ZnSO4 + H2
2. Change of state (of matter)- ie: KI + Pb(NO3)2 → PbI2 + 2KNO3
3. Change of colour- ie: C6H8O7 [citric acid]+ 3KMnO4 [potassium permanganate]→ 3HMnO4
[permanganic acid] + C6H5K3O7 [Potassium isocitrate]. Pink to colourless.
4. Change of temperature- ie: Water + heat → Vapour
- Law of conservation of mass- Mass can neither be created nor destroyed in a chemical reaction.
Total no. of atoms = Total no. of atoms & Total mass = Total mass.
[While balancing a Chemical equation, the formula of reactants and products should not be changed]
- Reactants- LHS, substances that undergo chemical change
- Products- RHS, New substance formed during the reaction
- Symbols-
↑ ↓ → ⇋ Δ + - [] (aq.)
Gas Precipitation Forward Reversible In the Absorbing Releasing Concentration Aqueous
release reaction reaction presence of energy energy
Chemical kinetics- the study of the rate of a chemical reaction under changing conditions. It’s to know
if reaction occurs at useful rates as per conditions (temperature/pressure /concentration).
Systems-: Open- Energy AND matter move
in and out freely. Ie: Body
Closed- Energy NOT matter moves in and
out freely. Ie: Saucepan with lid
Isolated- Energy NOR matter move in and
out freely. Ie: Thermos bottle
Exothermic= Reactants → Products
+ Energy. Ie: Neutralisation, fuel
combustion (burning), human
respiration, etc; Endothermic=
Reactants + Energy → Products. Ie:
Cooking, Photosynthesis, etc.
→ Collision theory:
- For a reaction to occur-
1. The reacting particles must collide with each other
2. The colliding particles must have the correct orientation at the time of collision
3. The particles must have the minimum kinetic energy required to initiate a reaction
→ Rate of Reaction:
- The speed at which reactants transform into products.
- Mean ROR=
ΔQuantity of Reactants used/Time taken OR ΔQuantity of products formed/Time taken
Where quantity is measured in grams or cm^3 and time is measured in seconds
- Alternatively, with the QR/TT we can calculate using moles/minute
- Real ROR=
GCSE Chemistry - Rates of Reaction
- Factors affecting ROR=
1. Surface Area
- Collisions take place at the surface of solids.
- Larger surface area = more frequent collisions (as it increases the chances)
- In reactions involving solids, powders react much faster than lumps or granules, which
react faster than lumps (all other variables kept constant). Powders > Granules>Lumps.
- Example: Compare the effect of surface area on the reaction between two equal masses
of calcium carbonate [CaCO3] and hydrochloric acid [HCl]. Notice how the initial
reaction rate is faster for the powdered solid because the acid can attack the surface of
the carbonate more easily. However, the outcome of the experiment is identical for both
conditions; the same amount of carbon dioxide, CO2 is eventually produced.
2. The temperature at which the reaction is carried out.
- When the temperature of the reaction mixture is increased, the reacting particles
(molecules, ions or atoms) move around at higher average speeds and hence have higher
average values of kinetic energy.
- This results in an increased rate of successful collisions and therefore an increased rate of
reaction plots the initial rate of a chemical reaction with temperature.
- The rate indicates the frequency of successful collisions.
3. Concentration of solution/Gas pressure
- As the concentration of a substance increases, the particles (usually ions or molecules)
are closer together. The number per unit volume increases. Concentration is usually
measured in moles per cubic decimeter (mol dm−3). The word ‘concentration’ is usually
used to describe a solution.
- In gases, increasing the pressure has the same effect as pushing more particles together
per unit volume. This produces an increased number of collisions, and so the reaction
rate increases. A gas at high pressure is more concentrated than a gas at low pressure.
4. Use of a catalyst, including enzymes.
- Catalysts provide an alternative reaction pathway or lower activation needed for the
transition state. (Reduced minimum energy required for initiation).
- The alternative pathway increases the rate at which products are formed. Catalysts can
also initiate reactions which would normally happen extremely slowly.
- This may be due to a steric effect (e.g. some enzymes hold the substrate in the correct
orientation for reaction), but it could also be because the presence of the catalyst leads to
a different reaction mechanism.
- The presence of the catalyst does not change the total amount of product formed, nor
does it affect the overall energy (enthalpy) change of a reaction.
- Enzymes function the same way. They lower the energy of the transition state. However,
because they are proteins, these very large molecules are also affected by intermolecular
attractions. Most enzymes are adapted to function best in particular, narrow conditions,
for example, at a particular temperature or within narrow ranges of pH.
GCSE Chemistry - Factors Affecting the Rate of Reaction
GCSE Chemistry - How to Calculate the Rate of Reaction - Measuring Rate of Reaction
→ Calculation of Bond enthalpy
- Bond forming: Releases energy
- Bond breaking: Requires/Absorbs energy to
overcome or break electrostatic attraction between
atoms or ions
- Bond Enthalpy (AKA Bond dissociation energy, E):
Energy required to break one mole of bonds in the gaseous state.
- Average bond enthalpy: Enthalpy change when one mole of bonds are broken in gaseous state,
averaged for the same bond in similar compounds.
- So the calculation will be energy in - energy out.
→ Reactions
- Reversible reactions: Forward → & Backward ← simultaneously in a closed system.
- Thermal dissociation: Reversible splitting up of a compound by heat to form products, which can recombine
on cooling. (ie- 2 Nitrogen Dioxide (g) ⇋ 2 Nitric Oxide + Oxide = 2NO2 → 2NO + O2; Ammonium Chloride (s) ⇋ Ammonia
(g) + HCl (g) = NH4Cl ⇋ NH 3+ HCl)
- Thermal decomposition: Splitting up a compound by heat to form products which don’t recombine on
cooling. Unlike thermal dissociation, more bonds are broken /rearranged and therefore these reactions aren’t
easily reversible. (ie- Pb(NO3)2 (s) →2PbO (s) + 4NO2 (g) + O2 (g))
→ Dynamic equilibrium
- Consider: Calcium Carbonate ⇋ Calcium Oxide + Carbon dioxide / CaCO3 (s) ⇋ CaO (s) + CO2 (g)
- In an OPEN container: The reactant decomposes, and the product formed (g) is
released. It continues until all of the reactant (s) is converted intro product (s); favors →.
- In a CLOSED container: The product (g) recombines with the product (s) to form
reactant (s). HOWEVER, the reaction to the right is faster, so as the reactant is used up,
the forward reaction becomes slower. Eventually, the rate of forward reaction = the rate of
backwards reaction.
Dynamic equilibrium = When forward reaction rate=backward reaction rate.
Dynamic equilibrium | Equilibrium | AP Chemistry | Khan Academy
→ Le Chatelier’s principle & the factors that affect it
- Le Chatelier’s principle: If a change is applied to a system in equilibrium, the system changes to counteract that
stress or change, by either rushing the reaction in the forward or backwards direction by changing the
conditions.
- Factor 1: Concentration
Fav Of Reactants Of products
→ If concentration of reactant is increased; Forward If the concentration of products is decreased, the forward
reaction speeds up to use up the reactants by turning reaction will speed up to produce more and use up more
them into products. reactants.
← Likewise, if the concentration of reactants decreases, the If concentration of products is increased; backward
backwards reaction speeds up, to product less. reaction spreads up; to produce less.
In simple words, produce more ≠Reactants used up less; Produce less
≠Reactants used up more.
- Factor 2: Temperature
Endothermic (ΔH=+ive / H + R ⇋ P) Exothermic (ΔH=-ive / R ⇋ P + H)
Products=Cold/require energy; Products=Hot/release energy;
Reactants=Hot/release energy Reactants=Cold/require energy
Increasing temperature speeds up Increasing temperature speeds up SO: If temperature is higher on one side, the
forward reaction → backward reaction ← reaction will be favoured to the other side.
- Factor 3 : Pressure (Reactants & Products must be gases)
Shift Of reactants Of products
← If pressure on reactants decreases; pressure on If pressure on products increases; Pressure on reactants
products increases; reaction shifts backwards because decreases; reaction shifts backward because product side
reactant side has less moles. has more moles.
→ If pressure on reactants increases, pressure on products If pressure on products decreases, Pressure on reactants
decreases; the reaction shifts forward because the increases; the reaction shifts forward because the
reactant side has more moles. product side has fewer moles.
Example:
SO: Equilibrium shifts to the side with fewer molecules. BTW
Pressure ∝ Volume.
Le Chatelier's Principle
→Haber’s process & the factors that affect it What Is The Haber Process | Reactions | Chemistry | FuseSchool
What? - The Haber process involves the reaction of 1 mol of nitrogen gas and 3 moles of hydrogen gas in the
presence of a catalyst: N2 (g) + 3H2 (g) 2NH3 (g) H = –92 kJ mol–1
- Haber’s genius was to investigate and propose the ideal conditions under which the highest yield possible
would be achieved in the shortest amount of time. He did this by determining the ideal temperature and
pressure for the reaction.
- Synthetic replication of the biological process of nitrification, which produces nitrogen (a finite resource,
but now is controlled by humans easily).
- As ammonia is an essential component used to provide nitrogen for crop fertilisers, today the annual
global production of ammonia stands at over 150 million tonnes and rising. Ammonia is mainly converted
into other important compounds such as nitric acid, HNO3, ammonium nitrate, NH4NO3 , ammonium
sulfate, (NH4)2SO4, and urea, NH2CONH2.
When? - Created in 1913; Used during World War One (1914-1918); Nobel Prize in 1918
- Mid-20th century era known as the “Green Revolution”
Who? - German chemist Fritz Haber (figured out how to maximise yield in the shortest time)
- German chemical engineer Carl Bosch (scaled it for industrial use)
Why? - To address the need for a reliable source of nitrogen for fertiliser production of crops as a result of a
growing global demand for food (due to an increasing population, it was initially 1.6 billion only).
- It was more reliable & cost-effective than natural processes of fertilisation, being applied to chemical
fertilisers, synthetic pesticides, and herbicides resulted in a higher crop yield.
- Moreover, nitrogen also helped produce a range of explosives for Germany during WWI.
How? - Obtaining hydrogen: Through a reaction between methane & steam, carbon monoxide is a byproduct
(CH4 (g) + H2O (g) → CO (g) + 3H2 (g)). Carbon monoxide affects ozone layer formation & atmospheric methane removal.
- Obtaining nitrogen: From the atmosphere through fractional distillation of liquid air, where H2 (g) meets
O2 (g) and leaves Nitrogen behind (as it makes up 77% of the air).
Liquefaction of Air: Cooled & compressed by passing it through a series of heat exchangers, at around -200°C.
Fractional Distillation: Fed into fractionating column with temperature gradient (bottom is warmer than top).
Separation of Gases: Nitrogen, having the lowest boiling point, vaporizes first and rises to top, where its collected.
Compression of Hydrogen & Nitrogen gas before entering the reactor.
- Making Ammonia: Nitrogen & Hydrogen react in gaseous form at proportions 1 : 3.
Pressure: Economically compromised higher pressure of 200 atm is applied to the reaction so that the reaction
shifts forwards faster (fewer moles).
Temperature: Forward reaction (exothermic); reactant temperature is lowered to a compromise of 400C °C for
speed. Too low is too slow, and too high will favour ammonia decomposition because it's exothermic.
Catalyst: Finely divided (for surface area) iron particles are used to speed up the rate of equilibrium.
Concentration: Higher for reactants to favour the forward reaction →.
- Unused ( 90-80%) nitrogen & hydrogen are recycled back to the reactor after heating and cooling for
liquefaction.
→ Nitrogen cycle
Understanding Our Soil: The Nitrogen Cycle, Fixers, and Fertilizer
The Nitrogen Cycle: Processes, Players, and Human Impact | Learn Science at Scitable
Why soil is one of the most amazing things on Earth | BBC Ideas
How Does The Use Of Fertilizer Affect The Nitrogen Cycle? - The Plant Enthusiast
- Nitrogen presence: It exists most abundantly in the atmosphere, making up roughly 78% of the gases in the air.
Atmospheric nitrogen is N2, which is LESS reactive.
- Nitrogen fixation: Occurs due to nitrogen-fixing bacteria (Rhizobium, Blue algae, Roots of leguminous
plants). It arrives in the form of ammonium, NH4+. It can occur from lightning as well.
- Nitrification: Mostly aerobic, carried out by prokaryotes (bacteria); Forms nitrite, NO2, which is in the soil.
Further nitrification forms NO3, nitrate, in the soil.
- Assimilation: Plants assimilate nitrogen and grow, and get eaten by herbivores.
- Ammonification: When a plant-eating animal dies, its carcass decays and forms ammonia, NH4; it gets
converted by bacteria into nitrite again through nitrification.
- Denitrification: Anaerobic bacteria, generally due to excessive nitrogen presence in soil or lack of oxygen,
denitrify the nitrate into nitrogen gas (N2) or nitrous oxide (N2O), which is a greenhouse gas.
→ Artificial fertilisers
Positive effects:
- Without the Haber process, farmers would only be able to produce food for 4 billion people. Thats 40%.
- 131 million metric tons are produced each year (80% is used in fertilisers, while the rest is used in industrial and
household cleaners, pharmaceuticals, plastic/synthetic fibres, refrigeration, etc).
- Increases crop yields, demands less land for farming (supporting higher population growth).
- Provides food security, especially in regions with limited arable land.
- The Haber process, in particular, is economically beneficial and allows for the production of other similar
products.
- Economic benefits, scientific and technological advantages, and more population growth are vital to boost
consumer spending and more.
Negative effects:
- Adds LOOSE, pure nitrogen in the soil.
- Population growth aint so good sometimes.
- Research finds that half of the nitrogen is not assimilated by plants.
- Eutrophication through leaching (Algae overgrowth; Reduces oxygen levels; Yamuna River)
- Excess reduces the presence of nitrogen-fixing bacteria in soil
- Pure nitrogen can undergo volatilisation by bacteria and come out as nitrous oxide (greenhouse)
- Over-reliance slows down natural nitrogen fixation in plants
- Decreased bacterial diversity can reduce soil fertility
- Irritates earth worms
- Removes carbon from soil
- Disrupts good fungus
- Imbalances in pH levels
- No more nitrogen left in soil; demands more fertiliser, SOIL DIES
- Nutritional decline in soil leading to nutritional decline in food leading to health issues (Bihar well issue)
- Plants are weaker, more prone to disease
- Nutrients also leached
Solutions:
- Using nitrogen-fixing plants (Pea family - 18000 members, examples like clover).
- Crop rotation (Changing the crops we grow on one land every once in a while).
- Biofertilisers
- Integrate trees (agroforestry)
- Conserve tillage to prevent erosion and leaching, and pollution of water.
- Bioremediation techniques like phytoremediation (plants that break down/absorb pollutants).
The chemical reaction that feeds the world - Daniel D. Dulek
Effective manipulation of finite resources to increase the efficiency of the rate of
production as well as increase/maximise the product yield
