C3 Rates and Energetics

C3 Rates and Energetics

 

Energetics

Exothermic is when the reaction releases heat towards its surroundings, potential energy decreases and that more bonds are formed than broken. Most chemical energy is transferred to thermal energy.

Endothermic is when the reaction absorbs heat from its surroundings, potential energy increases and more bonds are broken than formed. Most thermal energy (from surroundings) is transferred to chemical energy.

 

Rates of reaction

Use of catalysts, increasing temperature and concentration, increasing particle surface area increases rate of reaction.

 

Use gas syringe and stopwatch to measure reaction within a conical flask, such as HCl with Mg.

 

In reality

In mills, very fine powder will react with any spark from a machine. Combustion of fine powders can be prevented by having low temperatures and not having them scattered around oxygen, prevention of sparks from machines.

 

In mines, gases come together to become explosive at high concentrations. There should not be fire within mines or measures should be taken to reduce the concentration of these gases, such as infusing oxygen.

 

Industrial Processes

 

In the little real world, people love to eat vegetables. To make vegetables grow quicker, fertiliser is needed. Important ingrediants are nitrogen, phosphorus and potassium 氮磷钾 because:

Nitrogen helps leaves make proteins, absence will cause stunted growth

Phosphorus needed to make proteins and help cell division, absence will grow slower

Potassium controls opening and closing of stomata, also starch and protein. Absense will cause roots develop worse.

 

Making ammonia by Haber process

Ammonia is NH3, which can become a good fertiliser for nitrogen when reacted with an acid. There is 78% of nitrogen in the air.

Haber process produces ammonia industrially using hydrogen and nitrogen gas.

1. 1. Obtaining raw materials:

 

• Hydrogen from natural gas such as fossil fuels

• Nitrogen from fractional distillation of air.

 

2. Conditions for reaction:
   1. 400-450C temperature
   2. 200 atm pressure – high pressure allows particles to move faster
   3. Iron catalyst lowers activation energy
3. Reaction formula:
   1. N2 (g) + 3H2 (g) ←→ 2NH3 (g)
   2. The reaction is reversible.
4. The mixture is cooled so ammonia can be liquified. Remaining N and H are recycled.

 

Making sulphuric acid with Contact Process

Sulphuric acid can react with ammonia to make ammonium which is a good fertiliser.

1. Obtaining raw materials:

 

• Sulphur from underground deposits

• Air and water

 

1. Conditions for reaction:
   1. 450°C temperature
   2. 1-2 atm
   3. Vanadium oxide catalyst
2. Reaction formula/stages:
   1. Producing sulphur dioxide
      1. S(l) + O2(g)  → SO2 (g)
      2. Not reversible.
   2. Making sulphur trioxide
      1. 2SO2(g) + O2(g) ⇌ 2SO3(g)
      2. Reversible.
   3. Making oleum (not included in syllabus)
   4. Making sulphuric acid
      1. SO3 (g) + H2O(l) → H2SO4 (aq)
      2. Irreversible.