Topic 4 - Gases, Reversible Reactions and Ammonia Flashcards
What is Avogadro’s law?
One mole of any gas always occupies 24 dm^3 (24000cm^3, 24 l) at room temperature + atmospheric pressure (RTP - 25⁰C + 1 atmosphere), the molar volume of a gas
How do you work the volume of a gas?
mass of gas / M~r of gas [moles] x 24
How do you calculate volumes in reactions?
- Find the reacting mass (balance equation, write down M~r for each, divide to 1 then multiply)
- Use mass of gas / M~r of gas x 24 equation to work out volume
What is a reversible reaction?
One where the products of the reaction can themselves react to produce the original reactants
What is the symbol for a reversible reaction?
A+ B ⇌ C + D
When will an equilibrium be reached?
When a reversible reaction takes place in a closed system
What is an equilibrium?
The amount of reactants + products will reach a certain balance + stay there
What is a dynamic equilibrium?
Where a reversible reaction is taking place in both directions, but overall no effect as forward + reverse reactions cancel each other out. Reaction takes place at same rate in both directions
How can you alter the position of equilibrium (the relative amount of reactants and products)?
- Temperature
- Pressure
How can you use temperature to alter the position of equilibrium?
- All reactions are exothermic in one direction + endothermic in the other
- Increase temp: endothermic reaction favoured to use up extra heat
- Decrease temp: exothermic reaction favoured to give out more heat
How can you use pressure to alter the position of equilibrium?
- Many reactions have greater volume on 1 side (either products or reactants)
- Raise pressure: favour reaction producing less volume
- Lower pressure favour reaction producing more volume
How do catalysts speed up a reversible reaction?
- Speed up both forward + backward reaction by same amount
- So reaches equilibrium quicker but make same amount of product w/ out catalyst; doesn’t change position of equilibrium
How does nitrogen and hydrogen react to form ammonia?
-Nitrogen from air (78% N2)
-Hydrogen from natural gas
-Reversible reaction (occurs in both directions), not all nitrogen + hydrogen convert to ammonia: dynamic equilibrium
N2(g) + 3H2(g) ⇌ 2NH3(g)
What are the conditions used in the industrial manufacture of ammonia and why?
-Pressure, 200 atmospheres: high pressures favour forward reaction, so pressure high as possible to increase % yield without being too expensive to build
-Temp, 450⁰C: forward reaction is exothermic so increasing temp would move equilibrium wrong way (from ammonia towards N2 + H2) so ammonia yield lower but lower temp means slower rate of reaction (equilibrium reached more slowly), compromise to get acceptable yield in acceptable time.
-Iron catalyst: increases reaction but doesn’t affect % yield
(-Ammonia forms as gas but cools in condenser where liquefied and removed)
What is ammonia used for?
- Nitrogenous fertilisers
- Used as increase plant growth
