reversible reactions

reversible reactions examples

All are physical:

- freezing

- melting

- sublimation

- deposition

but a small selection of chemical reactions are reversible

equillibrium =

forward and reverse rates are equal

step 1 of reversible reaction

Initially, the rate of forward reaction is high, lots of a+b to collide

step 2 of reversible reaction

over time the forward rate decreases as less A+B but backwards rate starts to increase

step 3 of reversible reaction

eventually we reach equilibrium, when both rates are equal. The reaction continues, but we observe no further change.

- This only happens in a closed system

practice exam question:

Describe what happens to concentration and rate of reaction for the reaction A+B -_C+D

-at the start of the reaction, a+b had a high conc and a high rate of reaction, and the reverse c+d had no conc and the rate was 0

-As the reaction continued, the forward a+b were reducing in conc and rate, but for the reverse c+d reaction, conc was increasing

-When we reach equilibrium, a+b conc and rate was constant, and so was the reverse.

dynamic equilibrium

When the forward and backward rate of reaction are the same so they happen simultaneously, and we observe no further change.

chateliers principle

He states that if a change is applied to a system at a dynamic equilibrium, the equilibrium will shift in a direction that counteracts the change. Either left or right

- basically, we can alter the position of equilibrium to favour one of the reactions by altering one of three things

• temperature

• pressure (for gases)

• catalysts

chatelier 1 - temperature

- All reversible reactions are exothermic one way and endothermic the other

- If we increase the temperature, we favour the endothermic direction as there is more energy to take in.

-adjusting the temperature at which the reaction is taking place in the reaction will counteract the change

chatelier 2 - pressure (for gases)

-If we increase the pressure at which the reaction is happening at then the reaction will favour the reaction that produces fewer molecules

-increase the pressure favours the reaction that produces less molecules

chatelier 3 - catalyst

- speeds both reactions up equally

- Adding a catalyst doesn't impact the position of the equilibrium

- It just reaches the equilibrium faster

Haber process

making ammonia -

• Ammonia is an important chemical in the production of lots of other chemicals:

  -fertilizers

  -cleaning products

  -bombs/explosives

• Ammonia is made industrially by reacting nitrogen and hydrogen in the Haber process (it's reversible, so it is never complete)

• nitrogen+hydrogen ~ ammonia

• N^2 (g) + 3H^2 (g) ~ 2NH^3 (g)

Haber process (making ammonia)

air - nitrogen

-ammonia

methane/natty gas - hydrogen

haber compromise 1 - temperatur

- needs to be low to favour forward reaction

- but high enough so the rate is good

- but low temp=low rate

- compromise=450*

haber compromise 2 - pressure

dont understand so look carefully

- The forward reaction produces fewer molecules, so high pressure is needed

-but high pressure is difficult to maintain

-could explode

- cost a lot

- compromise=200atm (atmospheres)

- high enough to favour forward reaction

- but low enough so safer/pays less to workers

haber compromise 3 - catalyst

- Catalysts speed up both reactions equally, so we reach equilibrium faster.

- so we use an iron catalyst (no compromise)

- Use iron because it is a transition metal