3.6 CHAPTER 3. REACTION RATES
Open and closed systems ESCAW
An open system is one in which matteror energy can flow intoor out of the system.
In the liquid-gas demonstration we used, the first beaker was an example of an open
system because the beaker could be heated or cooled (a change in energy), and water
vapour (the matter) could evaporate from the beaker.
A closed system is one in which energycan enter or leave, butmatter cannot. The
second beaker covered by the bell jar is an example of a closed system. The beaker can
still be heated or cooled, but water vapour cannot leave the system because the bell
jar is a barrier. Condensation changes the gas toliquid and returns it to the beaker. In
other words, there is noloss of matter from the system.
DEFINITION: Open and closed systems
An open system is one whose borders allow the movement of energy
and matter into and outof the system. A closed system is one in which
only energy can be exchanged, but not matter.Reversible reactions ESCAX
Some reactions can takeplace in two directions.In one direction the reactants combine
to form the products. This is called the forward reaction. In the other, the products
react to form reactantsagain. This is called the reverse reaction. A special double-
headed arrow (�) is used to show this type of reversible reaction:
XY + Z� X + YZ
So, in the following reversible reaction:
H 2 (g) + I 2 (g)� 2HI(g)The forward reaction is H 2 (g) + I 2 (g)→ 2 HI(g). The reverse reaction is 2 HI(g)→ H 2 (g) + I 2 (g).