Consider these changes.
(a) hg(l)-> hg(g)
(b) 3o2(g)-> 2o3(g)
(c) cuso4*5h2o(s)-> cuso4(s)+5h2o(g)
(d) h2(g)+f2(g)-> 2hf(g)

at constant pressure, in which of the reactions is work done by the system on the surroundings? by the surroundings on the system? in which of them is no work done?

The mass has escaped into a gas which you can not see so the law stays true.

Firstly, the water in the dams hold gravitational potential energy as they are above ground level. the gravitational potential energy is then transferred to kinetic energy as it falls, spinning the turbinethe turbine then spins the generator, which is a large magnet surrounded by metal coils. by doing this, the generator generates electricity. in this action, the energy transfer taken place is kinetic to electric energy