constant pressure

[Mariane Sanchez 1E]

The book says: "In chemistry, though, we are more concerned with heat transfers at constant pressure: most chemical reactions take place in containers open to the atmosphere and therefore take place at a constant pressure of about 1 atm. Such systems are free to expand or contract. If a gas is evolved, the expanding gas needs to drive back the surround- ing atmosphere to make room for itself. Work is done, even though there is no actual piston." I'm just confused as how work was done at constant pressure?

[Curtis Tam 1J]

I think what it means is that chemical reactions may produce a net mol of gas which means there is expansion. If there is expansion, then you would have a nonzero value for deltaV and therefore you would also have a nonzero value for work. I'm confused on that as well :(

[Juanyi Tan 2K]

What I understand about this part is that the reaction is exposed to the atmosphere, which has a constant pressure of 1 atm. So when the system expand or contract, we can consider the work is done on the system or done by the system. Either way the work is done at a constant pressure since the chemical reaction occurs in the atmosphere.

[Joyce Lee 1C]

Here is a somewhat long explanation of how I interpreted it.
Let's consider gas in a piston. When the gas is heated, the kinetic energy of the gas molecules increases, and the individual molecules move faster, colliding more with the piston. The collisions with the piston transfer the energy to the piston and cause it to move against an external pressure, increasing the volume of the gas. Thus gas does work on its surroundings - including the piston and the area around it.
In an instance without a piston, when gas is heated, it expands like before but there is no piston that limits this expansion. So it still does work!