Thermodynamics: common misconceptions

If your child says… “A closed system can let matter escape.”

Ask together

Which arrow crosses the closed system’s boundary?

What’s true

A closed system allows energy exchange across its boundary, but no matter exchange.

If your child says… “Expanding gas does work, so its work must be positive.”

Ask together

When gas pushes its surroundings, which side receives energy?

What’s true

Expansion against outside pressure transfers energy away from the gas, giving work a negative sign.

If your child says… “The insulated cup absorbs no heat.”

Ask together

If the cup warms too, where does its energy come from?

What’s true

Insulation reduces outside heat exchange, but the cup itself can warm and absorb heat.

If your child says… “Doubling the reaction leaves its heat change unchanged.”

Ask together

If each mole releases heat, where would the second mole's contribution go?

What’s true

The enthalpy change belongs to the stated amount, so doubling that amount doubles the change.

If your child says… “Different reaction routes must give different total heat changes.”

Ask together

Which starting or finishing level changes when you take the route through the intermediate?

What’s true

Identical starting and finishing states give the same enthalpy change, whatever the route.

If your child says… “Both blocks’ entropy must increase for heat to flow naturally.”

Ask together

Which entropy arrow is larger: the gain or the loss?

What’s true

One block’s entropy can decrease while their combined entropy increases.

If your child says… “A negative Gibbs energy change means equilibrium.”

Ask together

If melting is still favoured, has the ice-water system reached equilibrium?

What’s true

At constant temperature and pressure, a negative reaction Gibbs energy change favours the forward change. A zero value means equilibrium.

If your child says… “The standard Gibbs energy change is zero at every equilibrium.”

Ask together

What happens to the logarithm if the equilibrium constant differs from one?

What’s true

The standard change is zero only when the equilibrium constant equals one.