First Law of Thermodynamics and Enthalpy

What is the first law of Thermodynamics?

Energy cannot be created or destroyed — only transferred or changed in form.

What is the formula for the First Law of Thermodynamics?

∆U = Q - W

(Change in internal energy = heat+ work)

What does “q” stand for in the formula ∆U = Q - W ?

q = heat (energy added or removed as heat)

What does “w” stand for in the formula ∆U = Q - W ?

w = work (energy added or removed as work)1

If a system absorbs heat, is q positive or negative?

Positive (+q)

If a system releases heat, is q positive or negative?

Negative (-q)

If work is done on the system, is w positive or negative?

Positive (+w)

If the system does work on the surroundings, is w positive or negative?

Negative (-w)

What does a positive ∆U mean?

The system gained energy.

What does a negative ∆U mean?

The system lost energy.

What’s the unit used for q,w, and ∆U?

Joules (J)

What happens to energy when gas expands?

It does work on the surroundings (w is negative) and loses energy.

What happens to energy when a gas is compressed?

Work is done on it (w is positive) and it gains energy.

What is a system in thermodynamics?

The part of the universes you’re studying — usually the chemicals in the beaker or reaction vessel.

What are the surroundings in thermodynamics?

Everything outside the system — like the air, container, or the lab environment.

What is enthalpy?

Enthalpy (H) is the heat content of a system at constant pressure.

What does the ‘triangle’ H represent?

It’s the change in enthalpy— how much heat is gained or lost in a reaction.

If H is positive, what kind of reaction is it?

Endothermic (heat is absorbed).

If H is negative, what kind of reaction is it?

Exothermic (heat is released).

What is the formula for H using enthalpies of products and reactants?

H = H(products) - H(reactants)

At constant pressure, what is the relationship between q and H?

q = H

Is enthalpy a state function?

Yes— it depends only on the start and end, not the path taken.

What’s an example of an endothermic process?

Boiling, melting, photosynthesis (absorbs heat)

What is PV work?

Work done by or on a gas during expansion or compression, calculate using W= -PV .

What happens to work when a gas expands?

Work is negative; the gas does work on the surroundings and loses energy

What happens to work when a gas is compressed?

Work is positive; the surroundings do work on the gas and it gains energy.

What does it mean if both Q (heat) and W (work) are negative?

Energy leaves the system, and the total internal energy change E is negative — the system cools down.

If Q and W have opposite signs, how do you determine the net energy change?

You compare their magnitudes — whichever is greater determines the sign of E.

When is H = Q a valid assumption?

When a reaction occurs pressure, like in an open container.

What happens if pressure is not constant during a reaction?

H and Q may not be equal, especially if gases are involved in the reaction.

What is a state function?

A property that depends only on the starting and ending state, not path ta