Question 1

Why is enthalpy change (ΔH) defined at constant pressure?

Accepted Answer

Most chemical reactions, especially in a school laboratory, are carried out in open containers, exposed to the atmosphere. This means they occur at a constant atmospheric pressure. At constant pressure, the heat transferred is exactly equal to the change in enthalpy (ΔH). If the pressure wasn't constant, we would also have to account for energy used to do work (expanding or contracting against the changing external pressure), which is more complex.

Question 2

What is the difference between heat and temperature?

Accepted Answer

They are related but distinct. Temperature is a measure of the average kinetic energy of the particles in a substance; it tells us how 'hot' or 'cold' something is. Heat is the energy that is transferred from a hotter object to a colder one. A large swimming pool at 25°C contains far more heat energy than a cup of coffee at 90°C, even though the coffee has a higher temperature, simply because there is so much more of it.

Question 3

In a calorimetry calculation, why do we use the specific heat capacity and mass of water, even when a solid is dissolved in it?

Accepted Answer

This is a key simplifying assumption. For dilute aqueous solutions, we assume that the solution has the same density (1 g cm⁻³) and specific heat capacity (4.18 J g⁻¹ K⁻¹) as pure water. This is a reasonable approximation because the solution is mostly water. In reality, the dissolved substance does slightly alter these properties, which is one source of inaccuracy in simple calorimetry experiments.

Enthalpy change, ΔH

What you need to know

Tracking Heat in Reactions