Chapter 12 Changes in Temperature and Phases

Goals Perform calculations with specific heat capacity. Interpret the various sections of a heating curve.

The specific heat capacity of a substance is defined as the energy required to change the temperature of 1 kg of that substance by 1°C. Also known as specific heat The subscript p indicates that the specific heat capacity is measured at constant pressure. In this equation,  T can be in degrees Celsius or in degrees Kelvin.

The equation for specific heat capacity applies to both substances that absorb energy from their surroundings and those that transfer energy to their surroundings.

Calorimetry is used to determine specific heat capacity. Calorimetry is an experimental procedure used to measure the energy transferred from one substance to another as heat.

Because the specific heat capacity of water is well known (c p,w = kJ/kg°C), the energy transferred as heat between an object of unknown specific heat capacity and a known quantity of water can be measured. energy absorbed by water = energy released by substance Q w = –Q x c p,w m w ∆T w = –c p,x m x ∆T x Subscript w will always stand for water Equation can be rewritten as Q w + Q x =0

Sample Problem A kg metal bolt is heated to an unknown initial temperature. It is then dropped into a calorimeter containing 0.15 kg of water with an initial temperature of 21.0°C. The bolt and the water then reach a final temperature of 25.0°C. If the metal has a specific heat capacity of 899 J/kg°C, find the initial temperature of the metal.

When substances melt, freeze, boil, condense, or sublime, the energy added or removed changes the internal energy of the substance without changing the substance’s temperature These changes in matter are called phase changes. The energy per unit mass that is added or removed during a phase change is called latent heat, abbreviated as L. Q = mL energy transferred as heat during phase change = mass  latent heat

During melting, the energy that is added to a substance equals the difference between the total potential energies for particles in the solid and the liquid phases. This type of latent heat is called the heat of fusion, abbreviated as L f. During vaporization, the energy that is added to a substance equals the difference in the potential energy of attraction between the liquid particles and between the gas particles. In this case, the latent heat is called the heat of vaporization, abbreviated as L v.