M6 Phy 1stSem Finals Heat and Gases Section 3 and 4

​Changes in Temperature and Phase

Objectives

• Perform calculations with a specific heat capacity.

• Interpret the various sections of a heating curve.

Each substance has a unique specific heat capacity, which determines how its temperature changes when energy is added or removed. This relationship is expressed mathematically.

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

This approach to determining a substance’s specific heat capacity is called calorimetry, and devices that are used for making this measurement are called calorimeters. Both contain a thermometer to measure the final temperature of substances at thermal equilibrium and a stirrer to ensure a uniform mixture of energy.

As substances undergo changes in temperature, they eventually reach the point of phase change.
Phase change is the physical change of a substance from one state (solid, liquid, or gas) to another at constant temperature and pressure.

Energy conservation can be used to calculate the specific heat capacity, cp,x , of the substance (indicated by the subscript x) as follows:
Energy absorbed by water = energy released by the substance

​Note that ∆T equals the final temperature minus the initial temperature.

​A 0.050 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.

​A 0.050 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.

​A 0.050 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.

​Latent heat is energy transferred during phase changes.
​During a phase change, the potential energy of particles, which is related to attractive bonds, changes. This change in potential energy causes particles to rearrange themselves without altering their average kinetic energy. The energy added or removed per unit mass during this process is called latent heat (L). The heat transferred during a phase change is the product of mass and latent heat.

Because phase and temperature can't change at the same time, latent heat accounts for the extra energy that's added or removed during phase change.

​Two points where phase changes occur latent heat will come into play in both of these locations In the first, it's known as the heat of fusion, which is the energy needed to transfer between the solid and liquid phases. When the water transitions to steam, we use the heat of vaporization. This is also called latent heat, but it's defined as the energy needed to transfer between the liquid and gas phases.

​During melting, the energy added to a substance equals the difference in potential energy between its solid and liquid phases, known as the heat of fusion (L_f). During vaporization, the energy added equals the difference in potential energy between its liquid and gas phases, called the heat of vaporization (L_v). These latent heats are specific to each substance.

Summary

• Specific heat capacity is a measure of the energy needed to change a substance’s temperature.

• By convention, the energy that is gained by a substance is positive, and the energy that is released by a substance is negative.

• Latent heat is the amount of energy required to change the phase of a substance.

​The First Law of Thermodynamics

Objectives

• Calculate heat, work, and the change in internal energy by applying the first law of thermodynamics.

• Apply the first law of thermodynamics to describe cyclic processes.

​The first law of thermodynamics, or the principle of energy conservation, includes a system’s internal energy, work, and heat. For a rapidly compressed balloon, work is done on the system, increasing its internal energy and temperature (W is negative). If the process is adiabatic, no heat is exchanged (Q = 0), and the change in internal energy equals the negative of the work done (∆U = −W). Later, heat is transferred out of the system, decreasing its internal energy (∆U = Q). Visualizing energy flow with arrows into or out of a system can help understand changes in internal energy.