Thermodynamics Heat Capacity Phase Changes, Study notes of Thermodynamics

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Thermodynamics

Heat Capacity

Phase Changes

Lana Sheridan

De Anza College

April 24, 2020

Last time

• finish applying the ideal gas equation
• thermal energy
• (^) introduced heat capacity

Warm Up Question

Quick Quiz 20.1^1 Imagine you have 1 kg each of iron, glass, and water, and all three samples are at 10◦C. (b) Rank the samples from greatest to least amount of energy transferred by heat if each sample increases in temperature by 20 ◦C. Heat capacities: glass – 837 J kg−^1 K−^1 iron – 448 J kg−^1 K−^1

(A) iron, glass, water (B) water, iron, glass (C) water, glass, iron (D) glass, iron, water

(^1) Serway & Jewett, pg 579.

Warm Up Question

Quick Quiz 20.1^1 Imagine you have 1 kg each of iron, glass, and water, and all three samples are at 10◦C. (b) Rank the samples from greatest to least amount of energy transferred by heat if each sample increases in temperature by 20 ◦C. Heat capacities: glass – 837 J kg−^1 K−^1 iron – 448 J kg−^1 K−^1

(A) iron, glass, water (B) water, iron, glass

(C) water, glass, iron ←

(D) glass, iron, water

(^1) Serway & Jewett, pg 579.

Heat Capacity

Heat Capacity, C of a sample of substance is the quantity of heat required to change the temperature of that sample by 1 degree C (or K).

Q = C ∆T

where ∆T is the change in temperature and Q is the heat.

Specific Heat Capacity, c of a substance is the quantity of heat required to change the temperature of a unit mass of that substance by 1 degree C (or K).

Q = cm ∆T

m is the mass of the object.

Heat and Temperature Change

Energy that causes a change in temperature does not have to enter our system as heat.

It can be a different form of energy transfer.

Examples:

• (^) in a microwave, energy TER enters the food as electromagnetic waves
• (^) work can cause a temperature change in two surfaces rubbed together, or as a bicycle pump pressurizes air in the bike tires, the air’s temperature rises

These energy transfers to our system will increase the internal energy of the system, Eint.

Units of Internal Energy and Heat: Calories

The units of both internal energy and heat are Joules, J.

However, heat was not always understood to be an amount of energy, so other units have been defined for it, and are still sometimes used.

1 calorie is the heat required to raise the temperature of 1 gram of water by 1 degree Celsius.

The “calories” listed on food labels are sometimes called “Calories” (capital C) because they are in fact kilocalories.

1 Calorie = 1 kilocalorie = the heat required to raise the temperature of 1 kilogram of water by 1 degree Celsius.

1 calorie = 4.18 Joules.

Calorimetry

Calorimetry a technique for determining the specific heat capacity of a sample by heating it to a known temperature, then transferring it to a known quantity of water and observing the temperature change in the water.

Steps: 1 sample of known mass mx is heated to temperature Tx 2 sample is moved to an isolated container of water, containing mass mw of water at temperature Tw < Tx 3 the sample and the water are allowed to reach thermal equilibrium 4 the final temperature of the water, Tf , is measured

Phase Changes

The processes by with matter changes from one state to another.

The different states of matter: solid, liquid, gas, plasma, are also called phases of matter.

Phase Changes

Phase changes tend to be dramatic.

If sudden, obvious changes in the properties and behaviors of a substance did not occur as we vary the temperature, we would probably have no need to refer to different states of matter!

Phase Changes

We know that as we heat a solid it will eventually melt to form a liquid and if we keep heating the liquid will boil off as a gas.

But how does the temperature change during these processes?

Phase Changes

During a phase change, temperature doesn’t change, even when heat is added!

Phase Changes

Why does this happen?

Where is the energy going?

It isn’t increasing the translational speed of the atoms, that would relate to an increase in temperature.

→ It goes into breaking bonds.

Latent Heat

latent heat of fusion, Lf The amount of energy (heat) per unit mass required to change a solid to a liquid. Q = mLf

where m is the mass of solid that is transformed into a liquid.

latent heat of vaporization, Lv The amount of energy (heat) per unit mass required to change a liquid to a gas. Q = mLv

where m is the mass of liquid that is transformed into a gas.

(^1) “Latent” from latere, “to lie hidden”.