Thermochemistry

Thermochemistry

Basic concepts of thermodynamics

– Energy is important for all living organisms to carry out their mental or muscular activities.
– Living organisms can get their energy from burning sugar inside their bodies.
-Heat energy is a form of energy that can be obtained from burning of natural gas.

Thermodynamics:

The science that deals with the study of energy and how it transfers.

Thermochemistry:

Branch of chemistry that studies the heat effects that accompanied the chemical reactions.

-There are different forms of energy as ( heat , light , electric, kinetic, ….) ,

-All these forms are related to each other by law of conservation of energy.

Law of conservation of energy:

Energy in any physical or chemical change can be neither created nor destroyed but it is transformed from one form to another.

What is the relation between chemical reaction and energy?

-All chemical reactions is associated with changing in energy either release or absorb energy -Energy exchange occurs between reaction mixture and surrounding.

System in Thermochemistry :

• It is the part of the substance under study.
• It is the part of the universe in which physical or chemical change occurs.

Surrounding:

• It is the part outside the system and exchange energy with it in the form of heat or work.

Types of systems:

Open system:

It freely exchange matter and energy with its surroundings.

Closed system:

It exchange energy but not matter with its surroundings in the form of heat or work.

Isolated system

It does not exchange neither energy or matter with its surroundings.

Here are examples illustrating the concepts of open, closed, and isolated systems:

Open System Example:

Consider a pot of boiling water on a stove. In this scenario:

System:

The pot and its contents (water) constitute the system.
Surroundings:

The environment outside the pot, including the air and surroundings.

Energy and Matter Exchange:

Heat from the stove transfers to the water, causing it to boil. As the water boils, steam (matter) escapes into the surroundings, and heat (energy) dissipates into the air. Additionally, water vapor may condense on the lid and drip back into the pot, representing a partial exchange of matter.
Conclusion: This represents an open system because both energy and matter can freely exchange between the system (pot and water) and the surroundings.

Closed System Example:

Consider a sealed thermos flask filled with hot coffee. In this scenario:

System: The thermos flask and the coffee inside constitute the system.
Surroundings: The environment outside the thermos flask.
Energy and Matter Exchange: Heat from the hot coffee is retained within the thermos flask due to its insulating properties, preventing significant heat exchange with the surroundings. Although the flask is sealed, there might be minimal heat loss through the walls of the thermos over time. However, no matter exchange occurs.
Conclusion: This represents a closed system because while there may be some minimal exchange of energy (heat), there is no exchange of matter between the system and the surroundings.

Isolated System Example:

Imagine a perfectly insulated and sealed container with a chemical reaction occurring inside. In this scenario:

System: The container and the chemicals undergoing the reaction inside constitute the system.
Surroundings: The external environment beyond the container.
Energy and Matter Exchange: Due to perfect insulation, there is no exchange of heat or matter between the system and the surroundings. The chemical reaction proceeds without any influence from external factors, and the total energy (including internal energy and any energy associated with the chemical reaction) remains constant.
Conclusion: This represents an isolated system because there is no exchange of energy or matter between the system and the surroundings.
These examples  shows the differences in energy and matter exchange between open, closed, and isolated systems, highlighting the principles of thermodynamics and system classification.

Give Reason:
The medical thermometer is considered as a closed system?
-Because it allows the exchange of energy only with the surrounding.

First law of thermodynamics:

The total energy of an isolated system is constant even the system is changed from one state to another.

-The relation of energy exchange between the system and surrounding

Universe = System + Surrounding -Change in universe energy

    = Change in system energy +Change in surrounding energy

ΔE Universe = ΔE System + Δ E surrounding

Any change in system energy is accompanied by similar change in the surrounding energy but with opposite sign to keep the total energy constant

ΔE system =-Δ E surrounding

Heat and temperature:

Heat flow from one position to another depending on the difference in temperature between the two positions.

First law of thermodynamics:

The total energy of an isolated system is constant even the system is changed from one state to another.

Temperature:

• It is indication of hotness or coldness of an object.
  Or
• It is measurement of the average kinetic energy of matter molecules.

-Matter consists of molecules or atoms , they are in continuous motion but they differ in speed according to their kinetic energy.
– When the system absorbs heat energy, kinetic energy increase the temperature increase.

Measuring units of quantity of heat:

Calorie:

It is the quantity of heat needed to raise the temperature of 1 g of water by 1o C

Joule

It is the quantity of heat needed to raise the temperature of 1 g of water by 1 / 4.18 c

Specific heat:

The quantity of heat needed to raise the temperature of one gram of the substance 10 C.
Unit: J/g
• Each substance has definite specific heat .
• The substance that has high specific heat need large quantity of heat to rise its temperature and also takes a long time to lose this heat again.
• Water has the highest specific heat.

•  Give reason Water causes a moderate climate in a coastal areas.
  Because it has high specific heat.

Calculating the quantity of heat:

The quantity of heat absorbed or released from the system calculated by this relation.

Quantity of heat = qp = m.c. Δ T

The quantity of heat at constant pressure.(joule) m
The mass of substance(gram)
c The specific heat(J/g.0C)
T= T2 – T1 ( final temperature – initial temperature) (c)

Example:

Using the calorimeter, 0.28 g of propanol was burned.

The temperature of water increased by 21.5 C if you knew that the mass of water in the calorimeter is 100 g , calculate the released quantity of heat from the burning of this amount of fuel.
Answer:

q_p = m.c. T = 100 × 4.18 x 21.5 =9030 J

Example:

Dissolve one mole of ammonium nitrates in an amount of water. Complete the solution volume to 100 ml of water. You notice that the temperature decreases from 250C to 170C calculate the quantity of absorbed heat.
Answer:
The mass of 100 ml water is 100 g
q_p = m.c. Δ T
q=100 x 4.18 x (17 – 25) = – 3344 J

The calorimeter:

It is an isolated system that allows us to measure the change in temperature of isolated system because it prevents lose or gain of heat or substance to the surroundings.

Types of calorimeter:

Coffee – cup calorimeter

Structure of  Coffee – cup calorimeter :
-Isolated container -Stirrer -Thermometer –Reactants

Uses of Coffee – cup calorimeter :

Measure the change in temperature

Bomb Calorimeter

Structure of Bomb Calorimeter:

-Isolated container -Stirrer -Thermometer -Reactants -Ignition wires

Uses :Measure the heat of combustion
Why Water is used in both types of Calorimeter?
Because it has high specific heat

Thermochemistry Quiz:

1-What is thermodynamics?

A. The study of heat effects in chemical reactions
B. The science of energy and its transformations
C. The measurement of temperature changes
D. The study of light energy
Explanation: Thermodynamics is the science that deals with the study of energy and how it transfers. So, option B is correct.

2-Which statement regarding energy and living organisms is true?

A. Living organisms get energy from burning natural gas.
B. Energy for living organisms is solely obtained from light.
C. Energy is important for mental and muscular activities.
D. Living organisms cannot obtain energy from chemical reactions.
Explanation: Energy is indeed crucial for mental and muscular activities in living organisms. Hence, option C is correct.

3-What is the relation between chemical reactions and energy exchange?

A. Chemical reactions release energy only.
B. Chemical reactions do not involve energy exchange.
C. Energy exchange occurs between reaction mixture and surroundings.
D. Chemical reactions absorb energy only.
Explanation: Chemical reactions involve energy exchange, either releasing or absorbing energy. So, option C is correct.

4-Why is a medical thermometer considered a closed system?

A. It allows energy exchange only with the surroundings.
B. It exchanges both energy and matter with the surroundings.
C. It does not allow any exchange of energy.
D. It allows free exchange of matter but not energy.
Explanation: A closed system exchanges energy but not matter with its surroundings. So, option A is correct.

5-What is the first law of thermodynamics?

A. Energy can be created but not destroyed.
B. The total energy of an isolated system is constant.
C. Energy is always lost during transformations.
D. The energy of a closed system increases over time.
Explanation: The first law states that the total energy of an isolated system remains constant. Hence, option B is correct.

6-What is temperature?

A. Measurement of hotness or coldness of an object.
B. The total energy of a system.
C. A form of energy exchange.
D. The quantity of heat needed to raise the temperature of water.
Explanation: Temperature indicates hotness or coldness of an object. So, option A is correct.

7-What is specific heat?

A. The quantity of heat needed to raise the temperature of a substance.
B. The total heat content of a system.
C. The energy needed to transform matter into light.
D. The heat capacity of water.
Explanation: Specific heat refers to the quantity of heat needed to raise the temperature of a substance. Thus, option A is correct.

8-Why does water cause a moderate climate in coastal areas?

A. Because it has low specific heat.
B. Because it absorbs less heat.
C. Because it has high specific heat.
D. Because it evaporates quickly.
Explanation: Water’s high specific heat allows it to absorb and release heat slowly, moderating climate. So, option C is correct.

9-Which unit is used to measure specific heat?

A. J/g°C
B. J
C. Calorie
D. Joule
Explanation: Specific heat is measured in Joules per gram per degree Celsius (J/g°C). Therefore, option A is correct.

10-Why is water used in both types of calorimeter?

A. Because it is easily available.
B. Because it has low specific heat.
C. Because it is a good conductor of heat.
D. Because it has high specific heat.
Explanation: Water’s high specific heat allows it to absorb heat effectively, making it suitable for calorimetry. Hence, option D is correct.

11-What is the law of conservation of energy?

A. Energy can be created but not destroyed.
B. Energy in any physical or chemical change can be neither created nor destroyed.
C. Energy can only be transferred as heat.
D. Energy is lost during energy transformations.
Explanation: The law of conservation of energy states that energy in any physical or chemical change can neither be created nor destroyed, only transformed from one form to another. Therefore, option B is correct.

12-What are the types of systems in thermodynamics?

A. Closed, open, and isolated systems
B. Organic, inorganic, and isolated systems
C. Static, dynamic, and closed systems
D. Liquid, gas, and solid systems
Explanation: The types of systems in thermodynamics are classified as open, closed, and isolated systems. So, option A is correct.

13-What is the role of a bomb calorimeter?

A. Measure the change in temperature of a solution.
B. Measure the heat of combustion.
C. Measure the specific heat of a substance.
D. Measure the energy content of food.
Explanation: A bomb calorimeter is used to measure the heat of combustion, making option B the correct choice.

14-What is the quantity of heat formula and its units?

A. q = mcΔT, units: J/g
B. q = mcΔT, units: J/g°C
C. q = mΔT, units: J
D. q = mΔT, units: J/g
Explanation: The formula for the quantity of heat is q = mcΔT, where q represents heat, m is mass, c is specific heat, and ΔT is the change in temperature. The correct units are J/g°C, making option B correct.

15-Why is the calorimeter considered an isolated system?

A. Because it allows energy exchange with the surroundings.
B. Because it prevents the exchange of heat or substance with the surroundings.
C. Because it is always at a constant temperature.
D. Because it only allows the exchange of matter with the surroundings.
Explanation: A calorimeter is considered an isolated system because it prevents the exchange of heat or substance with the surroundings, ensuring accurate measurement of energy changes within the system. Therefore, option B is correct.