Let's study a little bit of history:
In 1830, a Russian chemist and doctor known and Germain Henri Hess, formulate a law that it is still used today. It is called the Hess law.
According to Hess, the enthalpy change of a chemical process, only depend on the initial and final state, not on its pathway. The enthalpy change of the overall process is the sum of the enthalpy change of its individual steps.
domingo, 29 de setembro de 2013
Methods of Communicating Enthalpy Changes
Chemists have different ways to express the enthalpy change of a reaction. Which would be?
- Stating molar enthalpy of a specific reactant of the reaction;
- Stating the enthalpy change for a balanced equation.
- Including the enthalpy change as a part of the equation, being reactant or product, depending on the type of the reaction;
- Representing by a diagram.
They will provide us a better understanding on the topic.
Which is?
The change in the energy of substances after a reaction occur.
Which ones of these four methods are we can trust the most?
The first three ones, since they are empirical description. The fourth one, for being a diagram, is more of a theoretical description.
Let's talk about Method 1:
Well, the molar enthalpy is the measure of released or absorbed energy from or to a substance per mole. The substance and the reaction must be specified.
Do we have standards: Yes, we do. Standard molar enthalpy are expressed with a * subscript. It means that it represents the molar enthalpy of substance under ideal conditions.
Method 2:
Just write the enthalpy change next to the balanced equation. Iif we change the coefficient or the order of the equation, then the enthalpy change will also change.
Method 3:In endothermic reactions the energy is on the reactant side. in exothermic reaction the energy is on the product.
Calorimetry II
Alright, but let's think this through... If metals have specific heat capacity, wouldn't my result be affected if I did the procedure in a calorimetry made of Aluminium for example?
Yes, indeed. Calorimeter made of metal have a special attention by the chemists because they know it absorbs or release way too much energy to be ignored.
Thus, in our calculation, we need to use more information that a conventional procedure.
The temperature change of the calorimeter needs to be accounted for. If not stated, you can assume that the temperature change of the metal is the same as that of the water.
Lets not forget about other very special type of calorimetry: The combustion calorimetry.
It is usually done in a bomb calorimeter. With combustion, MASS ID NOT NECESSARY.
Calorimetry
Okay, you are probably thinking about: Where de heck is all those famous experiments used in chemistry?
Well, in thermochemistry we have an experimental method used to measure energy changes. It is called Calorimetry.
Before we start calorimetry, lets recall two very important laws related to this issue:
Calorimetry is the method that use calorimeter which is a device that is used to measure the amount of heat that was released or absorbed through a chemical reaction that occurred within it.
IDEALLY CALORIMETER IS AN ISOLATED SYSTEM! Yes, do not forget that! Because a isolated system means that the environment around the calorimetry DOES NOT affect the final result.
To use calorimeter, it is very important to have some assumption which are:
It is possible to find more currate results with aluminium calorimeters.
The best type is called the bomb calorimeter.
They all work based on the same principles.
Well, in thermochemistry we have an experimental method used to measure energy changes. It is called Calorimetry.
Before we start calorimetry, lets recall two very important laws related to this issue:
- Energy is never created or destroyed, only transformed.
- Heat flow from hotter to cooler substances until they reach the same temperature.
Calorimetry is the method that use calorimeter which is a device that is used to measure the amount of heat that was released or absorbed through a chemical reaction that occurred within it.
IDEALLY CALORIMETER IS AN ISOLATED SYSTEM! Yes, do not forget that! Because a isolated system means that the environment around the calorimetry DOES NOT affect the final result.
To use calorimeter, it is very important to have some assumption which are:
- All energy gained or lost by the calorimeter was gained or lost by the system.
- The calorimeter is completely isolated.
- The liquid in the calorimeter has the same physical proprieties as water.
- The thermal energy gained or lost by the other devices in the calorimeter, such as the container, thermometer, and lid, are too small to be not be ignored.
It is possible to find more currate results with aluminium calorimeters.
The best type is called the bomb calorimeter.
They all work based on the same principles.
Potential Energy - Enthalpy
The second form of energy studied in chemistry is called Potential energy.
Potential energy is commonly called Enthalpy in the chemistry world.
Can we measure potential energy like we measure kinetic energy?
Oh, actually Potential energy is equal to thermal energy, however we cannot measure by using the thermal equation, because it does not require temperature change.
Enthalpy energy is stored in the chemical bonds of molecules within a substance. When these bonds are broken or formed there will be a change in the total enthalpy of the substance.
Therefore we can measure enthalpy change by using the equation:
ΔH = nΔHm
Where ΔH is Enthalpy change (kJ), n (moles) and ΔHm (molar enthalpy).
Whoa whoa, what the heck is molar enthalpy?
Molar enthalpy is the change in the enthalpy when on per mole of substance undergoes a process.
Potential energy is commonly called Enthalpy in the chemistry world.
Can we measure potential energy like we measure kinetic energy?
Oh, actually Potential energy is equal to thermal energy, however we cannot measure by using the thermal equation, because it does not require temperature change.
Enthalpy energy is stored in the chemical bonds of molecules within a substance. When these bonds are broken or formed there will be a change in the total enthalpy of the substance.
Therefore we can measure enthalpy change by using the equation:
ΔH = nΔHm
Where ΔH is Enthalpy change (kJ), n (moles) and ΔHm (molar enthalpy).
Whoa whoa, what the heck is molar enthalpy?
Molar enthalpy is the change in the enthalpy when on per mole of substance undergoes a process.
Kinetic Energy
Kinetic Energy! One of the most abundant forms of energy on earth.
But, wait a minute, shouldn't we be studying this in a physics class?
Nope, actually kinetic energy is very present in chemistry.
Kinetic is the energy of motion. We know that particles are always in movement, thus it will generate energy in the form of movement.
You know how particles' movement influence and is influenced by the temperature temperature change? Well, this is simple to understand if we say that when the temperature increases, the kinetic energy increases as well, and vice versa.
Wait we know what kinetic energy is, but can we measure it?
The answer is no. However, we can measure temperature.
A change in temperature indicates that energy has left or entered a substance. However the amount of energy required to change the temperature of different substance by the same amount, varies with the type of substance.
For example: It actually take 4.19 J of energy to change the temperature of 1 ml of water by one degree, while it takes 0.897 J of energy to change the temperature of 1 g of aluminium by one degree.
These values quoted above are known as the specific heat of a substance. The amount of energy necessary to increase one gram of a substance by one degree Celsius.
It is very important to remember that the state of the substance influences its specific heat capacity.
Okay okay, lets not get out of focus here. We know that we can measure temperature. We also know we can be provided the specific heat capacity of a substance. Okay. Now, can we measure the mass of the substance?
If the answer is yes, we can use these three values and form a equation which will provide us the amount of energy that was exchanged.
Q=mc(tf-ti), where Q represents the amount of energy, m is equal to mass, c is equal to specific heat capacity.
Note that if the temperature change is negative, it will make the Q value be negative. Thus, this equation not only provide the amount of heat, but tells you if the reaction released or absorbed energy.
But, wait a minute, shouldn't we be studying this in a physics class?
Nope, actually kinetic energy is very present in chemistry.
Kinetic is the energy of motion. We know that particles are always in movement, thus it will generate energy in the form of movement.
You know how particles' movement influence and is influenced by the temperature temperature change? Well, this is simple to understand if we say that when the temperature increases, the kinetic energy increases as well, and vice versa.
Wait we know what kinetic energy is, but can we measure it?
The answer is no. However, we can measure temperature.
A change in temperature indicates that energy has left or entered a substance. However the amount of energy required to change the temperature of different substance by the same amount, varies with the type of substance.
For example: It actually take 4.19 J of energy to change the temperature of 1 ml of water by one degree, while it takes 0.897 J of energy to change the temperature of 1 g of aluminium by one degree.
These values quoted above are known as the specific heat of a substance. The amount of energy necessary to increase one gram of a substance by one degree Celsius.
It is very important to remember that the state of the substance influences its specific heat capacity.
Okay okay, lets not get out of focus here. We know that we can measure temperature. We also know we can be provided the specific heat capacity of a substance. Okay. Now, can we measure the mass of the substance?
If the answer is yes, we can use these three values and form a equation which will provide us the amount of energy that was exchanged.
Q=mc(tf-ti), where Q represents the amount of energy, m is equal to mass, c is equal to specific heat capacity.
Note that if the temperature change is negative, it will make the Q value be negative. Thus, this equation not only provide the amount of heat, but tells you if the reaction released or absorbed energy.
Energy
Welcome to the course of Chemistry 30.
Our first topic is ENERGY!!
Energy is essential for life. It provide us motion, fuel, light and many others!
But what is the exactly definition of energy?
Well, some say that energy is the strength necessary to sustain our physical and mental activities. Others, will say is the ability to do work.
However, in this course let's think energy as a amount of power. Power that is essential for life.
There are many forms of energy! Energy of motion, light energy, molecular energy, sound energy, nuclear energy and etc.
It is very important to know that energy originate from the sun!
Yes, the sun. Our biggest star is bale to convert hydrogen atoms into helium atoms, providing a enormous amount of energy to the earth.
This energy is converted into chemical energy into the plants by a process called Photosynthesis. This chemical energy is essential for plants to carry its metabolic life.
When animals eat the plants, this energy will now help the new organism to live its metabolic life. And it goes on and on.
When plants or animals consume energy, it is stored in the chemical bonds. When the plant and animal decompose, chemical reaction occur, since bonds are been broken. The energy can either be released or absorbed.
But how sun does not give nuclear energy for example. So how do they all originate there?
Well, to answer this question we have to understand the root of chemistry known as Thermochemistry.
In thermochemistry we study some laws called the laws of conservation of energy.
The first law explains that energy is never created or destroyed, however it is always transformed.
Energy originates from the sun in form of life, however it transforms into all the types of energy we know of.
In chemistry, energy is essential for chemical reactions to happen. Remember this: Bonds to be broken of formed, require energy.
There are many forms of energy! Energy of motion, light energy, molecular energy, sound energy, nuclear energy and etc.
It is very important to know that energy originate from the sun!
Yes, the sun. Our biggest star is bale to convert hydrogen atoms into helium atoms, providing a enormous amount of energy to the earth. This energy is converted into chemical energy into the plants by a process called Photosynthesis. This chemical energy is essential for plants to carry its metabolic life.
When animals eat the plants, this energy will now help the new organism to live its metabolic life. And it goes on and on.
When plants or animals consume energy, it is stored in the chemical bonds. When the plant and animal decompose, chemical reaction occur, since bonds are been broken. The energy can either be released or absorbed.
But how sun does not give nuclear energy for example. So how do they all originate there?
Well, to answer this question we have to understand the root of chemistry known as Thermochemistry.
In thermochemistry we study some laws called the laws of conservation of energy.
The first law explains that energy is never created or destroyed, however it is always transformed.
Energy originates from the sun in form of life, however it transforms into all the types of energy we know of.
In chemistry, energy is essential for chemical reactions to happen. Remember this: Bonds to be broken of formed, require energy.
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