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.
sábado, 28 de setembro de 2013
Energy and Efficiency
Guys, guys I have a question, what is efficiency?
Well, efficiency can be described in several forms. But when we relate it with energy, we say that efficiency is the ability to produce a desired effect with minimum energy expenditure.
For example, which one uses less energy to cook a potato: A microwave or a normal oven. Of course the microwave uses less energy, thus we say it is more efficient than a oven.
But let's put some math in this definition. Efficiency compares the input and output of energy. The ratio of useful energy produced (energy output) to energy used in its production (energy input), expressed as percentage.
But what it energy output and input?
Energy output is the work done, energy delivered to consumer in usable form. You can find it by calculating Q=mc(tf-ti) or enthalpy change equation.
Energy input is the solar energy, fuel energy, power energy and etc. it is found through Hess' Law.
LAB TIME:
To study energy efficiency, lets determine the efficiency of a barbecue why heat 5.10g of propane, and change the temperature of 250 g of water contained in a 500 g stainless steel pot (c=0.503 j/gxC) from 25C to 75C.
Well first of all lets find the energy output of this problem. By using Hess' Law we know that -236 kj.
Energy output can be figured by thermal energy equation which will give us a value of 65.0 kj
Now you take these two values, divide them and multiple by 100.
Well, efficiency can be described in several forms. But when we relate it with energy, we say that efficiency is the ability to produce a desired effect with minimum energy expenditure.
For example, which one uses less energy to cook a potato: A microwave or a normal oven. Of course the microwave uses less energy, thus we say it is more efficient than a oven.
But let's put some math in this definition. Efficiency compares the input and output of energy. The ratio of useful energy produced (energy output) to energy used in its production (energy input), expressed as percentage.
But what it energy output and input?
Energy output is the work done, energy delivered to consumer in usable form. You can find it by calculating Q=mc(tf-ti) or enthalpy change equation.
Energy input is the solar energy, fuel energy, power energy and etc. it is found through Hess' Law.
LAB TIME:
To study energy efficiency, lets determine the efficiency of a barbecue why heat 5.10g of propane, and change the temperature of 250 g of water contained in a 500 g stainless steel pot (c=0.503 j/gxC) from 25C to 75C.
Well first of all lets find the energy output of this problem. By using Hess' Law we know that -236 kj.
Energy output can be figured by thermal energy equation which will give us a value of 65.0 kj
Now you take these two values, divide them and multiple by 100.
Catalysts
Okay, we've learned that some reactions are very slow. They may take hours, days, weeks, and even years to be completed. But how does it affect the economy?
Yes, economy. We know that many things we use are produced through industrial application. Thus, the slowness of a reaction would make impossible to have any kind of profit in the production of these products.
So how on earth do the guys that work in industry manage to accelerate this reactions?
They use something very important for our daily-basis called catalyst.
But what is catalyst?
Well, catalyst is a substance that increases the rate of a chemical reaction without being consumed by the reaction.
Catalyst is not only important in industry, but it also play a very important role in our body.
And how does it work?
It basically provide an alternative pathway for a reaction to occur.
This pathway has a smaller activation energy requirement, even though it reacts the same substances and have the same enthalpy change as the uncatalyzed reaction.
This happens because the catalyst takes part of the reaction, but it is regenerated unchanged at the end of the reaction.
Yes, economy. We know that many things we use are produced through industrial application. Thus, the slowness of a reaction would make impossible to have any kind of profit in the production of these products.
So how on earth do the guys that work in industry manage to accelerate this reactions?
They use something very important for our daily-basis called catalyst.
But what is catalyst?
Well, catalyst is a substance that increases the rate of a chemical reaction without being consumed by the reaction.
Catalyst is not only important in industry, but it also play a very important role in our body.
And how does it work?
It basically provide an alternative pathway for a reaction to occur.
This pathway has a smaller activation energy requirement, even though it reacts the same substances and have the same enthalpy change as the uncatalyzed reaction.
This happens because the catalyst takes part of the reaction, but it is regenerated unchanged at the end of the reaction.
A Reaction
Okay, now let's take all we know and try to trace what happen in a reaction:
For this let's react BrCH3(aq) and OH(aq). This will be a exothermic reaction.
First of all, for a successful reaction to occur, the particles of BrCH and OH must collapse in the correct collision geometry.
If this occur there will be enough kinetic energy to be converted into potential energy, and stored in the bonds of the particles of the activated complex.
Because the activated complex is very unstable. It can either break down in products or it can be decomposed into reactant. Remember that it is a mid term?
If the activated complex turns into product, the potential energy that was stored in the bonds of the particles, now will turn into kinetic energy as the separate. This conversion will result in the decrease of the potential energy, and rise in the temperature, which are the main characteristics of an exothermic reaction.
CHEMISTRY!
For this let's react BrCH3(aq) and OH(aq). This will be a exothermic reaction.
First of all, for a successful reaction to occur, the particles of BrCH and OH must collapse in the correct collision geometry.
If this occur there will be enough kinetic energy to be converted into potential energy, and stored in the bonds of the particles of the activated complex.
Because the activated complex is very unstable. It can either break down in products or it can be decomposed into reactant. Remember that it is a mid term?
If the activated complex turns into product, the potential energy that was stored in the bonds of the particles, now will turn into kinetic energy as the separate. This conversion will result in the decrease of the potential energy, and rise in the temperature, which are the main characteristics of an exothermic reaction.
CHEMISTRY!
Activated Complex
Remember that hill that exist in an enthalpy change diagram? Now, let's go to the top of it.
In the peak of the hill, you will find many particles that are very unstable. They do not have any completed formed bonds, nor completed broken bonds.
This particles are neither reactants, nor products, they are the mid term, a transitional specie.
This particles are referred to as the activated complex.
Activation Energy and Enthalpy
Many of you may think that activation energy can be determined by the enthalpy change of a reaction.
Well, actually there is no way you can even predict the activation energy of a reaction from its enthalpy change.
A exothermic reaction that releases a lot of energy for example, may take a long time to be completed. As well as a endothermic reaction is able to happen in the blink of a eye.
Enthalpy change is the difference in potential energy of the reactants and products and it is independent of the pathway.
The activation energy, however, is determined by the rate of temperature in a reaction.
In general, the amount of energy required to start a reaction will determine if this reaction will take long or not to occur, independent of the type of reaction it is.
Let's answer that questions:
Well, actually there is no way you can even predict the activation energy of a reaction from its enthalpy change.
A exothermic reaction that releases a lot of energy for example, may take a long time to be completed. As well as a endothermic reaction is able to happen in the blink of a eye.
Enthalpy change is the difference in potential energy of the reactants and products and it is independent of the pathway.
The activation energy, however, is determined by the rate of temperature in a reaction.
In general, the amount of energy required to start a reaction will determine if this reaction will take long or not to occur, independent of the type of reaction it is.
Let's answer that questions:
- Activation energy for a reaction is the amount of energy required for a reaction occur. Without this energy, bonds cannot be broken nor formed.
- They are two distinct concepts. However, enthalpy change is only possible to happen if the activation energy is reached.
- The spark will provide enough energy for combustion to happen.
quarta-feira, 25 de setembro de 2013
Molecular Collision
Did you know you can represent the change in the energy generated by the collisions of particles using a potential energy diagram?
Yeah, but with one little difference: In addition to the enthalpy change of the reaction, there will be a small hill that will represent the activation energy.
Reaction with low activation energy will occur faster than those with higher activation energy.
A explosive reaction has a very low activation energy, for example.
Temperature and Activation Energy
Hold on... doesn't temperature influence the distribution of kinetic energy?
Well, yeah, and that is why when temperature increases, the number of collisions with enough energy increases as well.
And that is also the reason for the the high rate of successful collisions at high temperature.
Well, yeah, and that is why when temperature increases, the number of collisions with enough energy increases as well.
And that is also the reason for the the high rate of successful collisions at high temperature.
Activation Energy
Let's not forget that there are two criteria to be followed, in other to a reaction occur.
We've already learned about collision geometry.
Well, now, it's time for you to learn about the Activation Energy. But... what is it?
Activation energy, or Ea, is the minimum collision energy required for a successful reaction.
When two particles collide, they will need extra energy to break down their bonds, and to form other bond for their products.
In almost every reaction, only a small number of collisions have sufficient energy for a reaction to occur.
This collision energy is based on the kinetic energy of the particles that are being collided.
As you know, temperature is a the measure of the average kinetic energy.
If you plot the number of collisions in a substance at the given temperature against the kinetic energy of each collision, you get the distribution known as Maxwell-Boltzmann distribution.
The following graphic illustrate the M-B distribution.
The dotted line indicates the activation energy.
The shaded part of the graph is represents the collisions with equal or greater energy than the activation energy.
As you can observe, they are the minority.
We've already learned about collision geometry.
Well, now, it's time for you to learn about the Activation Energy. But... what is it?
Activation energy, or Ea, is the minimum collision energy required for a successful reaction.
When two particles collide, they will need extra energy to break down their bonds, and to form other bond for their products.
In almost every reaction, only a small number of collisions have sufficient energy for a reaction to occur.
This collision energy is based on the kinetic energy of the particles that are being collided.
As you know, temperature is a the measure of the average kinetic energy.
If you plot the number of collisions in a substance at the given temperature against the kinetic energy of each collision, you get the distribution known as Maxwell-Boltzmann distribution.
The following graphic illustrate the M-B distribution.
The dotted line indicates the activation energy.
As you can observe, they are the minority.
Orientation of Reactants
Well, we've learned that one of the criteria for a effective reaction occur, is that the particles must collide with the proper orientation.
As you can see, out of the five different types of collision, only one is geometrical, and will produce other particles.
This proper orientation is also known a collision geometry.
The following chemical equation illustrate why this criteria is essential:
NO(g) + NO3(g) = NO2(g) + NO2(g)
When the particles of NO(g) collides with particles of NO3(g), NO2(g) particles will be formed. However, not all the collisions between these particles will generate the product.
Analyse the following image
As you can see, out of the five different types of collision, only one is geometrical, and will produce other particles. Collision Theory
Today, we will learn about Collision Theory!
Well, before we start, just stop and think about all the reactions that are happening around you in this exact moment. It is impossible! Every second, there are millions of reactions occurring around us.
When we are driving a car for example, we can analyse two different reactions: the gasoline being combusted and the steel being rusted.
Even though, these two reactions happen to be occurring in the same vehicle, one takes longer to be completed than the other.
Scientifically speaking, the combustion of gasoline has a faster reaction rate than the rusting of steel. But what is reaction rate?
Reaction rate is the change in the amount of reactants consumed or products generated over time.
However, this leaves us with a question: Why does some reactions occur faster than others?
Collision Theory
Before we answer that question above, we need to know how a reaction occurs. What causes it?
A theory explains that a reaction occur when two particles collide with each other. These particles may be atoms, molecule, or ions.
This theory is called collision theory. However, does every collision result in a reaction?
The answer for that question is NO, and here is why:
Not every collision between reactants results in a reaction.
For example, in a lab, in 1 mL sample of gas there are several collisions of particles. This number is so high that all gases' reactions would be completed in less than a second. However, gas' reactions actually take a long time to occur.
For a collision between reactants actually result in a reaction, the collision must be effective.
A effective collision, which is the one that forms products, must satisfy two criteria:
Well, before we start, just stop and think about all the reactions that are happening around you in this exact moment. It is impossible! Every second, there are millions of reactions occurring around us.
When we are driving a car for example, we can analyse two different reactions: the gasoline being combusted and the steel being rusted.
Even though, these two reactions happen to be occurring in the same vehicle, one takes longer to be completed than the other.
Scientifically speaking, the combustion of gasoline has a faster reaction rate than the rusting of steel. But what is reaction rate?
Reaction rate is the change in the amount of reactants consumed or products generated over time.
However, this leaves us with a question: Why does some reactions occur faster than others?
Collision Theory
Before we answer that question above, we need to know how a reaction occurs. What causes it?
A theory explains that a reaction occur when two particles collide with each other. These particles may be atoms, molecule, or ions.
This theory is called collision theory. However, does every collision result in a reaction?
The answer for that question is NO, and here is why:
Not every collision between reactants results in a reaction.
For example, in a lab, in 1 mL sample of gas there are several collisions of particles. This number is so high that all gases' reactions would be completed in less than a second. However, gas' reactions actually take a long time to occur.
For a collision between reactants actually result in a reaction, the collision must be effective.
A effective collision, which is the one that forms products, must satisfy two criteria:
- The correct orientation of reactants (collision geometry);
- Sufficient collision energy (activation energy, Ea).
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