CHEMICAL REACTIONS - STOICHIOMETRY #4

Stoichiometry

Let's start with how to say this word. Five syllables: 
STOY-KEE-AHM-EH-TREE. It's a big word that describes a simple idea. Stoichiometry is the part of chemistry that studies amounts of substances that are involved in reactions. You might be looking at the amounts of substances before the reaction. You might be looking at the amount of material that is produced by the reaction. Stoichiometry is all about the numbers. 

All reactions are dependent on how much stuff you have. Stoichiometry helps you figure out how much of a compound you will need, or maybe how much you started with. We want to take the time to explain that reactions depend on the compounds involved and how much of each compound is needed. 

What do you measure? It could be anything. When you're doing problems in stoichiometry, you might look at...
- Mass of Reactants (chemicals before the reaction)
- Mass of Products (chemicals after the reaction)
- Chemical Equations
- Molecular Weights of Reactants and Products
- Formulas of Various Compounds 

Now, an example. Let's start with something simple like sodium chloride (NaCl). You start with two ions and wind up with an ionic/electrovalent compound. When you look at the equation, you see that it takes one sodium ion (Na⁺) to combine with one chlorine ion (Cl^-) to make the salt. When you use stoichiometry, you can determine amounts of substances needed to fulfill the requirements of the reaction. Stoichiometry will tell you that, if you have ten million atoms of sodium and only one atom of chlorine, you can only make one molecule of sodium chloride. Nothing you can do will change that. It's like this: 

10,000,000 Na + 1 Cl --> NaCl + 9,999,999 Na

Let's bump it up a level. When you mix hydrogen gas (H₂) and oxygen gas (O₂), nothing much happens. When you add a spark to the mixture, all of the molecules combine and eventually form water (H₂O). You would write it like this: 

2H₂ + O₂ --> 2H₂O

What does stoichiometry look at here? First, look at the equation. Four hydrogen atoms and two oxygen atoms are on each side of the equation. It's an important idea to see that you need twice as many hydrogen atoms as you do oxygen atoms. The number of atoms in the equation will help you figure out how much of each substance you will need to make the reaction happen. If you make this an extreme example and fill a sealed container with one million hydrogen molecules and only one oxygen molecule, the spark won't make an explosion. There is no monster reaction to be created when there is only one oxygen molecule around. You will make two water molecules and be done. 

CHEMICAL REACTIONS - MEASURING REACTION RATES #3

Measuring Reaction Rates

Scientists like to know the rates of reactions. They like to measure different kinds of rates too. Each rate that can be measured tells scientists something different about the reaction. We're going to take a little time to cover a few different measures of reaction rates. 

Forward Rate: The rate of the forward reaction when reactants combine to become products. 
Reverse Rate: The rate of the reverse reaction when products break apart to become reactants. 
Net Rate: The forward rate minus the reverse rate. 
Average Rate: The speed of the entire reaction from start to finish. 
Instantaneous Rate: The speed of the reaction at one moment in time. Some reactions can happen quickly at the start and then slow down. You have one average rate, but the instantaneous rates can tell you the whole story. 

Scientists measure all of these rates by finding out the concentrations of the molecules in the mixture. If you find out the concentration of molecules at two different times, you can find out what direction the reaction is moving toward and how fast it is going. Even if the concentrations are equal at the two points of measurement, scientists still learn something. If the concentrations are stable during two measurements, the reaction is at an equilibrium point. 

One Step at a Time

There is still more to know about measuring the rates of reactions. Since many reactions happen in several steps, the rate for each step needs to be measured. There will always be one step that happens at the slowest speed. That slowest step is called the rate-limiting step. That rate-limiting step is the one reaction that really determines how fast the overall reaction can happen. If you have six steps in your series of reactions and the third step goes incredibly slow, that is the rate-limiting step. As far as the overall reaction is concerned, none of the other rates really matter. If you want to speed up the overall reaction, you would focus on that slowest step. Don't forget that if you only speed up one step, another step may become the new rate-limiting step. You should always understand how all of the steps are involved in the overall reaction. 

CHEMICAL REACTIONS - RATE OF REACTION #2

Rate of Reaction
The rate of a reaction is the speed at which areaction happens. If a reaction has a low rate, that means the molecules combine at a slower speed than a reaction with a high rate. Some reactions take hundreds, maybe even thousands, of years while others can happen in less than one second. The rate of reaction depends on the type of molecules that are combining. If you want to think of a very slow reaction, think about how long it took dinosaur bones to become fossils through breakdown. You can thank chemical processes in bacteria for most of those dinosaur bones in the museum. 

There is another big idea for rates of reaction called collision theory. The collision theory says that as more collisions in a system occur, there will be more combinations of molecules bouncing into each other. If there are a higher number of collisions in a system, more combinations of molecules can occur. The reaction will go faster and the rate of that reaction will be higher. Even though they are both liquids, think about how slowly molecules move in honey when compared to your soda. There are a lower number of collisions in the honey. 

Reactions happen - no matter what. Chemicals are always combining or breaking down. The reactions happen over and over, but not always at the same speed. A few things affect the overall speed of the reaction and the number of collisions that can occur. 

Concentration: If there is more of a substance in a system, there is a greater chance that molecules will collide and speed up the rate of the reaction. If there is less of something, there will be fewer collisions and the reaction will probably happen at a slower speed. 
Sometimes you will mix solutions in ice so that the temperature of the system stays cold and the rate of reaction is slower. 

Pressure: Pressure affects the rate of reaction, especially when you look at gases. When you increase the pressure, the molecules have less space in which they can move. That greater density of molecules increases the number of collisions. When you decrease the pressure, molecules don't hit each other as often. The lower pressure decreases the rate of reaction. 

REACTIONS RATE - QUIZ

ENDOTHERMIC & EXOTHERMIC PROCESS

Endothermic and Exothermic Processes

Exothermic- the word describes a process that releases energy in the form of heat.

Forming a chemical bond  releases energy and therefore is an exothermic process.

Exothermic reactions usually feel hot because it is giving heat to you.

Endothermic - a process or reaction that absorbs energy in the form of heat.

Breaking a chemical bond requires energy and therefore is Endothermic.

Endothermic reactions usually feel cold because it is taking heat away from you.

Exothermic Processes	Endothermic Processes
freezing water solidifying solid salts condensing water vapor making a hydrate from an anhydrous salt forming an anion from an atom in the gas phase Annihilation of matter E=mc2 splitting of an atom	melting ice cubes melting solid salts evaporating liquid water making an anhydrous salt from a hydrate forming a cation from an atom in the gas phase splitting a gas molecule separating ion pairs cooking an egg baking bread

Endothermic and exothermic reactions involve the absorption and release, respectively, of energy to and from the environment.

Exothermic Reaction

An Exothermic Reaction releases energy upon completion.

KEY POINTS

• All chemical reactions involve the transfer of energy.
• Endothermic processes require an input of energy and are signified by a positive change in enthalpy.
• Exothermic processes release energy upon completion, and are signified by a negative change in enthalpy.

TERMS

• endothermic
  of a chemical reaction that absorbs heat energy from its surroundings
• exothermic
  of a chemical reaction that releases energy in the form of heat
• enthalpy
  In thermodynamics, a measure of the heat content of a chemical or physical system.

EXAMPLES

• An example of an exothermic reaction is the mixing of water and strong acids. In the presence of water, the acid will dissociate quickly and release heat.
• An example of an endothermic reaction is the melting of an ice cube. In order to melt the ice cube, heat is required.

1. 

   fig. 2

   Endothermic Reaction

   An endothermic reaction requires energy for completion because the energy of the reactants is less than that of the products.

2. Endothermic and Exothermic Reactions

    An energy diagram can be used to show energy movements in these reactions and temperature can be used to measure them macroscopically.

REVIEW


Many chemical reactions release energy in the form of heat, light, or sound. 
These are exothermic reactions. 

• Exothermic reactions may occur spontaneously and result in higher randomness or entropy (ΔS > 0) of the system. They are denoted by a negative heat flow (heat is lost to the surroundings) and decrease in enthalpy -  the amount of heat content used or released in a system at constant pressure. 
• In the lab, exothermic reactions produce heat or may even be explosive. 

• There are other chemical reactions that must absorb energy in order to proceed. These are endothermic reactions. 
• Endothermic reactions cannot occur spontaneously. Work must be done in order to get these reactions to occur. When endothermic reactions absorb energy, a temperature drop is measured during the reaction. 

Endothermic reactions are characterized by positive heat flow (into the reaction) and an increase in enthalpy (+ΔH). 

Examples of Endothermic and Exothermic Processes

• Photosynthesis is an example of an endothermic chemical reaction. In this process, plants use the energy from the sun to convert carbon dioxide and water into glucose and oxygen. 

•  An example of an exothermic reaction is the mixture of sodium and chlorine to yield table salt. 

ENDOTHERMIC & EXOTHERMIC QUIZ - turn into drawer

CHEMICAL REACTIONS #1

Chemical Reactions

Let's start with the idea of a reaction. In chemistry, a reaction happens when two or more molecules interact and the molecules change. That's it. What molecules are they? How do they interact? What happens? The possibilities are infinite. When you are trying to understand reactions, imagine that you are working with the atoms. Imagine the building blocks are right in front of you on the table, instead of billions of reactions in your beaker. Sometimes we do this using our chemistry toys to help us visualize the movement of the atoms. There are a few key points you should know about chemical reactions: 

1. A chemical change must occur. You start with one compound and turn it into another. That's an example of a chemical change. A steel garbage can rusting is a chemical reaction. That rusting happens because the iron (Fe) in the metal combines with oxygen (O₂) in the atmosphere. When a refrigerator or air conditioner cools the air, there is no reaction between the air molecules. The change in temperature is a physical change. When you melt an ice cube, it is a physical change. When you put bleach in the washing machine to clean your clothes, a chemical change breaks up your stains. 

2. A reaction could include ions, compounds, or molecules of a single element. We said molecules in the previous paragraph, but a reaction can happen with anything, just as long as a chemical change occurs (not a physical one). If you put pure hydrogen gas (H₂) and pure oxygen gas in a room, they can be involved in a reaction. The slow rate of reaction will have the atoms bonding to form water (H₂O) very slowly. If you were to add a spark, those gases would create a reaction that would result in a huge explosion. Chemists call that spark a catalyst. 

3. Single reactions often happen as part of a larger series of reactions. Take something as simple as moving your arm. The contraction of that muscle requires sugars for energy. Those sugars need to be metabolized. You'll find that proteins need to move in a certain way to make the muscle contract. A whole series (hundreds) of different reactions are needed to make that simple movement happen. In the case of your arm, some are physical changes and some are chemical. In the process of making sugars in a plant, you might have as many as a dozen chemical changes to get through the Calvin cycle which makes glucose (C₆H₁₂O₆) molecules.

SCIENTIFIC METHOD

WHY DON'T OIL & WATER MIX?

ELEMENTS, COMPOUNDS, & MIXTURES - INTRO

Elements, Compounds, and Mixtures Game from delokarl

SCIENCE NEWS - STUDENT PROJECT

SCIENCE NEWS

Since the start of the school year we have done a wide-range of SCIENCE NEWS to help us learn about the happenings in our world from the incredible people who research, develop, and change our lives with science.

You will research, design, & present a SCIENCE NEWS.

·       Topic:   must have current & detailed research on your science topic of your choice.

·       Format:  must enhance the presentation of facts (video, brochure, graphics, power point, Glogster, hands-on demonstration, audio, ETC…

·       Presentation:  must engage & inform your audience.  Facts delivered must be clear & informative

·       Time:  You will be allowed a maximum of 3 minutes to present your science news. 

·       You must have your topic approved by me before you begin

·       You must sign up for a date for your presentation

MIXTURES - #13

**** ADD THIS FLOW DIAGRAMS IN YOUR NOTES

Separating Mixtures

Many of the substances we use everyday were actually once part of a mixture. Someone somewhere separated that substance from the mixture so we could use it. It turns out that many compounds and elements aren't found in nature in their pure form, but are found as parts of mixtures. Separating substances from mixtures is an important part of chemistry and modern industry. 

Some important chemistry terms are used in this section including mixtures, suspensions, and solutions.  Read carefully.

Chemical Mixtures

One of the main aspects of chemistry is combining different substances. Sometimes combining substances can cause a chemical reaction and bonding which creates an entirely new substance called a compound. However, sometimes there is no chemical reaction or bonding. In this case, a mixture is formed from the combined substances. 

Mixture

A mixture is made when two or more substances are combined, but they are not combined chemically.

General properties of a mixture:

• The components of a mixture can be easily separated
• The components each keep their original properties
• The proportion of the components is variable 

 Types of Mixtures

There are two main categories of mixtures: homogeneous mixtures and heterogeneous mixtures. 

• In a homogenous mixture all the substances are evenly distributed throughout the mixture (salt water, air, blood). 
• In a heterogeneous mixture the substances are not evenly distributed (chocolate chip cookies, pizza, rocks)
  

 

Within the categories of homogeneous and heterogeneous mixtures there are more specific types of mixtures including solutions, alloys, suspensions, and colloids.
Solutions (homogeneous)

A solution is a mixture where one of the substances dissolves in the other. The substance that dissolves is called the solute. The substance that does not dissolve is called the solvent.

• An example of a solution is salt water. These components can be easily separated through evaporation and they each retain their original properties. However, the salt is dissolved into the water to where you can't see it and it is evenly distributed in the water. In this example the water is the solvent and the salt is the solute.

A saturated solution is one containing as much solute as possible without forming a precipitate. This is the maximum concentration of solute.

An unsaturated solution is a chemical solution in which the solute concentration is lower than its equilibrium solubility.

What is the difference between a solution and a mixture?

In chemistry a solution is actually a type of mixture. A solution is a mixture that is the same or uniform throughout. Think of the example of salt water. This is also called a "homogenous mixture." A mixture that is not a solution is not uniform throughout. 

• Think of the example of sand in water. This is also called a "heterogeneous mixture."

Alloys (homogeneous)

An alloy is a mixture of elements that has the characteristic of a metal. At least one of the elements mixed is a metal. 

• One example of an alloy is steel which is made from a mixture of iron and carbon.

Suspensions (heterogeneous)

A suspension is a mixture between a liquid and particles of a solid. In this case the particles do not dissolve. The particles and the liquid are mixed up so that the particles are dispersed throughout the liquid. They are "suspended" in the liquid. A key characteristic of a suspension is that the solid particles will settle and separate over time if left alone.

• An example of a suspension is a mixture of water and sand. When mixed up, the sand will disperse throughout the water. If left alone, the sand will settle to the bottom.

Colloids (heterogeneous)

A colloid is a mixture where very small particles of one substance are evenly distributed throughout another substance. They appear very similar to solutions, but the particles are suspended in the solution rather than fully dissolved. The difference between a colloid and a suspension is that the particles will not settle to the bottom over a period of time, they will stay suspended or float.

• An example of a colloid is milk. Milk is a mixture of liquid butterfat globules dispersed and suspended in water.

Colloids are generally considered heterogeneous mixtures, but have some qualities of homogeneous mixtures as well.

Interesting Facts about Mixtures

• Smoke is a mixture of particles that are suspended in the air.
• Tap water is a mixture of water and other particles. Pure water or H2O is generally referred to as distilled water.
• Many of the substances we come into contact with every day are mixtures including the air we breathe which is a mixture of gases like oxygen and nitrogen.
• Blood is a mixture that can be separated by a machine called a centrifuge into its two main parts: plasma and red blood cells.
• Mixtures can be liquids, gases, and solids.

SEPARATING MIXTURES 

Why do we want to separate mixtures? 

All the way back to Ancient History, industrious humans have separated mixtures in order to obtain the specific substances that they need. One example of this is extracting metal from ore in order to make tools and weapons. We'll discuss some other examples of separation below. 

Separation Processes 

The way in which different substances in a mixture are separated is called a process. There are a number of different processes used for separation. Many of them are very complex and involve dangerous chemicals or high temperatures. A lot of important industries in the world today are based on separation processes. 

Filtration 

One common method of separation is filtration. Filters are used everywhere. We use them in our houses to filter dust and mites out of the air we breathe. We use them to filter impurities from our water. We even have filters in our bodies such as our kidneys which act as filters to get bad stuff out of our blood. 

The filtration process is generally used to separate a suspension mixture where small solid particles are suspended in liquid or air. In the case of filtering water, the water is forced through a paper that is made up of a very fine mesh of fibers. The water that has been run through the filter is called the filtrate. The particles that are removed from the water by the filter are called the residue. 

Distillation 

Another common separation process is called distillation. Distillation uses boiling to separate mixtures of liquid solutions. It takes into account that different substances in the mixture will have different boiling points. 

For example, if you heat salt water the water in the solution will boil before the salt. The water will then evaporate leaving the salt behind. If the steam from the water is collected it will turn back into liquid as it cools. This cooled water will be pure water without any salt. 

Centrifuge In some cases, there are suspension mixtures where the solid particles are too fine to be separated with a filter. In these cases, sometimes a centrifuge is used. Centrifuges are mechanical devices that spin at very high speeds. These high speeds allow the solid particles in suspensions to settle very quickly.  For example, rather than wait for sand to slowly settle to the bottom of water, a centrifuge can cause the sand to settle in a matter of seconds. Some examples of how centrifuges are used include: separating blood into plasma and red cells separating cream from milk separating uranium isotopes for nuclear power plants.

The heavier particles move to the outside of the cylinder as the centrifuge spins allowing the mixture to be separated.

Other Processes 

There are many other separation processes such as sublimation, adsorption, crystallization, and chromatography. Sometimes it takes many stages of processes to get to the final result. 

• One example of this is the processing of crude oil. Crude oil uses many levels of fractional distillation to produce a number of different products including gasoline, jet fuel, propane gas, and heating oil. 

Interesting Facts about Separating Mixtures

• To separate liquid solutions where the substances have similar boiling points, a more complex version of distillation is used called fractional distillation.
• Painting uses the separation process of evaporation. The wet paint is a mixture of color pigment and a solvent. When the solvent dries and evaporates, only the color pigment is left.
• The separation process of winnowing was used in ancient cultures to separate the grain from the chaff. They would throw the mixture into the air and the wind would blow away the lighter chaff, leaving the heavier grain.
• High speed centrifuges can spin up to 30,000 times a minute.
• Many separation processes are occurring constantly in nature.