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.