Rate of Reaction - Article.pdf

Rate of Reaction The rate of a reaction is the speed at which a chemical reaction 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...

Rate of Reaction The rate of a reaction is the speed at which a chemical reaction 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. If you want to think of a very slow reaction, think about how long it takes plants and ancient fish to become fossils (carbonization). The rate of reaction also depends on the type of molecules that are combining. If there are low concentrations of an essential element or compound, the reaction will be slower. 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 you have more possible combinations there is a higher chance that the molecules will complete the reaction. The reaction will happen faster which means the rate of that reaction will increase. Think about how slowly molecules move in honey when compared to your soda even though they are both liquids. There are a lower number of collisions in the honey because of stronger intermolecular forces (forces between molecules). The greater forces mean that honey has a higher viscosity than the soda water. Factors That Affect Rate 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. Temperature: When you raise the temperature of a system, the molecules bounce around a lot more. They have more energy. When they bounce around more, they are more likely to collide. That fact means they are also more likely to combine. When you lower the temperature, the molecules are slower and collide less. That temperature drop lowers the rate of the reaction. To the chemistry lab! Sometimes you will mix solutions in ice so that the temperature of the system stays cold and the rate of reaction is slower. 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, when you are in a chemistry lab, you will add one solution to another. When you want the rate of reaction to be slower, you will add only a few drops at a time instead of the entire beaker. Pressure: 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 and the rate of reaction decreases. Pressure is also related to concentration and volume. By decreasing the volume available to the molecules of gas, you are increasing the concentration of molecules in a specific space. You should also remember that changing the pressure of a system only works well for gases. Generally, reaction rates for solids and liquids remain unaffected by increases in pressure. Catalyst: A catalyst is like adding a bit of magic to a chemical reaction. Reactions need a certain amount of energy in order to happen. If they don't have it, oh well, the reaction probably can't happen. A catalyst lowers the amount of energy needed so that a reaction can happen more easily. A catalyst is all about energy. If you fill a room with hydrogen gas (H2) and oxygen gas (O2), very little will happen. If you light a match in that room (or just produce a spark), most of the hydrogen and oxygen will combine to create water molecules (H2O). It is an explosive reaction. You can also add a catalyst to that room and get one little reaction started. In that situation, you could add a little palladium (Pd) to act as the catalyst. Surface Area: The rate of a chemical reaction can be raised by increasing the surface area of a solid reactant. This is done by cutting the substance into small pieces, or by grinding it into a powder. If the surface area of a reactant is increased: more particles are exposed to the other reactant. 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. Stoichiometry/Conservation of Mass 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. We often call this the conservation of mass. 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 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. Heat and Cold What are heat and cold? It's a pretty simple idea. When you think of heat, you probably think of fire. When you think of cold, you might think of an ice cube. It all has to do with kinetic energy in atoms. Heat has a lot of kinetic energy and gives it away. The cold doesn't have much energy and absorbs it from the surrounding area. Chemists measure heat in units called Joules. You may also hear about sinks and sources. If the temperature of an object is higher than the surrounding area, it is considered a heat source. If the temperature of an object is lower than the surrounding area, it is considered a heat sink. Thermochemistry There are two kinds of heat in chemistry. The first is caused by physical activity. As you get more kinetic energy, there is more activity in the system. This extra activity makes more molecular collisions occur. The collisions create the heat. This happens when you increase the pressure in a system. Chemical processes cause the second type of heat. Instead of exciting a system and feeling the heat, chemical bonds are made and broken, and the energy is then released. A release of energy charges up the system and the molecules bounce around faster, resulting in that physical activity we just explained. The opposite can also happen. Sometimes bonds are made and broken and energy is absorbed. The system then gets colder as the temperature goes down. Those emergency icepacks you see when people hurt their ankles are good examples of chemical reactions that absorb energy. There is energy all around us. Just as matter is all around us, energy is always there. Usually, you will feel this energy as heat. Let's say it's really hot out today. Why is it hot? One big reason is that there is a lot of heat/energy coming from the Sun. The Sun is a big furnace, and that furnace heats the Earth. When a lot of the Sun's radiant energy makes it to Earth, it transmits energy to the atoms and molecules in the air and ground. Those molecules heat up. The Sun makes your molecules more excited because of the energy hitting you. You should remember that only a small percentage of the Sun's energy makes it to Earth. We're talking about millionths of a percent. The Sun gives off more energy than you can imagine, and it doesn't end there. There are also millions of stars that are bigger than our Sun. There's a lot of energy in the Universe. Energy in Chemical Bonds We just talked about energy in a star. There is also energy stored in the bonds between atoms. How about when you burn a piece of wood? When you burn something, you release the energy from the chemical bonds in the wood. Where did the energy come from? The Sun. A plant needs the Sun to grow. Light hits the plant and is used by a process called photosynthesis. The plant captures the Sun's energy and stores it in the chemical bonds. You have probably heard of glucose (C6H12O6), which is one of the smallest sugar building blocks made by plants. The plant uses glucose to power certain processes, to manufacture the cellulose, and as a building block in the cellulose itself. When you burn a piece of wood, you are releasing all of the energy stored up. You experience that energy as heat and light (fire).