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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
Forward Rate: The rate of the forward reaction takes place when reactants combine to become products.
Reverse Rate: The rate of the reverse reaction takes place when products break apart to become reactants.
Net Rate: The forward rate minus the reverse rate.
Average Rate: refers to 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. If the concentrations are stable during two measurements, the reaction is at an equilibrium point.
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.
Reactions depend on the compounds involved and how much of each compound is needed. Stoichiometry is the part of chemistry that studies amounts of substances that are involved in reactions. 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.
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
Assuming you are making 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
When you mix hydrogen gas (H2) and oxygen gas (O2), nothing much happens. But 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
Look at the equation above . 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.
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.