Warmup 1/12/16 Imagine we have the following reaction: H 2 + I 2  2HI. If we double the amount of iodine, what do you think it will it do to the speed of the reaction? Will it do anything? Objective Tonight’s Homework To learn what affects the speed of a reaction pp 558: 1, 2, 3

Notes on Reaction Rates What makes a reaction go faster? To answer this question, we need to think about what’s physically happening when a reaction occurs. Let’s use the warmup as our example.

Notes on Reaction Rates What makes a reaction go faster? To answer this question, we need to think about what’s physically happening when a reaction occurs. Let’s use the warmup as our example. Here we have hydrogen and iodine combining to form hydrogen iodide. To combine, molecules need to actually collide. H H I I I H HI

Notes on Reaction Rates What can we do to get more collisions? 1) We can increase the concentrations If we cram in more molecules, we’ll get more collisions. More reactions! 2) We can increase the temperature If we make things hotter, we’re making the molecules move faster, and we’re bringing the total energy closer to the activation energy. Reactions happen easier! 3) We can do the right kind of reaction. Ionic reactions go faster than covalent ones.

Notes on Reaction Rates Let’s look at point 1 in more depth. Concentrations. We discussed how to do concentration last semester: Molarity (concentration) = moles Liters

Notes on Reaction Rates Let’s look at point 1 in more depth. Concentrations. We discussed how to do concentration last semester: Molarity (concentration) = If we wanted to discuss the concentration of, say, hydrogen iodide, we would write our chemical like this: This means we want to know its concentration moles Liters [HI]

Notes on Reaction Rates Let’s relate this to rate. We said that the more of each reactant we have, the faster the reaction should go. We would write this like this… rate = k[H 2 ][I 2 ] Where “k” depends on the size, speed, and type of molecules we have. We call “k” the specific rate constant. It’s a special number that helps tell us the speed of a reaction based specifically on what’s reacting.

Notes on Reaction Rates Let’s summarize this. The rate of a reaction depends on how concentrated the liquid or aqueous reactants are. If you’re pouring together really diluted chemicals, the reaction will go slowly. If you’re pouring together really concentrated chemicals, the reaction will go quickly.

Notes on Reaction Rates One more thing to note. Look at the following reaction: H 2 O 2 + 2HI  2H 2 O + I 2 We would write the rate for this as: rate = k[H 2 O 2 ][HI]

Notes on Reaction Rates One more thing to note. Look at the following reaction: H 2 O 2 + 2HI  2H 2 O + I 2 We would write the rate for this as: rate = k[H 2 O 2 ][HI] Why doesn’t the rate equation have [HI] 2 for 2 HIs? Because we can’t just look at this from a math perspective. The only way to know if we need a “2” or “3” or whatever in exponent is to get it through actual experimentation.

Example Problem Let’s see how we can calculate reaction rates. Given the data below, find the specific rate constant for the following reaction: H 2 O 2 + 2HI  2H 2 O + I 2 trial[H2O2][HI]Rate 10.1 M (mol/dm 3 )/s 20.1 M0.2 M (mol/dm 3 )/s 30.2 M0.1 M (mol/dm 3 )/s

Example Problem H 2 O 2 + 2HI  2H 2 O + I 2 What do we see here? - Between trials 1 and 2, [HI] doubles, so does the rate - Between trials 1 and 3, [H 2 O 2 ] doubles, so does the rate Since doubling either molecule doubles the rate, we know both are in our rate equation. If one of these hadn’t had an effect, we would have left it out of the rate equation. Rate = k[H 2 O 2 ][HI] trial[H2O2][HI]Rate 10.1 M (mol/dm 3 )/s 20.1 M0.2 M (mol/dm 3 )/s 30.2 M0.1 M (mol/dm 3 )/s

Example Problem H 2 O 2 + 2HI  2H 2 O + I 2 Rate = k[H 2 O 2 ][HI] If we want “k”, we just rearrange and solve. (To do this, use all the data from any one trial) k = k = k = 0.76 L/mols trial[H2O2][HI]Rate 10.1 M (mol/dm 3 )/s 20.1 M0.2 M (mol/dm 3 )/s 30.2 M0.1 M (mol/dm 3 )/s rate _ [H 2 O 2 ][HI] M/s__ (0.1 M)(0.1 M)

Practicing Reaction Rates Make sense? Let’s do a few more as a class to get some practice. Let’s go over #31 on page 569. After that, do 20, 21, 22, 23, and 24 on page 568.

Exit Question What things affect the speed of a reaction? a) Concentration b) The molecules involved c) Molecular size d) Temperature e) All of the above f) None of the above