Factors that affect reaction rates

Transcription

1 Factors that affect reaction rates

2 Nature of Reactants Chemical reactions require breaking of bonds between molecules. Forming bonds requires ENERGY or it can also release energy (breaking bonds)

3 Type of bonds It makes sense to think that if the bonds are weaker, then reaction can occur at a faster pace What are strong and weak bonds? Strong bonds covalent bonds - H 2, O 2 Weaker bonds ionic bonds Ionic compounds between non-metals and metals - NaCl

4 Nature of reactants For example, between these reactions which would be the fastest? H 2 (g) + I 2 (g) 2HI(g) Pb(NO3) 2 (aq) + 2NaI(aq) PbI 2 (s) + 2 NaNO 3 (aq) 1 st one has to break covalent bonds compared to ionic ones in the 2 nd. So 2 nd reaction will be faster

5 What if I have to break a lot of bonds? When comparing reactions with similar bonds, the more bonds that must be broken the slower the reaction.

6 Number of bonds For example, if we compare the following two reactions 2 NO(g) + O 2 (g) 2 NO 2 (g) 2 C 8 H 18 (g) + 25 O 2 (g) 16 CO 2 (g) + 18 H 2 O(g) Both reactions involve breaking covalent bonds. But the second one has many more bonds to break apart.

7 What about? What if they have similar bond types and similar number of bonds? Then we look at the state of the reactants Order of increasing rate Aqueous > Gases > Liquids > Solids Fastest Increasing Rate Slowest If the reactions have similar bonds and similar numbers of bonds, the state of the reactants is important.

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9 Temperature As temperature increases This Increases energy then increases speed then increases collisions then it can increase the probability of hitting at the correct orientations Therefore increasing the rate! Lowering the temperature reduces the average kinetic energy of the particles. This reduces the speed of the reactant particles and the number of reactant particles with activation energy. As a result, the frequency of effective collisions is decreased and the reaction rate decreases

10 Kinetic Energy The activation energy doesn t change!

11 Washing dishes If I need to wash grease off some dishes, which one would be more effective? 2 tablespoons in 1 cup of water 1 tsp 1 cup of water

12 Why the one with more? Because there is more detergent in the same volume, the dish detergent mixture is more concentrated. Increasing concentration of reactants increases the total number of particles. Increasing total number of particles more collisions

13 Eating and digestion Take a piece of bread, and chew chew chew. It will become sweet in your mouth because enzymes are breaking down the bread and changing in it into simple sugars. These enzymes are CATALYSTS

14 Catalyst A catalyst is a substance that speeds up or initiates a reaction without itself being permanently changed. Lowers the activation energy It helps by bringing the reactants together on to the active sites on the enzymes (catalysts)

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16 Catalysts The enzyme remains unchanged after the reaction and is not consumed by the reaction The reaction is a two-step mechanism: Enzyme + Substrate Enzyme-substrate complex Enzyme-substrate complex Enzyme + Product

17 Pressure Changes Recall Boyle s Law from Gr.11 Increase in pressure = decrease in volume

18 Pressure What are ways to increase the pressure? Add more product and/or reactant particles to the container. Increase or decrease the volume of the container Add an inert or unreactive gas.

19 How does pressure increase the rate? Increase in pressure = decrease in volume = concentration increases = increases the collisions = increased rate

20 Surface Area Dissolving sugar into coffee. Grains are more effective than cubes due to surface area. Increase in surface area = increases the number of particles of reactants in contact = increases frequency of collisions = increases rate

21 Summary The number and strength of the bonds which must be broken affect the rate of a reaction. Increasing temperature increases reaction rate by increasing the speed of reacting particles and the number of particles with activation energy. Increasing concentration of reactants increases reaction rates by increasing the number of particles with activation energy and increasing the frequency of collisions. Pressure changes only affect reactions containing gaseous reactants. Increasing pressure decreases the space between the particles, increasing the number of collisions. A catalyst is a substance which speeds up a reaction by reducing activation energy. A reduction in the activation energy increases the number of particles that can produce a successful collision. Grinding or powdering a solid reactant will increase reaction rates because of increased surface area in contact with other reactants.