5.3 - Factors that Affect Rates of Reactions

Factors that affect rate of reactions We will now focus on the factors that affect the rate of chemical reactions: 1. Nature of the Reactants 2. Temperature 3. Concentration and Pressure 4. Catalysts

1. Nature of Reactants A) Ionic Bonds/Ionic Compounds: Since chemical bonds are broken and reformed in a chemical reaction, the nature of the bonds changes the rate of reaction. That is, how readily the bonds are broken and formed. For example, a reaction which involves primarily the exchange of electrons tends to be very quick. ex) Ba(NO3)2(aq) + Na2CrO4(aq) → BaCrO4(s) + 2 NaNO3(aq) Therefore, the net ionic equation is: Ba2+(aq) + CrO42-→ BaCrO4(s) Ions have opposite charges. Opposite charges attract which will form new ionic bonds Reactions that involve ions in solution tend to be fast.

1. Nature of Reactants B) Covalent Compounds Reactants that involve covalent bonds tend to be very slow, unless they are highly exothermic (combustions). For example, the decomposition of hydrogen peroxide into hydrogen gas and oxygen gas happens very slowly.

1. Nature of Reactants C) Phase of Reactants The phase of the reactants is also important. That is, reactants in liquid, solution, or gas form react much more rapidly than solids.

1. Nature of Reactants D) Surface Area The exposed surface area also affects the rate of reaction. The greater the surface area, the faster the reaction. This is usually done by grinding into a powder or breaking into smaller pieces For example, kindling vs. logs when starting a fire

1. Nature of Reactants E) Mechanical Mixing Stirring also increases the reaction rate because it increases the frequency of collisions.

2. Temperature In general, if the temperature of a system increases, so does the rate of reaction. A rule of thumb is that the rate will double with an increase of 10˚C.

2. Temperature Remember that the curve represents all the particles in a system. All the particles have kinetic energy (because of their motion) but some have more than others.

2. Temperature Since a reaction cannot take place if the colliding particles do not have enough energy, why would increasing the temperature increase the reaction rate?

2. Temperature Note that increasing the temperature does not change the activation energy.

2. Temperature Instead, the number of particles with energy higher than the threshold energy increases. Thus, when comparing the curve at the first temperature (T1) and the second temperature (T2) we get:

2. Temperature Remember that temperature is the average kinetic energy in a system. This is why the curve is shifted, yet the activation energy remains the same. Notice the shaded area for T2 is larger than the area in T1.

2. Temperature There are two reasons why the rate increases: 1. Since particles are moving faster, there will be more collisions. 2. More particles have enough energy (the activation energy) to make successful collisions.

Temperature Potential energy curves will remain unchanged when we raise the temperature of a system. The curve says nothing about the rate, but if the system is at a higher temperature, more particles would be able to get ‘over the hill’ to make successful reactions.

3. Concentration and Pressure Increased reactant concentration will increase the rate of reaction More particles (increased concentration) means more collisions. More collisions means a faster reaction.

3. Concentration and Pressure As far as gases go, the concentration can be increased by decreasing the volume (putting the gas in a smaller container). This forces the particles to be closer together, increasing the concentration. Particles closer together means more collisions. This is also increasing the pressure.

4. Catalysts A catalyst is a substance that provides an alternate reaction mechanism with a lower activation energy. Catalysts do not get used up in the reaction. That is, they are introduced as a reactant and show up as a product. If we can lower the activation energy, the number of particles with enough energy to make successful collisions naturally increases.

4. Catalysts Solid line = uncatalyzed reaction Dotted line = catalyzed reaction Notice that for the diagram above, the ΔH = 15 J for both the catalyzed and uncatalyzed reaction. Therefore, the heat of reaction is independent of the pathway (Hess’s Law). However, the activation energy for the uncatalyzed reaction is 25 kJ, while it is only 10 kJ for the catalyzed reaction.

Enzymes Enzymes are biological catalysts Each enzyme has a specific shape that allows it to carry out a very specific function in living things Almost every chemical reaction in the body (or in most living things) is carried out by an enzyme Reactants Enzyme

4. Catalysts Example 4: Below is a different reaction mechanism for phosgene gas than what we saw before: a. Write the overall reaction equation. b. Identify any reaction intermediates. c. Identify any catalysts.