Reaction Rates and Equilibrium CHAPTER 12 Reaction Rates and Equilibrium 12.1 Reaction Rates

In Chapter 11 How much product can be formed? we discussed amounts of chemicals involved in a reaction: How much product can be formed? How much of a reactant is needed? How much excess reactant remains? Excess reactant Limiting reactant

In Chapter 11 In Chapter 12 How much product can be formed? we discussed amounts of chemicals involved in a reaction: How much product can be formed? How much of a reactant is needed? How much excess reactant remains? In Chapter 12 we will discuss the rate (speed) of a chemical reaction: How fast is a reactant used up? How fast does a product form? What are factors that affect how fast a reaction occurs?

What is a rate? How is it measured? The idea of rate Some reactions are fast Others are slow C(s) + O2(g) → CO2(g) 4Fe(s) + 3O2(g) → 2Fe2O3(s) What is a rate? How is it measured?

The idea of rate Average speed traveled by a typical race horse:

The idea of rate The speed is not constant throughout the race Average speed traveled by a typical race horse: The speed is not constant throughout the race Steeper slope: higher speed Most race horses finish at a faster speed than they start at

Can we measure time/distance? Rate in chemistry Consider the generic reaction: A → C Can we measure time/distance? How can we measure rates in chemistry?

Rate in chemistry Consider the generic reaction: A → C We monitor time How can we measure rates in chemistry?

the number of A or C particles in the system Rate in chemistry Consider the generic reaction: A → C We monitor time We monitor the number of A or C particles in the system (or concentration) How can we measure rates in chemistry?

Rate in chemistry Consider the generic reaction: A → C How can we measure rates in chemistry?

At time 0 seconds A → C Beginning: High concentration of A No C present A → C

End: High concentration of C Low concentration of A A → C

Why do the rates decrease over time? Steeper slope: faster rate “flatter” slope: slower rate A → C

Because there are fewer molecules of A to convert into C Why do the rates decrease over time? Because there are fewer molecules of A to convert into C A → C

Data for the reaction A → C Average rate of consumption of A Time (min) Moles of A Moles of C 1.00 0.00 10 0.74 0.26 20 0.54 0.46 30 0.40 0.60 40 0.30 0.70 Data for the reaction A → C in a 1 L container

Data for the reaction A → C Average rate of consumption of A: –0.02 moles/(L·min) Time (min) Moles of A Moles of C 1.00 0.00 10 0.74 0.26 20 0.54 0.46 30 0.40 0.60 40 0.30 0.70 Data for the reaction A → C in a 1 L container

Data for the reaction A → C Average rate of consumption of A: –0.02 moles/(L·min) Time (min) Moles of A Moles of C 1.00 0.00 10 0.74 0.26 20 0.54 0.46 30 0.40 0.60 40 0.30 0.70 Average rate of formation of C: Data for the reaction A → C in a 1 L container

Data for the reaction A → C Average rate of consumption of A: –0.02 moles/(L·min) Time (min) Moles of A Moles of C 1.00 0.00 10 0.74 0.26 20 0.54 0.46 30 0.40 0.60 40 0.30 0.70 Average rate of formation of C: 0.02 moles/(L·min) Data for the reaction A → C in a 1 L container

Data for the reaction A → C Average rate of consumption of A: –0.02 moles/(L·min) Time (min) Moles of A Moles of C 1.00 0.00 10 0.74 0.26 20 0.54 0.46 30 0.40 0.60 40 0.30 0.70 Average rate of formation of C: 0.02 moles/(L·min) 1 mole A ~ 1 mole C 1:1 ratio Data for the reaction A → C in a 1 L container The rate of consumption of A is equal to the rate of formation of C: Molecules A are consumed as fast as molecules C are formed Molecule A is consumed so its rate is a negative number

The decomposition of N2O5 follows the reaction The rate of decomposition of N2O5 was measured after 25 s and was found to be 5.60 x 10–6 M/s. What is the rate of formation of NO2? 2N2O5(g) → 4NO2(g) + O2(g)

The decomposition of N2O5 follows the reaction The rate of decomposition of N2O5 was measured after 25 s and was found to be 5.60 x 10–6 M/s. What is the rate of formation of NO2? 2N2O5(g) → 4NO2(g) + O2(g) Asked: Rate of NO2 formation Given: Rate of N2O5 decomposition = 5.60 x 10–6 M/s Relationships: 2 moles N2O5 ~ 4 moles NO2 Solve: Use mole ratios

The decomposition of N2O5 follows the reaction 2N2O5(g) → 4NO2(g) + O2(g) Asked: Rate of NO2 formation Given: Rate of N2O5 decomposition = 5.60 x 10–6 M/s Relationships: 2 moles N2O5 ~ 4 moles NO2 Solve: Use mole ratios Remember

The decomposition of N2O5 follows the reaction 2N2O5(g) → 4NO2(g) + O2(g) Asked: Rate of NO2 formation Given: Rate of N2O5 decomposition = 5.60 x 10–6 M/s Relationships: 2 moles N2O5 ~ 4 moles NO2 Solve: Use mole ratios

The decomposition of N2O5 follows the reaction 2N2O5(g) → 4NO2(g) + O2(g) Asked: Rate of NO2 formation Given: Rate of N2O5 decomposition = 5.60 x 10–6 M/s Relationships: 2 moles N2O5 ~ 4 moles NO2 Solve: Use mole ratios Answer: The rate of NO2 formation is 1.12 x 10–5 moles/(L·s)

The decomposition of N2O5 follows the reaction 2N2O5(g) → 4NO2(g) + O2(g) Asked: Rate of NO2 formation Given: Rate of N2O5 decomposition = 5.60 x 10–6 M/s Relationships: 2 moles N2O5 ~ 4 moles NO2 Solve: Use mole ratios Answer: The rate of NO2 formation is 1.12 x 10–5 moles/(L·s) Discussion: The mole ratio tells us that the rate of NO2 formation (by moles) is twice the rate of N2O5 decomposition. NO2 is formed twice as fast as N2O5 is decomposed (by moles).

Collision theory A + B → Products Chemical reactions take place at the molecular level, where molecules of reactants are colliding with each other

Collision theory The cue (white) ball collides with the orange ball and successfully sends it into the pocket

Collision theory But not all collisions are successful Collision alone does not guarantee success

Collision theory But not all collisions are successful Collision alone does not guarantee success The same is true in chemistry

Reaction profile Reaction: A + B → C + D ∆H < 0 Reactants Products

Reaction profile Reaction: A + B → C + D ∆H < 0 Energy is released as a result of the reaction Reactants Products

Reaction profile Reaction: A + B → C + D ∆H < 0 activation energy, Ea: the minimum amount of energy required for molecules to react. Reactants Products

Reaction profile Reaction: A + B → C + D ∆H < 0 activated complex, Ac: a high- energy state where bonds are being broken and reformed; also referred to as the transition state. Reactants Products

Reaction profile Reaction: A + B → C + D ∆H < 0 activated complex, Ac Ac is unstable and can: go back to A + B (reactants) proceed to C + D (products) Reactants Products

The reaction profile will look different Reaction: A + B → C + D ∆H < 0 EnergyRectants > EnergyProducts Consider the reverse reaction: Reaction: C + D → A + B ∆H > 0 EnergyRectants < EnergyProducts The reaction profile will look different

Reaction: C + D → A + B ∆H > 0 Energy is absorbed as a result of the reaction

A + B → C + D ∆H < 0 C + D → A + B ∆H > 0 Ea is larger! Exothermic process Endothermic process Exothermic reactions tend to be more common than endothermic reactions because the energy barrier is lower

Based on this energy curve, this reaction is an process exothermic endothermic

Based on this energy curve, this reaction is an process exothermic endothermic

Based on this energy curve, this reaction is an process Can you place the following labels on this reaction profile? Exothermic Endothermic Reactants Products ∆H Ea Ac

Based on this energy curve, this reaction is an process Can you place the following labels on this reaction profile? Ac Exothermic Endothermic Ea Products Reactants ∆H

If there is enough energy to overcome the energy barrier, Ea PROCEED Reactant Product If there is enough energy to overcome the energy barrier, Ea the activated complex becomes product

If there is not enough energy to overcome the energy barrier, Ea GO BACK Reactant Product If there is not enough energy to overcome the energy barrier, Ea the activated complex become reactants again

Very few collisions result in the actual formation of products GO BACK Reactant Product Very few collisions result in the actual formation of products

Oxygen and carbon molecules collide all the time, but the combustion of carbon will not start without an initial input of energy (such as a spark) GO BACK C(s) + O2(g) CO2(g) C(s) + O2(g) → CO2(g)

We saw earlier that the rates slow down over time This is because there are fewer molecules of reactants present as the reaction progresses A → C

The reaction rate is higher when more reactants are present More collisions occur when the concentration of reactants is higher More collisions means more reactions are possible The reaction rate is higher when more reactants are present

What else can affect the rate concentration of reactants affects the reaction rate What else can affect the rate of a chemical reaction? A → C

Temperature Energy barrier between reactants and products Temperature is a measure of the average kinetic energy of molecules The higher the kinetic energy, the higher the number of molecules that successfully overcome the energy barrier

The reaction rate increases when the temperature increases Energy barrier between reactants and products The reaction rate increases when the temperature increases

The reaction rate increases when the temperature increases This is why food goes bad faster when it is warm The reaction rate increases when the temperature increases

Increased surface area leads to a higher reaction rate Increased surface area means more particles are available for collisions Increased surface area leads to a higher reaction rate A More particles are exposed and available to collide with other particles to have a reaction

Based on this energy curve, this reaction is an process Can you place the following labels on this reaction profile? exothermic endothermic Ea Products Reactants ∆H

Factors that affect the reaction rate: Concentration of reactants The higher the concentration of reactants, the higher the rate Temperature The higher the temperature, the higher the rate Surface area The higher the surface area, the higher the rate

Factors that affect the reaction rate: Concentration of reactants The higher the concentration of reactants, the higher the rate Temperature The higher the temperature, the higher the rate Surface area The higher the surface area, the higher the rate Catalysts This will be discussed in Section 12.4