MU #1 Collision theory states that a reaction is most likely to occur if reactant particles collide with the proper energy and orientation.

Example

MU #2 The rate of a chemical reaction depends on several factors: temperature, concentration, nature of reactants, surface area, and the presence of a catalyst.

Temperature

As temperature increases the rate of the reaction increases because the KE of the molecules increases, therefore more effective collisions occur.

Concentration

As concentration of reactants increases the rate of reaction increases because there are more reactants, therefore more effective collisions occur.

Nature of reactants

The nature, strength, and number of bonds have an effect on the rate of reactions.

Nature of bonds

The rate of reactions involving hydrated ions is fast because free moving ions attract readily.

Example

Ag⁺(aq) + NO₃⁺(aq) + Na⁺(aq) + Cl^-(aq) --> AgCl(s) + Na⁺ (aq) + NO₃^-(aq)

Strength of bonds

The rate of reaction is usually slow when strong covalent bonds have to be broken.

Example

2 H₂(g) + O₂(g) -->  2 H₂O(l)

Number of bonds

The rate of reaction is slow if many bonds have to be broken.

Example

2 C₈H₁₈ + 25 O₂ --> 16 CO₂ +18 H₂O    

Surface Area

As the surface area of reactants increases the rate of reaction increases because more surface area is available for the reaction to take place.

Catalyst

The activation energy decreases and the rate of reaction increases. Catalysts are not consumed in reactions.

Example

The addition of a catalyst such as Pt(s) results in a very fast rate

TEST YOUR UNDERSTANDING

6/07

16 A 5.0-gram sample of zinc and a 50.-milliliter sample of hydrochloric acid are used in a chemical reaction. Which combination of these samples has the fastest reaction rate?

(1) a zinc strip and 1.0 M HCl(aq)                      (2) a zinc strip and 3.0 M HCl(aq)

(3) zinc powder and 1.0 M HCl(aq)    (4) zinc powder and 3.0 M HCl(aq)

17 For a given reaction, adding a catalyst increases the rate of the reaction by

(1) providing an alternate reaction pathway that has a higher activation energy         (2) providing an alternate reaction pathway that has a lower activation energy

(3) using the same reaction pathway and increasing the activation energy                   (4) using the same reaction pathway and decreasing the activation energy

8/07

46 Given the balanced equation representing a reaction: Zn(s) + 2HCl(aq) ---> H₂(g) + ZnCl₂(aq)

Which set of reaction conditions produces H₂(g) at the fastest rate?

(1) a 1.0-g lump of Zn(s) in 50. mL of 0.5 M HCl(aq) at 20.°C           (2) a 1.0-g lump of Zn(s) in 50. mL of 0.5 M HCl(aq) at 30.°C

(3) 1.0 g of powdered Zn(s) in 50. mL of 1.0 M HCl(aq) at 20.°C     (4) 1.0 g of powdered Zn(s) in 50. mL of 1.0 M HCl(aq) at 30.°C

8/02

Increasing the temperature increases the rate of a reaction by

(1) lowering the activation energy  (3) lowering the frequency of effective collisions between reacting molecules

(2) increasing the activation energy (4) increasing the frequency of effective collisions between reacting molecules

1/03

Which event must always occur for a chemical reaction to take place?

(1) formation of a precipitate                                              (2) formation of a gas

(3) effective collisions between reacting particles          (4) addition of a catalyst to the reaction system

8/03

35 In most aqueous reactions as temperature increases, the effectiveness of collisions between reacting particles
        (1) decreases (2) increases (3) remains the same

45 Based on the nature of the reactants in each of the equations below, which reaction at 25° C will
occur at the fastest rate?
        (1) C(s) + O₂ (g) --> CO₂ (g)                                     (3) CH₃OH(l) + CH₃COOH(l) --> CH₃COOCH₃ (aq) + H₂O(l)
        (2) NaOH(aq) + HCl(aq) --> NaCl(aq) + H₂O(l)     (4) CaCO₃ (s) --> CaO(s) + CO₂ (g)

6/04

19 Given the reaction at 25°C: Zn(s) + 2 HCl(aq) ---> ZnCl₂ (aq) + H₂ (g)

The rate of this reaction can be increased by using 5.0 grams of powdered zinc instead of a 5.0-gram strip of zinc because the powdered zinc has (1) lower kinetic energy (2) lower concentration (3) more surface area (4) more zinc atoms

8/04

20 A catalyst works by

                (1) increasing the potential energy of the reactants   (3) decreasing the potential energy of the products

                (2) increasing the energy released during a reaction  (4) decreasing the activation energy required for a reaction

6/05

19 Adding a catalyst to a chemical reaction results in 

                    (1) a decrease in activation energy and a decrease in the reaction rate 

                    (2) a decrease in activation energy and an increase in the reaction rate 

                    (3) an increase in activation energy and a decrease in the reaction rate 

                    (4) an increase in activation energy and an increase in the reaction rate 

45 A 1.0-gram piece of zinc reacts with 5 milliliters of HCl(aq). Which of these conditions of concentration and temperature would produce the greatest rate of reaction? 

                    (1) 1.0 M HCl(aq) at 20.°C       (2) 1.0 M HCl(aq) at 40.°C   (3) 2.0 M HCl(aq) at 20.°C       (4) 2.0 M HCl(aq) at 40.°C 

1/06

8/06

57 On the grid above, mark an appropriate scale on the axis labeled “Reaction Time (s).” An appropriate scale is one that allows a trend to be seen.

home