Directions for CST Released Questions 4/20 ELO

On another sheet of paper, show all work for the math problems. That's what will be given a stamp. See you Monday!

CST Released Questions 4/20 ELO #1-8

CST Released Questions #9-15

CST Released Questions 4/20 ELO #16-25

CST Released Questions 4/20 ELO 26-39

p 286 52c

p 282-286 4a, 13ae, 14 bf 52c

Ch 9 Review Sheet

Ch 17 Review Answers 1-4

1. According to the collision model, what determines whether colliding molecules will

    break bonds and form new products?

     Two substances must collide with sufficient energy to break bonds and allow new bonds to form.

2. Higher concentrations of reactants and higher temperatures increase the rate of formation of products. 

    How does the collision model explain these observations? ‘Increase the rate” means the reaction proceeds faster.

1. higher concentrations – Higher concentrations lead to more collisions and therefore to more reaction events.
2. Tigher temperatures – The speeds of the molecules increase with temperature. So at higher temperatures, the average collision is more energetic.

3. Sketch a graph of the progress of a chemical reaction showing the activation energy,  E_a, for the

    reaction.  Explain the graph for the decomposition of BrNO into NO and Br₂.

   

See p 539 Figure 17.3 If the energy contained in a collision of two BrNO molecules is greater than E_a, the colliding molecules bounce apart unchanged.

4. How does a catalyst increase the rate of a reaction?  Sketch a graph showing the effect of the catalyst

    on the E_a of the reaction.    See p 539 Figure 17.4

A catalyst provides a new pathway for the reaction with a lower activation energy than the original pathway, because more collisions will have enough energy to allow a reaction. This in turn leads to a faster reaction.

Ch 17 Review Answers 5-8

5. What do chemists mean by equilibrium?

The exact balancing of two processes, one of which is the opposite of the other. Rate of forward reaction = Rate of the Reverse Reaction

6. Why is equilibrium considered to be a dynamic state?

The concentration of the reactants and products do not change, but the formation of products and reactants is constant and ongoing.

7. What happens to the rates of forward and reverse reactions as a chemical system goes from start to

     equilibrium, starting with only reactants (no products).

At first, the rate of the forward reaction decreases and the rate of the reverse reaction increase. Equilibrium is reached when the forward rate and the reverse rate become the same.

8. Observe the following endothermic reaction:  2 NOCl(g)  « 2 NO(g) + Cl₂(g)

    What shift in equilibrium will occur when each of the following occurs?

a. Cl₂ gas is added (the concentration is increased) – left (shift away)

b. The volume is increased – right (larger number of gas moles)

c. NO(g) is removed – right (shift towards)

d. The temperature is decreased – left (endothermic, heat is on left side, shift towards heat)

e. The pressure is increased – left – (shift toward smaller number of gas moles)

Ch 17 Review Answers 1-8

1. According to the collision model, what determines whether colliding molecules will

    break bonds and form new products?

     Two substances must collide with sufficient energy to break (Ea) bonds and allow new bonds to form.

2. Higher concentrations of reactants and higher temperatures increase the rate of formation of products. 

    How does the collision model explain these observations? ‘Increase the rate” means the reaction proceeds faster.

1. higher concentrations – higher concentrations lead to more collisions and therefore to more reaction events
2. higher temperatures – the speeds of the molecules increase with temperature. So at higher temperatures, the average collision is more energetic.

3. Sketch a graph of the progress of a chemical reaction showing the activation energy,  E_a, for the

    reaction.  Explain the graph for the decomposition of BrNO into NO and Br₂.

   

See p 539 Figure 17.3 If the energy contained in a collision of two BrNO molecules is greater than E_a, the colliding molecules bounce apart unchanged.

4. How does a catalyst increase the rate of a reaction?  Sketch a graph showing the effect of the catalyst

    on the E_a of the reaction.    See p 539 Figure 17.4

A catalyst provides a new pathway for the reaction with a lower activation energy than the original pathway, because more collisions will have enough energy to allow a reaction. This in turn leads to a faster reaction.

5. What do chemists mean by equilibrium?

The exact balancing of two processes, one of which is the opposite of the other. The rate of the forward reaction is equal to the rate of the reverse reaction. 

6. Why is equilibrium considered to be a dynamic state?

The concentration of the reactants and products do not change, but there is a constant ongoing formation of products and reactants.

7. What happens to the rates of forward and reverse reactions as a chemical system goes from start to

     equilibrium, starting with only reactants (no products).

At first, the rate of the forward reaction decreases and the rate of the reverse reaction increase. Equilibrium is reached when the forward rate and the reverse rate become the same.

8. Observe the following endothermic reaction:  2 NOCl(g)  « 2 NO(g) + Cl₂(g)

    What shift in equilibrium will occur when each of the following occurs?

a. Cl₂ gas is added (the concentration is increased) – left (shift away)

b. The volume is increased – right (larger number of gas moles)

c. NO(g) is removed – right (shift towards)

d. The temperature is decreased – left (endothermic, heat is on left side, shift towards heat)

e. The pressure is increased – left – (shift toward smaller number of gas moles)

Ch 17 Review Answers 9-16

9. Which of the following reactions is homogeneous? (Multiple Choice)

a.   Zn(s) + HCl(aq) «ZnCl₂(aq) + H₂(g)

b.   CaCO₃(s) + HCl(aq) « CO₂(g) + CaCO(s) + H₂O(l) 

c.   CO(g) + NO₂(g) « CO₂(g) + NO(g) (all substances are the same state of matter)

10.  Explain how dissolving a slightly soluble salt to form a saturated solution is an equilibrium process.

        Only a small amount of a slightly soluble salt will dissolve. When the solution is saturated equilibrium is established. The total amount of solid dissolved remains constant, but at the surface of the solid, ions are going into solution at the same rate other ions are reforming into a solid.

11. List 3 catalysts from lecture, the book, or previous labs.

      Enzymes, carbonic anhydrase, Cl in ozone depletion, FeCl₃, MnO₂

12. Explain how Freon breaks down ozone in the upper atomosphere.

      The decomposition of Freon (CF₂Cl₂) releases Cl atoms. The Cl atoms combine with  O₃ to form ClO and O₂. An O atom combines with the ClO to produce Cl and O₂. The Cl atom is now free to break up another ozone molecule at a rate of 1 million per second.

13. Equilibrium can only be achieved in a __closed__ system. One with a lid, stopper, cap, etc.

14. Draw a picture representing carbon dioxide dissolved in Coke solution when pressure is on the system (it has a cap), and after the pressure is released (the cap is removed.)

In the capped bottle of Coke, CO2(aq) is dissolved in the solution. Trapped above the soda and cap, the CO2 is gaseous and remains at equilibrium with the aqueous CO2 dissolved in the Coke.

In the capped bottle, aqueous CO2 is dissolved in the Coke, but any gaseous CO2 existing above the Coke is released into the atmosphere and equilibrium does not occur. Gradually, all the aqueous CO2 escapes the solution to become gaseous CO2, and that’s how Coke goes flat.

15. Of the four changes that can shift equilibrium (pressure, volume, and temperature), which is the only one that will shift K? _____temperature______________

16. Draw and label the equilibrium diagram Figure 17.10 on p. 547. Describe what is happening in the diagram over time.

At first, the rate of the forward reaction decreases and the rate of the reverse reaction increases. Equilibrium is reached when the forward rate and the reverse rate become the same.