More Pretest 2.2 Type Questions

 

 

Combustion and Activation Energy

 

1. a. Write a balanced equation for the combustion of gallium, Ga.

b.                  Identify the fuel, the oxide(s), the agent of combustion and include heat on the appropriate side of the equation.

c.                   Does the oxide have a higher enthalpy than the oxygen and fuel combined?

 

Answers

 

a. 2Ga + 1.5 O2 Ga2O3 + heat ( Note that gallium is in aluminums family, so it acquires a +3 charge. Since oxygen acquires a 2 charge when reacting with metals, the formula of the product becomes Ga2O3.

b.                  fuel: Ga; agent of combustion: O2; oxide: Ga2O3.

c.                   No. Since all combustion reactions are exothermic, the oxides have a lower enthalpy than the reactants.

 

 

2.                  For combustion reactions, is there a difference between kindling point and activation energy? Explain.

 

Answer: Of course. The kindling point is the minimum temperature needed to get a fire going. Activation energy

is the minimum energy needed to get the fire going. Remember energy and temperature are not the same thing. Temperature is a measure of the average kinetic energy of molecules, and unlike energy, temperature does not depend on the total mass of molecules present.

 

3.                  If you wanted to sell kindling wood to help campers start fires in damp conditions, what kind of kindling point should the wood have? High? Or low?

Answer: low

 

4.                  Complete the table below for reaction A B:

 

Heat of reactants

50 kJ

Heat of products

 

Heat of activated complex

90 kJ

DH

22 kJ

Ae

 

 

Answers:

Heat of reactants

50 kJ

Heat of products

50 + 22 = 72kJ

Heat of activated complex

90 kJ

DH

22 kJ

Ae

90-50 = 40 kJ

 

5.                  Fill in the blank: Raising the temperature or adding a catalyst will increase the number of ___________collisions.

 

Answer: effective

 

6.                  If water does not react with the fuel or, as in the case of oil, if it does not allow it to float on top of itself, then water usually puts a fire. Why?

 

Answer: Water has a high specific heat so it absorbs a lot of energy and therefore lowers the temperature of the fuel.

 

Rate of Combustion

 

7.                  In each case explain what factor is slowing the rate of combustion.

 

a.                   A thick tree stands in the middle of a brush fire.

b.                  A small, poorly ventilated room with its windows closed is on fire.

c.                   A campfire at the mercy of elements at the South Pole.

d.                  When asked to gather fuel for the fire, a child throws in a soggy piece of wood.

 

Answers:

a. low surface area to weight ratio for the tree

b. lack of oxygen

c. low temperature; strong cold wind has cooling effect

d.                  Wrong choice of wood (nature of fuel)

 

Factors Influencing Rates

 

8.                  Pick the fastest and slowest reactions from the list below. All are at room temperature.

 

a. 2 H3PO4 + 3 Ca(OH)2 6H2O + Ca3(PO4)2

 

b. C4H10 + 6.5 O2 5 H2O + 4CO2

 

c. O2(g) + 2 Cl2(g) 2 OCl2(g)

 

Answers: fastest (a): neutralization reaction

Slowest (b): involves breaking the most and remaking the most covalent bonds

 

9.                  A cube of calcium is added to water. A second cube is first sliced parallel to one of its bases and then added to water. Which will react faster, and how much faster will it react, if all other conditions remain the same?

 

Answer: The second cube will react faster. How much faster? Well it its sliced in the manner described, we will be exposing 2 more squares. Picture it. That means that we will have a surface area of (6s2 + 2s2 ) versus an area of 6s2 for the unsliced cube of calcium. So the reaction will proceed (6s2 + 2s2 )/ 6s2 = 8/6 =1.33 times faster.

 

10.              If the rate of the reaction 2NO(g) + O2(g) 2NO2 (g) is described by the formula:

 

Rate = k[NO]2[O2] where [ ] means concentration, and k is a fixed number(constant), what will happen if you triple the concentration of NO gas?

 

Answer: The reaction will proceed [3]2 times or 9 times faster.

 

11.              In the example in which a bacteria attempts to make itself a cell wall, what acts as an agent which lowers the activation energy, and what raises the activation energy?

 

Answer: agent which lowers the activation energy = catalyst = enzyme.

agent which raises the activation energy = inhibitor = inhibitor.

 

12.              Which is the higher temperature? T1? Or T2? Why?

 

T2

 

 

Kinetic energy (kJ)

 

 

Answer: T2 because the area bound to the right of the Ae line and under the T2 curve is greater than the area bound to the right of the Ae line and under the T1 curve.

 

13. Given: 2 NO + O2 2NO2

 

Calculate the average rate of NO2 formation for the last 8 recorded minutes.

 

Time (minutes)

0

2

4

6

8

10

12

Amount of O2 disappearing

(moles)

0

3

4.5

5.0

5.3

5.4.

5.5

 

Avg. rate of oxygen disappearance = Dn/Dt = (5.5 - 4.5)/(12-4) = 0.125 moles O2/minute

0.125 moles O2/minute(2 moles NO2/mole O2)= 0.250 moles NO2/min

 

 

14. At 20 oC, the following reaction is extremely slow:

 

CH4 + 2 O2 CO2 + 2 H2O

 

 

Assuming that it takes 50 years for 160 g of CH4 to disappear, at what rate is water produced? Express your answer in ml/year.

 

(160g CH4/50 years)(mole CH4/16 g)(2 mole water/mole CH4) (18 g water /mole)(1ml/g) = 7.2 ml water /year