Review 6 Answers

1. B Remember any burning is always exothermic. But just cooking is endothermic.

 

2. If the reaction takes twice as long, (140 /70), then the rate is of the original:

  R2/R1 =

  2x3 (0.25) = 1

x3 = 2.

x =

 

[W] should be increased by a factor of 1.26

 

3. a. We need to calculate the density of air and of helium:

 

air:

There are two unknowns, n and V. There is no air in the balloon. The air in the formula is that of air surrounding the balloon.

rearranging PV = nRT, we obtain:

n/V = P/RT. But we want g/L(density)instead of moles/L . No problem!

Since (n/V)*molar mass = density, by substituting:

P/RT* avg.molar mass of air = density of air = 101.3/(8.31* [20.0 + 273]) * 28.9 = 1.20g/L.

 

Helium:

Use the same technique:

P/RT* molar mass of He= density of He = 101.3/(8.31* [20 + 273]) * 4.0 = 0.17 g/L.

 

,

= 10.0 (1.20 0.17) = 10.3 g.

 

b. Rearranging PV = nRT, we obtain:

n/V = P/RT. But we want g/L instead of moles/L . Since (n/V)*molar mass = density, which is the same as (P/RT)*molar mass = density

we can substitute for density in the mload formula and obtain:

 

mload = V(rair -rHe)

 

mload = V, where M = molar mass

 

or by factoring:

 

mload =

 

 

c. Attach a bunch of grapes to several balloons. Then start eating one grape at a time until the balloon neither rises nor sinks to the floor.

d. (BONUS-type question) According to the principle of buoyancy, the buoyant force is equal to the weight of the displaced fluid or gas ( For instance if a cork is forced under water, the buoyant force trying to propel it upward will be equal to the equivalent weight of water represented by the volume of the cork.). For something not to sink, the buoyant force has to overcome gravity. In our case, where the balloon is perfectly suspended, those forces are equal:

 

Fbuoyant = Fgravity

Weight of air displaced by helium = weight of string and plastic + weight of He

mairg = mloadg + mHeg where g = gravitational acceleration, but it cancels:

mair= mload + mHe

If we express mass as density*volume, we obtain:

Vrair = mload + VrHe where V = volume of balloon

By isolating m and factoring we get:

mload = Vrair - VrHe

 

mload = V(rair -rHe)

 

4. H2 + I2 2HI

 

Time (s)

Remaining Amount of Iodine (moles)

Amount of iodine that reacted(moles)

Amount of HI produced in moles ( see ratio)

0

2.6

0

0

10

1.9

2.6 - 1.9 = 0.7

0.7*2 = 1.4

20

1.4

2.6 - 1.4 = 1.2

1.2*2 = 2.4

30

1.2

2.6 - 1.2 = 1.4

1.4*2 = 2.8

 

Avg rate for the last 20 s =

 

5. O2(g) + H2(g) 2 OH(g) DH = 77.9 kJ (1)

O2(g) 2 O(g) DH = 495 kJ (2)

H2(g) 2 H(g) DH = 435.9 kJ (3)

 

target : O(g) + H(g) OH(g)

 

divide eq(1) by 2:

0.5 O2(g) + 0.5 H2(g) OH(g) DH = 77.9 kJ/2 = 38.95 kJ

reverse eq(2) and divide by 2:

O(g) 0.5 O2(g) DH = - 495 kJ/2 = - 247.5 kJ

reverse eq(3) and divide by 2:

H(g) 0.5 H2(g) DH = - 435.9 kJ/2 = -217.95 kJ

 

Add them up: O(g) + H(g) OH(g) DH = -426.5 kJ = -427 kJ

 

6. Using Boyles Law, we obtain V2 = 3V1.

 

 

 

 

 

7.

a.                   adding the preservative calcium propionate to bread to slow the growth of mold__yes____

b.                  treating a cut with iodine to inhibit the function of bacterial proteins._ yes ____

c.                   destroying enzymes by adding mercury and silver___ yes ____

d.                  producing chlorophyll in early spring__no___

e. adding lactase to milk to break down lactose____no___