3.1 Solutions Study Guide

CHAPTER 3

Strand 3: Stability and Change in Chemical Systems

Chapter Outline

• 3.1 Solutions (Chem.3.1)
• 3.2 Predicting Reactions (Chem.3.2)
• 3.3 Macroscopic Changes (Chem.3.3)
• 3.4 Conservation of Matter (Chem.3.4)
• 3.5 Conserving Resources (Chem.3.5)
• 3.6 Rates of Reactions (Chem.3.6)
• 3.7 Equilibrium (Chem.3.7)
• 3.8 Designed Chemicals (Chem.3.8)

Conservation of matter describes the cycling of matter and the use of resources. In both chemical and physical changes, the total number of each type of atom is conserved. When substances are combined, they may interact with each other to form a solution. The proportion of substances in a solution can be represented with concentration. In a chemical change, the atoms are rearranged by breaking and forming bonds to create different molecules, which may have different properties. Chemical processes can be understood in terms of the collisions of molecules and rearrangements of atoms. The rate at which chemical processes occur can be modified. In many situations, a dynamic and condition-dependent balance between a reaction and the reverse reaction determines the numbers of all types of molecules present. Chemists can control and design chemical systems to create desirable results, although sometimes there are also unintended consequences.

 

3.1 Solutions (Chem.3.1)
Explore this Phenomenon

A dump truck carries salt to transport it to different locations in a city in the winter.

1. Why do some cold weather locations put salt on the roads?
2. Do you think the amount or concentration of salt being added will change the effect?
3. Do you think sugar would have the same effect?
   
   

Standard Chem.3.1
Use mathematics and computational thinking to analyze the distribution and proportion of particles in solution. Emphasize proportional reasoning and the impact of concentration on solution properties, rather than algorithmic calculations. Examples of concentrations affecting solutions could include the Beer-Lambert Law, colligative properties, or pH. (PS1.A)

In this section, pay attention to the number, size, and types (proportion) of particles that make up a solution. It is critical to recognize how changes in the number, size and type of particles will affect the properties of that solution.

 

Concentration of Solutions
When the weather is nice, many people begin to work on their yards and homes. For many projects, sand is needed as a foundation for a walk or to add to other materials. You could order up twenty million grains of sand and have people really stare at you. You could order by the pound, but that takes a lot of time weighing out. The best bet is to order by the yard, meaning a cubic yard. The loader can easily scoop up a yard of sand and put it directly in your truck.

 

Avogadro’s Number
It certainly is easy to count bananas or to count elephants (as long as you stay out of their way). However, you would be counting grains of sugar from your sugar canister for a long, long time. Atoms and molecules are extremely small – far, far smaller than grains of sugar. Counting atoms or molecules is not only unwise, it is absolutely  impossible. One drop of water contains about 10²² molecules of water . If you counted 10 molecules every second for 50 years without stopping you would have counted only 1.6 × 10¹⁰ molecules. Put another way, at that counting rate, it would take you over 30 trillion years to count the water molecules in one tiny drop.

Chemists needed a name that can stand for a very large number of items. Amedeo Avogadro (1776 - 1856), an Italian scientist, provided just such a number. He is responsible for the counting unit of measure called the mole. A mole (mol) is the amount of a substance that contains 6.02 × 10²³ representative particles of that substance. The mole is the SI unit for the amount of a substance. Just like the dozen is 12, it is a name that stands for a number. There are therefore 6.02 × 10 23 water molecules in a mole of water molecules. There also would be 6.02 × 10 23 bananas in a mole of bananas, if such a huge number of bananas ever existed. This number, 6.02 × 10 23 is called Avogadro’s number. A mole of any substance contains Avogadro’s number (6.02 × 10²³) of representative particles.

 

Italian scientist Amedeo Avogadro, whose work led to the concept of the mole as a counting unit in  chemistry. 

 

 

 

 

 

 

 

Solutions
If you go to the store to buy apple juice you have many different options. If you buy juice concentrate some of the water has been removed and the directions on the back of the can tell you how much water to add to turn the concentrate into juice. Other bottles of apple juice may be ready to drink straight from the container.

Most juices are solutions (homogeneous mixtures of substances) . They are made of multiple compounds that are thoroughly mixed. Salt water is another example of a common household solution. The salt and the water are uniformly mixed at a particle level. Most of the time the different parts of the solution are not visible.

The water on the left looks like a pure substance, but it is actually a salt water solution. If you were to look at it on a molecular level (like the illustration on the right) you would see that the NaCl is evenly distributed.

The solvent and solute are the two parts of a solution. In the drawing above, H 2 O is the solvent (the substance present in the greatest amount). The solute, (the substance present in the least amount) is the NaCl. When you are making a cup of hot chocolate, you take a teaspoon of cocoa powder and dissolve it in a cup of hot water. Since much less cocoa powder is used than water, the cocoa powder is the solute and water is the solvent.

If you were to add a half teaspoon of salt to a cup of water, you would make a solution, but the composition of this solution would be different than if you tried to dissolve one-half cup of salt in a cup of water. What might happen? At this point, the solution has passed the limit of the amount of salt that can be dissolved in it, so it would no longer be a solution—salt would sink to the bottom of the container and never dissolve. As a result, solutions have a composition that can be varied up to a point. There are, however, limits to the amount of substance that can be dissolved into another substance and still remain evenly mixed.

If a scientist obtains a sample of water from the Atlantic Ocean and determined that the sample was about 3.5 percent dissolved salt the salt water from the Pacific Ocean would be approximately 3.5 percent also because it is a homogeneous mixture or a solution.

Concentration
Concentration (a measure of how much of a given substance is mixed with another substance) is one way that chemists describe solutions. Solutions can be said to be dilute or concentrated. A concentrated solution (one in which there is a large amount of solute in a given amount of solvent) has more particles dissolved in it than a dilute solution (one in which there is a small amount of solute in a given amount of solvent). A dilute solution is a concentrated solution that has been diluted or watered down. Think of the juice containers you buy in the grocery store. In order to make juice, you mix the frozen juice from inside these containers with about 3 or 4 times the amount of water. Therefore, you are diluting the concentrated juice. The terms “concentrated” and “dilute,” however, only provide a qualitative way of describing concentration. In this chapter, we will explore some quantitative methods of expressing solution
concentration.

In this image, the solution on the left is more concentrated (more solute particles compared to solvent particles) than the solution on the right.

As you move from left to right, the solutions become more concentrated.

Air in the atmosphere is a gaseous solution. It is a mixture that contains mainly nitrogen and oxygen gases, with very small amounts of several other gases. The circle graph shows the composition of air. Because air is a solution, it is homogeneous. In other words, no matter where you go, the air always contains the same proportion of gases that are shown in the graph.

 

Molarity
Of all the quantitative measures of concentration, molarity – (the number of moles of solute per liter of solution) is the one used most frequently by chemists. The symbol given for molarity is M, or moles/liter.

Example 1
A chemist wants to make a 2M solution of salt water. How could they do it?
Solution:

• First find the molar mass of NaCl using a periodic table.
• Next, multiply the molar mass by 2 since the solution asks for 2 moles.
• Lastly, weigh out the 2 moles of NaCl then add water until the total solution reaches a volume of 1 liter.

Molarity is very easy to calculate when making 1 liter of solution, but often times chemists want to make more or less of a solution that has the same concentration. What if you want to make 2 liters of 2M NaCl (the same concentration as Example 1)? Since you are adding twice as much water, you would have to add twice as much NaCl. What if you want to make 0.5 L of 2M NaCl?

Example 2
What is the concentration, in mol/L, when 2.34 moles of NaCl has been dissolved in 500.0 mL of H 2 O?
Solution:
The concentration of the NaCl solution is 4.68 mol/L or 4.68 M.

Example 3
A solution is prepared by dissolving 42.23 g of NH 4 Cl into enough water to make 500.0 mL of solution. Calculate its molarity.

Step 1 : List the known quantities and plan the problem.

Known: mass = 42.23 g NH 4 Cl; molar mass NH 4 Cl = 53.50 g/mol; volume solution = 500.0 mL = 0.5000 L

Unknown: molarity =? M

The mass of the ammonium chloride is first converted to moles. Then the molarity is calculated by dividing by liters. Note the given volume has been converted to liters.

Step 2: Solve.

Step 3: Think about your result.
The molarity is 1.579 M, meaning that a liter of the solution would contain 1.579 mol NH 4 Cl.

 

Colligative Properties
Any solute will lower the freezing point and raise the boiling point of any solvent. The greater the concentration of the solute the lower the freezing point and greater the boiling point.

You may have seen the trucks put salt on the roads when snow or ice is forecast. Why do they do that? When planes fly in cold weather, the planes need to be de-iced before liftoff. How is that done? It turns out that pure solvents differ from solutions in their boiling points and freezing points when a solute is added. Boiling and freezing point changes are both examples of colligative properties (properties of solutions that are due only to the number of particles in solution and not to the chemical  properties of the solute).

 

Boiling Point Elevation
At 1 atm of pressure, pure water boils at 100ºC, but salt water does not. When table salt is added to water, the resulting solution has a higher boiling point than water alone.

Essentially, the solute particles take up space in the solvent, physically blocking some of the more energetic water molecules from escaping into the gas phase. This is true for any solute added to a solvent. Boiling point elevation (increase in a solvent’s boiling temperature when a solute is added) is an example of a colligative property, meaning that the change in boiling point is related only to the number of solute particles in solution, regardless of what those particles are. A 0.20 m solution of table salt would have the same change in boiling point as a 0.20 m solution of KNO₃ .

Freezing Point Depression
The effect of adding a solute to a solvent has the opposite effect on the freezing point of a solution as it does on the boiling point. Recall that the freezing point is the temperature at which a liquid changes to a solid. At a given temperature, if a substance is added to a solvent (such as water), the solute-solvent interactions prevent the solvent from going into the solid phase, requiring the temperature to decrease further before the solution will solidify. This is called freezing point depression (decrease in a solvent’s freezing temperature when a solute is added).

Remember that colligative properties are due to the number of solute particles in the solution. Adding 10 molecules of sugar to a solvent will produce 10 solute particles in the solution. However, when the solute is an ionic compound, such as NaCl, adding 10 molecules of solute to the solution will produce 20 ions (solute particles) in the solution. Therefore, adding enough NaCl solute to a solvent to produce a 0.20 m solution will have twice the effect of adding enough sugar to a solvent to produce a 0.20 m solution. 

 

Putting It Together
Let us revisit this phenomenon:

A dump truck carries salt to transport it to different locations in a city in the winter.

1. What colligative property is being demonstrated in this example?
2. How do colligative properties of a solution change as the concentration of salt goes from .10 m NaCl to .20 m NaCl?
3. Which would have the larger change in colligative properties a .20 m table salt solution or a .20 m sugar solution? Explain why.