CHEM 1A Lecture Notes - Lecture 2: Ionic Compound, Chemical Equation, Chemical Formula

The Behavior of Substances in Water

Part 1:

1. Ammonia, NH₃, is a weak electrolyte. It forms ions in solution by reacting with water molecules to form the ammonium ion and hydroxide ion. Write the balanced chemical reaction for this process, including state symbols.
2. From everyday experience, you are probably aware that table sugar (sucrose), C₁₂H₂₂O₁₁, is soluble in water. When sucrose dissolves in water, it doesn’t form ions through any reaction with water. It just dissolves without forming ions, so it is a nonelectrolyte. Write the chemical equation for the dissolving of sucrose in water.
3. Both NH₃ and C₁₂H₂₂O₁₁ are soluble molecular compounds, yet they behave differently in an aqueous solution. Briefly explain why one is a weak electrolyte and the other is a nonelectrolyte.
4. Hydrochloric acid, HCl, is a molecular compound that is a strong electrolyte. Write the chemical reaction of HCl with water.
5. Compare the ammonia reaction with that of hydrochloric acid. Why are both of these substances considered electrolytes?
6. Explain why HCl is a strong electrolyte and ammonia is a weak electrolyte.
7. Classify each of the following substances as either ionic or molecular:

• KCl
• NH₃
• CO₂
• MgBr₂
• HCl
• Ca(OH)₂
• PbS
• HC₂H₃O₂  

8. For those compounds above that, you classified as ionic, use the solubility rules to determine which are soluble.
9. The majority of ionic substances are solids at room temperature. Describe what you would observe if you placed a soluble ionic compound and an insoluble ionic compound in separate beakers of water.
10. Write the chemical equation(s), including state symbols, for what happens when each soluble ionic compound that you identified above is placed in water. Are these substances reacting with water when they are added to water?
11. How would you classify the soluble ionic compounds: strong electrolyte, weak electrolyte, or nonelectrolyte? Explain your answer.
12. Sodium chloride, NaCl, is a strong electrolyte, as is hydroiodic acid, HI. Write the chemical equations for what happens when these substances are added to water. m
13. Are NaCl and HI strong electrolytes because they have similar behavior in an aqueous solution? If not, describe, using words and equations, the different chemical process that takes place in each case.

Part 2:

You have two hypothetical molecular compounds, AX and AY. AX is a strong electrolyte and AY is a weak electrolyte. The compounds undergo the following chemical reactions when added to water.

AX (aq) + H₂O (l) → AH₂O⁺ (aq) + X⁻ (aq)

AY (aq) + H₂O (l) → AH₂O⁺ (aq) + Y⁻ (aq)

1. Explain how the relative amounts of AX ( aq ) and AY ( aq ) would compare if you had a beaker of water with AX and a beaker of water with AY.
2. How would the relative amounts of X⁻ ( aq) and Y⁻ ( aq) in the two beakers compare? Be sure to explain your answer.

1) Consider the following equation: Rate = k [CH₃I]² [C₁₅H₁₁N]

i) If the concentration of C₁₅H₁₁N is doubled, does the rate constant:

A) Increase

B) Decrease

C) Remain unchanged

ii) If the concentration of of CH₃I is decreased by a factor of 3, the time it takes for the reaction to finish will:

A) Increase by a factor of 3

B) Increase by a factor of 9

C) Decrease by a factor of 9

D) Remain Unchanged

iii) If the temperature increases how will the activation energy change:

A) It will increase

B) It will decrease

C) It will remain unchanged

2) During a titration of a weak acid with a strong base you add an additional small amount of base after reaching the equivalence point.

i) How does the pH of the solution change?

A) Increase

B) Decrease

C) Remain unchanged

ii) How does the K_a of the acid change?

A) Increase

B) Decrease

C) Remain unchanged

3) You combine a solution of silver nitrate (AgNO₃) and a solution of sodium carbonate (Na₂CO₃) to form solid silver carbonate (Ag₂CO₃) and aqueous sodium nitrate (NaNO₃), the net ionic equation is listed below. You observe the white solid, Ag₂CO₃ in the test tube. You add a strong acid (HNO₃) to the mixture and it reacts with the carbonate ions as listed in the equations below. What would you expect to observe when adding the strong acid to the test tube?

2 Ag⁺_(aq) + CO₃^2-_(aq) <--> Ag₂CO_3(s)

2 H⁺_(aq) + CO₃^2-(aq) <--> H₂CO₃

H₂CO₃ -> H₂O_(l) + CO_2(g)

A) An increase in the amount of white solid

B) A color change of the solid

C) A decrease in the amount of white solid

D) No change in the amount of white solid

4) K_sp of BaF₂ at room temperature is 1.84 x 10^-7. It is also known that the reaction of dissolution of BaF₂ is endothermic. Imagine that you have a saturated solution of BaF₂ at equilibrium, that has some undissolved BaF₂ precipitated on the bottom of the test tube.

4.1) If the temperature is increased how would that affect:

i) The solubility of BaF₂

A) Increase

B) Decrease

C) Remain Unchanged

ii) The solubility constant of BaF₂

A) Increase

B) Decrease

C) Remain unchanged

4.2 An additional amount of BaF₂ was added to the solution how would that affect:

i) The solubility of BaF₂

A) Increase

B) Decrease

C) Remain unchanged

ii) The solubility constant of BaF₂

A) Increase

B) Decrease

C) Remain unchanged

4.3 If NaF is added to the solution how will it affect:

i) The solubility of BaF₂

A) Increase

B) Decrease

C) Remain Unchanged

ii) The solubility constant of BaF₂

A) Increase

B) Decrease

C) Remain unchanged