balance this equation zn hcl zncl h 2 2

HESI A2

Chemistry Hesi A2

1. Balance this equation: Zn + HCl → ZnCl + H2.

A. Zn + 2HCl → ZnCl + H2
B. Zn + HCl → 2ZnCl + H2
C. 2Zn + 2HCl → 2ZnCl + H2
D. Zn + 4HCl → ZnCl + H2

Correct answer: C

Rationale: The given unbalanced equation is Zn + HCl → ZnCl + H2. To balance it, we need to have equal atoms on both sides of the equation. The balanced equation is 2Zn + 2HCl → 2ZnCl + H2. This balanced equation shows that two atoms of Zn combine with two molecules of HCl to form two molecules of ZnCl and one molecule of H2. Choice A is incorrect because it does not balance the equation. Choice B is incorrect as it does not have the same number of atoms on both sides. Choice D is incorrect because it does not balance the equation properly, resulting in an unequal number of atoms on both sides.

2. What is the name of the force that holds ionic compounds together?

A. Covalent bonds
B. Ionic bonds
C. Hydrogen bonds
D. Metallic bonds

Correct answer: B

Rationale: Ionic bonds are the forces that hold ionic compounds together. In ionic compounds, positively and negatively charged ions are held together by electrostatic forces of attraction, forming a stable structure. Covalent bonds involve the sharing of electrons between atoms, not the transfer of electrons like in ionic bonds. Hydrogen bonds are a type of intermolecular force, not the primary force in holding ionic compounds together. Metallic bonds are found in metals and involve a 'sea of electrons' that hold metal atoms together, different from the electrostatic attraction between ions in ionic compounds.

3. Cobalt-60 has a half-life of 5 years. If you start with 20 g of cobalt-60, how much is left after 10 years?

A. 15 g
B. 10 g
C. 5 g
D. 2.5 g

Correct answer: C

Rationale: Cobalt-60's half-life of 5 years means that after 5 years, half of the initial amount remains. Therefore, after 10 years, a quarter (half of a half) of the initial amount will remain. Starting with 20 g, after 10 years, 5 g of cobalt-60 will be left. Choice A (15 g) is incorrect because it assumes a linear decrease, not considering the exponential decay characteristic of radioactive substances. Choice B (10 g) is incorrect as it overlooks that after 10 years, more decay has occurred. Choice D (2.5 g) is incorrect as it represents only an eighth of the initial amount after 10 years, not a quarter.

4. What is stoichiometry?

A. The study of energy changes in chemical reactions
B. The study of the mass relationships in chemical reactions
C. The study of atomic structure
D. The study of molecular geometry

Correct answer: B

Rationale: Stoichiometry is the branch of chemistry that deals with the quantitative relationships between reactants and products in chemical reactions. It focuses on the calculation of the amounts of substances consumed and produced in a reaction based on the balanced chemical equation. Choice A is incorrect because the study of energy changes in chemical reactions falls under thermodynamics. Choice C is incorrect as atomic structure is related to the arrangement of atoms within molecules. Choice D is incorrect as molecular geometry deals with the spatial arrangement of atoms within molecules.

5. What are the three types of intermolecular forces?

A. Ionic, covalent, hydrogen
B. Hydrogen bonding, dipole interactions, dispersion forces
C. Van der Waals, ionic, covalent
D. Hydrogen, Van der Waals, dispersion forces

Correct answer: B

Rationale: The three types of intermolecular forces are hydrogen bonding, dipole interactions, and dispersion forces. Option A includes ionic and covalent bonds, which are intramolecular forces, not intermolecular. Option C includes van der Waals forces, which encompass dipole interactions and dispersion forces, but also includes ionic and covalent bonds. Option D is close but misses dipole interactions, which are distinct from hydrogen bonding and dispersion forces. Therefore, option B is the correct choice as it includes the three specific types of intermolecular forces.