HESI A2
HESI A2 Chemistry
1. What is 0 K equal to in °C?
- A. -300°C
- B. -273°C
- C. -250°C
- D. -200°C
Correct answer: B
Rationale: 0 Kelvin, also known as absolute zero, is equal to -273°C. This is the point at which all molecular motion stops, making it the lowest possible temperature on the Kelvin scale. Choice A (-300°C) is incorrect as it is not the correct conversion of 0 K to °C. Choice C (-250°C) and Choice D (-200°C) are also incorrect as they do not correspond to the accurate conversion of 0 K to °C.
2. Which of the following best describes an endothermic reaction?
- A. A reaction that absorbs heat
- B. A reaction that releases heat
- C. A reaction that does not involve heat
- D. A reaction that remains at a constant temperature
Correct answer: A
Rationale: An endothermic reaction is a process that absorbs heat from its surroundings. When a reaction absorbs heat, it leads to a decrease in the temperature of the surroundings, making choice A the correct description. In an endothermic reaction, energy is taken in from the surroundings, resulting in a decrease in temperature around the reaction site. Choice B is incorrect because a reaction that releases heat is known as an exothermic reaction. Choice C is incorrect as all reactions involve heat to some extent. Choice D is incorrect as an endothermic reaction does not remain at a constant temperature but rather absorbs heat, leading to a temperature decrease in the surroundings.
3. What is the energy required to remove the outermost electron from an atom called?
- A. covalent bonding
- B. electronegativity
- C. atomic radius
- D. ionization energy
Correct answer: D
Rationale: Ionization energy is the energy needed to remove the outermost electron from an atom, resulting in the formation of a positively charged ion. The higher the ionization energy, the more difficult it is to extract an electron. Electronegativity, however, measures an atom's ability to attract shared electrons in a chemical bond. Atomic radius refers to the distance from the nucleus to the outermost electron. Covalent bonding involves sharing electron pairs between atoms to create a stable bond. Therefore, the correct answer is ionization energy as it specifically relates to the energy needed to remove an electron from an atom.
4. Which one is not a hydrocarbon?
- A. Methane (CH4)
- B. Pyridine (C5H5N)
- C. Ethane (C2H6)
- D. Propane (C3H8)
Correct answer: B
Rationale: The correct answer is B, Pyridine (C5H5N). Pyridine is not a hydrocarbon because it contains nitrogen (N) in its molecular structure, in addition to carbon (C) and hydrogen (H) atoms. Hydrocarbons consist solely of carbon and hydrogen atoms. Methane (CH4), ethane (C2H6), and propane (C3H8) are all examples of hydrocarbons as they only contain carbon and hydrogen atoms, making them organic compounds known for their combustion properties.
5. Which of the following types of matter changes in volume with changes in temperature and pressure?
- A. Liquid
- B. Gas
- C. Solid
- D. Plasma
Correct answer: B
Rationale: A gas changes its volume with changes in temperature and pressure due to the particles moving more rapidly at higher temperatures, and pressure affects the space between particles. Liquids have a relatively fixed volume but take the shape of their container. Solids have a definite shape and volume, so they do not change volume with changes in temperature and pressure. Plasma is a state of matter where particles are highly energized and do not have a fixed volume, but it does not exhibit volume changes with temperature and pressure variations.
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