Looking for Year 12 HSC Chemistry Practice Questions?

Section A: Multiple Choice Questions

1. Which of the following is NOT a type of intermolecular force?

a. Hydrogen bonding
b. Dipole-dipole interactions
c. Covalent bonding
d. London dispersion forces

Answer: c. Covalent bonding

2. Which of the following is a strong acid?

a. HNO3
b. H3PO4
c. H2CO3
d. HF

Answer: a. HNO3

3. Which of the following is a characteristic of an exothermic reaction?

a. Products have higher energy than reactants.
b. Heat is absorbed from the surroundings.
c. The reaction is endergonic.
d. The enthalpy change is negative.

Answer: d. The enthalpy change is negative.

4. What is the oxidation state of carbon in CO2?

a. +2
b. +4
c. +6
d. -4

Answer: b. +4

Section B: Short Answer Questions

5. Define electronegativity and explain how it affects chemical bonding.

Answer : Electronegativity is a measure of an atom’s ability to attract electrons in a chemical bond. The higher the electronegativity of an atom, the more strongly it attracts electrons. This affects chemical bonding because when two atoms with different electronegativities form a bond, the electrons in the bond will be pulled towards the atom with the higher electronegativity, creating a polar bond. This can lead to the formation of polar molecules and intermolecular forces, such as dipole-dipole interactions.

6. What is the difference between an acid and a base?

Answer : Acids are substances that donate protons (H+) in a chemical reaction, while bases are substances that accept protons or donate hydroxide ions (OH-) in a chemical reaction. Acids typically have a pH below 7 and taste sour, whilst bases typically have a pH above 7 and taste bitter. Acids and bases are often used in neutralisation reactions, where an acid and a base react to form a salt and water.

7. What is the difference between an exothermic and endothermic reaction?

Answer : An exothermic reaction gives off heat to the surroundings, whilst an endothermic reaction absorbs heat from the surroundings. In an exothermic reaction, the products have lower energy than the reactants, resulting in a negative enthalpy change (ΔH). In contrast, in an endothermic reaction, the products have higher energy than the reactants, resulting in a positive enthalpy change (ΔH). Enthalpy change measures the amount of heat absorbed or released during a reaction.

Section C: Extended Response Questions

8. Explain the concept of Le Chatelier’s principle and provide an example of how it can be applied in a chemical reaction.

Answer:

Le Chatelier’s principle states that when a system in equilibrium is subjected to a change, it will respond by shifting the equilibrium position to counteract the change and re-establish a new equilibrium. This principle can be used to predict the effect of changes in temperature, pressure, concentration, or the addition of a catalyst on a chemical reaction.

For example, consider the reaction:

N2(g) + 3H2(g) ⇌ 2NH3(g)

If the concentration of nitrogen gas is increased, according to Le Chatelier’s principle, the system will respond by shifting the equilibrium position to reduce the concentration of nitrogen gas. This means that the forward reaction (the formation of ammonia) will be favoured and the rate of the reverse reaction (the decomposition of ammonia) will decrease until a new equilibrium is established.

Similarly, if the temperature of the system is increased, the equilibrium position will shift to counteract the increase in temperature. In this case, the reaction is exothermic, meaning it releases heat. Therefore, the equilibrium position will shift towards the reactants (nitrogen and hydrogen gases) to absorb the excess heat and reduce the temperature.

Le Chatelier’s principle provides a useful framework for predicting the behaviour of chemical systems under different conditions and can be used to design and optimise chemical processes in industry.