Umar and Louis are observing a chemical reaction in their science class. Which statement is true if the free energy (ΔG) of the reaction they are studying is zero?

• The rate of the forward reaction is zero.

• The rate of the reverse reaction is zero.

• The reaction is approaching equilibrium.

• The reaction is at equilibrium.

Answer explanation

When the free energy (ΔG) of a reaction is zero, it indicates that the forward and reverse reactions occur at the same rate, meaning the system is at equilibrium.

During a chemistry experiment, Danny and Damilola add a catalyst to a reaction mixture that has reached equilibrium. The value of the equilibrium constant

• decreases

• increases

• remains the same

Answer explanation

A catalyst speeds up the rate of reaching equilibrium but does not change the position of equilibrium or the value of the equilibrium constant. Therefore, the equilibrium constant remains the same.

Jaeda and Umar are conducting an experiment. If they increase the temperature of a chemical system at constant pressure, the rate of the forward reaction will

• decrease

• increase

• remain the same

Answer explanation

Increasing the temperature at constant pressure typically favors the endothermic direction of a reaction. If the forward reaction is endothermic, its rate will increase with temperature, making 'increase' the correct choice.

Tags

NGSS.HS-PS1-5

During a chemistry experiment, Deshaun and Jaeda are observing a reaction in a closed container. If O₂ is added to the system at constant pressure and temperature, what will happen to the number of moles of HCl?

• decrease

• increase

• remain the same

Answer explanation

Adding O2 shifts the equilibrium of the reaction involving HCl, leading to a decrease in the concentration of HCl as it reacts to form products. Therefore, the number of moles of HCl will decrease.

Jaeda and Damilola are conducting an experiment with a chemical reaction at constant temperature. If they increase the pressure on the system, the value of the equilibrium constant for the reaction will

• decrease

• increase

• remain the same

Answer explanation

Increasing pressure shifts the equilibrium towards the side with fewer moles of gas. If the reaction produces more moles of gas, the equilibrium constant will increase, making the correct answer 'increase'.

Tags

NGSS.HS-PS1-6

During a science experiment, Umar and Jaeda are investigating phase changes at 1 atmosphere of pressure. Which equilibrium is correctly associated with the Kelvin temperature at which it occurs?

• ice-water equilibrium at 0 K

• ice-water equilibrium at 32 K

• steam-water equilibrium at 212 K

• steam-water equilibrium at 373 K

Answer explanation

The steam-water equilibrium occurs at 373 K (100°C) at 1 atmosphere pressure, which is the boiling point of water. The other options incorrectly associate temperatures with their respective equilibria.

Jaeda and Danny place a sealed flask of water at 298 K on their classroom desk. After some time, the water is in equilibrium with its vapor inside the flask. This is an example of:

• chemical equilibrium

• phase equilibrium

• solution equilibrium

• static equilibrium

Answer explanation

This scenario illustrates phase equilibrium, where liquid water and its vapor coexist in a sealed environment at a constant temperature. The system is balanced between the liquid and vapor phases.

In an experiment, Louis and Umar added radioactive Pb*(NO₃)₂(aq) to the following equilibrium system: [* indicates radioactive Pb²⁺ ions] PbCl₂(s) ↔ Pb²⁺(aq) + 2Cl⁻(aq). When equilibrium was re-established, some of the PbCl₂(s) was recovered from the system and dried. Testing showed the PbCl₂(s) was radioactive. Which statement is best supported by this result?

• At equilibrium, the rates of chemical change are equal.

• At equilibrium, the rates of chemical change are unequal.

• The process of dynamic equilibrium is demonstrated.

• The process of dynamic equilibrium is not demonstrated.

Answer explanation

The presence of radioactive Pb in the recovered PbCl2(s) indicates that Pb2+ ions were in equilibrium with the solid. This supports the idea of dynamic equilibrium, where both forward and reverse reactions occur at equal rates.

Louis and Jaeda are calibrating a Celsius thermometer at 1 atmosphere of pressure. The fixed temperature points they use are determined on the basis of

• the ice/water equilibrium temperature only

• the water/steam equilibrium temperature only

• both the ice/water and the water/steam equilibrium temperatures

• neither the ice/water nor the water/steam equilibrium temperatures

Answer explanation

The Celsius thermometer is based on two fixed points: the ice/water equilibrium temperature (0°C) and the water/steam equilibrium temperature (100°C) at 1 atmosphere of pressure, making the correct choice both these temperatures.

• increased solvent contact

• increased solute solubility

• the equilibrium to shift to the left

• the equilibrium to shift to the right

Answer explanation

Powdering CuSO4 increases its surface area, allowing more solvent to contact the solute. This enhances the dissolution rate, making the reaction proceed more towards the right, but the key reason is the increased solvent contact.

Tags

NGSS.HS-PS1-5