Question Video: Determining Graphically the Time Taken for a Reaction to Reach Completion Science • Third Year of Preparatory School

Video Transcript

The graph below shows the concentration of oxygen gas produced during the following chemical reaction: two H2O2 reacts to form two H2O plus O2. How long does this reaction take to reach completion? (A) 90 seconds, (B) 120 seconds, (C) 80 seconds, (D) 100 seconds, (E) 20 seconds.

In the reaction in this question, hydrogen peroxide decomposes to form water and oxygen gas. The concentration of oxygen gas has been graphed over time. We need to determine how long it takes this reaction to reach completion. At the start of the reaction, the concentration of oxygen is zero because no oxygen gas has been produced yet. Then, the concentration gradually increases until the concentration stops changing when it reaches a value of one mole per liter. When the concentration of the reactants and products stop changing, it means the reaction has reached completion. The graph shows that the concentration of oxygen stopped changing after 90 seconds. So answer choice (A) is the correct answer. It took 90 seconds for this reaction to reach completion.