Reactant A is placed in a beaker, to which Reactant B will be added. Reactants A and B will not react unless B gets to a certain concentration. Once the reaction starts, both concentrations decreases until B has been consumed . Which of the following graphs showing concentration in moles as a function of time in seconds, represent the reaction?

Options

A.
B.
C.
D.

Answer

To solve the problem, we need to analyze the behavior of the concentrations of reactants A and B over time as described in the question. Here’s a step-by-step breakdown of the solution:

### Step 1: Understand the Reaction Dynamics
- Reactant A is present in a beaker.
- Reactant B is added but will not react until it reaches a certain concentration.
- Once the reaction starts, both concentrations will decrease until B is completely consumed.

**Hint for Step 1:** Focus on the conditions under which the reaction begins and how the concentrations change over time.

### Step 2: Analyze the Graphs
- We need to look for graphs that show the behavior of the concentrations of A and B over time.
- Initially, the concentration of B must increase until it reaches the threshold for the reaction to start.

**Hint for Step 2:** Check for graphs where the concentration of B increases before any decrease occurs.

### Step 3: Eliminate Incorrect Options
- **Graph A:** Shows B as constant initially, which does not match the requirement that B must increase to start the reaction. **(Eliminate A)**
- **Graph B:** Shows B increasing to a certain point and then decreasing, which is consistent with the reaction dynamics.
- **Graph C:** Also shows B increasing and then decreasing, which is a potential candidate.
- **Graph D:** Shows B increasing and then leveling off at a value greater than zero, which means B is not fully consumed. **(Eliminate D)**

**Hint for Step 3:** Look for graphs that show B reaching zero after the reaction.

### Step 4: Check for Complete Consumption of B
- After the reaction starts, B must be completely consumed, meaning its concentration should reach zero.
- **Graph B:** B reaches zero, which is consistent with the requirement.
- **Graph C:** Also shows B reaching zero.

**Hint for Step 4:** Ensure that the graph shows B decreasing to zero, indicating complete consumption.

### Step 5: Consider the Behavior of Reactant A
- Reactant A is constant in volume and concentration until the reaction starts. Therefore, its concentration should not decrease until the reaction begins.
- **Graph B:** Shows A remaining constant until the reaction starts, which is correct.
- **Graph C:** Shows A decreasing from the beginning, which is not consistent with the problem statement. **(Eliminate C)**

### Conclusion
Based on the analysis, the correct graph that represents the reaction dynamics of reactants A and B is **Graph B**.

**Final Answer:** Graph B is the correct representation of the reaction.