The reaction `A to B` , `DeltaH=+24 kJ//"mole"`. For the reaction `B to C`, `DeltaH=-18 kJ//"mole"`. The decreasing order of enthalpy of `A`, `B`, `C` follow the order

Options

A. A,B,C
B. B,C,A
C. C,B,A
D. C,A,B

Explanation

To determine the decreasing order of enthalpy for substances A, B, and C based on the given reactions and their enthalpy changes, we can follow these steps:

### Step 1: Analyze the given reactions and their enthalpy changes
- The reaction from A to B has an enthalpy change (ΔH) of +24 kJ/mol.
- The reaction from B to C has an enthalpy change (ΔH) of -18 kJ/mol.

### Step 2: Determine the relationship between the enthalpies of A and B
Since the reaction A to B has a positive ΔH (+24 kJ/mol), this indicates that energy is absorbed when A is converted to B. Therefore, the enthalpy of B is greater than that of A:
\[ H_B > H_A \]

### Step 3: Determine the relationship between the enthalpies of B and C
The reaction from B to C has a negative ΔH (-18 kJ/mol), indicating that energy is released when B is converted to C. Thus, the enthalpy of C is less than that of B:
\[ H_C < H_B \]

### Step 4: Combine the relationships to find the order of enthalpy
From the above steps, we have:
1. \( H_B > H_A \) (B has higher enthalpy than A)
2. \( H_C < H_B \) (C has lower enthalpy than B)

### Step 5: Determine the relative positions of A, B, and C
From the relationships:
- We know \( H_A < H_B \)
- We also know \( H_C < H_B \)

Now, we need to determine the relationship between A and C. To do this, we can find the overall change in enthalpy from A to C:
- The total change from A to C can be calculated as:
\[ \Delta H_{A \to C} = \Delta H_{A \to B} + \Delta H_{B \to C} = +24 \text{ kJ/mol} - 18 \text{ kJ/mol} = +6 \text{ kJ/mol} \]
Since this value is positive, it indicates that the enthalpy of C is greater than that of A:
\[ H_C > H_A \]

### Step 6: Finalize the order of enthalpy
Now we can summarize the relationships:
- \( H_A < H_C < H_B \)

### Step 7: Write the decreasing order of enthalpy
Thus, the decreasing order of enthalpy is:
\[ H_B > H_C > H_A \]

### Final Answer
The correct decreasing order of enthalpy is:
**B > C > A**