You can select a particular zone id by assigning the pointer to point at the corresponding zone using the Lookup_Thread macro. You will need to get the corresponding zone ID. Here is the link from the Ansys UDF Manual regarding the macro.
3.2.6. Special Macros (ansys.com) -> Check section 220.127.116.11
To access the temperature for a cell, you can use the flow variable macro for temperature. The macro is C_T(c,t). Here is the link of the same from the UDF Manual.
3.2. Data Access Macros (ansys.com) -> Check section 18.104.22.168
For reference, here is the link of the definition of the macro DEFINE_VR_RATE. Please check out the examples below the theory of the macro for better understanding.
2.3. Model-Specific DEFINE Macros (ansys.com) -> Check section 2.3.53
Additionally, I see that you have defined mole weight and assigned it to the variable "mw1". However, you have not used this variable anywhere. You might want to remove unncessary variables from your UDF.
If you are not able to access the link, please refer to this forum discussion: Using Help with links (ansys.com)