At room temperature, a reaction is very fast if no bonds are broken while it takes place, and is slow if bonds have to be broken.
1 H2 + I2 → 2HI
2 Ag+ + Cl – → AgCl
3 ĊH3 + ĊH3 → C2H6
4 OH- + CH3Br → CH3OH + Br –
Which of the following are the two fastest reactions?

Answer:

22) D

1 – Slow

H-H and I-I bonds are broken. Molecules H₂ and I₂ split to form 2HI. Since bonds are broken, this will be a relatively slow reaction. 

2 – Fast

No bonds are broken. Ag+ and Cl- just react to form AgCl.

3 – Fast

Again, 2 molecules of CH₃ just bond together to form C₂H₆

4 – Slow

The C-Br bond is broken meaning this will be a relatively slow reaction. 

Reaction 2 and 3 are the fastest so D is the answer.