What is the rate of the reaction.

Question

Consider the following reaction: BrO3- + 5Br + 6H+ ===> 3Br2 + 3H2O

The rate law was experimentally determined and found to be Rate= k[BrO3-][Br][H+]2

What is the overall order of the reaction? (I know it is 4)

When Br- is decreasing at .2M/s how fast is Br2 increasing? (I know it is .12)

What is the rate of the reaction? (I need help with this)

Answer 1

Rate of reaction = 0.04 M/s

Your first two answers are both right (order 4, and Br₂ increasing at 0.12 M/s), so let me go straight at the third — because the thing tripping you up is a genuine ambiguity in the word "rate."

Two different things are called "rate"

• The rate of change of a species: d[Br₂]/dt, −d[Br⁻]/dt, and so on. These differ from one another, because the coefficients differ.
• The rate of reaction (sometimes "unique rate"): a single number for the whole reaction, obtained by dividing each species' rate by its coefficient.

For BrO₃⁻ + 5 Br⁻ + 6 H⁺ → 3 Br₂ + 3 H₂O:

rate = −d[BrO₃⁻]/dt = −(1/5) d[Br⁻]/dt = −(1/6) d[H⁺]/dt = (1/3) d[Br₂]/dt

Using your bromide data

Br⁻ is disappearing at 0.2 M/s, so −d[Br⁻]/dt = 0.2 M/s. Divide by its coefficient of 5:

rate = (1/5)(0.2 M/s) = 0.04 M/s

Check it against the number you already found

rate = (1/3) d[Br₂]/dt = (1/3)(0.12 M/s) = 0.04 M/s ✓

Both routes give the same value — which is the entire point of defining the reaction rate this way. It is the one number that does not depend on which species you happen to watch.

Why this matters

The rate law you were given, Rate = k[BrO₃⁻][Br⁻][H⁺]², has "Rate" on the left meaning this quantity — the coefficient-normalised one. So if you were solving for k, you would use 0.04 M/s, not 0.2 and not 0.12. Substituting a species rate into a rate law is a common way to get k wrong by a factor of 3 or 5.

The pattern to remember: bromide disappears 5× faster than the reaction "proceeds," and bromine appears 3× faster, because the equation says 5 bromides are consumed and 3 bromines produced per single turn of the reaction. Dividing by the coefficient converts any species rate to the shared reaction rate.