# Homework Help: Rate law expression with solids and liquids

1. May 12, 2014

### yolo123

Hello Forum!

I was wondering:

In the rate law expressions, I could never find an example with a solid or a liquid in the reactants in my textbook (Chemistry by Zumdahl). I searched Chemistry (Raymond Chang) and Principles of Molecular Chemistry, without any success.

What happens to solids and liquids who act as reactants in reactions? How do I incorporate them into the rate law? Are they ALWAYS already included in the k constant?

2. May 12, 2014

### Qube

Solids and liquids are not incorporated into the K constant because their concentrations are assumed to be constant (which is a pretty good approximation; as you consume either a solid or a liquid, you decrease the total amount but do not generally affect their concentrations).

3. May 13, 2014

### Staff: Mentor

k (kinetics) or K (equilibrium)?

For equilibrium presence of the solid is enough - no matter how much of the solid there is, the result (in terms of the final equilibrium position) is always the same.

For kinetics it is much more complicated, as reaction rate depends on the surface area of the solid. But I have never seen it addressed in the introductory courses.

4. May 13, 2014

### Qube

Oops, true, I should have specified that I meant equilibrium Ks only. As Borek says, Ks for kinetics differ.

5. May 13, 2014

### AGNuke

Not to mention, the rate law also depends on the reaction mechanism. So, it is impossible to guess on your own unless provided with some incentive.

6. May 13, 2014

### yolo123

I meant k as in the rate law: for example, rate=k[NO2]^2.

7. Dec 30, 2017

### Still Seeking

"For kinetics it is much more complicated, as reaction rate depends on the surface area of the solid. But I have never seen it addressed in the introductory courses."
Hi, it's three years too late but I'm hoping someone could elaborate on the complicated concept here. How do you include the order of the reaction of solid in the reaction rate expression? This is the only post on the internet where someone has hinted that there is an answer to this question. If you can't answer, it will still be helpful if you can point me to some resources as I have no idea what the answer to this is and I am tasked to find one. Any help will be appreciated, thank you.

(This is for both interest and homework.)​

8. Jan 1, 2018

### Ygggdrasil

In general, the reaction rate will depend on the surface area of the solid exposed to reactants. How the surface area changes over time will depend on the exact geometry of the material. Likely you would be able to find more information by searching for textbooks on surface chemistry or heterogeneous catalysis.