Reactions on surfaces

Key takeaways

Reactions on surfaces belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Reactions on surfaces to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Reactions on surfaces from memory before moving on to harder problems.

Reference excerpt

Reactions on surfaces are reactions in which at least one of the steps of the reaction mechanism is the adsorption of one or more reactants. The mechanisms for these reactions, and the rate equations are of extreme importance for heterogeneous catalysis. Via scanning tunneling microscopy, it is possible to observe reactions at the solid gas interface in real space, if the time scale of the reaction is in the correct range. Reactions at the solid–gas interface are in some cases related to catalysis.

Simple decomposition If a reaction occurs through these steps:

A + S ⇌ AS → Products where A is the reactant and S is an adsorption site on the surface and the respective rate constants for the adsorption, desorption and reaction are k1, k−1 and k2, then the global reaction rate is:

r = k 2 C A S = k 2 θ C S {\displaystyle r=k_{2}C_{\mathrm {AS} }=k_{2}\theta C_{\mathrm {S} }}

where:

r is the rate, mol·m−2·s−1

C A {\displaystyle C_{A}} is the concentration of adsorbate, mol·m−3

C A S {\displaystyle C_{\mathrm {AS} }} is the surface concentration of occupied sites, mol·m−2

C S {\displaystyle C_{\mathrm {S} }} is the concentration of all sites (occupied or not), mol·m−2

θ {\displaystyle \theta } is the surface coverage, (i.e. C A S / C S {\displaystyle C_{AS}/C_{S}} ) defined as the fraction of sites which are occupied, which is dimensionless

t {\displaystyle t} is time, s

k 2 {\displaystyle k_{2}} is the rate constant for the surface reaction, s−1.

k 1 {\displaystyle k_{1}} is the rate constant for surface adsorption, m3·mol−1·s−1

k − 1 {\displaystyle k_{-1}} is the rate constant for surface desorption, s−1

C S {\displaystyle C_{\mathrm {S} }} is highly related to the total surface area of the adsorbent: the greater the surface area, the more sites and the faster the reaction. This is the reason why heterogeneous catalysts are usually chosen to have great surface areas (in the order of a hundred m2/gram) If we apply the steady state approximation to AS, then:

d C A S d t = 0 = k 1 C A C S ( 1 − θ ) − k 2 θ C S − k − 1 θ C S {\displaystyle {\frac {dC_{\mathrm {AS} }}{dt}}=0=k_{1}C_{\mathrm {A} }C_{\mathrm {S} }(1-\theta )-k_{2}\theta C_{\mathrm {S} }-k_{-1}\theta C_{\mathrm {S} }} so θ = k 1 C A k 1 C A + k − 1 + k 2 {\displaystyle \theta ={\frac {k_{1}C_{\mathrm {A} }}{k_{1}C_{\mathrm {A} }+k_{-1}+k_{2}}}}

and

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Reactions on surfaces

Start with the simplest possible case. Write down what Reactions on surfaces claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Reactions on surfaces before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Reactions on surfaces ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Reactions on surfaces

In research: Reactions on surfaces appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Reactions on surfaces in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Reactions on surfaces is common in secondary-school and first-year university syllabi. It links to neighbouring topics Chemical kinetics, Chemical reaction engineering, Surface science, so understanding it makes those chapters shorter.
In everyday life: Look for Reactions on surfaces outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Reactions on surfaces in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Reactions on surfaces means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Reactions on surfaces out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Reactions on surfaces in simple terms?

Reactions on surfaces are reactions in which at least one of the steps of the reaction mechanism is the adsorption of one or more reactants. The mechanisms for these reactions, and the rate equations are of extreme importance for heterogeneous catalysis.

Why does Reactions on surfaces matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Reactions on surfaces?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Reactions on surfaces.