2.6.1 · D5Equilibrium
Question bank — Reversible reactions and dynamic equilibrium
Symbols used on this page (build them before you trap yourself)
Before any question, let's pin down every symbol so nothing is used before it is defined.
The graph below is the mental picture behind almost every trap on this page: two rates closing like scissors, and two concentrations levelling off once the rates meet.
True or false — justify
The word "equilibrium" means the reaction has stopped.
False — the rates are equal so the net change is zero, but forward and backward reactions both continue; it is dynamic, not static.
At equilibrium the concentrations of reactants and products are equal.
False — what is equal is the two rates (); concentrations settle at whatever values make those rates match, and are usually unequal.
A reaction can never reach equilibrium in an open beaker.
False in general — the true requirement is that no species escapes. If every species stays put (e.g. a nonvolatile solute reaction in an open flask), equilibrium can be established; it fails only when a volatile product (a gas) escapes so its backward rate can never build up.
At equilibrium the forward and backward rate constants become equal ().
False — the rates become equal, not the rate constants; and stay fixed (at a given temperature), and adjusts the concentrations to balance the rates.
Adding a catalyst increases the amount of product at equilibrium.
False — a catalyst speeds forward and backward reactions equally, so equilibrium arrives sooner but at exactly the same concentrations; is unchanged (see Catalysis).
You always end up at the same equilibrium mixture whether you start from pure reactants or pure products (same conditions).
True — depends only on temperature, so both routes converge on the same ratio of concentrations.
can be changed by adding more reactant.
False — adding reactant shifts the position of equilibrium (see Le Chatelier's principle) but itself only changes with temperature.
A large means the forward reaction is fast.
False — is a ratio; a large value means products are favoured, but says nothing about how quickly equilibrium is reached (that depends on the actual sizes of and ).
Spot the error
"At equilibrium, since nothing is changing, no molecules are reacting."
The error is confusing macroscopic stillness with molecular stillness — molecules keep reacting both ways; only the cancelling of the two rates () makes concentrations look frozen. Isotope experiments prove the motion.
"Because is at equilibrium, injecting radioactive will leave all the label in ."
Wrong — the label soon appears in HI too, because HI is still being formed and broken apart; this is the classic proof that equilibrium is dynamic.
"The reaction is reversible, so it must end with 50% reactants and 50% products."
There is no rule fixing a 50/50 split; the endpoint depends on . If products dominate, and vice versa.
"I removed the catalyst after equilibrium was reached, so now the equilibrium will shift."
A catalyst never sets the equilibrium position, so removing it does not shift anything — the mixture stays put; only the time to reach equilibrium would have differed.
"For the forward rate is ."
Only is a reactant of the forward step, so the forward rate is ; belongs to the backward rate .
"Equilibrium is reached when all the reactant has been used up."
If reactant were fully used up there would be no backward reaction possible, and could never equal a nonzero ; at true equilibrium both reactants and products are present and nonzero.
Why questions
Why must no species escape for equilibrium to be reached?
Because if any species leaves the system, its concentration cannot rise to feed the reverse reaction, so the forward rate and backward rate can never become equal.
Why does the forward rate fall while the backward rate rises as the reaction proceeds?
falls because is being consumed; rises because is being built up (rate concentration comes from Collision theory — more molecules, more collisions).
Why does depend only on temperature?
, and each rate constant changes only with temperature (via Effect of temperature on equilibrium); at fixed temperature both are fixed, so their ratio is fixed.
Why do we call the state "dynamic" rather than just "equilibrium"?
To stress that reactions are still occurring at the molecular level; "equilibrium" alone might suggest a dead, static balance, which is the exact misconception to avoid.
Why can a reaction reach the same equilibrium mixture from either direction?
Because equilibrium is defined by the condition , i.e. ; this target ratio is set by the rate constants, independent of the starting concentrations.
Why does a catalyst not change the equilibrium position even though it speeds the reaction?
It lowers the activation barrier for the forward and backward paths equally, multiplying both rate constants by the same factor, so their ratio — and therefore the equilibrium concentrations — is untouched (see Rate of reaction and rate constants).
Edge cases
If , what are the equilibrium concentrations of and in ?
Then , so at equilibrium — the one special case where reactant and product concentrations really are equal.
What does the "equilibrium" look like for an essentially irreversible reaction (like burning wood)?
The backward rate is negligibly small, so equilibrium lies overwhelmingly toward products; effectively the reaction runs to completion and is enormous.
At the very first instant when pure is added to , what is the backward rate?
Zero, because so ; only after starts accumulating does the reverse reaction begin.
Can a very large still take a long time to reach equilibrium?
Yes — (a ratio) tells you where equilibrium sits, not how fast it arrives; if both and are tiny, the balance is reached slowly even though products are heavily favoured.
If you double the volume of a gaseous equilibrium at constant temperature, does change?
No — depends only on temperature; changing volume alters the concentrations and may shift the position, but the constant itself stays the same.
Does removing all product mid-reaction destroy the equilibrium?
It temporarily breaks the balance ( drops toward zero), so the system is no longer at equilibrium and will react forward again until is re-established — provided no other species escapes.
Connections
- Equilibrium constant Kc and Kp — quantifies the ratio these traps circle around.
- Le Chatelier's principle — the correct framework for "shifting" the position.
- Catalysis — why catalysts leave alone.
- Effect of temperature on equilibrium — the only thing that changes .