Foundations — Reaction quotient Q vs K — direction of shift

This is the foundations page for the parent topic. Before you compare $Q$ and $K$, you must own every symbol that appears in them. We build each one from nothing, in an order where each idea leans only on the one before it — so we start with the raw ingredients (species, then how many of each) and only afterwards draw the reaction that uses them.

1. The chemical species — the "characters"

There is nothing to compute here — $A,B,C,D$ are just labels, like naming the kids on a seesaw. In a real problem $A$ might be $N_2$ (nitrogen gas), $B$ might be $H_2$ (hydrogen gas), and $C$ might be $N{H}_3$ (ammonia).

2. The small letters $a,b,c,d$ — stoichiometric coefficients

3. The reversible reaction — the picture we keep returning to

Now that we have named the species ($A,B,C,D$) and their counts ($a,b,c,d$), we can draw the reaction that ties them together.

Read this out loud in plain words: "$a$ particles of substance $A$ together with $b$ particles of substance $B$ can combine to give $c$ particles of $C$ and $d$ particles of $D$ — and the reverse also happens."

The top arrow is the forward direction (making products, the right-hand side). The bottom arrow is the backward direction (remaking reactants, the left-hand side). The topic's whole job is: which arrow currently wins?

4. The mole — how chemists count particles

5. Concentration and the square brackets $[]$

The two litre-boxes below make "crowdedness" concrete. Look at the figure: the left box holds only a few particles per litre (low $[X]$); the right box is packed (high $[X]$). Count the dots — same volume, more particles means higher concentration.

6. Partial pressure $p_X$ — concentration's cousin for gases

7. The fraction, and the exponents on it

7a. Why a fraction, and why products on top

7b. Coefficients become exponents

8. Activity, and why pure solids/liquids vanish

9. Naming the ratio: $Q$ (now) versus $K$ (target)

We have now built the fraction. Reading it at two different moments gives us the two stars of the topic.

Picture $Q$ as a slider that moves as the reaction runs, and $K$ as a fixed marker on the track. The reaction always drags $Q$ toward $K$:

• $Q$ to the left of $K$ ($Q<K$): too few products → slide forward.
• $Q$ to the right of $K$ ($Q>K$): too many products → slide backward.
• $Q$ on $K$ ($Q=K$): settled, no net motion — this is equilibrium.

10. The symbols in the thermodynamic "why"

The parent note explains why this rule is true using energy. Here are those symbols, from zero.

11. All cases, nothing left out

Prerequisite map

Equipment checklist

Cover the right side and test yourself — you are ready for the parent topic only if you can answer every one.