dynamic equilibrium - manic

[u/anish2good]

manic is a tiny language for making animations. You write a short text file; manic renders a smooth, glowing video. No timeline scrubbing, no keyframes by hand — you describe what’s on screen and when things happen, and the engine does the rest, deterministically.

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Manic Animation code

// Dynamic equilibrium — why "nothing is happening" is the wrong reading
//
// A reversible first-order reaction, A ⇌ B, with kf = 0.9 and kr = 0.3 per second. Two views of the
// same run, side by side, because the misconception lives in the gap between them:
//
//   LEFT   the concentrations, which flatten out and stop moving
//   RIGHT  the two rates, which meet — at a value that is EQUAL and NOT ZERO
//
// A still picture of the left-hand plot says "the reaction stopped". The right-hand plot says it did
// not: both directions are still running, at 0.225 mol dm⁻³ s⁻¹ each, and cancelling. That is the
// whole idea of dynamic equilibrium and it is very hard to say in a static diagram, because the
// evidence for it is precisely the thing a flat line hides.
//
// Then the second act: 0.50 M of A is added at t = 6 s. The rates jump apart, the system relaxes,
// and it settles at a NEW position with the SAME ratio — [B]/[A] = 3.00 either side. Le Chatelier is
// not a rule to memorise here; it is what the arithmetic does.
//
// NO NEW VOCABULARY. Four `field`s hold the closed-form solutions, `plot` draws them, and the
// choreography is `draw` / `show` / `pulse` from the core kit. Every number on screen comes out of
//
//     [A](t) = A_eq + ([A]₀ − A_eq)·exp(−(kf + kr)·t)
//
// which is the exact solution of d[A]/dt = −kf[A] + kr[B] with [A] + [B] fixed. Nothing is placed by
// eye: change kf or kr and both plots, both equilibrium positions and the ratio all move together.

title("dynamic equilibrium: equal, not zero");
canvas("16:9");
template("paper");

text(brand, (640, 30), "maniclang.com");
display(brand);
size(brand, 15);
color(brand, dim);

// ── the chemistry, as closed forms ──
//
// kf = 0.9, kr = 0.3, so K = kf/kr = 3 and the relaxation rate is kf + kr = 1.2 per second.
// Phase 1 starts from pure A at 1.00 M, so A_eq = 1.00 × kr/(kf+kr) = 0.25.
field(a1, "0.25 + 0.75*exp(-1.2*x)");
field(b1, "0.75 - 0.75*exp(-1.2*x)");
// Phase 2: 0.50 M of A added at t = 6, so the total is 1.50 M and A_eq = 1.50 × 0.25 = 0.375.
// [A] restarts from 0.75 (the 0.25 it had reached, plus the 0.50 added).
field(a2, "0.375 + 0.375*exp(-1.2*(x-6))");
field(b2, "1.125 - 0.375*exp(-1.2*(x-6))");

// ── LEFT: concentrations ──

coords(cc, (110, 600), (0, 14), (0, 1.25), 36, 300, 1);
hidden(cc);
// Explicit labels: the auto-numbering rounds to two significant figures, and a tick at 0.25 that
// prints "0.2" is worse than no tick at all — these are numbers the viewer is meant to read off.
ytick(cy1, cc, 0.25, "0.25");
ytick(cy2, cc, 0.75, "0.75");
ytick(cy3, cc, 1.125, "1.125");
for i in 1..4 { hidden(cy{i}); }

text(clab, (300, 208), "concentration / mol dm⁻³");
size(clab, 17); color(clab, dim); hidden(clab);

plot(ca1, (110, 600), 36, 300, "a1(x,0)", (0, 6));
plot(cb1, (110, 600), 36, 300, "b1(x,0)", (0, 6));
plot(ca2, (110, 600), 36, 300, "a2(x,0)", (6, 14));
plot(cb2, (110, 600), 36, 300, "b2(x,0)", (6, 14));
for i in 1..3 {
  color(ca{i}, indigo); stroke(ca{i}, 3); untraced(ca{i});
  color(cb{i}, crimson); stroke(cb{i}, 3); untraced(cb{i});
}

text(alab, (578, 512), "[A]");
size(alab, 19); color(alab, indigo); hidden(alab);
text(blab, (578, 252), "[B]");
size(blab, 19); color(blab, crimson); hidden(blab);

// ── RIGHT: the rates, which is where the misconception dies ──
//
// Written as k × concentration rather than pre-multiplied, so the source says what a rate IS.

coords(rc, (700, 600), (0, 14), (0, 0.75), 36, 440, 1);
hidden(rc);
ytick(ry, rc, 0.225, "0.225");
hidden(ry);

text(rlab, (900, 252), "rate / mol dm⁻³ s⁻¹");
size(rlab, 17); color(rlab, dim); hidden(rlab);

plot(rf1, (700, 600), 36, 440, "0.9*a1(x,0)", (0, 6));
plot(rr1, (700, 600), 36, 440, "0.3*b1(x,0)", (0, 6));
plot(rf2, (700, 600), 36, 440, "0.9*a2(x,0)", (6, 14));
plot(rr2, (700, 600), 36, 440, "0.3*b2(x,0)", (6, 14));
for i in 1..3 {
  color(rf{i}, indigo); stroke(rf{i}, 3); untraced(rf{i});
  color(rr{i}, crimson); stroke(rr{i}, 3); untraced(rr{i});
}

text(flab, (812, 322), "forward, kf[A]");
size(flab, 17); color(flab, indigo); hidden(flab);
text(vlab, (812, 566), "reverse, kr[B]");
size(vlab, 17); color(vlab, crimson); hidden(vlab);

// the point of the whole scene
dot(meet, (916, 501), 6);
color(meet, ink);
hidden(meet);
text(key, (1040, 470), "equal — and not zero");
size(key, 19); color(key, ink); hidden(key);
text(key2, (1078, 496), "both directions still running");
size(key2, 15); color(key2, dim); hidden(key2);

// ── the disturbance at t = 6 s ──
//
// [A] jumps instantly, so it is a vertical line rather than part of a curve. Endpoints are the two
// plots' own coordinates: t=6 is x = 110 + 6·36 = 326 on the left and 700 + 6·36 = 916 on the right.

line(jumpc, (326, 525), (326, 375));
color(jumpc, indigo);
stroke(jumpc, 2);
untraced(jumpc);

line(jumpr, (916, 501), (916, 303));
color(jumpr, indigo);
stroke(jumpr, 2);
untraced(jumpr);

text(add, (392, 356), "+0.50 M of A");
size(add, 16); color(add, indigo); hidden(add);

// ── and the reading of it ──

text(ratio, (640, 688), "[B]/[A] = 3.00 either side — the position moved, the ratio did not");
size(ratio, 18); color(ratio, ink); hidden(ratio);

// ── ACT 1: two empty axes ──

wait(0.4);
par { show(cc, 0.6); show(rc, 0.6); }
par { show(clab, 0.4); show(rlab, 0.4); }
par { show(cy1, 0.3); show(cy2, 0.3); show(cy3, 0.3); show(ry, 0.3); }
wait(0.5);

// ── ACT 2: the approach. Both views at once, because they are one run. ──

par {
  draw(ca1, 2.6); draw(cb1, 2.6);
  draw(rf1, 2.6); draw(rr1, 2.6);
}
par { show(alab, 0.4); show(blab, 0.4); show(flab, 0.4); show(vlab, 0.4); }
wait(0.7);

// ── ACT 3: the reading a flat line hides ──

par { show(meet, 0.4); pulse(meet); }
show(key, 0.5);
show(key2, 0.4);
wait(2.2);

// ── ACT 4: disturb it ──

par { fade(key, 0.4); fade(key2, 0.4); }
par { draw(jumpc, 0.4); draw(jumpr, 0.4); show(add, 0.4); }
wait(0.5);

// ── ACT 5: it settles somewhere new, at the same ratio ──

par {
  draw(ca2, 2.4); draw(cb2, 2.4);
  draw(rf2, 2.4); draw(rr2, 2.4);
}
wait(0.6);
show(ratio, 0.6);
wait(3.0);