WEBVTT

00:00:00.500 --> 00:00:03.975
Welcome to GCSE Edexcel Science revision.

00:00:04.125 --> 00:00:09.809
Unit S C 15: Dynamic equilibria and calculations involving volumes of gases.

00:00:12.467 --> 00:00:18.886
(Higher tier) At the same temperature and pressure, equal volumes of gases contain equal numbers of molecules.

00:00:19.036 --> 00:00:24.633
Balanced-equation coefficients therefore give reacting gas-volume ratios under matching conditions.

00:00:27.300 --> 00:00:37.111
(Higher tier) Use 24 cubic decimetres per mole, or 24000 cubic centimetres per mole, as the molar volume at room temperature and pressure when supplied.

00:00:37.261 --> 00:00:42.317
Amount equals gas volume divided by molar volume; keep the units consistent.

00:00:44.967 --> 00:00:48.167
(Higher tier) To connect a solid mass with a gas volume,

00:00:48.317 --> 00:00:53.946
calculate moles of the solid using amount equals mass divided by relative formula mass,

00:00:54.096 --> 00:00:56.677
apply the balanced-equation mole ratio,

00:00:56.827 --> 00:00:59.611
then multiply gas moles by molar volume.

00:01:02.267 --> 00:01:06.939
(Higher tier) For N 2 plus 3 H 2 reversibly produces 2 N H 3,

00:01:07.089 --> 00:01:13.391
one volume of nitrogen reacts with three volumes of hydrogen to form two volumes of gaseous ammonia,

00:01:13.541 --> 00:01:16.772
if conversion were complete at the same measurement conditions.

00:01:16.922 --> 00:01:22.777
All volumes are compared at the same temperature and pressure; complete conversion is a theoretical limit.

00:01:27.433 --> 00:01:35.080
(Higher tier) Use the numbers in the balanced equation to compare gas volumes only when the gases are measured at the same temperature and pressure.

00:01:35.230 --> 00:01:39.614
This method applies to gases, not to the volumes of solids or solutions.

00:01:42.267 --> 00:01:48.453
(Higher tier) Identify the limiting reactant by comparing available amounts with the equation ratio.

00:01:48.603 --> 00:01:53.829
An equilibrium reaction may produce less than the amount predicted for complete conversion.

00:01:56.500 --> 00:02:04.199
The Haber process combines nitrogen, obtained from air, with hydrogen, commonly obtained from natural gas, to form ammonia.

00:02:04.349 --> 00:02:11.537
The forward reaction is exothermic and reversible: N 2 plus 3 H 2 reversibly produces 2 N H 3.

00:02:14.200 --> 00:02:19.533
(Higher tier) Lower temperature favours the equilibrium yield of ammonia but slows the rate.

00:02:19.683 --> 00:02:28.769
A compromise temperature around 450 degrees Celsius gives a useful rate and yield; actual operating conditions vary between plants.

00:02:31.433 --> 00:02:36.894
(Higher tier) Higher pressure favours ammonia because the product side has fewer gas molecules.

00:02:37.044 --> 00:02:47.027
High pressure also increases collision frequency but requires stronger equipment and more energy; around 200 atmospheres is a typical GCSE example.

00:02:49.700 --> 00:02:56.664
(Higher tier) An iron catalyst speeds both forward and reverse reactions and reduces the time to reach equilibrium.

00:02:56.814 --> 00:03:03.233
It does not move the equilibrium position or change the equilibrium yield at a fixed temperature and pressure.

00:03:05.900 --> 00:03:12.544
(Higher tier) Ammonia is cooled and removed as a liquid, while unreacted nitrogen and hydrogen are recycled.

00:03:12.694 --> 00:03:17.971
Removing product favours further production and recycling reduces raw-material waste.

00:03:18.121 --> 00:03:22.337
Recycling and product removal improve overall use of reactants.

00:03:27.000 --> 00:03:33.921
(Higher tier) Increasing reactant concentration generally speeds approach to equilibrium by increasing collisions.

00:03:34.071 --> 00:03:41.685
Temperature, pressure, concentration and catalysts affect rates, but effects on equilibrium position must be considered separately.

00:03:44.367 --> 00:03:52.022
(Higher tier) Choose industrial conditions using rate, yield, raw-material supply, energy cost, safety and equipment cost.

00:03:52.172 --> 00:03:57.375
The conditions that maximise equilibrium yield are not necessarily the most economic.

00:04:00.033 --> 00:04:02.231
(Higher tier) For gas equilibria,

00:04:02.381 --> 00:04:06.349
increasing pressure favours the side with fewer gaseous molecules;

00:04:06.499 --> 00:04:08.739
if both sides have equal numbers,

00:04:08.889 --> 00:04:11.757
pressure does not change the equilibrium position.

00:04:11.907 --> 00:04:15.265
A temperature increase favours the endothermic direction.

00:04:17.933 --> 00:04:25.469
Fertilisers provide mineral nutrients, often nitrogen, phosphorus and potassium (NPK), to support plant growth.

00:04:25.619 --> 00:04:31.814
They supplement nutrients removed by harvesting; they do not replace light, water or carbon dioxide.

00:04:34.467 --> 00:04:42.935
Ammonia reacts with nitric acid to produce ammonium nitrate: N H 3 plus H N O 3 produces N H 4 N O 3.

00:04:43.085 --> 00:04:46.939
This salt supplies nitrogen and is used as a fertiliser.

00:04:49.600 --> 00:05:01.939
Ammonia solution reacts with sulfuric acid to produce ammonium sulfate: 2 N H 3 plus H 2 S O 4 produces open bracket N H four close bracket subscript two S O four.

00:05:02.089 --> 00:05:05.617
The equation requires two moles of ammonia per mole of acid.

00:05:08.300 --> 00:05:18.377
In a laboratory preparation, titrate to find reacting volumes, then repeat those volumes without indicator, concentrate the salt solution gently and crystallise it.

00:05:18.527 --> 00:05:24.211
Soluble reactants cannot be separated from a soluble salt by simply filtering the solution.

00:05:26.867 --> 00:05:35.857
Industrial production involves obtaining raw materials, making ammonia and sulfuric acid, then reacting and processing them on a much larger scale.

00:05:36.007 --> 00:05:46.106
Continuous operation, heat recovery and automatic controls differ from small laboratory batches; detailed sulfuric-acid manufacture is not required here.

00:05:48.767 --> 00:05:54.270
Evaluate fertiliser production using energy demand, costs and environmental impacts.

00:05:54.420 --> 00:06:01.159
Excess nutrients washed into water can contribute to eutrophication, so efficient application also matters.

00:06:03.833 --> 00:06:08.877
That completes Dynamic equilibria and calculations involving volumes of gases.

00:06:09.027 --> 00:06:12.373
Revisit the notes and test yourself on the revision website.