Equilibrium

In some reactions, the products can turn back into the reactants In some reactions, the products can turn back into the reactants. This is called a reversible reaction We use a special symbol to show this type of reaction

Anhydrous copper sulphate demo Thermochromic cord Anhydrous copper sulphate demo Description This unique polythene extrusion has some remarkable characteristics and may be used as a single-lesson introduction to smart materials. Below 27°C the cord is pink but above this temperature the colour disappears. The pink then reappears when the temperature drops below 27°C. The cord also exhibits highly unusual behaviour if stretched. When pulled steadily it suddenly necks down at random to a second smaller diameter - and gets stronger! Eventually the whole length of the material reduces to the second diameter as the long molecules are forced to align along the axis of the cord. As the cord stretches there is a temporary loss of colour; the mechanical effort of stretching the cord causes heating - thus neatly demonstrating energy transfer. Short lengths of smart cord may be made into wristbands. At very low cost these could be worn as a constant reminder of the material's unique properties and taken off for use in homework investigations. Cord diameter approximately 1.5mm.

If a reaction is exothermic in one direction, it is endothermic in the other The amount of energy in both directions will be the same

How have you shown that this is a reversible reaction? Page 139: Copy the word and symbol equation for the reversible reaction where ammonium chloride breaks down into ammonia and hydrogen chloride. How have you shown that this is a reversible reaction? A reaction between A and B makes C and D. The reaction is reversible. Write an equation for this reaction. A and C are liquids, B and D are gases. Explain what it means that this is a reversible reaction When A and B react together they release 71J of energy to the surroundings. What does this say about the reaction? What must be done in order to have C and D react?

Understanding of equilibrium as dynamic http://ed.ted.com/lessons/if-molecules-were-people-george-zaidan-and-charles-morton

Reaction starts. There is no yellow, only green and red atoms ↔ Reaction starts. There is no yellow, only green and red atoms

↔ Reaction is initially very fast, yellow is being produced rapidly as almost every collision is between a green and a red

↔ Rate of reaction slows as there are now fewer collisions between red and green atoms as there are fewer of them.

Yellow molecules start to dissociate to form green and red atoms again ↔ Yellow molecules start to dissociate to form green and red atoms again

Yellow molecules start to dissociate to form green and red atoms again ↔ Yellow molecules start to dissociate to form green and red atoms again

↔ However, red and green atoms are still colliding and forming yellow molecules.

↔ However, red and green atoms are still colliding and forming yellow molecules.

Yellow is being formed at the same rate as it is dissociating. Because these processes are happening at the same time the overall amount of yellow, red and green does not change. Yellow is being formed at the same rate as it is dissociating. Red and green are being used up at the same rate as they are being formed ↔

The system is in equilibrium The system is in equilibrium. The reactions are still taking place but at the same rate in both directions. ↔

Henry Louis Le Chatelier 1850-1936. Spent a lot of time on metallurgy. Almost managed to get the Haber process to work before Haber did, but blew up his equipment and nearly killed an assistant. His failings led to Haber being able to get it to work.

If a reaction is at equilibrium, changing the conditions will affect the position of the equilibrium The system will always act to counteract what you have changed

Different changes and their effects Different changes and their effects. Remember – the system will always try and oppose the change Removing product/adding reactant Changing the pressure Changing the temperature

There are five yellow molecules and five green and red atoms. The system is in equilibrium. The reactions are still taking place but at the same rate in both directions. There are five yellow molecules and five green and red atoms. ↔

My change was to take away yellow. There are now three yellows but five red and green. The system will try to oppose my change. It will do this by producing more yellow ↔

One green and red have now reacted and there are four yellows, four reds and four greens. ↔

If product is removed from the system, then the system will oppose that change by generating more product

Page 142-145 of new textbook all questions Page 230-31 all questions Q3 + 4 on page 236