Reversible reactions and chemical equilibrium
Sohail Awan
Greetings! I am Sohail, a dedicated educator hailing from the vibrant and culturally diverse land of Pakistan.
Some reactions can be reversed. For example, when blue, hydrated copper(II) sulfate is heated, it loses its water of crystallisation and changes to white anhydrous copper(II) sulfate.
CuSO4?5H2O(s) ? CuSO4 (s) + 5H2O(l)
This is called theforward reaction.?
When water is added to anhydrous copper(II) sulfate, the reaction is reversed.?
CuSO4 (s) + 5H2O(l) → CuSO4 ·5H2O(s)
This is called the backward (or reverse) reaction.?
We show these two reactions in the same equation by using two half arrows: ?
CuSO4?5H2O(s) ? CuSO4 (s) + 5H2O(l)
?Hydrated copper(II) sulfate (left) and anhydrous copper(II) sulfate (right).?
A reaction in which the products can react to re-form the original reactants is called a reversible reaction. In this case, heating and adding water are not being carried out at the same time. However, there is a type of chemical reaction in which the forward reaction and the backward reaction take place at the same time.?
In many chemical reactions, the reactants are not used up completely. Some products are formed but the maximum theoretical yield is not obtained. A mixture of products and reactants is formed. The products react together to re-form reactants at the same time as the reactants are forming products. This type of reversible reaction is called an chemical? ? ?equilibrium. We use the sign ? in equilibrium reactions to show that they are reversible.
?For example, consider the reaction between hydrogen and iodine carried out in a sealed glass tube at 400 °C:
?H2 (g) + I2 (g) ? 2HI(g)
Molecules of hydrogen iodide are breaking down to hydrogen and iodine at the same rate as hydrogen and iodine molecules are reacting together to form hydrogen iodide.
A snapshot of the dynamic equilibrium between hydrogen gas, iodine gas and hydrogen iodide gas.?
When fizzy drinks are made, carbon dioxide gas is dissolved in the drink under pressure. When you take the lid off a bottle of fizzy drink, bubbles of carbon dioxide suddenly appear. When you put the lid back on, the bubbles stop. This is because of the equilibrium reaction .
CO2 (g) ? CO2 (aq)?
The forward reaction happens during manufacture and the backward reaction happens on opening.
Characteristics of?chemical equilibrium
An equilibrium reaction has four particular features under constant conditions:
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It is dynamic
The phrase dynamic equilibrium means that the molecules or ions of reactants and products are continuously reacting. Reactants are continuously being changed to products and products are continuously being changed back to reactants.?
The forward and backward reactions occur at the same rate?
At equilibrium, the rate of the forward reaction equals the rate of the backward reaction. Molecules or ions of reactants are becoming products, and those in the products are becoming reactants at the same rate.?
The concentrations of reactants and products remain constant at equilibrium?
The concentrations remain constant because, at equilibrium, the rates of the forward and backward reactions are equal. The equilibrium can be approached from two directions. For example, in the reaction:
H2 (g) + I2 (g) ? 2HI(g)?
we can start by either:?
Figure shows what happens when 5.00 × 10 ?3 mol of hydrogen molecules and 5.00 × 10 ?3 mol of iodine molecules react at 500 K in a vessel of volume 1 dm3 . As time passes, the purple colour of the iodine vapour fades until equilibrium is reached. At equilibrium the mixture contains 0.68 × 10 ?3 mol of iodine, 0.68 × 10 ?3 mol of hydrogen and 8.64 × 10 ?3 mol of hydrogen iodide.?
Figure: The changes in the concentrations of reagents as 5.00 × 10?3 mol of each of hydrogen and iodine react to form an equilibrium mixture with hydrogen iodide in a vessel of volume 1 dm3.?
Figure shows that the same equilibrium can be achieved when 10.00 × 10 ?3 mol of hydrogen iodide molecules decompose to iodine and hydrogen. You can see that the same equilibrium concentrations of all three molecules are achieved.?
Figure: The changes in the concentrations of reagents as 10.00 × 10?3 mol of hydrogen iodide react to form an equilibrium mixture with hydrogen and iodine gases in a vessel of 1 dm3.?
Chemical??Equilibrium requires a closed system
A closed system is one in which none of the reactants or products escapes from the reaction mixture. In an open system some matter is lost to the surroundings. Figure shows the difference between a closed system and an open system when calcium carbonate is heated at a high temperature in a strong container.?
Many chemical reactions can be studied without placing them in closed containers. They can reach equilibrium in open flasks if the reaction takes place entirely in solution and no gas is lost.
Figure: a A closed system. No carbon dioxide escapes. The calcium carbonate is in equilibrium with calcium oxide and carbon dioxide. b An open system. The calcium carbonate is continually decomposing as the carbon dioxide is lost. The reaction eventually goes to completion.?
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9 个月Helpful article. Thank you.