CIE A Level Chemistry復習筆記1.7.1 Chemical Equilibria

Category: A-level課程, 教材筆記, 福利干貨 Date: 2022年8月11日下午6:57

Reversible Reactions & Dynamic Equilibrium

Reversible reaction

Some reactions go to completion where the reactants are used up to form the products and the reaction stops when all of the reactants are used up
In?reversible reactions?the products can react to reform the original reactants
To show a reversible reaction, two opposing half arrows are used: ?

1.7-Equilibria-Reversible-Reactions

The diagram shows an example of a forward and backward reaction that can be written as one equation using two half arrows

Dynamic equilibrium

In a?dynamic equilibrium?the reactants and products are?dynamic?(they are constantly moving)
In a dynamic equilibrium the?rate?of the?forward?reaction is the same as the rate of the?backward?reaction in a?closed system, and the?concentrations?of the?reactants?and?products?is?constant

1.7-Equilibria-Dynamic-Equilibrium

The diagram shows a snapshot of a dynamic equilibrium in which 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

1.7-Equilibria-Dynamic-Equilibrium-Starting-Reactants

The diagram shows that the concentration of the reactants and products does not change anymore once equilibrium has been reached (equilibrium was approached using reactants)

1.7-Equilibria-Dynamic-Equilibrium-Starting-Products

The diagram shows that the concentration of the reactants and products does not change anymore once equilibrium has been reached (equilibrium was approached using products)

A?closed system?is one in which none of the reactants or products escape from the reaction mixture
In an?open system, matter and energy can be lost to the surroundings
When a reaction takes place entirely in solution, equilibrium can be reached in open flasks as a negligible amount of material is lost through evaporation
If the reaction involves gases, equilibrium can only be reached in a closed system

1.7-Equilibria-Closed-System

The diagram shows a closed system in which no carbon dioxide gas can escape and the calcium carbonate is in equilibrium with the calcium oxide and carbon dioxide

1.7-Equilibria-Open-System

The diagram shows an open system in which the calcium carbonate is continually decomposing as the carbon dioxide is lost causing the reaction to eventually go to completion

Exam Tip

A common misconception is to think that the concentrations of the reactants and products are?equal.?However, they are?not equal?but they?remain constant?at dynamic equilibrium (i.e. the concentrations are not changing).The concentrations will change as the reaction progresses, only?until the equilibrium?is reached.

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