Chemical reactions can be fast or slow – just think about the difference between exploding dynamite and a rusting iron gate. What’s the main difference? The rate of chemical change. One happens in a split second while the other can take decades before it’s fully completed.
Another main difference is the extent of chemical reaction, meaning how much of the reactant is consumed to form the end-product. Continue reading for a GCSE chemistry recap on rates of reaction, the mechanisms behind them, and how reversible reactions work.
Rates of Reaction
In chemistry, the rate of reaction refers to the time it takes to complete a chemical reaction given a proportion of reactants under certain conditions. It’s defined by the change in the concentration of a substance over the change in time, as expressed by the formula below:
The extent of a reaction refers to the completion of the reaction, i.e. when a chemical equilibrium or balance has been achieved. Usually when this happens, one of the reactants is completely consumed to form the final product of the reaction. For instance, if you light a match, the flame will continue to burn until it completely consumes the match, turning it into carbon and water vapour.
Several factors affect the rate and extent of chemical reactions. Chemical kinetics is the study of the rate and extent of chemical reactions under different conditions. It also deals with the mechanisms behind how chemical reactions occur, such as:
- Reactant concentration or proportion: Reactants that are concentrated or have higher proportions typically react faster than reactants with lower concentrations. The main reason for this is the higher probability of molecules bumping into each other, like a crowded shop where people are all moving around.
- High temperature: Temperature is directly proportional to the rate of a chemical reaction. As temperature increases, molecules move faster, meaning that the probability of the molecules of the reactants bumping into each other is higher. At the same time, some molecules easily break their bonds at higher temperatures.
- Physical state of the reactants: The surface area of solid reactants is crucial to the rate of reaction. For instance, sawdust is potentially more flammable than lumber because the latter has greater surface area.
- Presence of a catalyst or inhibitor: While a catalyst doesn’t participate in a chemical reaction, it does make the reaction occur faster. For example, potassium iodide and manganese (IV) oxide are commonly used as catalysts in the decomposition of hydrogen peroxide, such as in the elephant toothpaste experiment. Naturally, hydrogen peroxide decomposes very slowly, but in the presence of these catalysts, its decomposition is sped up rapidly. Meanwhile, inhibitors slow down chemical reactions or prevent them from occurring at all. For example, enzymes can be prevented from binding with substrates in the presence of inhibitors.
- Presence of light: The most common example of a chemical reaction that’s triggered by the presence of light is the imprinting of an image onto a photographic film, which contains silver halide. The intensity and wavelength of light also have an effect on the reaction. High-energy wavelengths, like ultraviolet rays, have stronger effects, such as in the case of the synthesis of vitamin D in the skin from cholesterol.
A reversible reaction is exactly what it sounds like: a reaction that can be reversed back into its original reactants. But it wasn’t until 1803 that scientists even realised chemical reactions could be reversed. We have Claude Louis Berthollet, a French chemist, to thank for the concept of reversible chemical reactions.
Berthollet knew that the reaction between sodium carbonate and calcium chloride produced calcium carbonate and sodium chloride. However, he also observed at the edge of a salt lake that the salts in the evaporating water reacted with calcium carbonate to form sodium carbonate. This was an indication that reversed chemical reactions are possible.
The balanced chemical equations below illustrate what Berthollet observed in the salt lake:
(1) 2NaCl + CaCO3 → Na2CO3 + CaCl2
(2) Na2CO3 + CaCl2→ 2NaCl + CaCO3
In a closed system, chemical equilibrium is achieved when there are equal amounts of reactants and products. Otherwise, it would be an infinite loop, which is impossible as some energy will inevitably be lost. It’s only in this case that true chemical equilibrium is achieved. This isn’t possible in an irreversible chemical reaction: since the reaction occurs only in one direction, the products and the reactants cannot be perfectly equal in amounts.
Technically, all chemical reactions can be reversed, but it would require an impractical amount of energy. For example, sodium and chlorine readily react with each other, but the resulting product, sodium chloride, a.k.a. table salt, is very stable. This means that sodium chloride can’t be easily split into its constituent elements, sodium and chlorine, without energy being applied, such as heat or electricity. Even if the reversed reaction is achieved, the elements will once again react to become salt as they won’t be in a stable state.
Many reversible chemical reactions are decomposition-combination reactions, such as the decomposition of solid ammonium chloride into ammonia and hydrogen chloride when heated. When the two gas products are cooled, they combine to form solid ammonium chloride. Therefore, in a closed system, the chemical equilibrium can be written as follows:
NH4Cl(s) ⇌ NH3(g) + HCl(g)
In the five types of chemical reactions, there are some examples of reversible reactions. However, many are irreversible. For example, in the case of single replacement reactions of metals, the more reactive metals always replace the less reactive ones, such as shown in this chemical reaction:
Mg (s) + Cu(NO3)2 (aq) → Mg(NO3)2 (aq) + Cu (s)
As you can see in the example above, the copper in the aqueous solution of copper (II) nitrate is replaced by magnesium to produce magnesium nitrate and solid copper. The reverse is not possible without using complicated processes involving high energy. If you refer to the periodic table of elements, you can predict which metals can be easily replaced by other metals in a chemical reaction based on their electron configuration.
When you’re revising for GCSE chemistry on the topic of the rate and extent of chemical reactions, it helps to refer to the periodic table. Chemical reactions are generally predictable when you look at this, allowing you to make an educated guess whether a reaction is easily reversible or not.
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