Synthesis and Decomposition Reactions
HSC Chemistry Syllabus
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Conduct investigations to predict and identify the products of a range of reactions, for example:
Synthesis and Decomposition Reaction
This video explains the distinguishing characteristics of two types of chemical reactions: synthesis and decomposition.
What Are Synthesis Reactions?
A synthesis reaction is a reaction which involves the combination of chemical substances to form a larger compound.
The general equation for synthesis reactions is given as:
$$A + B \rightarrow AB$$
Beyond forming simple binary compounds like AB, synthesis can combine various elements and compounds to form larger structures. E.g.
$$AB + BC \rightarrow AB_2C$$
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Bond formation: Synthesis reactions are not limited by the bond types which are formed. Both ionic and covalent substances can be synthesised.
- Energy involved: Synthesis type reactions are usually exothermic in nature as they involve bond formations which release energy.
Example: Formation of Ammonia
$$3H_2(g) + N_2(g) \rightarrow 2NH_3(g)$$
In this reaction, covalent bonds are formed between separated nitrogen and hydrogen atoms.
This synthesis reaction involves both the formation and breaking of bonds. `N-H` bonds are formed, but the bonds between `H_2` and `N_2` must first be broken.
If bond breaking is endothermic, and bond formation is exothermic, why then is this synthesis reaction still overall exothermic? This is because the bonds formed are stronger than those broken. As such, the bond formation (N–H) step releases more energy than that initially used to break the bonds in reactant molecules.
Example: Reaction Between Metal Oxide and Water
The reaction between calcium oxide and water produces calcium hydroxide:
$$CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq)$$
In this reaction, both ionic and covalent bonds are either being broken or formed.
Decomposition Reactions
Decomposition reactions are the opposite of synthesis reactions. They involve the breakdown of a compound into simpler products.
The general equation for decompositions reactions is given as the opposite of synthesis reactions:
$$AB \rightarrow A + B$$
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Bond breakage: Decomposition reactions like synthesis reactions, are not limited by the bond types which are formed. Both ionic and covalent bonds can be broken.
- Energy involved: Decomposition type reactions involve the breaking of bonds. Energy is required to be absorbed in order to break those bonds so decomposition reactions are endothermic.
Example: Heating of Metal Carbonates e.g. Calcium Carbonate
A common example of decomposition that will also be explored in HSC Chemistry is the thermal decomposition of calcium carbonate:
$$CaCO_3(s) \rightarrow CaO(s) + CO_2(g)$$
The decomposition of calcium carbonate involves the breaking and forming of both ionic and covalent bonds using heat. The energy required to break bonds is greater than that released by the bonds formed. As such, the overall process is endothermic.
Example: Electrolysis Water
Using electrical energy, water can be decomposed into elemental forms of its constituents: hydrogen and oxygen gas.
$$2H_2O(l) \rightarrow 2H_2(g) + O_2(g)$$
This reaction only involves the breaking and formation of covalent bonds. The energy required to break the covalent bonds in water is greater than the energy released during formation of bonds in hydrogen and oxygen gas molecules. This results in an endothermic reaction.
Example: Photolysis of Silver Salts e.g. AgCl
Silver salts are ionic compounds that contain silver and one of the halide ions. Photolysis refers to the decomposition of silver salt into pure silver metal and halogen gas in the presence of light energy.
For example, the photolysis of silver chloride produces silver metal and chlorine gas:
$$2AgCl(s) \xrightarrow{\text{UV light}} 2Ag(s) + Cl_2(g)$$
Photographic film utilises the decomposition of AgCl to capture and preserve images. In essence, photolysis of silver chloride in photographic films is the initial step in capturing and forming images through the interaction of light with light-sensitive silver halide crystals, ultimately leading to the creation of visible photographs after development and processing.
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