calculate the molecular weight of calcium carbonate, water,ammonia
, sulphuric acid, and sodium chloride
Answers
Energy considerations
Energy plays a key role in chemical processes. According to the modern view of chemical reactions, bonds between atoms in the reactants must be broken, and the atoms or pieces of molecules are reassembled into products by forming new bonds. Energy is absorbed to break bonds, and energy is evolved as bonds are made. In some reactions the energy required to break bonds is larger than the energy evolved on making new bonds, and the net result is the absorption of energy. Such a reaction is said to be endothermic if the energy is in the form of heat. The opposite of endothermic is exothermic; in an exothermic reaction, energy as heat is evolved. The more general terms exoergic (energy evolved) and endoergic (energy required) are used when forms of energy other than heat are involved.
A person's hand pouring blue fluid from a flask into a beaker. Chemistry, scientific experiments, science experiments, science demonstrations, scientific demonstrations.
BRITANNICA QUIZ
Ins and Outs of Chemistry
How many electrons does a sodium atom have?
A great many common reactions are exothermic. The formation of compounds from the constituent elements is almost always exothermic. Formation of water from molecular hydrogen and oxygen and the formation of a metal oxide such as calcium oxide (CaO) from calcium metal and oxygen gas are examples. Among widely recognizable exothermic reactions is the combustion of fuels (such as the reaction of methane with oxygen mentioned previously).
The formation of slaked lime (calcium hydroxide, Ca(OH)2) when water is added to lime (CaO) is exothermic.
CaO(s) + H2O (l) → Ca(OH)2(s)
This reaction occurs when water is added to dry portland cement to make concrete, and heat evolution of energy as heat is evident because the mixture becomes warm.
Not all reactions are exothermic (or exoergic). A few compounds, such as nitric oxide (NO) and hydrazine (N2H4), require energy input when they are formed from the elements. The decomposition of limestone (CaCO3) to make lime (CaO) is also an endothermic process; it is necessary to heat limestone to a high temperature for this reaction to occur.
CaCO3(s) → CaO(s) + CO2(g)
The decomposition of water into its elements by the process of electrolysis is another endoergic process. Electrical energy is used rather than heat energy to carry out this reaction.
2 H2O(g) → 2 H2(g) + O2(g)
Generally, evolution of heat in a reaction favours the conversion of reactants to products. However, entropy is important in determining the favourability of a reaction. Entropy is a measure of the number of ways in which energy can be distributed in any system. Entropy accounts for the fact that not all energy available in a process can be manipulated to do work.