Chemistry · Chapter 2
Study notes aligned to the official NEB syllabus.
Stoichiometry (from the Greek stoicheion, meaning element, and metron, meaning measure) is the branch of chemistry that deals with the quantitative relationships between reactants and products in a chemical reaction. It lets us answer questions such as: how much product can a given mass of reactant give, how much of one reactant is needed to react completely with another, and how much of a reagent is left over.
All stoichiometric calculations rest on two ideas: the conservation of mass (atoms are neither created nor destroyed in a chemical change) and the mole concept, which links the counting of particles to measurable masses and volumes.
In 1808 John Dalton proposed the first scientific atomic theory. Its main postulates are:
Limitations. Modern chemistry has modified Dalton's theory: the atom is divisible (into electrons, protons, neutrons); atoms of the same element can differ in mass (isotopes); atoms of different elements may have equal masses (isobars); and some compounds combine in non-simple or non-fixed ratios (non-stoichiometric compounds). Despite this, the theory remains the foundation of the laws of chemical combination.
Statement: In a chemical reaction, matter is neither created nor destroyed; the total mass of the products equals the total mass of the reactants.
For example, when $\ce{10.0 g}$ of calcium carbonate is decomposed:
$$\ce{CaCO3(s) ->[\Delta] CaO(s) + CO2(g)}$$
the $\ce{5.6 g}$ of $\ce{CaO}$ and $\ce{4.4 g}$ of $\ce{CO2}$ produced add up to the original $\ce{10.0 g}$.
Statement: A given chemical compound always contains the same elements combined in the same fixed proportion by mass, regardless of its source or method of preparation.
For example, pure water $\ce{H2O}$ always contains hydrogen and oxygen in the mass ratio $1 : 8$, whether it comes from a river, a well, or is made in the laboratory.
Statement: When two elements combine to form more than one compound, the different masses of one element that combine with a fixed mass of the other bear a simple whole-number ratio to one another.
Carbon and oxygen form $\ce{CO}$ and $\ce{CO2}$. For a fixed $\ce{12 g}$ of carbon, the masses of oxygen are $\ce{16 g}$ (in $\ce{CO}$) and $\ce{32 g}$ (in $\ce{CO2}$). The ratio $16 : 32 = 1 : 2$, a simple whole number ratio.
Statement: When two different elements combine separately with a fixed mass of a third element, the ratio of their masses is either the same as, or a simple multiple of, the ratio in which they combine with each other.
Stoichiometry (from the Greek stoicheion, meaning element, and metron, meaning measure) is the branch of chemistry that deals with the quantitative relationships between reactants and products in a chemical reaction. It lets us answer questions such as: how much product can a given mass of reactant give, how much of one reactant is needed to react completely with another, and how much of a reagent is left over.
All stoichiometric calculations rest on two ideas: the conservation of mass (atoms are neither created nor destroyed in a chemical change) and the mole concept, which links the counting of particles to measurable masses and volumes.
In 1808 John Dalton proposed the first scientific atomic theory. Its main postulates are:
Limitations. Modern chemistry has modified Dalton's theory: the atom is divisible (into electrons, protons, neutrons); atoms of the same element can differ in mass (isotopes); atoms of different elements may have equal masses (isobars); and some compounds combine in non-simple or non-fixed ratios (non-stoichiometric compounds). Despite this, the theory remains the foundation of the laws of chemical combination.
Statement: In a chemical reaction, matter is neither created nor destroyed; the total mass of the products equals the total mass of the reactants.
For example, when of calcium carbonate is decomposed:
the of and of produced add up to the original .
Statement: A given chemical compound always contains the same elements combined in the same fixed proportion by mass, regardless of its source or method of preparation.
For example, pure water always contains hydrogen and oxygen in the mass ratio , whether it comes from a river, a well, or is made in the laboratory.
Statement: When two elements combine to form more than one compound, the different masses of one element that combine with a fixed mass of the other bear a simple whole-number ratio to one another.
Carbon and oxygen form and . For a fixed of carbon, the masses of oxygen are (in ) and (in ). The ratio , a simple whole number ratio.
Statement: When two different elements combine separately with a fixed mass of a third element, the ratio of their masses is either the same as, or a simple multiple of, the ratio in which they combine with each other.