..... WHAT THEY CAN TELL US, AND WHAT THEY CANNOT TELL US
KEY IDEAS
Balanced chemical equations can tell us the following:
Chemical equations tell us about relative amounts of substances, and not about actual (absolute) reacting amounts.
Take, for example, the balanced chemical equation for the combustion of methane (natural gas):
- What the reactant substances are, and what the product substances are.
- For however much reaction happens, the relative amounts (in moles) of the reactant substances that are “consumed” and product substances that are produced.
Chemical equations tell us about relative amounts of substances, and not about actual (absolute) reacting amounts.
Take, for example, the balanced chemical equation for the combustion of methane (natural gas):
This equation tells us that for every 1 mol of the gaseous substance methane that reacts, 2 mol of the substance oxygen gas also reacts and the resultant products are 1 mol of the gaseous substance carbon dioxide and 2 mol of the substance water vapour.
This relationship applies, regardless of how much methane actually reacts.
The equation enables us to calculate, for example, the amount (or weight) of gaseous carbon dioxide that is formed if a specified amount (or weight) of methane is burned. This is the field of stoichiometry.
Chemical equations tell us about relative amounts of substances, and not about relative numbers of atoms or molecules or ions – although the relative numbers of these can often be deduced.
Although the relative numbers of molecules of those substance are the same as the relative amounts of the substances, the equation should not be taken as a portrayal (leading to a mental image) of 2 molecules of oxygen reacting with 1 molecule of methane.
[We use equations with different meanings when we consider the interaction of individual molecules (or atoms or ions) of reactants when we discuss the mechanisms of reactions (How they happen: what molecules need to collide, which bonds break, which bonds are formed ..... ). On these occasions, we focus on single collisions between molecules (of the trillions upon trillions of collisions that happen during a reaction), and the formulas in the equation represent single molecules.]
This relationship applies, regardless of how much methane actually reacts.
The equation enables us to calculate, for example, the amount (or weight) of gaseous carbon dioxide that is formed if a specified amount (or weight) of methane is burned. This is the field of stoichiometry.
Chemical equations tell us about relative amounts of substances, and not about relative numbers of atoms or molecules or ions – although the relative numbers of these can often be deduced.
Although the relative numbers of molecules of those substance are the same as the relative amounts of the substances, the equation should not be taken as a portrayal (leading to a mental image) of 2 molecules of oxygen reacting with 1 molecule of methane.
[We use equations with different meanings when we consider the interaction of individual molecules (or atoms or ions) of reactants when we discuss the mechanisms of reactions (How they happen: what molecules need to collide, which bonds break, which bonds are formed ..... ). On these occasions, we focus on single collisions between molecules (of the trillions upon trillions of collisions that happen during a reaction), and the formulas in the equation represent single molecules.]
Balanced chemical equations CANNOT tell us any of the following:
- How much reaction actually happened in any situation – that is, how many moles of substances reacted, or were formed. It can only tell us about relative amounts.
- How much of each reactant species was in the reaction chamber initially.
- Whether the reaction that it represents is exothermic or endothermic.
- How fast the reaction represented proceeds (that is, its rate of reaction – how many moles of substances react per unit of time).
- Anything about the reaction mechanism (that is, the sequence of events that happen at the sub-microscopic level).
In summary:
A balanced reaction equation tells us only what the substances are that react and are formed, and the relative amounts (in moles) that react or are formed.
A balanced reaction equation tells us only what the substances are that react and are formed, and the relative amounts (in moles) that react or are formed.
ADDENDUM
Neither can a balanced chemical equation tell us anything about how far a reaction goes before coming to equilibrium.
It allows us to deduce the form of the reaction quotient, Q, whose value at equilibrium (at defined temperature) is called the equilibrium constant, K (Module 1103: Equilibrium constants). But it can tell us nothing about the magnitude of K.
For example, for the reaction represented by the equation
Neither can a balanced chemical equation tell us anything about how far a reaction goes before coming to equilibrium.
It allows us to deduce the form of the reaction quotient, Q, whose value at equilibrium (at defined temperature) is called the equilibrium constant, K (Module 1103: Equilibrium constants). But it can tell us nothing about the magnitude of K.
For example, for the reaction represented by the equation
we can deduce from the balanced equation that the mathematical expression of reagent concentrations that we call the reaction quotient, Q, is
The numerical value of Q is the same in all reaction mixtures in which this reaction is at equilibrium (at the same temperature) – which is not true of any other function of the concentrations.
However, the balanced chemical equation can give us no sense at all of the value of Q in reaction mixtures at equilibrium (that is, of the magnitude of the equilibrium constant, K).
However, the balanced chemical equation can give us no sense at all of the value of Q in reaction mixtures at equilibrium (that is, of the magnitude of the equilibrium constant, K).
SELF CHECK
Which of the following statements about reaction of nitrogen gas and hydrogen gas to form ammonia gas can be deduced from the balanced chemical equation shown here?
A The initial amounts of reactants are 1 mol of ammonia gas and 3 mol of hydrogen gas.
B Regardless of how much nitrogen gas, hydrogen gas and ammonia gas are in a reaction vessel initially, if 0.6 mol of hydrogen reacts with nitrogen, 0.4 mol of ammonia is produced.
C The reaction happens as a result of collisions between one nitrogen molecule and three hydrogen molecules simultaneously.
D Reaction to form ammonia is exothermic.
E The reaction is slow because the likelihood of simultaneous collision of one nitrogen molecule and three hydrogen molecules is very low.
F If, over a period of time, the mount of ammonia gas in a reaction vessel increases by 0.6 mol, in that time the amount of hydrogen gas decreases by 0.9 mol.
G At equilibrium, the “backward” reaction of ammonia to form nitrogen and hydrogen occurs as a result of collisions each involving two ammonia molecules.
H The amounts of reactant species initially present are: 1 mol of nitrogen, 3 mol of hydrogen, and 2 mol of ammonia.
I The amounts of reactant species present when the reaction reaches equilibrium are: 1 mol of nitrogen, 3 mol of hydrogen, and 2 mol of ammonia.
J Because the reaction, the basis of the Haber process, is used to produce ammonia fertiliser that can dramatically increase crop production, it is regarded as one of the most important reactions known.
B Regardless of how much nitrogen gas, hydrogen gas and ammonia gas are in a reaction vessel initially, if 0.6 mol of hydrogen reacts with nitrogen, 0.4 mol of ammonia is produced.
C The reaction happens as a result of collisions between one nitrogen molecule and three hydrogen molecules simultaneously.
D Reaction to form ammonia is exothermic.
E The reaction is slow because the likelihood of simultaneous collision of one nitrogen molecule and three hydrogen molecules is very low.
F If, over a period of time, the mount of ammonia gas in a reaction vessel increases by 0.6 mol, in that time the amount of hydrogen gas decreases by 0.9 mol.
G At equilibrium, the “backward” reaction of ammonia to form nitrogen and hydrogen occurs as a result of collisions each involving two ammonia molecules.
H The amounts of reactant species initially present are: 1 mol of nitrogen, 3 mol of hydrogen, and 2 mol of ammonia.
I The amounts of reactant species present when the reaction reaches equilibrium are: 1 mol of nitrogen, 3 mol of hydrogen, and 2 mol of ammonia.
J Because the reaction, the basis of the Haber process, is used to produce ammonia fertiliser that can dramatically increase crop production, it is regarded as one of the most important reactions known.
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Answer: B and F only
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Prof Bob (2018)