The Chemical Equation and Balancing It
Every time you turn on the gas stove at home, methane reacts with the oxygen in the air and turns into carbon dioxide and water vapor. That transformation can be written as a chemical equation: a short sentence, made of formulas, that tells you exactly what goes in and what comes out.
Reactants and products
When you mix vinegar and baking soda in a glass, it bubbles up intensely, the white powder disappears, and a new gas is released. How can we neatly show which substances we had at the start and which ones appeared?
When lighting a gas ring on a cooker, methane ($\text{CH}4$) reacts with oxygen in the air and turns into carbon dioxide and water vapour. The diagram shows the…
When lighting a gas ring on a cooker, methane ($\text{CH}_4$) reacts with oxygen in the air and turns into carbon dioxide and water vapour. The diagram shows the particles of each substance:
Identify the reactants and products of this reaction.
Complete the sentence with the missing words: 'In a chemical equation, the substances that are transformed are called $\dots$ and are written to the $\dots$ of the…
Complete the sentence with the missing words: 'In a chemical equation, the substances that are transformed are called $\dots$ and are written to the $\dots$ of the arrow; the new substances formed are called $\dots$ and are written to the $\dots$.'
Nadia says: 'When paper burns, matter disappears because the ash weighs much less than the original paper.' Is she right? Reason your answer considering all the…
Nadia says: 'When paper burns, matter disappears because the ash weighs much less than the original paper.' Is she right? Reason your answer considering all the substances involved and formed, including gases.
Subscripts and coefficients: they are not the same thing
Read equation $\text{N}2 + 3\ \text{H}2 \rightarrow 2\ \text{NH}3$ carefully and explain in your own words what it tells us: how many molecules of each substance react…
Read equation $\text{N}_2 + 3\ \text{H}_2 \rightarrow 2\ \text{NH}_3$ carefully and explain in your own words what it tells us: how many molecules of each substance react and how many molecules are formed.
What does the subscript $2$ in $\text{H}2$ mean? And the coefficient $2$ in front of $2\ \text{H}2\text{O}$? Explain in your own words the difference between a subscript…
What does the subscript $2$ in $\text{H}_2$ mean? And the coefficient $2$ in front of $2\ \text{H}_2\text{O}$? Explain in your own words the difference between a subscript and a coefficient.
How many atoms of each element are there in total in $3\ \text{CO}2$? And in $2\ \text{H}2\text{O}$? The figure shows these molecules: Explain how atoms are counted when…
How many atoms of each element are there in total in $3\ \text{CO}_2$? And in $2\ \text{H}_2\text{O}$? The figure shows these molecules:
Explain how atoms are counted when a formula has a coefficient in front of it.
To 'balance' the equation $\text{H}2 + \text{O}2 \rightarrow \text{H}2\text{O}$, a student changes the product and writes…
To 'balance' the equation $\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}$, a student changes the product and writes $\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}_2$. Explain why this trick is not allowed: what substance $\text{H}_2\text{O}_2$ actually is and what changing a subscript in a formula means.
States of matter in the equation
An equation can also tell you what state each substance is in. A letter is written in parentheses, like a small subscript, right after the formula.
| Symbol | Means | Example |
|---|---|---|
| (s) | solid | $\text{NaCl(s)}$, table salt |
| (l) | liquid | $\text{H}_2\text{O(l)}$, liquid water |
| (g) | gas | $\text{H}_2\text{O(g)}$, water vapor |
| (ac) | aqueous solution (dissolved in water) | $\text{NaCl(ac)}$, salt water |
How many atoms of each element are there in total in $3\ \text{CO}2$? And in $2\ \text{H}2\text{O}$? The figure shows these molecules: Explain how atoms are counted when…
How many atoms of each element are there in total in $3\ \text{CO}_2$? And in $2\ \text{H}_2\text{O}$? The figure shows these molecules:
Explain how atoms are counted when a formula has a coefficient in front of it.
Why we have to balance: the Law of Conservation of Matter
If you burn $16\text{ g}$ of methane with $64\text{ g}$ of oxygen in a closed container, the scale reads $80\text{ g}$ before and after the combustion. How is the fact that no mass is gained or lost shown at the scale of atoms?
And if the ashes weigh less when you burn paper, was matter lost? No: part of the paper left as gases (carbon dioxide and water vapor) and you didn't weigh them. In a closed container, the total mass does not change. The same thing happens with a candle: in open air the scale reads less because the gases escape; under an airtight bell jar it reads the same.
Move the coefficients until the table says "balanced" for every element. Notice that hydrogen and oxygen do not even out at the same time with the same number.
Why must chemical equations be balanced before using them? Name the law on which balancing is based and explain it in a simple sentence about what happens to atoms in a…
Why must chemical equations be balanced before using them? Name the law on which balancing is based and explain it in a simple sentence about what happens to atoms in a reaction.
Count the hydrogen and oxygen atoms on each side of equation $\text{H}2 + \text{O}2 \rightarrow \text{H}2\text{O}$, represented with particles in the figure, and state,…
Count the hydrogen and oxygen atoms on each side of equation $\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}$, represented with particles in the figure, and state, without changing it yet, whether it is balanced or not. Justify your answer.
In a closed container, $12\ \text{g}$ of carbon are completely burned with $32\ \text{g}$ of oxygen, forming carbon dioxide. The diagram represents the closed container…
In a closed container, $12\ \text{g}$ of carbon are completely burned with $32\ \text{g}$ of oxygen, forming carbon dioxide. The diagram represents the closed container before and after the reaction:
What mass of carbon dioxide is obtained? Which law allows you to state this without needing to weigh it?
In a closed container, $10\ \text{g}$ of a substance A react completely with $5\ \text{g}$ of a substance B to produce $12\ \text{g}$ of a substance C together with a…
In a closed container, $10\ \text{g}$ of a substance A react completely with $5\ \text{g}$ of a substance B to produce $12\ \text{g}$ of a substance C together with a certain mass of a substance D. The diagram summarises the known masses:
- a) What mass of D has been formed?
- b) Justify your calculation by stating the law you applied.
We place a lit candle on a precision balance in two situations: a) In the open air: the reading on the balance decreases as the candle is consumed. b) Inside an airtight…
We place a lit candle on a precision balance in two situations:
- a) In the open air: the reading on the balance decreases as the candle is consumed.
- b) Inside an airtight glass bell jar: the reading barely changes.
The figure shows the two set-ups:
Explain both results using the law of conservation of mass, and reason whether matter really 'disappears' in either case.
Explain, using the particle model, exactly what happens to atoms in a chemical reaction: what happens to the atoms of the reactants, why their total number does not…
Explain, using the particle model, exactly what happens to atoms in a chemical reaction: what happens to the atoms of the reactants, why their total number does not change, and how a chemical reaction differs from simply mixing substances. Illustrate your explanation with the reaction of hydrogen with oxygen to form water, represented using particles in the figure:
Balancing by trial and error
Balance the equation for the formation of water by inspection: $\text{H}2 + \text{O}2 \rightarrow \text{H}2\text{O}$. Write the coefficients needed for each substance.
Balance the equation for the formation of water by inspection: $\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}$. Write the coefficients needed for each substance.
The natural gas in many homes is methane. Balance its combustion by inspection: $\text{CH}4 + \text{O}2 \rightarrow \text{CO}2 + \text{H}2\text{O}$. State the…
The natural gas in many homes is methane. Balance its combustion by inspection: $\text{CH}_4 + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}$. State the coefficients in the order in which the substances appear.
Ammonia is manufactured industrially from nitrogen and hydrogen and then used to make fertilisers. Balance the equation: $\text{N}2 + \text{H}2 \rightarrow \text{NH}3$.
Ammonia is manufactured industrially from nitrogen and hydrogen and then used to make fertilisers. Balance the equation: $\text{N}_2 + \text{H}_2 \rightarrow \text{NH}_3$.
The magnesium in sparklers and fireworks burns with a very intense white light, forming magnesium oxide. Balance the equation:…
The magnesium in sparklers and fireworks burns with a very intense white light, forming magnesium oxide. Balance the equation: $\text{Mg} + \text{O}_2 \rightarrow \text{MgO}$.
When iron rusts completely, iron(III) oxide is formed. Balance the equation: $\text{Fe} + \text{O}2 \rightarrow \text{Fe}2\text{O}3$. Then check by counting the atoms of…
When iron rusts completely, iron(III) oxide is formed. Balance the equation: $\text{Fe} + \text{O}_2 \rightarrow \text{Fe}_2\text{O}_3$. Then check by counting the atoms of each element on both sides.
The butane in gas cylinders, $\text{C}4\text{H}{10}$, burns with oxygen in the air, forming carbon dioxide and water. a) Balance the equation:…
The butane in gas cylinders, $\text{C}_4\text{H}_{10}$, burns with oxygen in the air, forming carbon dioxide and water.
- a) Balance the equation: $\text{C}_4\text{H}_{10} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}$.
- b) Verify, by counting the carbon, hydrogen and oxygen atoms on both sides, that your balanced equation satisfies the conservation of atoms.
Hydrogen peroxide from the first-aid kit, $\text{H}2\text{O}2$, gradually decomposes into water and oxygen. a) Balance the equation:…
Hydrogen peroxide from the first-aid kit, $\text{H}_2\text{O}_2$, gradually decomposes into water and oxygen.
- a) Balance the equation: $\text{H}_2\text{O}_2 \rightarrow \text{H}_2\text{O} + \text{O}_2$.
- b) The bottle of hydrogen peroxide states that it should be kept tightly closed and in a cool, dark place. Relate these instructions to the reaction above and explain what would happen if they were not followed.
The thin layer that protects aircraft aluminum parts from corrosion is formed when the metal reacts with oxygen in the air. a) Balance the equation:…
The thin layer that protects aircraft aluminum parts from corrosion is formed when the metal reacts with oxygen in the air.
- a) Balance the equation: $\text{Al} + \text{O}_2 \rightarrow \text{Al}_2\text{O}_3$.
- b) Explain why, when balancing by inspection, it is advisable to start with an element that appears in only one substance on each side of the arrow.
What the balanced coefficients mean
The balanced equation for the combustion of methane is $\text{CH}4 + 2\ \text{O}2 \rightarrow \text{CO}2 + 2\ \text{H}2\text{O}$. If $5$ methane molecules are burned: a)…
The balanced equation for the combustion of methane is $\text{CH}_4 + 2\ \text{O}_2 \rightarrow \text{CO}_2 + 2\ \text{H}_2\text{O}$. If $5$ methane molecules are burned:
- a) How many oxygen molecules are consumed?
- b) How many carbon dioxide molecules and how many water molecules are formed?
The balanced equation for the formation of water is $2\ \text{H}2 + \text{O}2 \rightarrow 2\ \text{H}2\text{O}$. Someone suggests dividing all coefficients by two and…
The balanced equation for the formation of water is $2\ \text{H}_2 + \text{O}_2 \rightarrow 2\ \text{H}_2\text{O}$. Someone suggests dividing all coefficients by two and writing $\text{H}_2 + \frac{1}{2}\ \text{O}_2 \rightarrow \text{H}_2\text{O}$.
- a) Would conservation of atoms still hold?
- b) What is the problem with referring to 'half a molecule' of oxygen?
- c) Justify why, by convention, the coefficients of a balanced equation are the simplest possible integers.
Some rockets use hydrogen as fuel, which burns with oxygen to form water vapour. a) Write the equation for the reaction and balance it. b) If $100$ hydrogen molecules…
Some rockets use hydrogen as fuel, which burns with oxygen to form water vapour.
- a) Write the equation for the reaction and balance it.
- b) If $100$ hydrogen molecules are burned in a ground test, how many oxygen molecules are consumed and how many water molecules are formed?
- c) How many hydrogen atoms are there in total before the reaction? And after? Relate your answer to the law of conservation.
Back to the beginning: the combustion of the methane in the stove is written with the balanced equation $\text{CH}_4 + 2\ \text{O}_2 \rightarrow \text{CO}_2 + 2\ \text{H}_2\text{O}$: for each molecule of methane you need exactly two from the oxygen in the air to get one of carbon dioxide and two of water vapor.