Every time you light the gas hob in the kitchen, the methane in the gas reacts with the oxygen in the air and turns into carbon dioxide and water vapour. That transformation can be written as a chemical equation: a short sentence, made of formulae, that tells you exactly what goes in and what comes out.
When you mix vinegar and bicarbonate of soda in a glass, there is intense fizzing and the white powder disappears while a new gas is released; how can we represent, in an orderly and concise way, which substances we had at the start and which ones have appeared?
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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
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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.
If you burn $16\text{ g}$ of methane with $64\text{ g}$ of oxygen in a hermetically sealed container, the scales show exactly $80\text{ g}$ before and after the combustion; how is it shown at the atomic scale that no mass is gained or lost?
Move the coefficients until the table says "balanced" for every element. Notice that hydrogen and oxygen do not balance at the same time with the same number.
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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?
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.
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.
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?
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 on the hob is written with the balanced equation $\text{CH}_4 + 2\ \text{O}_2 \rightarrow \text{CO}_2 + 2\ \text{H}_2\text{O}$: for every molecule of methane you need exactly two of oxygen from the air to turn into one of carbon dioxide and two of water vapour.
Summary
Let's practice! 8 medium exercises in this section