Combustion reactions
The hob in your kitchen, a car engine and a power station all do the same thing: they burn a fuel with the oxygen in the air. This everyday reaction has a fixed structure that, once you understand it, lets you write, balance and carry out calculations for any combustion, from the butane in a gas cylinder to the glucose that every cell in your body "burns".
What a complete combustion is
If you cover a lit candle with a glass upside down, the flame gets smaller until it goes out within a few seconds, while the inside walls mist up: which substances does a fire need in order to burn, and which are produced during the reaction?
In the gas cooker at Lucía's home, butane reacts with oxygen from the air while dinner is heating up. Complete the general scheme of complete combustion with the missing…
In the gas cooker at Lucía's home, butane reacts with oxygen from the air while dinner is heating up. Complete the general scheme of complete combustion with the missing words: "In any complete combustion, a $\dots$ reacts with the $\dots$ in the air to form $\dots$ and $\dots$".
Methane ($CH_4$) is the natural gas that reaches many kitchens. Write the equation for its complete combustion (the products are carbon dioxide and water) and balance it.
Methane ($CH_4$) is the natural gas that reaches many kitchens. Write the equation for its complete combustion (the products are carbon dioxide and water) and balance it.
General balancing of the combustion of a hydrocarbon CₓHᵥ
To burn $1\ \text{mol}$ of methane ($\text{CH}_4$) completely, $2\ \text{mol}$ of $\text{O}_2$ are needed, but for $1\ \text{mol}$ of propane ($\text{C}_3\text{H}_8$) you need $5\ \text{mol}$ of $\text{O}_2$: what rule lets you work out the exact oxygen required by any hydrocarbon without memorising each reaction?
This pattern is always the same, with $x$ atoms of C and $y$ of H: the oxygen coefficient needed turns out to be $x + \frac{y}{4}$, which is a whole number only when $y$ is a multiple of $4$ (as in methane, $\text{CH}_4$); in the other hydrocarbons a fraction appears, which forces you to double the whole equation.
Propane ($C_3H_8$) is the fuel for many portable heaters and caravans. Write and balance the equation for its complete combustion.
Propane ($C_3H_8$) is the fuel for many portable heaters and caravans. Write and balance the equation for its complete combustion.
The butane ($C_4H_{10}$) in orange gas cylinders burns with oxygen to form carbon dioxide and water. Write the equation and balance it; you will get a fractional…
The butane ($C_4H_{10}$) in orange gas cylinders burns with oxygen to form carbon dioxide and water. Write the equation and balance it; you will get a fractional coefficient for oxygen, which you will need to eliminate by multiplying the entire equation by two.
Bioethanol fireplaces burn ethanol ($C_2H_6O$) producing carbon dioxide and water. Write and balance the equation for its complete combustion. Hint: take into account…
Bioethanol fireplaces burn ethanol ($C_2H_6O$) producing carbon dioxide and water. Write and balance the equation for its complete combustion. Hint: take into account that the ethanol molecule itself already contains an oxygen atom.
Incomplete combustion: when there isn't enough oxygen
In the laboratory, the flame of a Bunsen burner is blue when the air hole is open, and yellow and luminous when it is closed, as shown in the figure. One of these two…
In the laboratory, the flame of a Bunsen burner is blue when the air hole is open, and yellow and luminous when it is closed, as shown in the figure.
One of these two flames leaves soot on test tubes. State which of the two leaves soot and explain what type of combustion (complete or incomplete) takes place in each case.
When a gas heater operates in a poorly ventilated room, the combustion of butane becomes incomplete. Name the two substances that can then form instead of carbon dioxide…
When a gas heater operates in a poorly ventilated room, the combustion of butane becomes incomplete. Name the two substances that can then form instead of carbon dioxide and explain why one of them is particularly dangerous to people in the room.
A poorly ventilated gas heater burns methane ($CH_4$) with limited oxygen, and instead of carbon dioxide, carbon monoxide ($CO$) is formed, along with water. Write and…
A poorly ventilated gas heater burns methane ($CH_4$) with limited oxygen, and instead of carbon dioxide, carbon monoxide ($CO$) is formed, along with water. Write and balance the equation for this incomplete combustion of methane.
Calculating with combustion: moles, masses and the limiting reactant
The balanced equation for the combustion of propane is $C_3H_8 + 5\ O_2 \rightarrow 3\ CO_2 + 4\ H_2O$. a) How many oxygen molecules are needed to burn one molecule of…
The balanced equation for the combustion of propane is $C_3H_8 + 5\ O_2 \rightarrow 3\ CO_2 + 4\ H_2O$.
- a) How many oxygen molecules are needed to burn one molecule of propane?
- b) How many moles of oxygen are required to burn $0,4\ \text{mol}$ of propane?
- c) How many moles of carbon dioxide will be formed in that case?
Charcoal on a barbecue is almost pure carbon and its complete combustion is $C + O_2 \rightarrow CO_2$. a) How many moles of carbon dioxide are formed when burning…
Charcoal on a barbecue is almost pure carbon and its complete combustion is $C + O_2 \rightarrow CO_2$.
- a) How many moles of carbon dioxide are formed when burning $3\ \text{mol}$ of carbon?
- b) The molar masses of carbon and carbon dioxide are $12\ \text{g/mol}$ and $44\ \text{g/mol}$, respectively. Calculate how many grams of $CO_2$ are emitted when burning $36\ \text{g}$ of charcoal.
A camping gas cylinder contains $5,8\ \text{kg}$ of butane ($C_4H_{10}$, molar mass $58\ \text{g/mol}$) and its complete combustion is…
A camping gas cylinder contains $5,8\ \text{kg}$ of butane ($C_4H_{10}$, molar mass $58\ \text{g/mol}$) and its complete combustion is $2\ C_4H_{10} + 13\ O_2 \rightarrow 8\ CO_2 + 10\ H_2O$.
- a) Calculate the moles of butane contained in the gas cylinder.
- b) Calculate the moles of $CO_2$ that will be emitted when burning it completely.
- c) Using the molar mass of $CO_2$, $44\ \text{g/mol}$, calculate the kilograms of $CO_2$ emitted and comment on why these emissions are a concern even if combustion is complete.
Fuels, emissions and the role of cellular respiration
A petrol car emits on average $120\ \text{g}$ of $CO_2$ per kilometre and a diesel car, $105\ \text{g}$ per kilometre. a) Calculate the emissions of each vehicle on a…
A petrol car emits on average $120\ \text{g}$ of $CO_2$ per kilometre and a diesel car, $105\ \text{g}$ per kilometre.
- a) Calculate the emissions of each vehicle on a journey of $250\ \text{km}$.
- b) The petrol car makes that journey with $4$ passengers and the diesel car, on another day, with only the driver. Calculate the emissions per passenger in each case and comment on the result.
A car's catalytic converter converts some of the toxic gases from the exhaust pipe into less harmful ones before releasing them into the air, as shown in the diagram.…
A car's catalytic converter converts some of the toxic gases from the exhaust pipe into less harmful ones before releasing them into the air, as shown in the diagram.
One of those reactions is $2\ CO + O_2 \rightarrow 2\ CO_2$.
- a) Explain what happens to carbon monoxide in this reaction and why it improves urban air quality.
- b) Despite this, the $CO_2$ coming out of the exhaust is still a problem: explain why.
- c) Another reaction in the catalytic converter is $2\ NO \rightarrow N_2 + O_2$. Check that it is balanced and explain why it is beneficial for the air we breathe.