Combustion Reactions

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 $\dots$".

State which of these equations correspond to complete combustion and explain why:

1. a) $CH_4 + 2\ O_2 \rightarrow CO_2 + 2\ H_2O$
2. b) $2\ H_2O \rightarrow 2\ H_2 + O_2$
3. c) $C + O_2 \rightarrow CO_2$
4. d) $HCl + NaOH \rightarrow NaCl + H_2O$

After finishing a barbecue, Jorge covers the embers with sand and the fire goes out. The combustion diagram can be represented like this:

Explain, using the combustion diagram, which "ingredient" of the reaction is missing when the embers are covered and why the fire cannot continue.

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.

Propane ($C_3H_8$) is the fuel for many portable heaters and caravans. Write and balance the equation for its complete combustion.

A classmate has written the complete combustion of ethane ($C_2H_6$) in their notebook as: $2\ C_2H_6 + 7\ O_2 \rightarrow 4\ CO_2 + 6\ H_2O$. Count the carbon, hydrogen and oxygen atoms on each side of the arrow and check whether the equation is correctly balanced. Justify your answer.

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 that the ethanol molecule itself already contains an oxygen atom.

When you run, your muscles "burn" glucose ($C_6H_{12}O_6$) with the oxygen you breathe, forming carbon dioxide and water. This is cellular respiration, a slow combustion inside your cells. Write and balance the equation for this reaction.

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 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 balance the equation for this incomplete combustion of methane.

The soot that accumulates in chimneys is carbon that has not burned completely.

1. a) Write the balanced equation for the complete combustion of carbon, which forms carbon dioxide.
2. b) Write the balanced equation for its incomplete combustion when carbon monoxide is formed.
3. c) Compare the oxygen consumed by each for every $2$ carbon atoms and explain why soot and $CO$ appear when air is scarce.

Methane can burn in two ways depending on the available oxygen.

1. a) Write the balanced equations for its complete combustion (products $CO_2$ and $H_2O$) and for its incomplete combustion when carbon monoxide ($CO$) and water are formed.
2. b) Calculate how many moles of $O_2$ each reaction consumes for every $2\ \text{mol}$ of methane.
3. c) Using these numbers, explain why a gas cooker left on for a long time in a small, unventilated room can end up producing carbon monoxide.

The balanced equation for the combustion of propane is $C_3H_8 + 5\ O_2 \rightarrow 3\ CO_2 + 4\ H_2O$.

1. a) How many oxygen molecules are needed to burn one molecule of propane?
2. b) How many moles of oxygen are required to burn $0.4\ \text{mol}$ of propane?
3. 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 $C + O_2 \rightarrow CO_2$.

1. a) How many moles of carbon dioxide are formed when burning $3\ \text{mol}$ of carbon?
2. 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 $2\ C_4H_{10} + 13\ O_2 \rightarrow 8\ CO_2 + 10\ H_2O$.

1. a) Calculate the moles of butane contained in the gas cylinder.
2. b) Calculate the moles of $CO_2$ that will be emitted when burning it completely.
3. 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.

A bioethanol fireplace burns $92\ \text{g}$ of ethanol ($C_2H_6O$, molar mass $46\ \text{g/mol}$) in an evening.

1. a) Write and balance the equation for its complete combustion.
2. b) Calculate the moles of ethanol burned and the moles of oxygen consumed.
3. c) Using the molar mass of $O_2$, $32\ \text{g/mol}$, calculate the mass of oxygen consumed in the room and comment on why the manufacturer recommends ventilating the room.

A gas car emits on average $120\ \text{g}$ of $CO_2$ per kilometer and a diesel car, $105\ \text{g}$ per kilometer.

1. a) Calculate the emissions of each vehicle on a trip of $250\ \text{km}$.
2. b) The gas car makes that trip 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.

One of those reactions is $2\ CO + O_2 \rightarrow 2\ CO_2$.

1. a) Explain what happens to carbon monoxide in this reaction and why it improves urban air quality.
2. b) Despite this, the $CO_2$ coming out of the exhaust is still a problem: explain why.
3. 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.

To find out what products are formed by a candle flame, its gases are passed through a tube with anhydrous copper sulfate, which is white, and then bubbled through limewater (dissolved calcium hydroxide), as shown in the figure.

It is observed that the copper sulfate turns blue and the limewater turns cloudy.

1. a) Which combustion product is indicated by each of the two observed changes?
2. b) The cloudiness is due to the reaction $CO_2 + Ca(OH)_2 \rightarrow CaCO_3 + H_2O$, which forms a white precipitate of calcium carbonate. Check that the equation is balanced.
3. c) What can you conclude about which elements candle wax contains? Justify your answer.

The coal in some thermal power stations contains sulfur as an impurity.

1. a) Write and balance the equation for the combustion of sulfur, forming sulfur dioxide ($SO_2$).
2. b) In the atmosphere, $SO_2$ reacts with water in clouds to form sulfurous acid ($H_2SO_3$): write and balance this reaction.
3. c) Relate these facts to the phenomenon of acid rain and explain why many power stations install desulfurisation units in their chimneys.

Rockets such as Ariane use liquid hydrogen as fuel, which burns like this: $2\ H_2 + O_2 \rightarrow 2\ H_2O$.

1. a) Why is hydrogen said to be a "clean" fuel at the point of combustion?
2. b) Calculate how many moles of water are formed when burning $5\ \text{mol}$ of hydrogen.
3. c) Today, most hydrogen is produced from natural gas in a process that emits $CO_2$ ("gray" hydrogen), but it can also be produced by electrolysis of water using renewable electricity ("green" hydrogen). Explain the criteria used to distinguish one hydrogen from another and why only one of them truly helps combat climate change.

Two widely used fuels are coal (almost pure carbon) and natural gas (almost pure methane).

1. a) Write balanced equations for the complete combustion of carbon and methane.
2. b) Compare how many moles of $CO_2$ each emits per mole of fuel burned.
3. c) Given also that burning $1\ \text{mol}$ of methane releases significantly more energy than burning $1\ \text{mol}$ of carbon, explain why replacing coal with natural gas in a power station reduces $CO_2$ emissions per kilowatt-hour produced.