Problem 33 - Chemistry

Question

A piece of red-hot magnesium ribbon will continue to burn in an atmosphere of \(\mathrm{CO}_{2}\) even though \(\mathrm{CO}_{2}\) does not support combustion. Explain.

Step-by-Step Solution

Answer

Magnesium can continue to burn in an atmosphere of carbon dioxide because it is able to thermally decompose the carbon dioxide molecule, obtaining the oxygen it needs to continue burning. The heat from the burning magnesium breaks the carbon dioxide down into magnesium oxide and carbon, with the magnesium oxide being the white ash that forms during the burning process. This is an exothermic reaction, and it is the reason why magnesium can burn even in an atmosphere that does not typically support combustion.

Step 1: Understand the Chemical Behaviour of Magnesium

Magnesium is a highly reactive metal. If magnesium is heated, it will burn with a bright white flame. If burning magnesium is placed in an atmosphere of carbon dioxide, it will continue to burn.

Step 2: Explain Why CO2 Does Not Usually Support Combustion

Carbon dioxide does not usually support combustion. This is because combustion needs oxygen to occur, and carbon dioxide is commonly used as an extinguishing agent because it starves a fire of oxygen, preventing the combustion reaction from continuing.

Step 3: Understand the Reaction Between Magnesium and Carbon Dioxide

Magnesium can break apart carbon dioxide molecules at high temperatures. It reacts with the carbon dioxide to form magnesium oxide and carbon. This is demonstrated through the chemical equation: \( \mathrm{2Mg} + \mathrm{CO}_{2} \rightarrow \mathrm{2MgO} + \mathrm{C} \). This is an exothermic reaction, meaning it gives off heat.

Step 4: Explain Why Magnesium Continues to Burn in CO2

In this process, the magnesium actually obtains the required oxygen for burning (combustion) from the carbon dioxide itself, allowing it to continue to burn even in the presence of carbon dioxide. So, the key reason why magnesium can burn in CO2 is that it is able to decompose the \( \mathrm{CO}_{2} \) to obtain the oxygen it needs to continue burning, even though \( \mathrm{CO}_{2} \) does not typically support combustion.

Key Concepts

Chemical Reaction

Chemical reactions involve the transformation of substances through the breaking and forming of chemical bonds. In the context of magnesium combustion, we focus on the reaction between magnesium and carbon dioxide. When magnesium burns, it reacts with nearby substances. This particular reaction between magnesium and carbon dioxide is fascinating because it involves a unique process where magnesium decomposes carbon dioxide to release oxygen.
This decomposition allows magnesium to continue burning, showing that the outcome of a chemical reaction can sometimes defy usual expectations. In general, the key aspects involve:

• Reactants: Starting materials, here magnesium and carbon dioxide.
• Product: The new materials formed, in this case, magnesium oxide and carbon.
• Energy Change: The reaction provides energy, making it exothermic.

Understanding these components helps us grasp why magnesium continues to burn in carbon dioxide, a scenario where we typically wouldn't expect combustion.

Carbon Dioxide

Carbon dioxide ( CO_2 ) is commonly known as a compound that does not support combustion. It is a gas composed of one carbon and two oxygen atoms. This property makes it very effective as a fire extinguishing agent. The reason is that fires require oxygen to keep burning, and carbon dioxide can displace the oxygen around a flame, thus halting the combustion process.
However, carbon dioxide plays an unexpected role when magnesium is burning. While it doesn't support combustion directly, magnesium is able to use high heat to decompose carbon dioxide. This process splits carbon dioxide into oxygen and carbon. The key takeaways about carbon dioxide involve:

• Non-supportive of typical combustion.
• Acts as an oxygen source in specific reactions like with magnesium.
• Used in fire extinguishers due to its ability to displace oxygen.

Understanding these characteristics of carbon dioxide helps reveal why magnesium can burn in its presence despite usual expectations.

Combustion Reaction

A combustion reaction is a form of chemical reaction where a substance combines with oxygen to produce heat and usually light. Combustion is typically associated with burning that requires oxygen. For most materials, having enough oxygen is crucial for the fire to keep burning.
In the case of magnesium burning, the combustion reaction takes an interesting turn. Even though carbon dioxide surrounds it, the magnesium still finds a way to continue burning by breaking down carbon dioxide for oxygen. This is what allows it to combust even in an environment typically thought to inhibit such reactions. Important points about combustion reactions include:

• Required for substances to continue burning.
• Normally needs free oxygen from the air.
• Energy release, in the form of heat and light.

When magnesium burns in carbon dioxide, it showcases how some combustion reactions can adapt and continue by accessing alternative oxygen sources.

Magnesium Oxide

Magnesium oxide ( MgO ) is one of the products formed when magnesium combusts, especially in the presence of carbon dioxide. This compound results from the reaction of magnesium with oxygen, which it obtains by decomposing carbon dioxide.
Magnesium oxide is a white solid and is often used in various applications due to its properties such as high thermal conductivity and ability to withstand high temperatures. Here’s what makes magnesium oxide important:

• Result of magnesium combustion, confirming a successful reaction.
• Forms a stable compound that is quite inert under normal conditions.
• Has practical applications in industries due to its heat resistance.

Understanding the formation of magnesium oxide helps underscore the chemical transformations taking place when magnesium continues to burn in unusual atmospheres like carbon dioxide.