Laws of Chemical Reactions Quiz

Lei de Lavoisier (Conservação de Massa)

• In a closed system, the sum of the masses of the reactants equals the sum of the masses of the products.
• A closed system is a sealed container that does not allow the exchange of material with the outside environment.
• Mass is conserved, but the number of moles, the number of molecules, and the number of atoms are not always conserved.

Lei de Proust (Proporções Fixas)

• In a given chemical reaction, substances always react in the same mass proportion.
• The proportion is usually expressed by small whole numbers.
• The proportion is constant, regardless of the amount of reactants used.
• Example: In the combustion reaction of carbon, the mass proportion between carbon and oxygen is always the same, regardless of the amount of carbon or oxygen used.

Lei de Dalton (Proporções Múltiplas)

• When two chemical elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in the ratio of small whole numbers.
• Example: Sulfur and oxygen can combine to form sulfur dioxide (SO2) or sulfur trioxide (SO3).
• In the formation of SO2, 32 g of sulfur combines with 32 g of oxygen. To form SO3, 32 g of sulfur combines with 48 g of oxygen.
• The ratio of the masses of oxygen that combine with the same mass of sulfur is 32:48, which simplifies to 2:3.

Proportions in Chemical Reactions

• The ratio of reactants and products in a chemical reaction is called the proportion.
• The proportion is constant for any given reaction, regardless of the amount of reactants used.
• The smallest possible whole number ratio for a chemical reaction is called the smallest whole number proportion.
• For example, in the reaction 2H₂ + O₂ → 2H₂O the smallest whole number proportion is 2:1:2.
• This means that for every two moles of hydrogen (H₂), one mole of oxygen (O₂) is consumed to produce two moles of water (H₂O).
• The proportion can be interpreted in terms of moles, mass, or volume.
• The proportion is important because it allows us to predict the amount of products that will be formed from a given amount of reactants.

The Law of Multiple Proportions

• The law of multiple proportions states that when two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in ratios of small whole numbers.
• For example, carbon and oxygen can combine to form carbon dioxide (CO₂) and carbon monoxide (CO).
• In CO₂, the ratio of carbon to oxygen is 1:2, while in CO, the ratio is 1:1.
• This means that the mass of oxygen that combines with one gram of carbon in CO₂ is twice the mass of oxygen that combines with one gram of carbon in CO.

Gay-Lussac's Law of Combining Volumes

• Gay-Lussac's Law of Combining Volumes states that when gases react, they do so in volumes that are in a simple whole number ratio.
• This law is a direct consequence of Avogadro's Law, which states that equal volumes of gases at the same temperature and pressure contain the same number of molecules.
• For example, in the reaction N₂ + 3H₂ -> 2NH₃, the ratio of volumes of nitrogen, hydrogen, and ammonia is 1:3:2.
• This means that one liter of nitrogen gas will react with three liters of hydrogen gas to produce two liters of ammonia gas.
• This law applies only to gases, not liquids or solids.

Key Takeaways

• Proportions are crucial for understanding chemical reactions.
• Proportions can be expressed in terms of moles, mass, or volume.
• The Law of Multiple Proportions and Gay-Lussac's Law of Combining Volumes are essential laws governing chemical reactions.

Reactions in a Closed System

• When carbon (coal) reacts with oxygen in combustion, it releases carbon dioxide (CO2) as a gas.
• The CO2 gas escapes the system, causing a mass decrease on the side containing the burning coal. This leads to the scale tilting toward the burning coal.

Reactions in an Open System

• When iron (steel wool) reacts with oxygen in combustion, it forms iron oxide (Fe2O3 or Fe3O4).
• The mass of the iron oxide is greater than the initial mass of the iron, as oxygen is incorporated.
• The scale tilts toward the side with the burning steel wool because the mass has increased.

Law of Constant Proportions

• The law states that the same chemical compound always contains the same elements in the same proportion by mass.
• This law is used to determine if samples consist of the same compound by comparing the ratios of the weights of different elements within them.
• Variations in these ratios suggest different chemical compounds.