Question

State in your own words the law of conservation of mass. State the law in its modern form.

Short answer

Answer: The law of conservation of mass states that, in a closed system, the total amount of mass remains constant during a chemical reaction. This means that the mass of the reactants equals the mass of the products. The conservation of mass ensures that the matter is neither created nor destroyed during chemical reactions.

Step-by-step solution

Introduction to the Law of Conservation of Mass

The law of conservation of mass states that, in a closed system (i.e., when there is no exchange of matter with the surroundings), the total amount of mass remains constant. This means that the mass of the reactants in a chemical reaction equals the mass of the products.

Modern Form of the Law of Conservation of Mass

The modern form of the law of conservation of mass takes into account the concept of energy. The total mass-energy of a closed system remains constant. This is also known as the conservation of mass-energy. In the famous equation $E=m{c}^2$, Albert Einstein showed the relationship between mass and energy, where $E$ is energy, $m$ is mass, and $c$ is the speed of light.

Explanation and Example

To understand the law of conservation of mass, consider the following example: Suppose we have a reaction between hydrogen gas ($H_2$) and oxygen gas ($O_2$) to form water ($H_{2}O$). The balanced chemical equation for this reaction is: $2H_2+O_2\to 2H_{2}O$ Let's assume we have 4 grams of $H_2$ (mass of $H_2$) and 32 grams of $O_2$ (mass of $O_2$) as reactants. According to the law of conservation of mass, the total mass of the reactants should equal the total mass of the products. 1. Calculate the total mass of the reactants: Mass of reactants = mass of $H_2$ + mass of $O_2$ Mass of reactants = 4 grams + 32 grams = 36 grams 2. Carry out the reaction (in a closed system): $2H_2+O_2\to 2H_{2}O$ 3. Determine the total mass of the products: Since the conservation of mass states that the mass of reactants equals the mass of products, the mass of 2 $H_{2}O$ molecules (mass of products) should be equal to 36 grams. This example illustrates that, in a closed system, the mass of the reactants remains the same before and after a chemical reaction, fulfilling the law of conservation of mass.

Key concepts

Chemical Reaction

A chemical reaction is a process where substances, known as reactants, undergo a transformation to form different substances called products. This transformation involves the breaking and forming of chemical bonds, which rearranges atoms to create new chemical compounds.

During a chemical reaction, the fundamental principle to keep in mind is that the total mass of the reactants is conserved; it must equal the total mass of the products formed. This is guided by the law of conservation of mass, making sure that in a closed system, no mass is lost even though matter changes form. For instance, when hydrogen gas reacts with oxygen to form water, the mass of hydrogen and oxygen before the reaction will be the same as the mass of the water produced. This ensures that all atoms are accounted for, maintaining balance throughout the reaction.

Key points to remember about chemical reactions include:

• Reactions involve rearranging atoms, not creating or destroying them.
• The mass of reactants equals the mass of the products.
• Reactions can absorb or release energy, although the mass remains constant in a closed system.

Closed System

A closed system is fundamental when discussing the law of conservation of mass because it ensures that no matter or energy can enter or leave the system during a reaction. In practical terms, this means the boundaries are sealed so that all substances involved in the reaction remain contained.

In a closed system, what you start with is what you end with in terms of mass. For example, if you start a reaction with 100 grams of reactants, you will end up with 100 grams of products, although the substances might have changed form. This is vital for experiments and calculations because it allows for precise measurements and observations, without any external interference altering the mass.

Closed systems are important for:

• Ensuring accurate measurement of mass before and after a reaction.
• Conducting controlled experiments where only the intended variables are active.
• Preventing contamination or loss of reactants and products during chemical reactions.

Mass-Energy Conservation

The modern understanding of mass conservation goes beyond just mass itself; it encompasses mass-energy conservation. Albert Einstein's famous equation, $E=m{c}^2$, explains the interchangeability of mass and energy, indicating that mass can be converted to energy and vice-versa. However, in most chemical reactions at ordinary conditions, this conversion is negligible, and mass alone appears conserved.

Mass-energy conservation is crucial for comprehending how matter behaves in processes that might involve significant energy changes, such as nuclear reactions. In these cases, the energy released or absorbed manifests as a measurable change in mass. But for everyday chemical reactions, we assume conventional mass conservation since the energy changes are not enough to cause perceivable differences in mass.

Essential points regarding mass-energy conservation:

• Mass and energy are forms of the same thing and can be converted, although observable changes in mass are rare in typical chemical reactions.
• Conservation of mass-energy is a broader principle that includes energy exchange, essential in high-energy reactions.
• In routine applications, considering mass conservation helps predict and balance chemical reactions efficiently.