READ: Conservation of Mass in Reactions

READ: Conservation of Mass in Reactions

Law of Conservation of Mass

By the late 1700s, chemists accepted the definition of an element as a substance that cannot be broken down into a simpler substance by ordinary chemical means. It was also clear that elements combine with one another to form more complex substances called compounds. The chemical and physical properties of these compounds are different than the properties of the elements from which they were formed. There were questions about the details of these processes.

In the 1790s, a greater emphasis began to be placed on the quantitative analysis of chemical reactions. Accurate and reproducible measurements of the masses of reacting elements and the compounds they form led to the formulation of several basic laws. One of these is called the law of conservation of mass which states that during a chemical reaction the total mass of the products must be equal to the total mass of the reactants. In other words, mass cannot be created or destroyed during a chemical reaction, but must always be conserved.

As an example, consider the reaction between silver nitrate and sodium chloride. These two compounds will dissolve in water to form silver chloride and sodium nitrate. The silver chloride does not dissolve in water, so it forms a solid that we can filter off. When we evaporate the water, we can recover the sodium nitrate formed. If we react 58.5 grams of sodium chloride with 169.9 grams of silver nitrate, we start with 228.4 grams of materials. After the reaction is complete and the materials separated, we find that we have formed 143.4 grams of silver chloride and 85.0 grams of sodium nitrate, giving us a total mass of 228.4 grams for the products. So, the total mass of reactants equals the total mass of products, a proof of the law of conservation of mass.

Follow this link to watch a video which illustrates conversation of mass:

Lavoisier and Conservation of Mass

How do scientists know that mass is always conserved in chemical reactions? Careful experiments in the 1700s by a French chemist named Antoine Lavoisier led to this conclusion. Lavoisier carefully measured the mass of reactants and products in many different chemical reactions. He carried out the reactions inside a sealed jar. In every case, the total mass of the jar and its contents was the same after the reaction as it was before the reaction took place. This showed that matter was neither created nor destroyed in the reactions.

Q: Lavoisier carried out his experiments inside a sealed glass jar. Why was sealing the jar important for his results? What might his results have been if he hadn’t sealed the jar?

A: Sealing the jar was important so that any gases produced in the reactions were captured and could be measured. If he hadn’t sealed the jar, gases might have escaped detection. Then his results would have shown that there was less mass after the reactions than before. In other words, he would not have been able to conclude that mass is conserved in chemical reactions.

Mass-Mass Calculations

A mass-mass calculation would allow you to solve one of the following types of problems:

1. Determine the mass of reactant necessary to produce a given amount of product.
2. Determine the mass of product that would be produced from a given amount of reactant.
3. Determine the mass of reactant necessary to react completely with a second reactant.

As was the case for mole ratios, it is important to double check that you are using a balanced chemical equation before attempting any calculations.

Many chemists prefer to solve stoichiometry problems with a single line of math instead of writing out the multiple steps. This can be done by using dimensional analysis, also called the factor-label method. Recall that this is simply a method that uses conversion factors to convert from one unit to another. In this method, we can follow the cancellation of units to obtain the correct answer.

For instance: 15.0 g of chlorine gas is bubbled through liquid sulfur to produce disulfur dichloride. How much product is produced in grams?

Step 1: As always, the first step is to correctly write and balance the equation:

Cl2(g)+2 S(l)→S2Cl2(l)

Step 2: Identify what is being given (for this question, 15.0 g of Cl₂ is the given) and what is asked for (grams of S₂Cl₂).

Step 3: Next, use the correct factors that allow you to cancel the units you don’t want and get the unit you do want:

Example 1

Consider the thermite reaction again. This reaction occurs between elemental aluminum and iron(III) oxide, releasing enough heat to melt the iron that is produced. If 500.0 g of iron is produced in the reaction, how much iron(III) oxide was placed in the original container?

Solution:

Step 1: Write and balance the equation:

Fe2O3(s)+2 Al(s)→2 Fe(l)+Al2O3(s)

Step 2: Determine what is given and what needs to be calculated:

given=500 g of Fecalculate=grams of Fe2O3

Step 3: Set-up the dimensional analysis system:

500 g Fe⋅1 mol Fe55.85 g Fe⋅1 mol Fe2O32 mol Fe⋅159.7 g Fe2O31 mol Fe2O3=717 g Fe2O3

Example 2

Ibuprofen is a common painkiller used by many people around the globe. It has the formula C₁₃H₁₈O₂. If 200 g of Ibuprofen is combusted, how much carbon dioxide is produced?

Solution:

Step 1: Write and balance the equation:

2 C13H18O2(s)+33 O2(g)→26 CO2(g)+18 H2O(l)

Step 2: Determine what is given and what needs to be calculated:

given=200 g of ibuprofencalculate=grams of CO2

Step 3: Set-up the dimensional analysis system:

200 g C13H18O2⋅ 1 mol C13H18O2206.3 g C13H18O2⋅26 mol CO22 mol C13H18O2⋅44.01 g CO21 mol CO2=555 g CO2

Example 3

If sulfuric acid is mixed with sodium cyanide, the deadly gas hydrogen cyanide is produced. How much sulfuric acid must be placed in the container to produce 12.5 g of hydrogen cyanide?

Solution:

Step 1: Write and balance the equation:

2NaCN(s)+H2SO4(aq)→Na2SO4(aq)+2 HCN(g)

Step 2: Determine what is given and what needs to be calculated:

given=12.5 g HCNcalculate=grams of H2SO4

Step 3: Set-up the dimensional analysis system:

12.5 g HCN⋅1 mol HCN27.0 g HCN⋅1 mol H2SO42 mol HCN⋅98.06 g H2SO41 mol H2SO4=22.7 g H2SO4

Summary

• Antoine Lavoisier did careful experiments to discover the law of conservation of mass in chemical reactions.
• The balanced reaction gives the mole ratios of reactants and products
• Using the molar masses of reactants and products and a balanced equation, it is possible to calculate the mass of product produced from a given amount of reactant.

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