Chemistry Unit 4 Lesson 2

Dynamite explosively demonstrates energy released during chemical reactions

In the early nineteenth century, chemists recognized that unstable nitroglycerin (C3H5(NO3)3) could be used as an explosive. Mixtures of gunpowder and nitroglycerin were used, but they were too dangerous to handle because nitroglycerin was so unstable. This hindered the use of this explosive both in mining and in warfare. However, in 1866 Swedish chemist Alfred Nobel found that if nitroglycerin was mixed with diatomaceous earth or sawdust it could be handled and shaped safely. Nobel's invention of dynamite in 1867 immediately replaced gunpowder as the explosive of choice. Although Nobel became rich from an invention that is sometimes used in warfare, he was a peaceful man. He set aside money in his will to establish the Nobel Prizes in science, peace, and literature upon his death.

An instant heat pack produces heat through a chemical reaction.

True

Chemical reactions are driven by changes in energy. Physicists and chemists in the late eighteenth and early nineteenth centuries measured the energy changes (in the form of heat) that occurred during chemical reactions. They formulated an understanding of thermal energy, how it changes during reactions and phase changes, and how to predict which reactions will occur based on energy changes. Chemists thus learned a lot about chemical thermodynamics. In this unit, you will learn about the concepts of thermal energy, heat, enthalpy, specific heat, and the laws of thermodynamics.

True

Combustion reactions give off heat so they are exothermic.

True

Hand warmers are another example of exothermic reactions.

True

Sodium reacting with water is an exothermic reaction.

True

chemical reactions involve energy and mass

energy releases exothermic energy absorbs endothermic

heat exits from the environment

exothermic

Dynamite gets its energy from

nitroglycerin (C3H5(NO3)3).

Dynamite releases vast amounts of heat and light energy when it explodes.

Alfred Nobel invented dynamite in 1866. The explosion of dynamite forcefully demonstrates the vast potential energy contained in chemical bonds of unexploded dynamite. Besides releasing energy, chemical reactions can also absorb energy to form chemical bonds. The changes in energy during chemical reactions are an important part of chemical thermodynamics. Here you will learn about such changes in energy and how the total energy of chemical reactions is conserved.

Consider melting ice. The solid ice absorbs heat energy to become liquid water. What type of reaction is this and why?

Because this change absorbs energy, we call it an endothermic reaction. Other changes of state are exothermic, such as when a liquid changes to a solid.

Energy changes occur during chemical reactions.

Chemical energy is stored in the chemical bonds between atoms in molecules. Chemical reactions that release energy, usually in the form of heat, are called exothermic reactions. Chemical reactions that absorb energy, usually in the form of heat, are called endothermic reactions. Many types of reactions can be endothermic or exothermic. Changes of state also can be endothermic or exothermic. In any system, the total energy before and after a reaction remains the same. This is the law of conservation of energy.

Combustion releases energy, while melting absorbs energy.

Consider a burning match. The combustion of the match releases energy in the form of heat and light as the wood burns. Because the chemical reaction releases heat energy, we call it an exothermic reaction. Some other chemical reactions are endothermic. Chemical reactions can be exothermic or endothermic. Changes of state, such as melting ice or boiling water, can also be exothermic or endothermic. What is important is determining whether energy, usually in the form of heat, is given off (exothermic) or absorbed (endothermic).

For any chemical reaction, the total energy in the reaction remains the same.

Consider a chemical reaction like the oxidation reaction in the disposable hand warmer. Although the reaction was exothermic, the heat energy did not appear out of the blue. The energy was stored in the bonds of the reactants. So the total energy before the reaction and the total energy after the reaction is the same. This principle is called the law of conservation of energy. It is similar to what you learned about the mass of reactants and products in stoichiometry (conservation of mass). In fact, the more precise description is that for any chemical reaction, the mass and energy of the products must equal the mass and energy of the reactants. This is called the law of conservation of mass and energy.

Endothermic reactions absorb energy.

Explore what happens during an endothermic reaction between two chemicals through a thermal imager.

Exothermic reactions release energy.

Explore what happens during an exothermic reaction between two chemicals through a thermal imager.

Cold packs absorb heat by dissolving salts.

Many injuries, such as sprains, require cold packs to reduce swelling. Cold packs have a thin inner bag of water surrounded by crystals of ammonium nitrate or ammonium chloride. When you break the inner bag, the water dissolves the ammonium salts in one of the following chemical reactions: NH4NO3 + heat → NH4+ + NO3- NH4Cl + heat → NH4+ + Cl- Both reactions are endothermic. The bag absorbs heat from its surroundings and cools the damaged body tissue. The energy absorbed by ammonium nitrate is more than that absorbed by ammonium chloride, so ammonium nitrate cold packs will get colder than ammonium chloride ones. However, ammonium nitrate packs can explode when overheated, so ammonium chloride cold packs are safer.

Where does the energy of dynamite come from?

The chemical bonds between the atoms of the nitroglycerin molecule are not still. They constantly move; they bend, stretch, and rotate. These bonds have potential energy. When nitroglycerin's bonds break, energy is released—enormous amounts of energy in the form of heat and light—in the following reaction: 4C3H5N3O9(l) →6N2 (g) + 12CO2 (g) + 10H2O (g) + O2 (g)

Here are some examples of endothermic and exothermic processes.

The following processes absorb heat (are endothermic): Separating ion pairs Changes of state: solid → liquid → gas These processes can release energy (are exothermic): Forming ionic salts Combustion/oxidation Mixing some salts with water Mixing water with concentrated acids Changes of state: gas → liquid → solid

Some hand warmers consist of a cellulose bag, iron powder, water, and vermiculite to absorb the water. The water is contained in a thin inner bag surrounded by the vermiculite-iron powder mixture. When you break the inner bag, the water and oxygen from the air react with the iron powder and begin to oxidize, or rust, the iron. Once begun, the reaction (below) cannot be reversed and the hand warmer must be thrown away. 4Fe + 3O2 → Fe2O3 + heat Another type of hand warmer uses liquid sodium acetate encased in a plastic bag with a metal disc. When you press the metal disc, it creates a crystal of sodium acetate. As the liquid sodium acetate becomes solid, the temperature inside rises to the melting point of 54°C. The reaction is exothermic. The bag releases heat energy and warms your hand. This reaction can be reversed by boiling the bag. The sodium acetate will again return to a liquid state and the hand warmer can be reused.

True

Exothermic reactions release energy, while endothermic reactions absorb energy.

When a chemical reaction releases energy, the energy is usually in the form of heat. This type of reaction is called an exothermic Opens in modal popup window reaction. A common way to measure the release of heat is as an increase in the temperature of the surroundings. When a chemical reaction absorbs energy, the energy is usually in the form of heat. This type of reaction is called an endothermic Opens in modal popup window reaction. A common way to measure the absorption of heat is as a decrease in the temperature of the surroundings.

mass is

conserved

heat enters from the environment

endothermic