chemical reaction overview

energy in and energy out

All chemical reactions involve energy. Energy is used to break bonds in reactants, and energy is released when new bonds form in products. In terms of energy, there are two types of chemical reactions: endothermic reactions and exothermic reactions. In exothermic reactions, more energy is released when bonds form in products than is used to break bonds in reactants. These reactions release energy to the environment, often in the form of heat or light. In endothermic reactions, more energy is used to break bonds in reactants than is released when bonds form in products. These reactions absorb energy from the environment. Q: When it comes to energy, which type of reaction is the burning of wood? Is it an endothermic reaction or an exothermic reaction? How can you tell? A: The burning of wood is an exothermic reaction. You can tell by the heat and light energy given off by a wood fire.

same atoms new bonds

The reactants and products in a chemical reaction contain the same atoms, but they are rearranged during the reaction. As a result, the atoms are in different combinations in the products than they were in the reactants. This happens because chemical bonds break in the reactants and new chemical bonds form in the products. Consider the chemical reaction in which water forms from oxygen and hydrogen gases. The Figure 1.2 represents this reaction. Bonds break in molecules of hydrogen and oxygen, and then new bonds form in molecules of water. In both reactants and products there are four hydrogen atoms and two oxygen atoms, but the atoms are combined differently in water.


types of chemical reactions

The chemical reaction in the Figure 1.2, in which water forms from hydrogen and oxygen, is an example of a synthesis reaction. In this type of reaction, two or more reactants combine to synthesize a single product. There are several other types of chemical reactions, including decomposition, replacement, and combustion reactions. The Table 1.1 compares these four types of chemical reactions. Type of Reaction Synthesis Decomposition General Equation A+B C AB A + B Example 2Na + Cl2 2NaCl 2H2 O 2H2 + O2 Type of Reaction Single Replacement Double Replacement Combustion General Equation A+BC B+ AC AB+ CD AD + CB fuel + oxygen carbon dioxide + water Example 2K + 2H2 O 2KOH + H2 NaCl+ AgF NaF + AgCl CH4 + 2O2 CO2 + 2H2 O Q: The burning of wood is a chemical reaction. Which type of reaction is it? A: The burning of woodor of anything elseis a combustion reaction. In the combustion example in the table, the fuel is methane gas (CH4 ). Click image to the left or use the URL below. URL:

what is a chemical reaction

A chemical reaction is a process in which some substances change into different substances. Substances that start a chemical reaction are called reactants. Substances that are produced in the reaction are called products. Reactants and products can be elements or compounds. Chemical reactions are represented by chemical equations, like the one below, in which reactants (on the left) are connected by an arrow to products (on the right). Reactants Products Chemical reactions may occur quickly or slowly. Look at the two pictures in the Figure 1.1. Both represent chemical reactions. In the picture on the left, a reaction inside a fire extinguisher causes foam to shoot out of the extinguisher. This reaction occurs almost instantly. In the picture on the right, a reaction causes the iron tool to turn to rust. This reaction occurs very slowly. In fact, it might take many years for all of the iron in the tool to turn to rust. Q: What happens during a chemical reaction? Where do the reactants go, and where do the products come from? A: During a chemical reaction, chemical changes take place. Some chemical bonds break and new chemical bonds form.


instructional diagrams

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a chemical reaction changes some substances into other substances with different chemical properties.

-->  a. true

b. false

the substances involved in a chemical reaction may be elements or compounds.

-->  a. true

b. false

during a chemical reaction

-->  a) chemical bonds break.

b) matter is destroyed.

c) new atoms form.

d) two of the above

which of the following represents a synthesis reaction?

-->  a) 2h2 + o2  2h2o

b) 2k + 2h2o  2koh + h2

c) 2nacl  2na + cl2

d) none of the above

which type of reaction is represented by the following chemical equation? ch4 + 2o2 co2 + 2h2o

-->  a) combustion reaction

b) decomposition reaction

c) single replacement reaction

d) double replacement reaction

diagram questions

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