Chemical reactions
Physical Science - Middle School
Chemical reactions
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Study Guide Chemical reactions Physical Science - Middle School
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CHEMICAL REACTIONS Reactants and Products When a chemical reaction occurs, the original substances put together, called reactants, lose their chemical properties and become different substances called products with a different set of chemical properties. Look at the example below: H2 + O2 H20 Reactants product In this example, an explosive gas, Hydrogen, combines with a gas that supports combustion, Oxygen, to make a substance that puts out fires, water. Lesson Checkpoint: What is a reactant? Energy in Reactions There are two types of evidence to show that a chemical reaction occurs. • The first is the simple fact that the products of the reaction have different chemical properties than the reactants. The fact that Hydrogen and Oxygen combine to make water is a clear example of this. • The second type of evidence has to do with energy. In some cases energy has to be absorbed in order for the reaction to occur and in other reactions, energy is released. When energy is absorbed, it is called an endothermic reaction. • Reactions where energy is released are called exothermic reactions. An example of an exothermic reaction would be a rocket taking off. The burning of fuel provides the heat which causes the rocket to lift off. Lesson Checkpoint: What is the difference between an endothermic and an exothermic chemical reaction? © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
Types of Chemical Reactions There are four types of chemical reactions: • In a synthesis reaction, two elements combine to make a compound. See the example below. • In a decomposition reaction, a compound is broken down into simpler substances. In the example above, the reactant, hydrogen peroxide, is broken down into water and oxygen. • In a single replacement reaction, an element takes the place of another element in a compound. See the example below. In this example, the element Fluorine has taken the place of Chlorine in the original compound • In a double replacement reaction, elements in one compound exchange positions with elements in the other compound. See the example below. In this example Sodium(Na) has taken the place of H and H has taken the place of Na. Lesson Checkpoint: In which type of chemical reaction does one element take the place of another element? © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
Mass Remains Constant According to the law of conservation of mass, mass can not either be destroyed or created in a chemical reaction. This means that the total mass of each element does not change during the reaction. See the example below. H2 + 02----H20 If you count the atoms of each element, you will notice that there are more oxygen atoms on the left side than on the right side. To correctly write this reaction we need to use a coefficient or number placed in front of the chemical formula. The reaction below has been balanced with the use of coefficients. In this balanced reaction, there are four atoms of Hydrogen on each side of side of the reaction and two atoms of Oxygen on each side of the reaction. All chemical reactions need to be balanced to properly represent how the reaction actually occurs. Lesson Checkpoint: What is the Law of Conservation of Mass? Affecting Reaction Rate Some reactions require an initial burst of energy to get the reaction going. This is called activation energy. The outcome of a reaction using activation energy could still be endothermic or exothermic. Once a chemical reaction gets going there are several factors that control the rate at which the reaction occurs. • Increasing the surface area of the reactants or increasing the temperature usually speeds up a reaction. • Another way to speed up a reaction is by increasing the concentration or amount of reactants. • Still another way to move the reaction along is to use a catalyst or enzyme. These are substances added to the reaction just to speed it up but they are not involved like the reactants. An enzyme brings reactants into contact to speed up the reaction. The enzyme does not change. Lesson Checkpoint: Name two steps you can take to speed up a chemical reaction. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
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