8.0 Chemical Reactions
What are the reactants and products in a chemical reaction?
The reactants are the substances that start the chemical reaction. The products are the substances that are produced in the chemical reaction.
Tina made a "volcano" by pouring vinegar over a "mountain" of baking soda. The wet baking soda bubbled and foamed. Did a chemical reaction occur? How do you know?
Yes; a chemical reaction occurred. You know because bubbles of gas were produced and the production of a gas is a sign that a chemical reaction has occurred.
Apply lesson concepts to classify the following chemical reactions: (a) Zn + 2HCl → H2 + ZnCl2
single replacement reaction] (b) 2KClO3 → 2KCl + 3O2 • [decomposition reaction] (c) 2KI + Cl2 → 2KCl + I2 • [single replacement reaction] (d) AgNO3 + KCl → AgCl + KNO3 • [double replacement reaction]
Explain why it is necessary to balance chemical equations.
It is necessary to balance chemical equations because matter cannot be created or destroyed in a chemical reaction. Therefore, in every chemical reaction, the same number of each type of atom must appear in the products as started in the reactants.
Balance this chemical equation: Hg + O2 → HgO.
The balanced equation is 2Hg + O2 → 2HgO.
What is a chemical equation? Give an example.
A chemical equation is a symbolic representation of a chemical reaction. Examples may vary. Sample answer: C + O2 → CO2
Define chemical reaction
A chemical reaction is a process in which some substances change into different substances.
What is a coefficient? How are coefficients used in chemistry?
A coefficient is a number placed in front of a chemical symbol or chemical formula. It shows how many atoms or molecules of the substance are involved in a reaction. Coefficients are used in chemistry to balance chemical equations.
Compare and contrast the four types of reactions described in this lesson. Include an example of each type of reaction.
A synthesis reaction is one in which two or more reactants combine to form a single product. It can be represented by A + B → C. An example of a synthesis reaction is the combination of sodium and chlorine to produce sodium chloride. A decomposition reaction is one in which one reactant breaks down into two or more products. This is the reverse of a synthesis reaction. It can be represented by AB → A + B. An example of a decomposition reaction is the breakdown of water to hydrogen and oxygen. A replacement reaction is one in which ions change places in one compound (single replacement) or two compounds (double replacement). A single replacement reaction can be represented by A + BC → B + AC. A double replacement reaction can be represented by AB + CD → AD + CB. An example of a (single) replacement reaction is potassium reacting with water to form potassium hydroxide and hydrogen. Combustion reaction is one in which a substance, usually referred to as a fuel, reacts quickly with oxygen to form carbon dioxide and water. Energy is also released. A combustion reaction is commonly called burning. It can be represented by fuel + O2 → CO2 + H2O. An example is the burning of methane.
Define activation energy.
Activation energy is the energy needed to start a chemical reaction.
Compare and contrast endothermic and exothermic chemical reactions.
An endothermic reaction is one that absorbs energy. In an endothermic reaction, it takes more energy to break bonds in the reactants than is released when new bonds form in the products. As a result, the reaction needs a constant input of energy to keep going. The general equation for an endothermic reaction is Reactants + Energy → Products. An exothermic reaction is one that releases energy. In an exothermic reaction, it takes less energy to break bonds in the reactants than is released when new bonds form in the products. As a result, the reaction releases enough energy for the reaction to keep going without a constant input of energy. The general equation for an exothermic reaction is Reactants → Products + Energy.
Describe how Antoine Lavoisier showed that matter is conserved in chemical reactions.
Antoine Lavoisier carefully measured the mass of reactants and products in many different chemical reactions. He carried out the reactions inside a sealed jar. As a result, any gases involved in the reactions were captured and measured. 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.
What form of energy is needed for the endothermic reaction called photosynthesis?
Energy in the form of light is needed for photosynthesis.
Explain the meaning of the term "equilibrium" as it applies to a chemical reaction. How can you tell when a chemical reaction has reached equilibrium?
Equilibrium is a balance between opposing changes. In a chemical reaction, equilibrium is the point at which the forward and reverse reactions occur at the same rate. You can tell when a chemical reaction has reached equilibrium because there is no further change in the amounts of reactants and products.
What evidence shows that combustion reactions are exothermic?
Evidence that combustion reactions are exothermic is the heat and light energy they produce.
List four common signs that a chemical reaction has occurred.
Four common signs that a chemical reaction has occurred are a change in color, a change in temperature, the production of a gas, and the formation of a precipitate.
List four factors that affect the rates of chemical reactions.
Four factors that affect the rates of chemical reactions are temperature, concentration, and surface area of reactants; and presence of catalysts
Suppose you put a whole antacid tablet in one glass of water and a crushed antacid tablet in another glass containing the same amount of water at the same temperature. Both tablets would start reacting and producing bubbles of gas. Use lesson concepts to predict which tablet would stop producing bubbles first. Explain your prediction. Then, with the permission of an adult, do the activity. Do your results agree with your prediction?
Sample answer: I predict that the crushed tablet would stop producing bubbles first. The crushed tablet has more surface area, so it should react more quickly. If students do the activity, they should get this result
Draw a sketch that shows how atoms are rearranged in the chemical reaction represented by equation
Sketches may vary but should reflect an understanding how atoms are rearranged in the reaction 2H2 + O2 → 2H2O. Sketches should show that bonds break between hydrogen atoms and between oxygen atoms in the reactants, and that bonds form between hydrogen and oxygen atoms in the products.
sketch a simple graph to show how energy changes in an exothermic reaction. Include activation energy in your graph.
Students' graphs may vary but should reflect an understanding of how energy changes in an exothermic reaction. They should include an initial burst of energy labeled "activation energy." They should also show that there is a net release of energy, with products having less energy than reactants. The graphs should resemble the exothermic reaction graph in the FlexBook® lesson.
Describe what happens to the atoms involved in a chemical reaction.
The atoms involved in a chemical reaction are rearranged during the reaction. As a result, the atoms are in different combinations in the products than they were in the reactants.
What happens to the energy that is absorbed in an endothermic reaction?
The energy that is absorbed in an endothermic reaction is stored as chemical energy in the bonds of the products.
Write an equation for the chemical reaction in which hydrogen reacts with oxygen to form water. What type of reaction is this?
The equation for the reaction in which hydrogen reacts with oxygen to form water is 2H2 + O2 → 2H2O. This is a synthesis reaction.
In the general equation in question 3, what do the individual letters represent?
The individual letters represent ions.
Write an equation for the reverse of the reaction in question 1. What type of reaction is this?
The reverse of the reaction in question 1 is 2H2O → 2H2 + O2. This is a decomposition reaction
Name the type of reaction represented by this general equation: AB + CD → AD + CB
The type of reaction represented by equation is a double replacement reaction.] In the general equation in question 3, what do the individual letters represent? • [The individual letters represent ions.
equilibrium
balance between opposing changes, such as the forward and reverse directions of a chemical reaction
combustion reaction
chemical reaction in which a substance reacts quickly with oxygen, producing carbon dioxide, water, and energy; commonly called burning
replacement reaction
chemical reaction in which ions switch places in one compound (single replacement) or in two compounds (double replacement)
synthesis reaction
chemical reaction in which two or more reactants combine to form a single product
endothermic reaction
chemical reaction that needs a constant input of energy to continue because it takes more energy to break bonds in the reactants than is released when new bonds form in the products
exothermic reaction
chemical reaction that releases energy because it takes less energy to break bonds in the reactants than is released when new bonds form in the products
activation energy
energy needed to start a chemical reaction
In an exothermic reaction, which has more stored chemical energy: the reactants or the products?
in an exothermic reaction, the reactants have more stored chemical energy than the products
law of conservation of energy
law stating that matter cannot be created or destroyed in chemical reactions • reaction rate: speed at which a chemical reaction occurs
What are the reactants and products in a combustion reaction?
n a combustion reaction, the products are some type of fuel, such as methane or glucose, and oxygen. The products are carbon dioxide and water. Energy is also released.
concentration
number of particles of a substance in a given volume
chemical reaction
process in which some substances, called reactants, change chemically into different substances, called products
decomposition reaction chemical
reaction in which one reactant breaks down into two or more products
product
substance produced in a chemical reaction
catalyst
substance that increases the rate of a chemical reaction but is not changed or used up in the reaction
reactant
substance that starts a chemical reaction
chemical equation
symbolic representation of a chemical reaction