When a substance undergoes a chemical change, it takes part in a chemical reaction

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Chapter 8 Notes. Recognizing Chemical Reactions. When a substance undergoes a chemical change, it takes part in a chemical reaction. After it reacts, it no longer has the same chemical identity. Recognizing Chemical Reactions. - PowerPoint PPT Presentation

Transcript of When a substance undergoes a chemical change, it takes part in a chemical reaction

  • When a substance undergoes a chemical change, it takes part in a chemical reaction.Recognizing Chemical ReactionsAfter it reacts, it no longer has the same chemical identity.Chapter 8 Notes

  • Recognizing Chemical ReactionsWhile it may seem amazing that a substance can undergo a change and become part of a different substance, chemical reactions occur around you all the time.Many important clues indicate when chemical reactions occur.None of them alone proves that such a change occurs because some physical changes involve one or more of these signs.

  • Writing Chemical EquationsIn order to completely understand a chemical reaction, you must be able to describe any changes that take place.Part of that description involves recognizing what substances react and what substances form.

  • Writing Chemical EquationsA substance that undergoes a reaction is called a reactant.When reactants undergo a chemical change, each new substance formed is called a product.

  • Writing Chemical EquationsFor example, a familiar chemical reaction involves the reaction between iron and oxygen (the reactants) that produces rust, which is iron(III) oxide (the product).The simplest reactions involve a single reactant or a single product, but some reactions involve many reactants and many products.

  • Word EquationsThe simplest way to represent a reaction is by using words to describe all the reactants and products, with an arrow placed between them to represent change.Reactants are placed to the left of the arrow, and products are placed to the right.Plus signs are used to separate reactants and also to separate products.

  • Word EquationsVinegar and baking soda are common names.The compound in vinegar that is involved in the reaction is acetic acid, and baking soda is sodium hydrogen carbonate.These scientific names can also be used in a word equation.

  • Chemical EquationsWord equations describe reactants and products, but they are long and awkward and do not adequately identify the substances involved.Word equations can be converted into chemical equations by substituting chemical formulas for the names of compounds and elements.

  • Chemical EquationsThe equation for the reaction of vinegar and baking soda can be written using the chemical formulas of the reactants and products.By examining a chemical equation, you can determine exactly what elements make up the substances that react and form.

  • Chemical EquationsIt may also be important to know the physical state of each reactant and product.How can we indicate the bubbles we see during this reaction are CO2?Symbols in the parentheses are put after formulas to indicate the state of the substance.Solids, liquids, gases, and water (aqueous) solutions are indicated by the symbols (s), (l), (g), and (aq).

  • Chemical EquationsThe following equation shows these symbols added to the equation for the reaction of vinegar and baking soda.

  • Chemical EquationsNow the equation tells us that mixing an aqueous solution of acetic acid (vinegar) with solid sodium hydrogen carbonate (baking soda) results in the formation of an aqueous solution of sodium acetate, liquid water, and carbon dioxide gas.

  • Energy and Chemical EquationsNoticeable amounts of energy are often released or absorbed during a chemical reaction.Some reactions absorb energy. If energy is absorbed, the reaction is known as an endothermic reaction.For a reaction that absorbs energy, the word energy is sometimes written along with the reactants in the chemical equation.

  • Energy and Chemical EquationsFor example, the equation for the reaction in which water breaks down into hydrogen and oxygen shows that energy must be added to the reaction.Reactions that release heat energy are called exothermic reactions.

  • Energy and Chemical EquationsWhen writing a chemical equation for a reaction that produces energy, the word energy is sometimes written along with the products.Some of this energy is in the form of light.

  • Energy and Chemical EquationsYou may have also noticed that the word energy is not always written in the equation.It is used only if it is important to know whether energy is released or absorbed.

  • Balancing Chemical EquationsThe mass of the products is always the same as the mass of the reactants that react to form them.The law of conservation of mass summarizes these findings. Matter is neither created nor destroyed during a chemical reaction.

  • Balancing Chemical EquationsRemember that atoms dont change in a chemical reaction; they just rearrange.The number and kinds of atoms present in the reactants of a chemical reaction are the same as those present in the products.When stated this way, it becomes the law of conservation of atoms.

  • Balancing Chemical EquationsFor a chemical equation to accurately represent a reaction, the same number of each kind of atom must be on the left side of the arrow as are on the right side.If an equation follows the law of conservation of atoms, it is said to be balanced.

  • Balancing Chemical EquationsThe easiest way to count atoms is to practicefirst with a simple reaction and then with some that are more complex.For example, consider the equation that represents breaking down carbonic acid into water and carbon dioxide.

  • Balancing Chemical EquationsBecause a subscript after the symbol for an element represents how many atoms of that element are found in a compound, you can see that there are two hydrogen, one carbon, and three oxygen. All of the atoms in the reactants are the same as those found in the products.

  • Balancing Chemical EquationsExamine the equation for the formation of sodium carbonate and water from the reaction between sodium hydroxide and carbon dioxide.

  • Balancing Chemical EquationsOne carbon atom is on each side of the arrow, but the sodium, oxygen, and hydrogen atoms are not balanced.The equation, as written, does not truly represent the reaction because it does not show conservation of atoms.

  • Balancing an EquationTo indicate more than one unit taking part or being formed in a reaction, a number called a coefficient is placed in front of it to indicate how many units are involved.Look at the previous equation with a coefficient of 2 in front of the sodium hydroxide formula.

  • Balancing an EquationIs the equation balanced now? You should be able to find four on each side. How about hydrogen atoms?Two sodium atoms are on each side. How many oxygen atoms are on each side?Now two are on each side.Because one carbon atom is still on each side, the entire equation is balanced; it now represents what happens when sodium hydroxide and carbon dioxide react.

  • Balancing an EquationThe balanced equation tells us that when sodium hydroxide and carbon dioxide react, two units of sodium hydroxide react with each molecule of carbon dioxide to form one unit of sodium carbonate and one molecule of water.

  • Major Classes of ReactionsIn one type of reaction, two substanceseither elements or compoundscombine to form a compound.Whenever two or more substances combine to form a single product, the reaction is called a synthesis reaction.If you can classify a reaction into one of five major categories by recognizing patterns that occur, you already know a lot about the reaction.

  • A Synthesis ReactionWhen iron rusts, iron metal and oxygen gas combine to form one new substance, iron(III) oxide.The balanced equation for this synthesis reaction shows that there is more than one reactant but only one product.

  • A Synthesis Reaction

  • Major Classes of ReactionsIn a decomposition reaction, a compound breaks down into two or more simpler substances.The compound may break down into individual elements, such as when mercury(II) oxide decomposes into mercury and oxygen.

  • Major Classes of ReactionsThe products may be an element and a compound, such as when hydrogen peroxide decomposes into water and oxygen.The compound may break down into simpler compounds.

  • A Decomposition ReactionWhen ammonium nitrate is heated to a high temperature, it explosively breaks down into dinitrogen monoxide and water.The decomposition reaction taking place is represented by a balanced equation that shows one reactant and more than one product.

  • A Decomposition Reaction

  • Major Classes of ReactionsIn a single-displacement reaction, one element takes the place of another in a compound.The element can replace the first part of a compound, or it can replace the last part of a compound.

  • Single DisplacementIf an iron nail is placed into an aqueous solution of copper(II) sulfate, the iron displaces the copper ions in solution, and copper metal forms on the nail.

  • Single Displacement

  • Major Classes of ReactionsIn double-displacement reactions, the positive portions of two ionic compounds are interchanged.For a double-displacement reaction to take place, at least one of the products must be a precipitate or water.

  • Double DisplacementWhen clear aqueous solutions of lead(II) nitrate and potassium iodine are mixed, a double-displacement reaction takes place and a yellow solid appears in the mixture.This solid is lead(II) iodine, and it precipitates out because it is insoluble in water, unlike the two reactants and the other product.

  • Double Displacement

  • Major Classes of ReactionsA combustion reaction is one in which a substance rapidly combines with oxygen to form one or more oxides.

  • CombustionWhen welding is done with an acetylene torch, acetylene combines with oxygen to form carbon dioxide and water