Presentation on theme: "Chapter 21 Chemical reactions are taking place all around you and even within you. A chemical reaction is a change in which one or more substances are."— Presentation transcript:
Chemical reactions are taking place all around you and even within you. A chemical reaction is a change in which one or more substances are converted into new substances. Describing Chemical Reactions A chemical reaction occurs when you bake a cake.
Change that alters the form of a substance but not the composition Tearing of paper Chopping wood Melting ice Change of state Dissolving
Result of a chemical reaction Breaking of bonds and formation of new bonds Atoms are rearranged Energy is released or absorbed
Combustion – substance and oxygen are combined and release heat or light Metabolism – chemical change that releases energy through digestion Fermentation – break down of carbohydrates by living things in the absence of oxygen Corrosion- Rusting Electrolysis
Rapid release of energy– heat, light, sound Production of a gas– bubbles Forming a precipitate– a solid that doesn’t dissolve in liquid Substance getting colder Examples: Fire, antacid tablet in water, rusting, silver tarnish
More energy is produced than it takes to break the bonds of the reactants Energy is released from the reaction Gives off heat and/or light Fire MRE’s – wrapped in special sleeve with Mg – addition of water starts reaction that heats food Takes more energy to break bonds in the reactants than is released by forming products Energy has to be put in for the reaction to occur Gets colder Instant ice pack – water and ammonium nitrate Rusting iron
Conservation of Mass For example, the mass of the candles and oxygen before burning is exactly equal to the mass of the remaining candle and gaseous products. The French chemist Antoine Lavoisier established that the total mass of the products always equals the total mass of the reactants.
One of the questions that motivated Lavoisier was the mystery of exactly what happened when substances changed form. Lavoisier's Contribution He began to answer this question by experimenting with mercury.
Lavoisier placed a carefully measured mass of solid mercury (II) oxide, which he knew as mercury calx, into a sealed container. Lavoisier's Contribution When he heated this container, he noted a dramatic change. The red powder had been transformed into a silvery liquid that he recognized as mercury metal, and a gas was produced.
When he determined the mass of the liquid mercury and gas, their combined masses were exactly the same as the mass of the red powder he had started with. Lavoisier's Contribution Lavoisier also established that the gas produced by heating mercury(II) oxide, which we call oxygen, was a component of air.
the mass of the reactants will always equal the mass of the products in a closed system(you have to “catch all of the products” 2H 2 + O 2 = 2H 2 O H – 1.01 amu O – 16 amu ReactantsProducts 4H – 4.042H 2 O – 36.04 2O – 32 amu Total – 36.04 amu
the number of atoms in the reactants will always equal the number of atoms in products in a closed system 2H 2 + O 2 = 2H 2 O ReactantsProducts 4H4H 2O2O
Scientists have developed a shorthand method to describe chemical reactions. Writing Equations A chemical equation is a way to describe a chemical reaction using chemical formulas and other symbols. “Recipe” for chemical reactions Bonds are breaking and reforming CH 4 + O 2 → CO 2 + H 2 O + heat + light Methane and oxygen yield carbon dioxide and water One or more substance (reactants) change into one or more new substances (products)
Reactants to the left of the arrow Products to the right of the arrow Arrow is yields Reactants yield the products Some of the symbols used in chemical equations are listed in the table. Example: (aq) stands for aqueous, which means dissolved in water Writing Equations
Balanced Equations Lavoisier's mercury(II) oxide reaction can be written as: Notice that the number of mercury atoms is the same on both sides of the equation but that the number of oxygen atoms is not the same.
Start with a skeleton equation like Lavoisier’s Each side of the equation must have the same number of atoms of each element We use coefficients to balance equations – (whole number in front of the chemical formula) H 2 + O2 → H 2 O 2H 2 + O2 → 2H 2 O
Two things you CANNOT do when balancing an equation You cannot change a subscript You can only write the coefficient in front of a chemical formula
Limiting Reactant – the reactant that is used up first limits the amount of product Excess Reactant – the reactant that is not completely used up because there is extra If I have 1 cup flour and 2 eggs but only need 1 egg for my recipe – the flour is the limiting reactant and the egg is the excess reactant
Catalysts speed up reactions. They do not change what goes in or comes out, but they make the reactions happen faster- example- enzymes Catalysts are often affected by other factors such as pH and temperature Inhibitors slow down reactions– such as food preservatives that help keep food from spoiling too quickly
A chemical reaction in which something is burned Lavoisier was one of the first to observe this type of reactions A combustion reaction occurs when a substance reacts with oxygen to produce heat and/or light C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 0 Products are always the same
Also called addition Two or more substances combine to form another substance General Formula: A + B AB Example: 2H 2 + O 2 2H 2 0
A substance breaks down, or decomposes into its components General Formula: AB A + B Example: 2H 2 0 + 2H 2 + O 2
Also called single replacement One atom displaces, or replaces another to form a new substance General Formula: A + BC AC + B Example: Cu (s) + 2AgNO 3 (aq) Cu (NO 3 ) 2 + 2Ag (s)
Also called double replacement The positive ion of two compounds switch with each other, making two entirely new compounds General Formula: AB + CD AD + CB Example: Ba (NO 3 ) 2 (aq) + K 2 SO 4 (aq) BaSO 4 (s) + 2KNO 3 (aq)