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Unit 7: Properties of Matter Lesson 4: Chemical Reactions Signs of a Chemical Reaction • What is a chemical reaction? Signs of a Chemical Reaction • What is a chemical reaction? – The process in which atoms are rearranged and chemical bonds are broken and formed to produce a chemical change of a substance • What are the signs that a chemical reaction is occuring? Signs of a Chemical Reaction • What is a chemical reaction? – The process in which atoms are rearranged and chemical bonds are broken and formed to produce a chemical change of a substance • What are the signs that a chemical reaction is occuring? – – – – Formation of a precipitate Color change Change in odor Appearance of gas bubbles Modeling Chemical Reactions • What is a chemical formula? Modeling Chemical Reactions • What is a chemical formula? – a combination of chemical symbols and numbers to represent a substance Modeling Chemical Reactions • The chemical symbols in a formula tell what types of elements make up a substance • The ________________ in a chemical formula tell how many of each type of atom are in a molecule Modeling Chemical Reactions • The chemical symbols in a formula tell what types of elements make up a substance • The ________________ in a chemical formula tell how many of each type of atom are in a molecule • Example: The chemical formula for rust, or iron oxide, is Fe2O3. How many iron atoms and how many oxygen atoms are in one unit of iron oxide? Modeling Chemical Reactions • What are reactants and what are products in terms of a chemical reaction? Modeling Chemical Reactions • What are reactants and what are products in terms of a chemical reaction? – Reactatnt: a substance that participates in a chemical reaction – Product: a substance that forms in a chemical reaction Modeling Chemical Reactions • In the chemical formula C + O2 CO2 What are the reactants and what is the product? Modeling Chemical Reactions • In the chemical formula C + O2 CO2 What are the reactants and what is the product? – Carbon and oxygen are the reactants, and carbon dioxide is the product. One carbon atom and two oxygen atoms combine to form one carbon dioxide molecule. Balancing Equations • Because of the law of conservation of mass, chemical equations must show the same numbers and kinds of atoms on both sides of an arrow. • To balance an equation, ________________ are placed in front of a chemical formula. Only ____________, not subscripts, can be changed when balancing a chemical equation. Balancing Equations • Examples: _____ Mg + ____ O2 _____ K + _____ O2 _____ H2O2 _____ MgO _____ K2O ______ H2O + ______ O2 Energy can be Absorbed • What is an endothermic reaction? Energy can be Absorbed • What is an endothermic reaction? – A chemical reaction that requires the input of energy – The energy taken in from the surroundings is absorbed from the surroundings as ____________ – Therefore, endothermic reactions often feel ____________ – An example of an endothermic reaction is __________________ Energy Can be Released • What is an exothermic reaction? Energy Can be Released • What is an exothermic reaction? – A chemical reaction in which energy is released to the surroundings – Exothermic reactions often feel __________ because energy is released as heat – An example of an exothermic reaction is _______________ Energy is Always Conserved • What does the law of conservation of energy state? Energy is Always Conserved • What does the law of conservation of energy state? – Energy cannot be created or destroyed. However, energy can change form. What affects the rates of reactions? • Reactions can occur in less than a second or over a matter of days. • Reaction rates are affected by how often particles _______________. • The factors that affect reaction rates are _________________ , ______________ _____________, _____________, and the presence of a ________________. Concentration • Higher concentration = _____________ reaction rate • Why? Concentration • Higher concentration = _____________ reaction rate • Why? – There are more reactants in a given volume, therefore they are more likely to collide and react Surface Area • Higher surface area = ______________ reaction rate • Why? Surface Area • Higher surface area = ______________ reaction rate • Why? – More reactant particles are exposed to one another, so more collisions occur Temperature • Higher Temperature= _________________ reaction rate • Why? Temperature • Higher Temperature= _________________ reaction rate • Why? – At higher temperatures, particles move more quickly, and are more likely to collide and react Catalyst • A catalyst is a substance that changes the rate of a chemical reaction without being used up or changed very much. • Catalysts usually ____________ reaction rate by bringing together reactants • _____________ are an example of a catalyst found in living things