Science

Study Guide Unit 5 - Chemical Reactions


Physical Properties and examples

  • Can be observed or measured without changing the composition of the matter

Ex:

  1. States of matter(solid, liquid, gas)

  2. Appearance (size, shape, color)

  3. Texture

  4. Smell

  5. Density

  6. boiling point, melting point, freezing point

Chemical Properties and examples

  • Relates to a substance’s ability to undergo changes that transform it into a different substance

Ex:

  1. Rustable

  2. Flammability

  3. Reactivity

  4. Toxicity 

Chemical Reactions

  • The process by which substances interact to form new substances with different compositions 

Chemical changes and examples

  • A change in matter that produces one or more new substances

  • Chances on a molecular level

Ex:

  1. Cooking an egg

  2. A nail rusting

  3. Burning wood

  4. fireworks

Signs of a chemical change and examples

  • Smoke or fire

  1. A chemical reaction between substances, usually including oxygen accompanied by giving off heat or light

  2. This is a permanent change that cannot be undone

  3. Ex:

  4. Wood burning

  5. Candle burning

  6. Burning fossil fuels

  • Bubbles or fizzing  

  1. When gasses produced in a chemical reaction are released 

  2. Ex:

  3. Baking soda and vinegar

  • Unexpected temperature change

    If a substance gets warmer or cooler

  • When the temperature increases it is a sign that energy was released during a chemical reaction

  • When the temperature decreases, it is a sign that energy was absorbed during a chemical reaction 

  • Unexpected color change

  1. New molecules created in a chemical reaction radiate light differently, producing new colors

  2. Ex:

  3. Fireworks

  4. Leaves changing color

  • Rusting

  1. Iron reacts with water and oxygen to form rust (iron oxide Fe2 O3)

  2. A chemical reaction that causes rust is called oxidation

  • Precipitate is formed

  1. When a solid suddenly appears in a solution and settlers to the bottom, collects on another object, or makes the solution cloudy

  2. Ex:

  3. Milk souring

  4. Bathtub rings

  • New substance is formed 

  • Change in odor

  1. When two or more compounds or elements are mixed and a scent or odor is present, a chemical reaction has taken place

  2. Food burns or rots - a rotten egg

Chemical equations and chemical formulas (recognize what they are)

  • yup

Reactants and products

  • In a chemical reactions, you start with reactants that are combined to make products 

  • The reactants are the starting substances

  • The products are the new substances which result from the chemical reaction

Reaction symbols (what do they mean)

  • The small symbols in the parentheses (s, l, g, aq) next to each chemical formula indicates the phase of each substance in the reaction

Subscripts and coefficients (recognize and know how to use them)

  • Subscript:

  1. Tells you the amount of atoms of that element in each molecule

  2. If an atom has no subscript that means there is 1 atom

  • Coefficient:

  1. Tells you how many molecules of that substance there are

The Law of Conservation of Mass

  • Antoine Lavoisier

  1. French chemist

  2. Studies the nature of combustion

  3. Found out that combustion involved a reaction with oxygen

  4. Combined the mass of the burning wood and oxygen is converted into carbon dioxide and water

  5. He showed that a closed system must be used when studying chemical reactions

  6. Mass is neither created nor destroyed during chemical reactions or physical reactions

Why is it important to use a closed system when studying chemical reactions?

  • When chemicals are reacted in a closed container, you can show that the mass before and after the reaction is the same

How to Balance equations

  1. Count the number of of each element to see if it is balanced or not

  2. Add coefficients to balance our equation

Types of chemical reactions and be able to recognize them. 

  • Synthesis reactions

  1. Two or more substances combine to form a new compound

  2. A + B -> AB

  • Decomposition reactions

  1. A chemical reaction in which a single compound is broken down into two or more smaller compounds

  2. AB -> A + B

  • Single displacement reaction

  1. Also called single replacement

  2. When one element replaces a similar element in a compound 

  3. AX + B -> BX + A

  • Double displacement reactions

  1. Ions form two compounds in a solution exchange places to produce two new compounds

  2. One of the compounds formed is usually:

  3. A precipitate that settles out of the solution

  4. A gas that bubbles out of the solution 

  5. Or a molecular compound like water

  • Combustion reaction

  1. The reaction of an element or compound (usually a hydrocarbon) with oxygen to form CO2, water and heat

Endothermic vs. Exothermic Reaction and examples

  • Chemical energy

  1. Energy stored in the chemical bonds of a substance

  2. Chemical reactions always involve energy changes

  3. Making and breaking bonds involve energy changes

  • Activation energy

  1. The energy required to break the bonds in the reactants for a chemical reaction to occur. 

  2. Energy needed to get a reaction started

  3. We must supply energy to break bonds

  • Energy of chemical reactions

  1. Energy must be supplied or absorbed to break chemical bonds of reactants

  2. Energy is released when new chemical bonds are made in the products

  • Endothermic reactions

  1. A reaction is endothermic if more energy is absorbed than released

  2. Energy is absorbed

  3. Endo = into

Ex:

  1. Cooking pancakes

  2. Ice melting/ice pack

  3. Evaporation 

  4. photosynthesis

  • Signs of endothermic reactions

  1. Heat (energy) is absorbed

  2. Temperature drops

  3. Surroundings feel cold

  • Exothermic reactions

  1. A reaction is exothermic if more energy is released then supplied

  2. Energy is released

  3. Exo = exit

  4. Heat (energy) is given off

  5. Temperature of the surroundings rises

  6. Surroundings feel hot

            Ex:

  1. Burning of gasoline

  2. Yeast and hydrogen peroxide reaction

  3. Handwarmer

  4. Condensation 

  5. Combustion of fuels

  • You may see an endothermic reaction written like this:

  1. Reactants + energy -> products 

Or

  1. Reactants + heat -> products 

  • You may see exothermic reactions written like this:

  1. Reactants -> products + energy

  2. Reactants -> products + heat

  • Based on the type of energy (heat) change involved, chemical reactions are either exothermic or endothermic