Chemical Changes and Conservation of Matter

What happens during a chemical change?

Chemical change means Molecules change

• Molecules are made of elements.• Example: Water (H2O) is made of

Hydrogen (H) and Oxygen (O).• During a chemical change the elements of

at least one molecule are rearranged to make a molecule that was not present before the change.

• During a physical change, the molecules stay the same afterwards.

Specific Chemical Changes

• Synthesis►reactant + reactant product

• Decomposition►reactant product + product

• Single Replacement►reactant + reactant product + product

• Double Replacement►reactant + reactant product + product

• Combustion►fuel + oxygen water + carbon dioxide

Synthesis

• Happens when 2 molecules combine to form one molecule

• Example: Rust (Oxidization)

• Iron + Oxygen + Water Iron Oxide (Iron oxide is the brown stuff we call rust.)

+ +

Decomposition

• Happens when a molecule breaks apart to form two or more molecules

• Example: Carbonic Acid (pop fizz)

• Carbonic Acid Water + Carbon Dioxide

+

Single Replacement

• Happens when an element is exchanged from one molecule to another

• Example: Hydrogen and Magnesium

• Magnesium + Hydrochloric Acid Magnesium Chloride + Hydrogen

+

+

Double Replacement

• Happens when two elements from a pair of molecules switch places

• Example: Neutralization (Acid + Base)

• Sodium Hydroxide + Hydrochloric Acid Water + Sodium Chloride (Salt)

+ +

Combustion

• Happens when a hydrocarbon (a molecule made from hydrogen and carbon) is burned in the presence of oxygen

• Example: Burning gas

• Gas + Oxygen + Flame Water + Carbon Dioxide + Heat

+ + +

+

Quiz! Name the type of Reaction

1.When wood is burned, it combines with oxygen to make water and carbon dioxide.

2.When electricity is passed through water, it releases hydrogen and oxygen.

3.When iron and sulfur are combined it makes iron (II) sulfide.

4.When zinc is combined with hydrochloric acid, it makes zinc chloride and hydrogen.

5.When silver nitrate and sodium chloride are mixed, silver chloride and sodium nitrate are produced.

Exothermic vs. Endothermic

• If a reaction either absorbs heat or gives off heat, that is an indication that a chemical change has happened.

• “Thermic” means heat (think “thermos”).• “Exo” means it gives off heat (think “exit”).• “Endo” means it absorbs heat (think

“enter”).• Examples: Combustion is exothermic and

Photosynthesis is endothermic.

Conservation of Matter

• In any of the examples seen so far, did an element ever go missing or disappear?

• IDEA: In any chemical change, the same elements present beforehand are present after the reaction. That way, matter is conserved.

• The total mass of the reactants will always equal the total mass of the products.

Quiz 2! Find the Missing Masses

1. A solid has a mass of 35 g. When it is mixed with a solution, a chemical reaction occurs. If the final total mass of products is 85 g, what was the mass of the solution?

2. Solution A has a mass of 60 g. Solution B has a mass of 40 g. When they are mixed, a chemical reaction occurs in which gas is produced. If the mass of the final mixture is 85 g, what mass of gas was produced?

Overview

• Chemical changes involve rearranging elements to form new molecules.

• We looked at 5 types: synthesis, decomposition, single replacement, double replacement and combustion.

• Many chemical changes either give off heat (exothermic) or absorb heat (endothermic).

• In all chemical changes, mass is conserved. This means the total mass of the reactants is equal to the total mass of the products.

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