Science 9: Unit B – Matter Science 9: Unit B – Matter and Chemical Changeand Chemical Change

Topic 7: Writing Chemical Topic 7: Writing Chemical EquationsEquations

BackgroundBackground All compounds are created, changed, or All compounds are created, changed, or

broken apart through chemical reactions. broken apart through chemical reactions. Signs that a chemical change have Signs that a chemical change have

occurred include a color change, occurred include a color change, bubbling, a PRECIPITATE (solid) forming bubbling, a PRECIPITATE (solid) forming in a liquid, and heat and light being in a liquid, and heat and light being produced or absorbed.produced or absorbed.

In a chemical reaction, the In a chemical reaction, the BONDSBONDS between atoms are broken and new between atoms are broken and new bonds are formed so that atoms are bonds are formed so that atoms are rearranged to form new compounds.rearranged to form new compounds.

Eg. hydrogen + oxygen = waterEg. hydrogen + oxygen = water 2H2 + O2 → 2H2O2H2 + O2 → 2H2O or: H-H + H-H + O-O = H-O-H + H-O-Hor: H-H + H-H + O-O = H-O-H + H-O-H

Reactants and ProductsReactants and Products In chemical reactions, the chemicals that In chemical reactions, the chemicals that

undergo the reaction are called the undergo the reaction are called the reactantsreactants. These are your original . These are your original materials. In the above example hydrogen materials. In the above example hydrogen and oxygen are your reactants.and oxygen are your reactants.

The chemicals that come out of a chemical The chemicals that come out of a chemical reaction are called the reaction are called the productsproducts. In the . In the above example, two water molecules are above example, two water molecules are the product.the product.

Note that when we write down a chemical Note that when we write down a chemical reaction, we are writing down the lowest reaction, we are writing down the lowest RATIO RATIO of molecules that will react together. of molecules that will react together. In reality, billions of molecules are reacting In reality, billions of molecules are reacting with each other.with each other.

Signs of a Chemical ReactionSigns of a Chemical Reaction

Products are new chemicals with Products are new chemicals with different chemical properties than the different chemical properties than the reactants that make them. Water is a reactants that make them. Water is a liquid at room temperature; hydrogen liquid at room temperature; hydrogen and oxygen are gases at room and oxygen are gases at room temperature.temperature.

There are signs that indicate a chemical There are signs that indicate a chemical reaction has taken place: color change, reaction has taken place: color change, bubbles forming, heat absorbed or bubbles forming, heat absorbed or given off, and a new material formed all given off, and a new material formed all point towards a chemical reaction.point towards a chemical reaction.

Writing Chemical ReactionsWriting Chemical Reactions

To describe what occurs during a To describe what occurs during a chemical reaction, scientists use chemical reaction, scientists use chemical equations.chemical equations.

Chemical equations involve the Chemical equations involve the following parts:following parts:

The Parts of a Chemical ReactionThe Parts of a Chemical Reaction1. The reactants on the left side, using the 1. The reactants on the left side, using the

symbols of elements for each atom. These symbols of elements for each atom. These symbols are the same ones from the symbols are the same ones from the periodic table. For each molecule indicate periodic table. For each molecule indicate in brackets behind it whether it is a solid in brackets behind it whether it is a solid (s), liquid (l), or gas (g). An ‘+’ is used to (s), liquid (l), or gas (g). An ‘+’ is used to separate the different reactants.separate the different reactants.

2. An arrow 2. An arrow pointing to the rightpointing to the right separating separating the reactants from the products.the reactants from the products.

3. The products on the right side of the 3. The products on the right side of the equation are labeled in the same manner equation are labeled in the same manner as the reactants.as the reactants.

Eg. Eg. CH4(g) + O2(g) → CO2(g) + H2O(l) CH4(g) + O2(g) → CO2(g) + H2O(l)

Exothermic and Endothermic Exothermic and Endothermic ReactionsReactions

Exothermic reactions RELEASE energy when reacting. Exothermic reactions RELEASE energy when reacting. Reactants → Products + EnergyReactants → Products + Energy Endothermic reactions ABSORB energy from the Endothermic reactions ABSORB energy from the

surrounding environment.surrounding environment. Reactants + Energy → ProductsReactants + Energy → Products An exothermic example is the starting of a barbecue An exothermic example is the starting of a barbecue

involves the igniting of propane and oxygen. This involves the igniting of propane and oxygen. This releases a large amount of energy.releases a large amount of energy.

CC33HH88(l) + 5O(l) + 5O22(g) → 3CO(g) → 3CO22(g) + 4H(g) + 4H22O(g) + EnergyO(g) + Energy An endothermic reaction example is the splitting up of An endothermic reaction example is the splitting up of

water into hydrogen and oxygen molecules using an water into hydrogen and oxygen molecules using an electric current.electric current.

2H2H22O(l) + Energy → 2HO(l) + Energy → 2H22(g) + O(g) + O22(g) (g)

An Important Note about An Important Note about Endothermic and Exothermic Endothermic and Exothermic

Reactions Reactions An important note is that if a reaction is An important note is that if a reaction is

exothermic then the products are MORE exothermic then the products are MORE stable than the reactants. If the stable than the reactants. If the reaction is endothermic the products reaction is endothermic the products are LESS stable than the reactants. are LESS stable than the reactants. Since all elements are trying to become Since all elements are trying to become more stable exothermic reactions are more stable exothermic reactions are more natural while endothermic more natural while endothermic reactions need an outside energy reactions need an outside energy source.source.