Section 6.2: Describing chemical reactions Key concepts: What information does a chemical equation contain? What does the principle of conservation of mass state? What must a balanced chemical equation show? What are the three categories of chemical reactions?

Key Terms: Chemical equation Reactant Product Conservation of mass Open system Closed system Coefficient Synthesis Decomposition Replacement

What are chemical equations? A chemical equation is a short easy way to show a chemical reaction. They use chemical formulas and other symbols to summarize a reaction.

Formulas in an equation All chemical equations use formulas to represent the substances involved in a reaction. A chemical formula (remember ?) is a combination of symbols that represents the elements in a compound (like CO2 for carbon dioxide)

Structure of an equation A chemical equation shows you what substances you start with and what you get at the end. Substances you start with are called reactants. Substances you end with are called products. Reactant + reactant  product + product (the arrow is read as “yields”) Numbers of products and reactants varies

Conservation of mass Conservation of mass – during a chemical reaction, matter is not created or destroyed All the atoms present at the start are present at the end (just in a different configuration) The total mass of reactants MUST equal total mass of products A CHEMICAL EQUATION MUST SHOW THE SAME NUMBER OF EACH TYPE OF ATOM ON BOTH SIDES OF THE EQUATION

Open and closed systems Open system – matter can enter from or escape to the surroundings (e.g. a wood fire – gas escapes into the air) Closed system – matter is not allowed to enter or leave (e.g. a pear decaying in a glass jar) Why does it matter? You want to know what could be interfering within your chemical reactions. An open system is more difficult to monitor than a closed system.

Balancing Chemical Equations A CHEMICAL EQUATION MUST SHOW THE SAME NUMBER OF EACH TYPE OF ATOM ON BOTH SIDES OF THE EQUATION How do we do this?

The Steps! 1. Write the equation H2 + O2  H2O 2. Count the atoms! But omg like how like omg this looks hard omg 2. Count the atoms! Um well like, dude, there was like 2 hydrogen atoms and stuff and like 2 oxygen and stuff in the reactants. Then there’s like 2 hydrogen but 1 oxygen in the products n stuff

The steps! continued 3. Use coefficients to balance atoms Use coeawhaaa? Coefficients: numbers placed in front of chemical formulas in equations H2 + O2  H2O Does it? We need another oxygen atom… what do we do?

Let’s add an oxygen atom!! H2 + O2  2H2O Does that work? Is it all balanced?

H2 + O2  2H2O Well now we have 2 oxygen atoms on both sides, but we have 4 hydrogen atoms on the product side and 2 on the reactant side. How do we have 4? We now have 2 molecules of water, and each molecule has 2 hydrogen atoms. That gives us 4 hydrogen atoms. So NOW what? Keep balancing! 2H2 + O2  2H2O

Classifying chemical reactions Chemical reactions can be classified in one of three categories: Synthesis Decomposition replacement

Synthesis When two or more elements or combounds combine to form a more complex substance, the process is called synthesis. Hydrogen and oxygen to make water is synthesis.

Decomposition Decomposition breaks down compounds into simpler products. Hydrogen peroxide (the kind your granny likes to put on your cuts) decomposes into water and oxygen gas

Replacement When one element replaces another in a compound, or when two elements in different compounds trade places, the process is called replacement. 2 Cu2O + C  4 Cu + CO2 Copper metal can be obtained by heating copper oxide with carbon. Carbon then takes the place of copper, and binds with oxygen.

Single vs double In a single replacement reaction, one element replaces another. In a double replacement reaction, elements in one compound appear to “trade places” with elements in another compound. ex of a double: FeS + 2 HCl  FeCl2 + H2S