Chapter 5-- Section 2Chapter 5-- Section 2

Writing Chemical EquationsWriting Chemical Equations

What you should learnWhat you should learn

What does a chemical equation tell What does a chemical equation tell you?you?

How does mass change during a How does mass change during a chemical reaction?chemical reaction?

What are the three categories of What are the three categories of chemical reactions?chemical reactions?

Vocabulary: Chemical equation, Vocabulary: Chemical equation, subscripts, products, reactants. subscripts, products, reactants.

What is a chemical What is a chemical equation?equation?

A CHEMICAL EQUATION is a shorter, A CHEMICAL EQUATION is a shorter, easier way to show chemical reactions, easier way to show chemical reactions, using symbols instead of words.using symbols instead of words. • Ex. “Hydrogen molecules react with Ex. “Hydrogen molecules react with

oxygen molecules to form water oxygen molecules to form water molecules” would be written 2Hmolecules” would be written 2H22 + O + O22 --> --> 2H2H22O.O.

Symbols express a concept in a shorter Symbols express a concept in a shorter form.form.

Writing Chemical FormulasWriting Chemical Formulas

A formula (like HA formula (like H220) identifies the elements in 0) identifies the elements in the compound and shows the ratios of the compound and shows the ratios of different atoms that make up that substance.different atoms that make up that substance.

Notice that the numbers in a formula are Notice that the numbers in a formula are written smaller and lower that the letter written smaller and lower that the letter symbols. These numbers are subscripts. symbols. These numbers are subscripts. SUBSCRIPTS show the ratio of the atoms of SUBSCRIPTS show the ratio of the atoms of different elements in a formula. different elements in a formula. • If a letter symbol is a chemical formula doesn’t have a If a letter symbol is a chemical formula doesn’t have a

subscript, the number 1 is understood.subscript, the number 1 is understood.

• Ex. HEx. H220 means 2 hydrogen and 1 oxygen0 means 2 hydrogen and 1 oxygen

Structure of an EquationStructure of an Equation

Chemical equation summarizes a reaction.Chemical equation summarizes a reaction. The materials you have at the beginning are The materials you have at the beginning are

called called REACTANTS.REACTANTS. When the reaction is done, you have When the reaction is done, you have

different materials, called the different materials, called the PRODUCTS.PRODUCTS. A chemical equation uses symbols to show A chemical equation uses symbols to show

the reactants and the products of a reaction.the reactants and the products of a reaction.• Ex. CaCOEx. CaCO33 --> CaO + CO --> CaO + CO2.2.

• Reactant Product ProductReactant Product Product

Conservation of MassConservation of Mass

No matter how many reactants and products No matter how many reactants and products are involved, all of the atoms at the start of are involved, all of the atoms at the start of the reaction are present at the end of the the reaction are present at the end of the reaction. reaction.

The amount of matter involved in a chemical The amount of matter involved in a chemical reaction does not change. The total mass of reaction does not change. The total mass of the reactants must equal the total mass of the the reactants must equal the total mass of the products.products. • This principle, called the This principle, called the CONSERVATION OF MASSCONSERVATION OF MASS, ,

means that during a chemical reaction, matter is means that during a chemical reaction, matter is not created or destroyed.not created or destroyed.

Balancing Chemical Balancing Chemical EquationsEquations

Steps for balancing equations:Steps for balancing equations:• Write the word equation.Write the word equation.• Write the chemical equation.Write the chemical equation.• Count the number of atoms or each Count the number of atoms or each

element on each side of the equation. element on each side of the equation. • Chose coefficients to balance the Chose coefficients to balance the

equation.equation.• Think about it.Think about it.

Now try a few.Now try a few.

Example ProblemsExample Problems HH22 + O + O22 --> H --> H22OO

Mg + OMg + O22 --> MgO --> MgO

C + ClC + Cl22 --> CCl --> CCl44

AlAl22OO33 --> Al + O --> Al + O22

AnswersAnswers HH22 + O + O22 --> H --> H22OO

• 22HH22 + O + O22 --> --> 22HH22OO

Mg + OMg + O22 --> MgO --> MgO

• 22Mg + OMg + O22 --> --> 22MgOMgO

C + ClC + Cl22 --> CCl --> CCl44

• 44C + C + 22ClCl22 --> CCl --> CCl44 AlAl22OO33 --> Al + O --> Al + O22

• 22AlAl22OO33 --> --> 44Al + Al + 33OO22

Classifying Chemical Classifying Chemical ReactionsReactions

Many Chemical reactions can be classified in Many Chemical reactions can be classified in one of three Categories: synthesis, one of three Categories: synthesis, decomposition, or replacementdecomposition, or replacement..• When two or more substance combine to form a more complex When two or more substance combine to form a more complex

substance is a process called substance is a process called synthesissynthesis..– Ex. Ex. 2 Mg + O2 Mg + O22 --> 2 MgO --> 2 MgO

• DecompositionDecomposition breaks down compounds into simpler products. breaks down compounds into simpler products.– Ex. Ex. 2 Al2 Al22OO33 --> 4 Al + 3 O --> 4 Al + 3 O22

• ReplacementReplacement is when one element replaces another in a is when one element replaces another in a compound or when two element in different compounds trade compound or when two element in different compounds trade places.places.

– Ex. 2 CuO + C --> 2 Cu + CoEx. 2 CuO + C --> 2 Cu + Co22

Answer these Questions. Answer these Questions.

What does a chemical equation tell What does a chemical equation tell you?you?

How does mass change during a How does mass change during a chemical reaction?chemical reaction?

What are the three categories of What are the three categories of chemical reactions?chemical reactions?