Chapter 11 What is a chemical reaction?

Reactions involve rearrangements of atoms. Reactants are converted into products. The law of conservation of mass states that mass cannot be created nor destroyed. This means that the number of atoms in the product must be equal to the original reactants.

Atoms do not become other kinds of atoms, nor do they appear or disappear.

Chemical reactions release or absorb energy Endothermic reaction- reaction in which energy is absorbed. - Bond formation always requires energy. Exothermic reaction- reaction in which energy is released. –Bond breaking always releases energy.

Particles must collide for a chemical reaction to occur. Example, when a safety match is lit, the reaction begins when the two substances are brought together by striking the match head across the striking surface. If this collision happens with enough energy, the bonds in the reactants are broken, allowing new bonds to form between the atoms.

Balancing Equations

Balancing requires patience To satisfy the law of conservation of mass, you should insert coefficients into the chemical equation in order to balance the number of elements. There must be equal numbers of atoms for each element on each side of the equation when an equation is balanced.

Testing an equation for balance Reactants Products Balance? Unbalanced Formula equation CH 3 CH 2 OH +O 2 CO 2 + H 2 O Carbon atoms 2 1 No Hydrogen atoms 6 2 No Oxygen atoms 3 3 Yes

To balance this equation, there are two carbon atoms in the reactant column and only one in the product column. To balance the number of carbon atoms you need to double the number of carbon dioxide molecules in the products.

CH 3 CH 2 OH + ? O 2 2CO 2 + ? H 2 O The 2 is called a coefficient and indicates that there are two carbon dioxide molecules. Coefficients are normal whole numbers, when a coefficient is equal to 1, the 1 is not written for simplicity. Coefficients are written in front of the formula and multiply the entire formula.

Testing an equation for balance Reactants Products Balance? Unbalanced Formula equation CH 3 CH 2 OH +O 2 2CO 2 + H 2 O Carbon atoms 2 2 Yes Hydrogen atoms 6 2 No Oxygen atoms 3 5 No

The carbon atoms are now balanced by the hydrogen atoms are not. In addition the oxygen atoms have been thrown out of balance. Let’s start with hydrogen, there are six in the reactant column, so we must add three to the product column.

Testing an equation for balance Reactants Products Balance? Unbalanced Formula equation CH 3 CH 2 OH +O 2 2CO 2 + 3H 2 O Carbon atoms 2 2 Yes Hydrogen atoms 6 6 Yes Oxygen atoms 3 7 No

The oxygen atoms are still unbalanced. Multiplying the number of oxygen molecules by three results in an equal number of oxygen atoms.

Testing an equation for balance Reactants Products Balance? Unbalanced Formula equation CH 3 CH 2 OH + 3O 2 2CO 2 + 3H 2 O Carbon atoms 2 2 Yes Hydrogen atoms 6 6 Yes Oxygen atoms 7 7 Yes

Now you have written a balanced chemical equation. Replacing the question marks in the formula equation with the right coefficients now give a correct chemical equation.

Tips for balancing equations 1.Save the elements that appear in several reactants or products for last. (usually oxygen and hydrogen ) 2.Balance lone elements last. 3.Balance the equation from left to right. 4.For ionic equations, be sure that charges are balanced.

Testing an equation for balance Reactants Products Balance? Unbalanced Formula equation NH 3 + O 2 NO + H 2 O Nitrogen atoms 1 1 Yes Oxygen atoms 2 2 Yes Hydrogen atoms 3 2 No

Testing an equation for balance Reactants Products Balance? Unbalanced Formula equation NH 3 + O 2 NO + H 2 O Nitrogen atoms Oxygen atoms Hydrogen atoms

Testing an equation for balance Reactants Products Balance? Unbalanced Formula equation 4NH 3 + 5O 2 4NO + 6H 2 O Nitrogen atoms 4 4 Yes Oxygen atoms Yes Hydrogen atoms Yes