Balancing Chemical Equations 8 th gr. Chemistry

Ice cubes in a Bag You are having an argument with your friend about what happens to the mass when matter changes from one form to another. To prove your idea, you put three ice cubes in a sealed plastic bag and record the mass of the ice in the bag. You let the ice cubes melt completely. Ten minutes later you record the mass of the water in the bag. Which of the following best describes the result? You are having an argument with your friend about what happens to the mass when matter changes from one form to another. To prove your idea, you put three ice cubes in a sealed plastic bag and record the mass of the ice in the bag. You let the ice cubes melt completely. Ten minutes later you record the mass of the water in the bag. Which of the following best describes the result? A. The mass of the water in the bag will be less than the mass of the ice cubes in the bag. B. The mass of the water in the bag will be more than the mass of the ice cubes in the bag. C. The mass of the water in the bag will be the same as the mass of the ice cubes in the bag.

Law of conservation of mass- Lavoisier showed through his experiment that the combined mass of the reactants before an experiment and the combined mass of the products after the experiment are equal to one another. Law of conservation of mass- Lavoisier showed through his experiment that the combined mass of the reactants before an experiment and the combined mass of the products after the experiment are equal to one another.

Chemical reaction- is a change in which one or more substances are changed into new substances Chemical reaction- is a change in which one or more substances are changed into new substances Chemical equation- an expression using chemical symbols to represent a chemical reaction. Chemical equation- an expression using chemical symbols to represent a chemical reaction.

Reactant- Reactant- in a chemical reaction, the substance that reacts (you begin with) in a chemical reaction, the substance that reacts (you begin with) Product- Product- in a chemical reaction, the new substance that is formed. in a chemical reaction, the new substance that is formed.

Below is the equation for photosynthesis, label the reactants and products in the equation: Below is the equation for photosynthesis, label the reactants and products in the equation: CO 2 (g) + H 2 O (l) + energy  C 6 H 12 O 6 (s) + O 2 (g) CO 2 (g) + H 2 O (l) + energy  C 6 H 12 O 6 (s) + O 2 (g) Reactant Product Reactant Product

Subscripts are numbers placed behind an element symbol and are used to indicate the number of atoms present for each element. Subscripts are numbers placed behind an element symbol and are used to indicate the number of atoms present for each element. CO 2 CO 2 H 2 0 H 2 0 CaCl 2 CaCl 2

Coefficients must be added in front of elements and compounds to make a balanced chemical reaction Coefficients must be added in front of elements and compounds to make a balanced chemical reaction 2H 2 O 2H 2 O 3CO 2 3CO 2

Subscripts and coefficients are to be multiplied. Subscripts and coefficients are to be multiplied. 2H 2 O 2H 2 O 3CO 2 3CO 2 So if the compound were 4CO 2 there would be _____ atoms of carbon and _____ atoms of oxygen. So if the compound were 4CO 2 there would be _____ atoms of carbon and _____ atoms of oxygen.

H 2 + O 2  H 2 O Make a list of the elements in the equation. Make a list of the elements in the equation. HO

H 2 + O 2  H 2 O Tally the number of atoms of each element for the left side of the equation. Tally the number of atoms of each element for the left side of the equation. 2 H 2 O

H 2 + O 2  H 2 O Tally the number of atoms of each element for the right side of the equation. Tally the number of atoms of each element for the right side of the equation. 2 H 2 2 O 1

H 2 + O 2  H 2 O Check to see if the number of atoms is equal for each element. Check to see if the number of atoms is equal for each element. 2 H 2 2 O 1

If they are not equal write a coefficient in FRONT of the element or compound that needs more atoms. If they are not equal write a coefficient in FRONT of the element or compound that needs more atoms. H 2 + O 2  2H 2 O

Check to see if the number of atoms is equal for each element. Check to see if the number of atoms is equal for each element.2H42O2 If they are not, place another coefficient to make them even. If they are not, place another coefficient to make them even.

H 2 + O 2  2H 2 O 2H 2 + O 2  2H 2 O Recheck your tally Recheck your tally4H42O2

2H 2 + O 2  2H H 4 2 O 2 If they are equal your equation is balanced. If not add another coefficient to a different element or compound to make them even. If they are equal your equation is balanced. If not add another coefficient to a different element or compound to make them even.

Rules to remember! Remember to multiply the coefficient and the subscript by one another. Remember to multiply the coefficient and the subscript by one another. Coefficients MUST be WHOLE numbers! Coefficients MUST be WHOLE numbers! 3CO 2 NOT 1.5CO 2 3CO 2 NOT 1.5CO 2 You can NEVER EVER add in or change a subscript! You can NEVER EVER add in or change a subscript! 2H 2 0 NOT H 2 O 2 2H 2 0 NOT H 2 O 2

Coefficients are the same in math class. Distributive property! Coefficients are the same in math class. Distributive property! 3(x-1)= 3x-3 3(x-1)= 3x-3 3H 2 0 means that there are 3H 2 0 means that there are ____ Hydrogen atoms ____ Hydrogen atoms 6 ______ Oxygen atoms ______ Oxygen atoms 3 Multiply the coefficient and the subscript! Multiply the coefficient and the subscript!

3Mg(OH) 2 Mg atoms Mg atoms 3 O atoms O atoms 6 H atoms H atoms 6

If an element appears twice on the SAME side of an equation you will ADD them together. If an element appears twice on the SAME side of an equation you will ADD them together. CH 4 + O 2  CO 2 + H 2 O So on the right there are ___ oxygen atoms? So on the right there are ___ oxygen atoms? 3