Chemical Formulas and Equations

Transcription

1 Chemical Formulas and Equations 8.5D recognize that chemical formulas are used to identify substances and determine the number of atoms of each element in chemical formulas containing substances; 8.5F recognize whether a chemical equation containing coefficients is balanced or not and how that relates to the law of conservation of mass

2 Chemical Compounds Chemists have identified over 10 million chemical compounds Some are molecular like the proteins and hormones in your body Some are ionic compounds such as the salts in your body fluids A chemical formula shows the kinds and numbers of atoms in the smallest representative unit of the substance (recall this from the last lesson?)

3 Formulas H2O shows that a water molecule has 2 hydrogen and 1 oxygen atom CO2 carbon dioxide 1 carbon and 2 oxygen atoms C2H6 ethane (natural gas) 2 carbon and 6 hydrogen atoms

4 What they don t show: If we look at NH3, it does show how many atoms and elements and the ratio that make up that molecule. What a formula doesn t show is the structure You had a bit of practice with this idea during the bonding practice

5 So why the formula? There are many reasons, but the best reason is to demonstrate on paper, chemical reactions! Chemical reactions are shown with chemical equations. Learning how to read one is very important in chemistry

6 Chemical formulas practice: Make sure to complete the front page and ask questions for now. When everyone is done, we will continue on with equations.

7 Chemical Equations A chemical equation is a symbolic representation of a chemical reaction 2 2

8 Equation Example: The burning of methane gas in oxygen is: CH O 2 CO H 2 O

9 Reading Chemical Equations Each side of an equation represents a combination of chemicals The combination is written as a set of chemical formulas, separated by + symbols. CH O 2 CO H 2 O

10 Reading Chemical Equations The two sides of the equation are separated by an arrow The combination of chemicals before the reaction are on the left side of the arrow The right side indicates the combination of chemicals after the reaction

11 Language of Chemical Equations Yields 4Na + O 2 2Na 2 O Reactants Arrow (yields) Products Reactants Products In this reaction, sodium (Na) and oxygen (O 2 ) react to produce a single molecule, Na 2 O (this together is known as sodium oxide

12 Conservation of Mass and Balancing Equations 8.5F recognize whether a chemical equation containing coefficients is balanced or not and how that relates to the law of conservation of mass

13 Law of Conservation of Matter Mass is neither created nor destroyed (during a chemical reaction)

14 Conservation of Mass and/or Matter What this means: 1. Matter is conserved type of atoms does not change Nothing is created or destroyed 2. Mass is conserved amount of atoms cannot change Nothing is created or destroyed

15 So IN A CHEMICAL REACTION: Elements can combine to form new substances Substances can be broken down into simpler substances (down to the elemental level) Atoms are re-arranged; NOT created or destroyed

16 In a chemical formula this law is Reactants Products demonstrated Mass of Reactants = Mass of Products

17 Balancing Equations To show conservation of mass We Balance equations Make sure there are the same number of each type of atom in the products (right side) and on the reactants side (left)

18 Take a look at this reaction. How many hydrogen atoms are there on each side of the reaction? Now, look at oxygen.

19 Balancing Equations If there is not a subscript or coefficient number shown, then it is understood to be 1 CH 4 + 2O 2 CO 2 + 2H 2 O = C 1 H 4 + 2O 2 C 1 O 2 + 2H 2 O 1 1 C C 4 H 4 H 4 O 4 O

20 Complete (and correct) equations signifying reactions are always balanced

21 Are the following balanced? Let s look at the number of atoms on each side of the reaction and then decide if the equation is balanced.

22 Counting Atoms remember the rules from the chemical formulas practice? 2CuO2 Copper - 2 Oxygen - 4 2NH 4 Nitrogen - 2 Hydrogen - 8 3S 2 O 2 Sulfur - 6 Oxygen - 6

23 Counting Atoms remember the rules from the chemical formulas practice? 4Mg 2 O 4 H 2 Magnesium 8 Oxygen 16 Hydrogen - 8 2NaN 3 HO Sodium 2 Nitrogen 6 Hydrogen 2 Oxygen - 2

24 BALANCED or NOT BALANCED? Now, in your notes, there is a space to write examples and to explain why the examples are balanced or not. Cu + O2 2Cu + O S + O2 Mg + O2 2Mg + O2 CuO 2CuO SO2 MgO 2MgO Not Balanced Not Balanced Balanced Not Balanced Balanced

25 Steps: 1. Work with a partner to fill out this page 2. When done, check your answers with another group and then with the teacher Now let s practice!