4.3 Conservation of Mass and Chemical Equations

1. 4.3 4.3 Conservation of Mass and Chemical Equations

2. 4.3 Conservation of Mass in Chemical Changes • A chemical change always involves the conversion of substances (reactants) into other substances (products) • Properties of the products are different from the properties of the reactants (even though the products have the same atoms as the reactants) • In a chemical reaction, atoms are conserved – they are not created or destroyed!

3. 4.3 • In a chemical reaction, atoms are rearranged to form new substances • Chemical Reaction: One or more chemical changes that occur at the same time • In the 1700’s French chemist Antoine Lavoisier was making very careful measurements of mass for chemical reactions

4. 4.3 • He worked with Mercury (II) oxide which forms mercury and oxygen when heated • He always found that the total mass of the products equaled the mass of the products Law of Conservation of Mass: In a chemical reaction, the total mass of the products is always the same as the total mass of the reactants

5. Conservation of Mass The law of conservation of mass: • In any given chemical reaction, the total mass of the reactants equals the total mass of the product. • Matter cannot be created or destroyed

6. 4.3 Writing Chemical Equations • A chemical equation is used to represent a chemical reaction • There are 3 forms of chemical equations: • 1) Word equations • 2) Skeleton Equations • 3) Balanced chemical equations

7. 4.3 Word Equations • In a word equation, the name of each reactant is written to the left of an arrow and the name of each product to the right of the arrow • A plus sign on the reactants side means “reacts with” • A plus sign on the products side means “and” • The arrow stands for “yields” or “reacts to produce” hydrogen + oxygen water

8. 4.3 Skeleton Equations • Word equations show the reactants and products, but do not provide information about the chemical composition of the substances • Replacing the words with chemical formulas produces a skeleton equation H2 + O2 H2O

9. 4.3 Skeleton Equations

10. 4.3 Balanced Chemical Equations • Skeleton equations show composition of each substance in the reaction, but does not show the units of reactants and units of products • A balanced chemical equation demonstrates the law of conservation of mass – which requires the same number of atoms to appear on both sides of the equation • Coefficients: Numbers placed in front of a formula in a balanced chemical equation to show how many units of a substance is involved in the chemical reaction 2H2 + O2 2H2O

11. Chemical Reactions Chemical reaction: the process by which one or more substances becomes different substances • (e.g.) hydrogen gas and oxygen gas react to form water We can represent chemical reactions with a chemical equation that can be: • Word equation Only contain names magnesium + hydrochloric acid  magnesium chloride + hydrogen • Skeletal equation only contains the chemical formulas and has not been balanced Mg(s) + HCl(aq)  MgCl2(aq) + H2(g) • Balanced chemical equation the number of atoms of each element is the same for both sides of the equation Mg(s) + 2HCl (aq)  MgCl2(aq) + H2(g)

12. 4.3 Balanced Chemical Equations !! The only way to balance a chemical equation is by changing the coefficients to a new value……DO NOT CHANGE SUBSCRIPTS!!

13. 4.3 Balancing Chemical Equations STEP #1

14. 4.3 Balancing Chemical Equations STEP #2

15. 4.3 Balancing Chemical Equations STEP #3

16. 4.3 Balancing Chemical Equations Tips for Balancing Chemical Equations: • Remember the diatomic molecules (I2, Br2, Cl2, F2, O2, N2, H2) • Balance compounds first, then elements last • Balance hydrogen and oxygen last • Treat polyatomic ions as a single unit (they usually do not get split up)

17. Example Write a balanced chemical equation for: magnesium + oxygen magnesium oxide Step 1:Write the skeleton equation by replacing the chemical names with symbols Mg + O2MgO

18. Step 2: Count the number of atoms on either side of the arrow Mg + O2MgO

19. Step 3: Use coefficients to adjust the number of particles Mg + O2MgO

20. Step 3: Use coefficients to adjust the number of particles Mg + O2MgO Mg + O2MgO

21. Step 3: Use coefficients to adjust the number of particles Mg + O2MgO Mg + O2MgO The balanced chemical equation is: 2Mg + O22MgO

22. 4.3 Balancing Chemical Equations • After you have balanced a chemical equation, verify that it is balanced by counting the atoms of each element one more time for both sides of the equation • If you find you need to use higher and higher coefficients to balance, double check that the chemical formula is correct – An incorrect chemical formula will prevent an equation from balancing! • It is OK to use fractions as coefficients! (you will often need them when balancing diatomic molecules)

23. 4.3 Showing the State of a substance • To complete a balanced chemical equation, the states of the reactants and products must be included 2H2 (g) + O2(g) 2H2O(l)

24. 4.3 Practice Balancing Chemical Equations • Example: Ammonia (NH3(g)) is produced from the reaction of nitrogen gas and hydrogen gas. Write a balanced chemical equation for this reaction

25. 4.3 4.3 Practice Balancing Chemical Equations • Example: Ammonia (NH3(g)) is produced from the reaction of nitrogen gas and hydrogen gas. Write a balanced chemical equation for this reaction Solution: Word Equation: nitrogen + hydrogen ammonia Skeleton Equation: N2 + H2 NH3 Balanced Equation: N2(g) + 3H2(g) 2NH3(g) Do Questions #1-2 on page 165

26. Do Questions #1-3 & 5-8 on page 168