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# Balancing equations

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### Transcript

• 1. You only get out what you put inWrite this title in your notebooks
• 2. Law of conservation of mass Matter is neither created nor destroyed – it is just changes form.For example, if you react hydrochloric acid with magnesium: 2HCl + 2Mg  H2+ 2MgClYou mix a liquid and a solid and get a gas a liquid – but nothing disappeared, it’s just different.Write the Law of Conservation of Mass in your notebook
• 3. Reaction equationsEquations are like the recipes you use to cookThey tell you what to put in – ReactantsAnd what you’ll get out – Products Write definitions of reactants and products in your notebook
• 4. Equations and balancingLook back at the equation on slide two.Copy the equation into your notebookCount the H, Mg, and Cl atoms in the Reactants and the Products.Reactants: _____ H, _____ Mg, _____ ClProducts: _____ H, _____ Mg, _____ Cl
• 5. Equations and balancingLook back at the equation on slide two.Copy the equation into your notebookCount the H, Mg, and Cl atoms in the Reactants and the Products.Reactants: __2__ H, __2__ Mg, __2__ ClProducts: __2__ H, ___2_ Mg, ___2_ Cl
• 6. There were the same number of atoms of each element in the products as the reactants You only get out what you put in.
• 7. Balancing equationsWhenever we write or use an equation in chemistry it MUST be balanced and obey the law of conservation of mass You only get out what you put in.
• 8. Balancing equationsSome equations are harder to balance than others, use the following technique and you’ll be fine.First write the equation BIG – so it goes all the way across the page.Balance this equation in your notebook as you watch a step by step demonstration.
• 9. ____Cu2O + ____C → ____Cu + ____CO2 Draw a line in front of each compound This is where you will write the Coefficient that tells you how many of each substance you have. Write a definition of Coefficient in your notebook.
• 10. ____Cu2O + ____C → ____Cu + ____CO2 Draw a line under the arrow so that you divide the area into two parts, Reactants and Products
• 11. ____Cu2O + ____C → ____Cu + ____CO2 Cu - Cu - O- O- C- C- List the elements on each side. List them in the same order
• 12. ____Cu2O + ____C → ____Cu + ____CO2 Cu - 2 Cu - O- 1 O- C- 1 C- Now count how many of each element there are on each side. I’ve done the Reactants – you do the Products.
• 13. ____Cu2O + ____C → ____Cu + ____CO2 Cu - 2 Cu - 1 O- 1 O- 2 C- 1 C- 1 Compare the numbers. Is it balanced? Are the same number of atoms going in as going out?
• 14. ____Cu2O + ____C → ____Cu + ____CO2 Cu - 2 Cu - 1 O- 1 O- 2 C- 1 C- 1 No there aren’t There are more copper atoms on the Reactant side than on the Product side. We need more copper on the Product side.
• 15. ____Cu2O + ____C → ____Cu + ____CO2 Cu - 2 Cu - 1 O- 1 O- 2 C- 1 C- 1 Important rule – we can only change the Coefficients – the number in front of the compound
• 16. ____Cu2O + ____C → __2_Cu + ____CO2 Cu - 2 Cu - 1 2 O- 1 O- 2 C- 1 C- 1 Now we have to update our element count. Look again – is the equation balanced?
• 17. ____Cu2O + ____C → __2_Cu + ____CO2 Cu - 2 Cu - 1 2 O- 1 O- 2 C- 1 C- 1 No it’s not. There is more Oxygen on the Product side than on the Reactant side.
• 18. __2_Cu2O + ____C → __2_Cu + ____CO2 Cu - 2 Cu - 1 2 O- 1 O- 2 C- 1 C- 1 We can only change the coefficients. So the only way to get more O on the reactant side is to add another Cu2O
• 19. __2_Cu2O + ____C → __2_Cu + ____CO2 Cu – 2 4 Cu - 1 2 O- 12 O- 2 C- 1 C- 1 Now we update our element count again. Look again – is the equation balanced?
• 20. __2_Cu2O + ____C → __2_Cu + ____CO2 Cu – 2 4 Cu - 1 2 O- 12 O- 2 C- 1 C- 1 No it’s not – there is more copper on the Reactant side than the product side.
• 21. __2_Cu2O + ____C → __4_Cu + ____CO2 Cu – 2 4 Cu - 1 2 O- 12 O- 2 C- 1 C- 1 We can only change the coefficients. Change the Cu to 4.
• 22. __2_Cu2O + ____C → __4_Cu + ____CO2 Cu – 2 4 Cu - 1 2 4 O- 12 O- 2 C- 1 C- 1 Now we update our element count again. Look again – is the equation balanced?
• 23. __2_Cu2O + ____C → __4_Cu + ____CO2 Cu – 2 4 Cu - 1 2 4 O- 12 O- 2 C- 1 C- 1 Yes! It is balanced. 2Cu2O + C  4Cu + CO2
• 24. PracticeNow it’s your turn.Follow the steps we just used and balance the following equations: 1. Write the equation big 2. Draw a line under the arrow 3. List the elements 4. Count the elements – is it balanced? 5. If not, change the Coefficients 6. Count again and repeat from step 4.
• 25. Practice 1/5 ___Na + ___Cl2  ___NaCl
• 26. Practice 2/5 ___H2 + ___O2  ___H2O
• 27. Practice 3/5 ___SnO2 + ___H2  ___Sn + ___H2O
• 28. Practice 4/5 ___NaCl + ___F2  ___NaF + ___Cl2
• 29. Practice 5/5 ___CH4 + ___O2  ___CO2 + ___H2O
• 30. Balancing polyatomicsDo you remember polyatomic ions?Ions that have more than one element in them  SO42+  NO3 -  OH-There’s a shortcut…
• 31. __H2SO4 +__Zn  __ZnSO4 + __H2 H-2 H-2 S- 1 S- 1 O- 4 O- 4 Zn - 1 Zn - 1 Instead of writing out all of the S and O separately, notice that SO4 is together on both sides.
• 32. __H2SO4 +__Zn  __ZnSO4 + __H2 H-2 H-2 SO4 - 1 SO4 - 1 Zn - 1 Zn - 1 We can count the SO4‘s instead.
• 33. Practice 1/2 __Al2(SO4)3 + __Ca(OH)2 → __Al(OH)3 + __CaSO4
• 34. Practice 2/2__AgNO3 + __K3PO4 → __Ag3PO4 + __KNO3
• 35. The end.