# General Chemistry – Unit 7 Chemical Equations

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General Chemistry – Unit 7 Chemical Equations
Chapter 7

What is a chemical rxn? A process where one or more substances changes to one or more other substances. Exchange of electrons!!!

Parts of a chemical reaction
Reactants the original substances Products the resulting substances

ASK YOURSELF: Which are the products and which are the reactants in these equations? Na+ + Cl- NaCl 2H2O  2H2 + O2

Quick Demo Magnesium

A word equation Magnesium metal reacts with oxygen gas to form white magnesium oxide solid. We must learn how to translate these!

Balancing equations Tips
__Fe2O3 + __H2SO4  __Fe2(SO4)3 + __H2O If possible treat polyatomic ions as 1 unit Balance hydrogen 2nd last Balance oxygen last If the last element does not balance double everything

Ways to Represent Reactions
Chemical equation: symbols that describe a chemical reaction and show what atoms (and how many, relatively speaking) are involved. Word equation: the names of each product/reactant are written out. Doesn’t show how many of each

Write out the word equation
Magnesium metal reacts with oxygen gas to form magnesium oxide solid. Mg + O  MgO

Remember BrINClOFH Br2 – a liquid I2 - a solid N2 - a gas Cl2 - a gas
O2 - a gas F2 - a gas H2 - a gas

Check for BrINClOFH Any time you see 1 of the 7 diatomic elements alone, write them with a 2 subscript Mg + O2  MgO

Balance the charges of ionic comp
In our reaction we have already done this, but make sure to use proper formulas (for molecular comp too!): MgO = Mg2+ O2- Mg + O2  MgO

Balance number of atoms/ions on each side
In ordinary chemical changes, atoms just rearrange This is the LAW OF CONSERVATION OF MATTER We do this with coefficients 2Mg O2  2MgO

Example If you take Ca(NO3)2 and change it to 2Ca(NO3)2, how many of each element do you have?

Put in physical states Symbols used in chemical equations: (s) solid
(l) liquid (g) gas (aq) aqueous solution (dissolved in H2O)

Put in physical states I will give you physical states in word problems: Magnesium metal reacts with oxygen gas to form magnesium oxide solid 2Mg(s) O2(g)  2MgO(s)

Word Equation example Hydrogen gas and Oxygen gas combine to form liquid water. 2H2(g) + O2(g)  2H2O(l)

Example: Sodium solid reacts with chlorine gas to form sodium chloride solid. 2Na(s) + Cl2(g)  2NaCl(s)

Try this one: Dicarbon dihydride reacts with oxygen gas to produce carbon dioxide gas and water

Evidence of rxns All rxns involve chemical change
Look for evidence of those changes: Light, heat, gas, color change, precipitate

Other Symbols  precipitate forms  gas forms  “produces” or “yields”
 products can reform in to reactants – end result is a mix of the two. Δ – means that heat was added

Chemistry – Chemical Equations
Part II

Get the proper safety gear, and the proper equipment Do steps 1-8, then set the solid aside to dry (step 9) Return to your seat – while the solid dries, you can work on A7 At the end of class – record the mass of your beaker and solid.

Balanced equations show RATIOS
The coefficients of a balanced equation reveal the RATIO of each reactant to each product: 2H2 + O2  2H2O means there is a 2:1:2 ratio of H2 to O2 to H2O. This is the same as a 4:2:4 ratio, etc. etc.

The activity series Writing an equation doesn’t mean it will actually happen How do we know it will happen? Single replacement reactions are predicted with the activity series This activity series has been created through experiment.

The activity series Things on the list will displace anything below but not above other things on the list There are two lists – one for elements that become positive ions, and one for elements that become negative ions.

Element Reactivity Li Rb K Ba Ca Na React with cold H2O and acids, replacing hydrogen F2 Cl2 Br2 I2 Listed from most reactive to least reactive Mg Al Mn Zn Cr Fe React with acids or steam but usually not liquid water, to replace hydrogen Ni Sn Pb All react with acids but not water, to replace hydrogen H2 Cu Hg All react with oxygen to form oxides Ag Pt Au Mostly unreactive

? When a rxn releases energy, where does it come from?
You have to put energy in to break bonds If bonding is stronger in the products they will be more stable More stable = lower energy Lower energy means energy must be RELEASED This is an exothermic reaction Is this an example of an ENDOthermic or EXOthermic reaction?

If bonding is weaker in the products, they will be less stable, have more energy, and will therefore absorb it. Is this an endo or exothermic rxn?