General Chemistry – Unit 7Chemical 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

Experiment 17

Please read over steps 1-8 carefully 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 Element Reactivity

LiRbKBaCaNa

React with cold H2O and acids, replacing hydrogen

F2

Cl2Br2

I2

Listed from most reactive to least reactive

MgAlMnZnCrFe

React with acids or steam but usually not liquid water, to replace hydrogen

NiSnPb

All react with acids but not water, to replace hydrogen

H2

CuHg

All react with oxygen to form oxides

AgPtAu

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 RELEASEDIs 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?