a

Introduction to Chemical Reactions

when two hydrogen atoms collide they

join together to make an h2 molecule

will show this join as a bond between

the atoms you can also see the unused

bonds on the separate hydrogen atoms

that haven't collided yet when another

hydrogen atom crashes into the molecule

it doesn't stick because all the bonds

inside the molecule are used up a box

full of hydrogen atoms will quickly turn

into a box full of h2 molecules it's

similar with oxygen atoms inside a box

except that oxygen makes two bonds with

other atoms when they collide they join

to each other twice to form a double

bond a box full of oxygen atoms will

quickly turn into a box full of o2

molecules h2 and o2 molecules will stay

that way unless we do something to

change them what if we put one hydrogen

molecule and one oxygen molecule in a

box together in reality a box filled

with gases would contain trillions of

trillions of molecules but we just want

to look at the action of individual ones

what will they do well we know that they

will move in straight lines and they

will crash into the walls and bounce off

again but what happens when they collide

with each other

nothing they just bounce off well that's

not surprising because the molecules are

using all their bonds and don't have any

left over to reach out and join to other

atoms but what if we light the gases

with a flame whoa what happened there

the hydrogen and oxygen molecules busted

apart into atoms and then the atoms

rejoined to form a different molecule

h2o and a leftover oxygen atom on its

own why did the flame make a difference

and why did we get a new substance we'll

replay this and slow it down so we can

follow it

look at the molecules when we bring in

the flame

they speed up that's what heat is the

hotter the faster and when these faster

molecules crash into the walls or each

other they bust apart into atoms

now that the atoms are on their own they

are free to join two other atoms and

they can join with different ones here

one of the oxygen atoms has bonded to

both of the hydrogen atoms and because

it's valency is 2 it has no unused bonds

left over but the other oxygen atom

missed out on getting any hydrogen's at

all so it's got two unused bonds and is

on the lookout for other atoms to join

to and would run out of hydrogen atoms

because we only started with two and the

other oxygen got them both

but we can fix this by putting an extra

h2 into the box at the beginning let's

do that and light this candle

nice that was even louder

now both oxygens have the two hydrogen

atoms that they need and we end up with

two h2o molecules water wasn't there

before

so a new substance has been made not

only that the old substances have

disappeared

this rearrangement of atoms to form a

new substance is called a chemical

reaction will replay this chemical

reaction to show three snapshots in time

as it goes through different stages at

the beginning there were two h2

molecules and 102 molecule these

starting substances are called the

reactants when the flame busted them

apart they were four separate hydrogen

and two separate oxygen atoms then each

oxygen atom joined with two hydrogen

atoms to make two h2o molecules which

are called the products of the reaction

will now show how the old bonds break

and the new bonds are made to do this

we'll use structural formulas which show

the molecules as symbols joined by

sticks when the reactant molecules are

pulled apart the bond

inside each h2 and the double bond

inside the o2 are broken when the h2o

products are formed four new bonds are

made inside their molecules the h2 and

o2 have disappeared and are replaced by

the h2o a new substance has been formed

so this qualifies as a chemical reaction

but the molecules in chemical reactions

are too small to see so how can we tell

if one is happening well any evidence

that shows that a new substance is being

made or that an old substance is

disappearing indicates that a chemical

reaction is going on

telltale signs include a color change

a cloudy precipitate forming

bubbles of gas

a new smell

or a solid dissolving

the breaking of old bonds in the forming

of new bonds changes the energy inside

the molecules to so heat light and noise

can also be signs if the bonds in a

reaction release energy this goes into

the surroundings and heats them up a

thermometer shows this as a rising

temperature a reaction that releases

energy is called exothermic

if the bonds absorb energy this is

pulled out of the surroundings and cools

them down a thermometer shows a falling

temperature instead

a reaction that absorbs energy is called

endothermic each reaction is unique it

has its own combination of sines

different chemical reactions are

happening everywhere all the time right

now the bonds of the molecules inside

your body are being broken and new bonds

are being made old substances are

disappearing and new substances are

being formed

in fact chemical reactions are keeping

you alive