Post on 19-Jul-2020
Ch. 9 Review Chemical Reactions
•When substances undergo chemical changes, they form new substances.
•Atoms are rearranged, because bonds are broken and reformed
Remembering RXNs
•
What are Reactants?•
What are Products?
•
Why do we balance and how?•
Energy (exo-
or endothermic)
•
Signs of reactions
Endothermic reaction
•
Reaction where heat energy is absorbed.
•
Surroundings get colder•
chemical energy of products greater than reactants
•
▲H is positive
Exothermic Reaction
•
Reaction where heat energy is released
•
surrounding get hotter•
Chemical energy of reactants greater than products
•
▲H is negative
Signs of Chemical Changes1.heat or light
2.Color Change
Signs of Chemical Changes
3.Gases emitted
4.Precipitate (solid)
DON’T WORRY IN MY CLASS WE WEAR SAFETY GOGGLES
Reaction Rates
Ch. 22.1
Chemical Reactions
•
Reactions normally happen in a series of steps.
•
These steps determine the speed of the reaction.
•
Reactions can sometimes be reversible.
•
Reversible reactions then can reach equilibrium.
What is a Rate?•
Fixed ratio between two things
•
It measure how fast or slow
a reaction or an action takes place.
•
It does not measure time!!!!!
Kinetics and Reaction Rate
•
Kinetics is the area of chemistry dealing with the speed
or rate
at
which reactions take place.•
Reactions take place at different
speeds.•
Some reactions act quickly. (air bags)
•
Some reactions react slowly. (rusting, food spoiling)
Why Rates Important?
•
It is important to understand rates in order to control the reaction.
Fires have fast rates, that must be controlled
Fire Triangle
Understanding Rates
•Once there was a king whose daughter was kidnapped by the evil dragon lord.
•So the bravest knight (Sir Glennolot) sent out to save the princess
Understanding Rates
•
Once slaying the dragon, he returned the princess to her father.
•
So overcome with joy, the king offered anything in his kingdom to the knight.
•
The knight only wanted one thing. One piece of wheat. And asked they double the amount for each space on a chess board. (or 63 days)
•
King agreed to what he thought was a silly request.
Understanding Rates
•
2, 4, 8, 16, 32, 64, 128, etc (amounts quickly increase)
•
After the 63rd
day, the knight now
controlled all the food in the kingdom, and became the king himself.
•
And all lived happily ever after.•
Well except the dragon
•
And I guess the king didn’t end up so well.
Understanding Rates
•
Chemists, engineers, and everyday chefs need to be aware how reaction rates can change when conditions for the reaction change.
•
Ex. Double the concentration of reactants can quadruple the rate.
Understanding Rates•
Ex. Raising the temperature only 100C will double the reaction rate
Understanding Rates
•
Ex. Changing the dosage or the surface area can make a rate dramatically and dangerously fast.
Calculating Rate
Rate = Δ [ x ]Δ t
Change in amount
Change in time
Rate: examples?
•
beats/min, •
mi/hr,
•
pills/day•
Time is always on the bottom
RR ExperimentUse the pre-measured amount of both baking soda and acetic acid.
Combine the two reactants. Answer the following questions
CHEMICAL REACTIONS AND RATES
1.
What phases are the reactants?2.
What phases are the products?
3.
How did you know there was a chemical reaction taking place?
4.
How did you know when the reaction was over?
5.
When was the most gas evolved?6.
What equations would be used to represent the rate of this reaction?
7.
What measurements would you need to make to calculate a rate for this reaction?
8.
Describe three ways you could experimentally increase the rate of this reaction.
REACTION RATES ! !
RR = Δ [reactants ]Δ t
RR = Δ [products ]Δ t
RATE CALCULATIONS
1.
John takes 10 weeks
to earn $150. However, Mary earns $150
in 30 days. Calculate the rate at which they both earn money. Which has the larger rate?
•
Mary
John•
Amount
Amount•
Time
Time•
Rate ($/day)
Rate ($/day)•
Larger =2.
Kyle takes 8 minutes
to ride his bike 0.5 Km
to school. Erin takes 20 minutes
to ride 5 Km
to school. At what rate does each of them travel? Who is faster?KYLE
ERINDistance
DistanceTime
TimeRate (Km/min) Rate (Km/min)
•
Larger =
$150 $15030 days 70 days
$5/day $2/dayMary
0.5km 5 km
20 minutes8 minutes
Erin
= 0.06 = 0.25
RATE CALCULATIONS3.
Time released Tylenol has 500 mg each that last 12 hours. However, CVS acetaminophen, last 4 hours and contains 220 mg each. At what rate is the medication released for each? Which dissolves at a larger rate?
•
TYLENOL
CVS•
Amount
Amount•
Time
Time•
Rate =
Rate
=•
Larger =
4.
A four pound roast cooks in the grill in about 2.5 hours. However, a 0.25 pound hamburger take only 15 minutes to cook. Which one cooks at a slower rate?
ROAST HAMBURGERA = A=T= T=Rate = Rate =
Slower =
= 500 mg= 12 hr42 mg/hr
= 220 mg= 4 hr55 mg/hr
•CVS
burger
4 lb2.5 hr1.6 lb / hr
0.25 lb.25hr
1 lb / hr
Collision Theory
•Reactions and the rate of reactions taking place depend on two or more molecules colliding in such a way that old bonds break and new bonds form.
COLLISION THEORY
•
C
= collisions
•
E
= energy
•
O
= orientation
Collisions
•
Reactants must be in contact in order to react.
Energy•
Every reaction needs energy to get things started. (Activation Energy)
•
This energy is needed to break the chemical bonds of the reactants.
•
Energy to form activated complex
Energy Diagram
Orientation•
Reactants must be lined up properly when colliding.
Factors Affecting RXN Rates
•
Nature of Reactants•
Temperature
•
Concentration•
Surface Area
•
Catalysts/inhibitors
Nature of Reactants
•
Not all materials react with each other.
•
If reactants don’t have enough energy, no reaction can take place.
•
If reactants don’t collide in the proper orientation, no reaction takes place.
Nature of reactants
Collisions
Collisions
EnergyCollisions
Energy
Orientation
NO
NO
YES
Temperature•Higher the temperature the faster the reaction.
•Higher temperature causes particles to move faster, thus they collide more often.
Concentration (M)
•
The more concentrated a solution the faster the reaction will be.
•
If the number of reactant particles per unit volume increases, then the chance of successful collisions increase.
Which has a higher concentration of reactants?
More concentrated
Surface Area (particle exposure)
•
The greater the surface area the faster the reaction.
•
More surface area means more reactants are in contact, which of course leads to more collisions.
Surface AreaMore it can be broken down more area
MORE
kindle
groundLESS Whole bean
Logs
Catalyst•
A substance that changes the rate of chemical reactions without being permanently changed itself.
•
Brings energy to reaction•
Lowers the activation energy required.
•
Ex. yeast, enzymes
Inhibitors
•Inhibitors slow down reactions by tying up the reactants
•Ex. preservatives
Reversible Reactions and Equilibrium
Ch 22.2
Reactions Going Completion
• products are removed from
the reacting system (3 products)
1. Forming a insoluble
precipitate (no ions left)2.
Forming a gas
3.
Water neutralized
Removing a Product•
gas formed (g)
•
2H2
O 2H2 (g) + O2 (g)
•
Precipitate formed (s)CuSO4
+Na2
S Na2SO4(aq) + CuS(s)
•
Water is formed (H2
O)•
NaOH
+ HCl NaCl + H2O
Product formed
Reactants used up
Going to Completion
Reversible Reactions
3H2
+ N2
↔
2NH3
•
the products remain in the system so they may react back to original reactants.
•
Two reactions where the only difference is the direction.
•
Reactions are occurring at the same time.
Examples: Reversible Reactions.•
Unopened Soda
•
Breathing cycle
•
Rechargeable batteries
•
Mood rings
Equilibrium•The state in which a chemical reaction and its reverse reaction occur at the same rate.
1. Equilibrium is reached when rats are equal
2. No change in the amounts.
Equilibrium
Reactants
Products
Equilibrium reached when concentrations become constant
Le Chatelier’s
Principle
STRESS to SYSTEM
Three stress’s are 1.Temperature
2.Concentration3. Pressure.
(change in volume)
Le Chatelier’s
Principle
•If a system at equilibrium is stressed, it will react to undo the stress.
Must form more reactant to balance again, so this is a shift to the left.
TEMPERATURE
Le Chatelier’s
Principle•
If disturbed by a change by a stress, a net shift will occur to offset the change.
•
A shift left or right will occur•
A shift is represented by the formation of more reactants or products.
•
Shifts are sometimes observed by a color change
When a stress is applied the reaction will reestablish equilibrium
Equilibrium = No change in amount over time
1. Concentration
Concentration
•
If you
ADD
reactants
, The system
will REMOVE
reactants
by making more products.
•
If you
REMOVE
reactants
, The system
will ADD
reactants
by reacting the products.
2. Temperature
3. Pressure
The side withleast moles
The side withmost moles
LEFT: RIGHT:
↔