1. Calculating Reaction Rates

2. Interesting Chemistry:
By the end of the Unit we will see that Mother nature has an easier time with making Ammonia (NH3) than chemical companies!
Doesn’
Fertilizer generated from ammonia produced by the Haber process is estimated to be responsible for sustaining one-third of the Earth's population

3. LEGO trucks – example for calculating rates
Lee made 6 trucks in 3 minutes.
Can you make this a rate?
Legoland

4. Calculating rates Reaction rate=
change in Amount of Reactant or Product
Change in Time
State
Amount
Unit
Example:
(not legitimate chemistry )
Objects (lego trucks)
Number
Trucks
Solid
Mass
Grams
Gas
Volume
Liters
Aqueous (liquid)
Concentration M

5. Time Units Rate Units minutes M/min Months mL/month (moles/L) /Hour h
Kg/sec s

6. Measuring Reaction Rates
Reactants decrease
Products increase
2 NO2(g) → NO(g) O2 (g)

7. 4 (Lego) blocks → 1 (Lego) truck
Can you see why chemists need to measure in MOLES
Could it be?
Products decrease
Reactants increase
4 things =
4moles
4 (Lego) blocks → (Lego) truck
YES
2 NO2(g) → NO(g) O2 (g)
1 thing =
1mole

8. Calculating rates convert to MOLES
Reaction rate=
change in CONCENTRATION of Reactant or Product
Change in Time
State
Amount
Unit
Concentration
Solid
Mass
Grams -> moles
Moles/L
Moles
Gas
Volume
Liters -> moles
Aqueous (liquid)
M -> moles

9. always a positive number
Time Units
Rate Units
minutes
M/min
(moles/L) /Hour h s
M/sec s
(moles/L) /sec
moles/s
Change is most always in measured in concentration= Molarity=moles/L
Change = difference =
always a positive number

10. Example: Zinc in Hydrochloric acid
Zn(s) + 2HCl(aq)  H2(g) + ZnCl2(aq)
r = D mass of Zn
D time
r = D [ HCl ]
D time
[ ] = molar concentration
You need to capture the gas
r = Dvolume H2
D time

11. 1. Measure Gas by change in Volume
Zn(s) HCl(aq) → H2(g) ZnCl2(aq)
Volume (mL)
4.0
5.1
5.9
6.8
7.6
8.3
Time (s) 15 30 45 60 75
WHY can we use volume in this reaction to measure the rate?
H2(g) is the only gas in the reaction!

12. Let’s practice: 1 mole H2 Volume (mL) 4.0 5.1 5.9 6.8 7.6 8.3 Time (s)15 30 45 60 75
Rate in mL H2 /s = mL
=0.057 ml/s s
Rate in mole STP
0.057 mL s
x 1 L
x mole
x 60 s
= 1.5 x mole/min
1000 mL
22.4 L
1 min
Rate in g HCl/h
1.5 x mole H2
min
x 2 moles HCl
x g
x 60 min
= g/h
1 mole H2
1 mole
1 h

13. Let’s practice: What did we measure? Rate in volume: mL H2 /s =
Rate in volume: mL H2 /s = mL
=0.057 ml/s s
Rate in moles: STP
0.057 mL s
x 1 L
1000 mL
x mole
22.4 L
x 60 s
1 min
= 1.5 x mole/min
OVER-ALL REACTION RATE!
If temperature and concentration stays the same can the reaction rate change?
Think about the lego demo… if we did it for 30 minutes would the reaction rate remain constant?
Reactants are used up
And the concentration may decrease

14. Initial, Instantaneous and average reaction rates
9/20/2018
rise
run
Slope (rise/run) is the reaction rate in (mol/L)/s…

15. Reaction rates Reaction rate can vary through a reaction:
Initial reaction rate (start)
Instantaneous reaction rate (at any point)
Average reaction rate (over long time period)
What will be most accurate?
Which one would you expect to be the fastest?

16. Pressure could also be measured!
Why?
Pressure in a closed container
Pressure
Volume
Ca(s) + 2HOH(l) → Ca(OH)2(aq) H2(g)
P=1/V

17. We can measure weight in an open system.
CaCO3(s) + 2HCl(aq) → CO2(g) CaCl2(aq) + H2O(l)
WHY?
CaCO3(s) + 2HCl(aq) → CO2(g) CaCl2(aq) + H2O(l)
Solid Aqueous gas Aqueous liquid
Does dissolving sugar in water change the weight of your coffee? Nah…… So Aqueous solutions have little/no effect on the weight. The weight change is the CaCO3(s)
Mass (g)
82.07
81.84
81.71
81.66
81.64
81.63
Time (s) 15 30 45 60 75

18. 1. Calculate the average rate in units of grams CO2/s.
CaCO3(s) + 2HCl(aq) → CO2(g) CaCl2(aq) + H2O(l)
Solid Aqueous gas Aqueous liquid
Mass (g)
82.07
81.84
81.71
81.66
81.64
81.63
Time (s) 15 30 45 60 75
1. Calculate the average rate in units of grams CO2/s.
( ) g
Rate =
= g/s
75 s
Keep all sig figs on calculator
2. Calculate the rate in grams CaCO3/hour
g CO2 s
x 1 mole
x 1 mole CaCO3
x g
x s
= 48 g/h
1 mole CO2
1 mole
1 h
44.0 g
Round final answer

19. All of the above examples what did we take a close look at to see what we would look at?
GAS
LIquid
Solid

20. PHASE affects rate: Homogeneous =same Heterogeneous reactions
two or more phases (g) and (l)
therefore are not thoroughly mixed
(two solids do not mix)
Heterogeneous reactions
Homogeneous =same
Same what?
All reactants are in the same phase (ag), (g), (l)
And are mixed thoroughly
Homogenous reactions

21. How about this reaction?
CaCO3(s) + 2HCl(aq) → CO2(g) CaCl2(aq) + H2O(l)
Calcium carbonate and acid -> carbon dioxide
…. Sound like a little environmental issue you know about? Limestone and acid rain….
How could calculating the rate of reaction help environmental action?
How could you weight a huge limestone rock?
Measure the amount depleted with a ruler. Estimate the volume and then the weight in grams and calculate.

22. You already know this:
Why would you stir hot-chocolate powder to make it dissolve quicker? (mixing!)
Next class we will get into WHY… and
HOW!
We will look how the molecules collide
Would a orange juice drink (aqueous sugar ) and water mix faster than a frozen juice ice cube and water?
Chemistry is all around….
If you step back… you know it already.

23. Group Practice Problems
Small groups will work through practice problems together.
We will go over the answers – I will pull a name from the hat and that people will walk us through the problem.
The complete answer set will be posted on the class website
Exit slip: Make a connection with changing or calculating differing rates of chemical reaction in your daily life.
Example: Ms. Daley
I use kindling to start my campfires instead of logs. Chemistry would say I am increasing the surface area of the reactant.

24. Extra example if time permits: Sulfuric acid is neutralized with a base
If g of H2SO4 is neutralized with 0.50 M NaOH in s, what is the reaction rate in moles NaOH /min.
All g is used up and some of the 0.50 M is consumed.
H2SO NaOH →
0.895 g H2SO4
x 1 mole
x 2 moles NaOH
Rate =
98.1 g
1 mole H2SO4
0.500 min
= moles NaOH/min

25. Spectator ions
Mg(s) + 2HCl(aq)  H2(g) + MgCl2(aq) Aqueous Reactions may be written in ionic form to see if ions change concentration. IONIC FORM: dissociate all the aqueous (aq) compounds Mg(s) + 2H+(aq) + 2Cl-(aq)  H2(g) + Mg2+(aq) + 2Cl-(aq) SINCE [Cl -] does not change concentration then it is a spectator ion