Calculating Reaction Rates Interesting Chemistry By the end

Calculating Reaction Rates

Interesting Chemistry: By the end of the Unit we will see that Mother nature has an easier time with making Ammonia (NH 3) than chemical companies! Fertilizer generated from ammonia produced by the Haber process is estimated to be responsible for sustaining one-third of the Earth's population

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

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

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

Measuring Reaction Rates Reactants decrease 2 NO 2(g) → 2 NO(g) + O 2 (g) Products increase

Could. Canityoube? see why chemists need to measure in MOLES Products decrease 4 things = 4 moles 24 NO (Lego) 2(g) blocks→ Reactants increase YES 2 NO + O 2 truck → (g) 1 (Lego) (g) 1 thing = 1 mole

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 Moles/L Moles Aqueous (liquid) Concentration M -> moles Moles/L

Time Units Rate Units minutes M/min h (moles/L) /Hour 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

Example: Zinc in Hydrochloric acid Zn(s) + 2 HCl(aq) H 2(g) + Zn. Cl 2(aq) r= D mass of Zn D time r = D [ HCl ] D time [ ] = molar concentration ture p a c o t d You nee r = Dvolumeth. He 2 gas D time

1. Measure Gas by change in Volume Zn(s) + 2 HCl(aq) → H 2(g) + Zn. Cl 2(aq) Volume (m. L) 4. 0 5. 1 5. 9 6. 8 7. 6 8. 3 Time (s) 0 15 30 45 60 75 WHY can we use volume in this reaction to measure the rate? H 2(g) is the only gas in the reaction!

Let’s practice: Volume 4. 0 5. 1 5. 9 6. 8 7. 6 8. 3 (m. L) Time (s) 0 15 8. 3 - 4. 0 m. L Rate in m. L H 2 /s = 30 45 60 75 =0. 057 ml/s 75 - 0 s Rate in mole H 2/min @ STP 0. 057 m. L x x 1 L 1000 m. L s 1 mole 22. 4 L x 60 s 1 min = 1. 5 x 10 -4 mole/min Rate in g HCl/h 1. 5 x 10 -4 mole H 2 min x 2 moles HCl 1 mole H 2 x 36. 5 g 1 mole x 60 min 1 h = 0. 67 g/h

Let’s practice: What did we measure? = 8. 3 - 4. 0 m. L Rate in volume: m. L H 2 /s 75 - 0 s Rate in moles: H 2/min @ STP 0. 057 m. L s x 1 L 1000 m. L =0. 057 ml/s x OVER-ALL REACTION RATE! 1 mole 22. 4 L x 60 s 1 min = 1. 5 x 10 -4 mole/min If temperature and concentration stays the same can the reaction rate change? Reactants are used up Think about the lego demo… if we did it for 30 minutes would the And the concentration may decrease reaction rate remain constant?

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

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?

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

We can measure weight in an open system. Ca. CO 3(s) + 2 HCl(aq) → CO 2(g) + Ca. Cl 2(aq) + H 2 O(l) Ca. CO 3(s) + 2 HCl(aq) → Solid Aqueous WHY? CO 2(g) + Ca. Cl 2(aq) + H 2 O(l) gas Aqueous liquid Mass (g) Time (s) 82. 07 81. 84 81. 71 81. 66 81. 64 81. 63 0 15 30 45 60 75 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

Ca. CO 3(s) + 2 HCl(aq) → Solid Aqueous Mass (g) Time (s) 1. CO 2(g) + Ca. Cl 2(aq) + H 2 O(l) gas Aqueous liquid 82. 07 81. 84 81. 71 81. 66 0 15 30 45 60 75 Calculate the average rate in units of grams CO 2/s. Rate = (82. 07 - 81. 63) g 75 s 2. 81. 64 81. 63 = 0. 0059 g/s Keep all sig figs on calculator Calculate the rate in grams Ca. CO 3/hour 0. 0059 g CO 2 x 1 mole Ca. CO 3 x 100. 1 gx 3600 s = 48 g/h 1 mole 1 h s 44. 0 g 1 mole CO 2

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

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

How about this reaction? Ca. CO 3(s) + 2 HCl(aq) → CO 2(g) + Ca. Cl 2(aq) + H 2 O(l) Calcium carbonate and acid -> carbon dioxide …. Sound like a little environmental issue you know about? Limestone and acid rain…. 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. How could calculating the rate of reaction help environmental action?

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 cube and water? Chemistry is all around…. If you step back… you know it already.

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.

Extra example if time permits: Sulfuric acid is neutralized with a base If 0. 895 g of H 2 SO 4 is neutralized with 0. 50 M Na. OH in 30. 0 s, what is the reaction rate in moles Na. OH /min. All 0. 895 g is used up and some of the 0. 50 M is consumed. H 2 SO 4 + 2 Na. OH → 0. 895 g H 2 SO 4 Rate = x 1 mole 98. 1 g 0. 500 min = 0. 0365 moles Na. OH/min x 2 moles Na. OH 1 mole H 2 SO 4

Spectator ions Mg(s) + 2 HCl(aq) H 2(g) + Mg. Cl 2(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) + 2 H+(aq) + 2 Cl-(aq) H 2(g) + Mg 2+(aq) + 2 Cl-(aq) SINCE [Cl -] does not change concentration then it is a spectator ion