Calculating Reaction Rates

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

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: (not legitimate chemistry ) Objects (lego trucks) Number Trucks Solid Mass Grams Gas Volume Liters Aqueous (liquid) Concentration M

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

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

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 → 1 (Lego) truck YES 2 NO2(g) → 2 NO(g) + O2 (g) 1 thing = 1mole

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

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

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

1. Measure Gas by change in Volume Zn(s) + 2HCl(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!

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 = 8.3 - 4.0 mL =0.057 ml/s 75 - 0 s Rate in mole H2/min @ STP 0.057 mL s x 1 L x 1 mole x 60 s = 1.5 x 10-4 mole/min 1000 mL 22.4 L 1 min Rate in g HCl/h 1.5 x 10-4 mole H2 min x 2 moles HCl x 36.5 g x 60 min = 0.67 g/h 1 mole H2 1 mole 1 h

Let’s practice: What did we measure? Rate in volume: mL H2 /s =   Rate in volume: mL H2 /s = 8.3 - 4.0 mL =0.057 ml/s 75 - 0 s Rate in moles: H2/min @ STP 0.057 mL s x 1 L 1000 mL x 1 mole 22.4 L x 60 s 1 min = 1.5 x 10-4 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

Initial, Instantaneous and average reaction rates 9/20/2018 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! Why? Pressure in a closed container Pressure Volume Ca(s) + 2HOH(l) → Ca(OH)2(aq) + H2(g)   P=1/V

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

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. (82.07 - 81.63) g Rate = = 0.0059 g/s 75 s Keep all sig figs on calculator 2. Calculate the rate in grams CaCO3/hour 0.0059 g CO2 s x 1 mole x 1 mole CaCO3 x 100.1 g x 3600 s = 48 g/h 1 mole CO2 1 mole 1 h 44.0 g Round final answer

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

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

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.

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.

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 H2SO4 is neutralized with 0.50 M NaOH in 30.0 s, what is the reaction rate in moles NaOH /min. All 0.895 g is used up and some of the 0.50 M is consumed. H2SO4 + 2NaOH → 0.895 g H2SO4 x 1 mole x 2 moles NaOH Rate = 98.1 g 1 mole H2SO4 0.500 min = 0.0365 moles NaOH/min

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