RATE IS A MEASURE OF THE SPEED OF ANY CHANGE WITHIN AN INTERVAL OF TIME

Transcription

1 REACTION RATES

2 WHAT IS RATE? RATE IS A MEASURE OF THE SPEED OF ANY CHANGE WITHIN AN INTERVAL OF TIME Ex. Speed is change in distance in a given time interval A sprinter runs 100 meters in 11.5 seconds The speed aka rate is 11m/11.5 sec In chemistry, the rate or speed of a chemical reaction is expressed as the amount of reactant changing per unit of time. Ex. 0.5 moles of iron will rust in 1 year Rate = 0.5 mol/yr

3 Note at the start of the chemical reaction, the reactant (purple particles) concentration is high, there is no product present (green particles) As the reaction progresses, the reactant (purple) gets used up, so concentration decreases. More and more product particles are generated, so product (green) concentration increases. When reaction finishes, only product particle concentration is high. Reactant concentration is low. What is the rate of this reaction? 50 mol/60 sec

4 RATE CAN VARY SOME REACTIONS OCCUR SLOWLY - the rate milk spoils is slow -the rate iron rusts is slow SOME REACTIONS OCCUR QUICKLY -the rate bleach kills mold in the shower is fast -the rate vinegar and baking soda react to make carbon dioxide bubbles in a volcano model is fast

5 FACTORS THAT AFFECT REACTION RATE 1. TEMPERATURE 2. CONCENTRATION 3. PARTICLE SIZE 4. AGITATION 5. USE OF A CATALYST 6. NATURE OF REACTANTS (identity)

6 TEMPERATURE RAISING TEMPERATURE increases reaction rate LOWERING TEMPERATURE slows reaction rate The higher the temperature, the higher the kinetic energy, the faster the particles move. more movement = mole collisions so more product forms Ex. A starter flame is needed to help charcoal and oxygen atoms collide and cross activation energy barrier. The heat of reaction (ΔH) released will then supply energy for more reactants to combust.

7 REACTION RATE INCREASES AS TEMPERATURE INCREASES PRODUCT FORMS FASTER AT HIGHER TEMPERATURE NOTE THAT PRODUCT GRAPH END AT THE SAME PLACE, ONE JUST GETS THERE FASTER.

8 CONCENTRATION HIGHER CONCENTRATION INCREASES REACTION RATE LOWER CONCENTRATION DECREASES REACTION RATE THE HIGHER THE REACTANT CONCENTRATION, THE MORE PARTICLES THAT CAN COLLIDE WHICH INCREASES REACTON RATE

9 HIGHER CONCENTRATION REACTION OCCURS FASTER (HIGHER RATE) LOWER CONCENTRATION REACTION OCCURS SLOWER (LOWER RATE) NOTE THAT BOTH GRAPHS END AT THE SAME PLACE, ONE JUST GETS THERE FASTER.

10 PARTICLE SIZE SMALLER PARTICLES REACT FASTER LARGER PARTICLES REACT SLOWER Ex. Sugar dust in air will combust faster then roasting a marshmellow Smaller particles have more surface area, which means more atoms are exposed to other reactants and more likely to collide. The larger block has several atoms hidden on the interior. Those interior atoms cannot contact other reactants, cannot react.

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12 AGITATION INCREASING AGITATION INCREASES REACTION RATE DECREASING AGITATION DECREASES REACTION RATE STIRING OR SHAKING REACTANTS INCREASES MOVEMENT & COLLISIONS, WHICH INCREASES REACTION RATE

13 USE OF A CATALYST USING A CATALYST INCREASES REACTION RATE NO CATALYST, REACTION OCCURS SLOWER CATALYST substance that increases reaction rate without being used up itself. A catalyst will lower the amount of energy needed aka activation energy for reaction to occur. Ex. The decomposition of hydrogen peroxide uses a catalyst of manganese (II) oxide. H 2 O 2 MnO 2 H 2 O + O 2

14 The green dotted line shows that less energy is added to reactants (bond breaking) when a catalyst is used in the formation of products (bond making). The red line shows an uncatalyzed reaction requiring more energy (higher on y-axis).

15 NATURE OF REACTANTS SOME ELEMENTS ARE MORE REACTIVE THEN OTHERS, SO THOSE REACTIONS WOULD OCCUR AT A FASTER RATE. Ex. Metals are more reactive the nonmetals

16 CALCULATING REATION RATE Reaction rate = - change in concentration change in time = -ΔM Δt - [Concentration reactant] at time 2 [Concentration reactant] at time 1 t 2 t 1 T 2 Time 2 T 1 Time 1 [Concentration] = Molarity

17 CALCULATING REACTION RATE Consider the decomposition reaction below. What is the rate of disappearance of the reactant AB from time 40 seconds to 80 seconds? SOLUTION: AB A + B TIME (seconds) CONCENTRATION of AB (moles/liter) RATE = - [0.17M] - [0.51M] = 0.34 M = M/s 80s -40s 40s RATE IS molar per second

18 CALCULATING REACTION RATE Consider the decomposition of hydrogen peroxide in the reaction below. Calculate the amount of O 2 formed at 60 seconds? SOLUTION: 2 H 2 O 2 2 H 2 O (l) + 1 O 2 (g) First, calculate how much H 2 O 2 reacted at 60s. TIME (seconds) CONCENTRATION of H 2 O 2 (moles/liter) M M = 0.92 M reacted Now use mole ratio (coefficients) of the balanced equation to calculate how much product was made mol H 2 O 2 x 1 mole O 2 = 0.46 mole O 2 will be 2 mole H 2 O 2 made