Rates of Reaction - Chp 16 nf

Formula for average rate of reaction:

Average rate = Total vol. given off (cm³)/Total time for reaction to reach completion (mins)

Rate of Reaction

the change in concentration per unit of time of any one reactant of product

Instantaneous rate of reaction

the rate of reaction at any one particular time during the reaction

How do you find the instantaneous rate of reaction?

Find the slope of the tangent at the time

• plot results on a graph

• draw a tangent to the time

  • find the rise/run = slope

What are the 5 factors affecting the rate of reaction

• nature of reactants

• particle size

• concentration

• temperature

• catalysts

How does the nature of reactants affect the rate of reaction?

• reactions occur more quickly when ions exist as free ions in solution

• this is because the oppositely charged ions are coming together + no bonds need to be broken

• reactions involving covalent compounds occur much more slowly

• the covalent bonds must first be broken so new bonds can be formed in the reaction

How does the particle size affect the rate of reaction?

• the smaller the particle size the greater the overall surface area is

  • (this means that there can be more successful collisions)

Draw a diagram + describe an experiment that shows how particle size affects rate of reaction

1. find the mass of a known mass of marble chips in a conical flask and a known volume of dilute HCl in a graduated cylinder using a mass balance

2. a cotton wool ball is placed in the mouth of the conical flask to prevent loss of spray + allows CO₂ to escape

3. the dilute HCl is added to conical flask and the cotton wool is replaced, the stopclock is started

4. mass is recorded every 30s

5. the difference in mass is the mass of carbon dioxide produced

What is a dust explosion?

a build up of very fine dust in the air that can lead to an explosion

Draw a diagram for an experiment to illustrate materials in a powdered state undergo very fast reactions

What conditions are needed for a dust explosion to take place?

1. must occur in an enclosed space

2. particles must be combustible

3. must be a source of ignition

4. particles must be dry

5. must be oxygen present

How does the concentration affect the rate of reaction?

• the higher the concentration the faster the reaction occurs

• there are more reactants colliding so there is a higher chance of successful collisions

How does the temperature affect the rate of reaction?

• the higher the temperature the more energy the particles have

• this leads to more collisions and a higher chance of successful collisions

Catalyst

a substance that alters the rate of a chemical reaction but is not consumed in the reaction

What are the general properties of catalysts?

• they are recovered chemically unchanged at the end of the reaction

• they are specific: they will work for one particular reaction or a class of similar reactions

• needed only in small amounts

• can be destroyed by certain substances called catalyst poisons

Enzyme

a substance that is produced by a living things and acts as a biological catalyst

What are the types of catalysis?

• homogenous catalysis

• heterogenous catalysis

• autocatalysis

Homogenous catalysis

catalysis in which both the reactants and catalyst are in the same phase/there is no boundary between them

Give an example of an experiment to demonstrate homogenous catalysis:

The decomposition of hydrogen peroxide with a potassium iodide catalyst

• concentrated potassium iodide solution is added to washing up liquid

• hydrogen peroxide solution is added + decomposes into oxygen + water

• oxygen released and carries foam from the washing-up liquid

Heterogenous catalysis

catalysis in which the reactants and catalysts are in different phases/there is a boundary between them

Give an example + describe an experiment to demonstrate heterogenous catalysis:

Catalytic oxidation of methanol by platinum

Method

• some methanol is placed in a conical flask

• a T shaped piece of cardboard covered in aluminium foil is put in conical flask

• in the fume cupboard the methanol is warmed

• a spiral of platinum wire is heated until red hot using a Bunsen burner

• the hot wire is hung inside the conical flask

Observation

• the hot wire glows red hot (exothermic reaction), popping sounds (H₂ in the absence of sufficient oxygen) are heard

• a cycle takes place; the wire glows, popping, the wire cools then repeats

• smell of methanol and a flame may be observed

Explanation

• T piece acts as a chimney to allow waste gases to leave + more air to flow into the conical flask

• T covered in foil to prevent it from burning

• glowing ceases for a while as CO is formed due to incomplete combustion which poisons the catalyst: after the CO leaves the platinum, it starts working as a catalyst again and glows again

• the flame is caused by the wire igniting the methanol vapour

Autocatalysis

catalysis in which one of the products of the reaction acts as a catalyst for the reaction

Give an example of an experiment to demonstrate autocatalysis:

Reaction between permanganate ions and iron (II) ions

What are the 2 mechanisms of catalysts?

• The Intermediate Formation Theory

• The Surface Adsorption Theory

Describe the intermediate formation theory:

• one or more of the reactants combine with the catalyst to form an intermediate compound

• the intermediate then reacts with the other reactant to give the final produce and regenerate the catalyst

Give an example for the intermediate formation theory:

Oxidation of the tartrate ion using hydrogen peroxide catalysed by Co²⁺ ions

• the solution is pink at the start due to the presence of cobalt ions

• after adding hydrogen peroxide fizzing is observed due to the formation of carbon dioxide

• solution changes to green due to the formation of the intermediate

• the fizzing stops and the solution returns to pink as the cobalt ions have not been used up in the reaction + has been regenerated

Describe using an example the surface adsorption theory:

1. adsorption stage - the hydrogen and oxygen molecules diffuse towards the surface and adsorb to the surface of the platinum; they are held on the surface by temporary bonds

2. reaction on surface - the higher concentration of the molecules on the surface of the catalyst makes it more likely that the molecules will collide with eachother; covalent bonds holding the atoms together are weakened by the formation of these new temporary bonds

3. desorption stage - products/gaseous water leave the surface of the catalyst and diffuse away from it, then more reactants adsorb to its surface

Catalytic converter

a device in the exhaust system of a motor vehicle which contains catalysts to convert pollutants in the exhaust gases to less harmful substances

What fumes are released by motor vehicles prior to being catalysed?

CO

NO

NO₂

unburned hydrocarbons

Why are fumes are released by motor vehicles prior to being catalysed more harmful?

• CO is highly poisonous

• NO_x compounds dissolve in water to produce nitric acid/acid rain

• unburned hydrocarbons contribute to smog

What metals are found in a catalytic converter?

platinum, paladium, rhodium

Catalyst poison

a substance that makes a catalyst inactive

What gives the catalytic converter a large surface area?

the ceramic honeycomb surface

Write an equation for the experiment for the rate of production of oxygen from hydrogen peroxide using a manganese dioxide catalyst

Describe the experiment for the rate of production of oxygen from hydrogen peroxide using a manganese dioxide catalyst

• dilute hydrogen peroxide is placed in a conical flask

• the delivery tube is connected to an inverted graduate cylinder filled with water in a basin of water

• a small amount (a tip of a spatula) of manganese dioxide is added to the conical flask

• the stopper is inserted quickly and the stopwatch is started and the solution is swirled

• the volume of gas produced is measured at regular intervals

• the results are graphed

Write an equation for the experiment to show the effect of concentration on the reaction rate using sodium thiosulfate and hydrochloric acid

Describe the experiment to show the effect of concentration on the reaction rate using sodium thiosulfate and hydrochloric acid

• using a graduation cylinder a known volume of 0.2M sodium thiosulfate solution is poured into a conical flask

• the flask is placed on a white sheet of paper which has been marked with a cross

• using another graduated cylinder another known volume of HCl solution is quickly added

• the flask is swirled and the stopclock is started

• the cross is observed, the solution in the flask becomes more milky, a creamy yellow colour is observed

• the stopclock is stopped when the cross is no longer visible

• the equipment is rinsed

• 0.2 M sodium thiosulfate is diluted to different degrees and the process is repeated

• graph the results

Describe the experiment to show the effect of temperature on the reaction rate using sodium thiosulfate and hydrochloric acid

• using a graduated cylinder a fixed quanitity of 0.05M sodium thiosulfate is poured into a conical flask

• the flask is placed on a white sheet of paper which has been marked with a cross

• using another graduated cylinder another known volume of HCl solution is quickly added

• the flask is swirled and the stopclock is started

• the cross is observed, the solution in the flask becomes more milky, a creamy yellow colour is observed

• the stopclock is stopped when the cross is no longer visible

• the equipment is rinsed

• the same quantity of sodium thiosulfate and HCl solution is heated to a higher temperature

• graph the results