Chemistry ( ) Rates of Reaction © SSER Ltd.

Chemical Reactions Chemistry is effectively the study of the changes which occur when particles collide resulting in the making or breaking of chemical bonds. All chemical reactions involve the making or breaking of bonds and new substances being formed, e.g... The animation illustrates the combustion of methane... CH 4 + 2O 2  2H 2 O + CO 2

Chemical Reactions Chemistry is effectively the study of the changes which occur when particles collide resulting in the making or breaking of chemical bonds. All chemical reactions involve the making or breaking of bonds and new substances being formed, e.g. The animation illustrates the combustion of methane... CH 4 + 2O 2  2H 2 O + CO 2

Reactants and Products This is an example of a combustion reaction. CH 4 + 2O 2  2H 2 O + CO 2 The reactants are methane and oxygen. The products are water and carbon dioxide. In the chemical reaction shown in the equation above, methane (CH 4 ) and oxygen (O 2 ) react to form water (H 2 O) and carbon dioxide (CO 2 ).

Fast and Slow Reactions Some chemical reactions happen very quickly.

Fast and Slow Reactions Some chemical reactions happen very slowly.

Collisions and Reactions Chemical reactions can only take place when the reactant molecules collide with one another.

Collisions and Reactions If the collision is energetic enough, then a reaction may occur. Chemical reactions can only take place when the reactant molecules collide with one another. The energy needed to bring about a chemical reaction is called the Activation energy.

Any factors which can make the collisions more violent, or make them happen more frequently will speed up a chemical reaction. Collisions and Reactions

The quantity of reactants The concentration of fluid reactants The surface area of solid reactants The temperature of the reaction Light The presence of a catalyst Factors which Affect the Rate of Chemical Reactions Rate of Reaction

Concentration - Solutions If more molecules of a reactant are dissolved in solution, then more collisions are likely to occur, and the reaction takes place quicker. The concentration of solutions is measured in mol/dm 3. Solutions of the same concentration contain the same number of particles.

Concentration - Gases If the gas pressure of a reactant is increased, then more collisions are likely to occur, and the reaction takes place quicker. Gases at the same pressure and temperature contain the same number of particles.

Surface Area In a reaction between a solid and a liquid, the reaction takes place at the surface of the solid particles. Liquid particles cannot all make contact with the solid to react at the same time. If large particles of solid are broken into smaller particles, there is a larger area of the solid exposed to the liquid - more collisions will occur per second and the reaction is therefore faster. Number of particles in contact with the surface. 3 8

Catalysis The addition of a catalyst to a chemical reaction can affect the rate of reaction. A catalyst speeds up a chemical reaction, but is unchanged itself. Only specific catalysts will work for certain reactions, e.g. Platinum is used as a catalyst in the catalytic converters found in modern car exhaust systems. The platinum helps to convert harmful exhaust gases such as carbon monoxide and oxides of nitrogen, into harmless nitrogen, oxygen and carbon dioxide.

Temperature If the particles are moving faster, then collisions will occur more often and be more violent, and the reaction is more likely to occur. Move the slide bar to see the effect of increasing temperature. The Kinetic Theory explains why an increase in temperature makes particles move faster.

Temperature If the particles are moving faster, then collisions will occur more often and be more violent, and the reaction is more likely to occur. Move the slide bar to see the effect of increasing temperature. The Kinetic Theory explains why an increase in temperature makes particles move faster.

An increase in temperature will increase the rate of reaction - an increase of 10ºC will double the rate of many reactions. Reaction at 10 ºC Reaction at 20 ºC Temperature

The low temperatures, in a refrigerator (4ºC), slow down the chemical reactions which cause food to decay. Therefore the foods ‘last longer’. Temperature

Some chemical reactions such as photosynthesis are activated by light. Increased light intensity tends to speed up the reaction. Light energy can make reactant particles more energetic, and therefore more likely to react upon collision. Light

The chemicals impregnated on photographic film react only when they are exposed to light. Light

Rate of Reaction Industrial processes, and food preparation and cookery, involve chemical reactions whose timing needs to be carefully controlled. Chemists, therefore, study rates of reaction very carefully. The time taken for a chemical reaction to occur is an important issue in many everyday situations.

Rate of Reaction The rate of a chemical reaction is defined as the rate at which reactants are used up, or products are formed. This can be represented by the calculation: Rate of Reaction = Amount of reactant used Time Rate of Reaction = Amount of product produced Time OR

Rate of Reaction Rate of Reaction = Amount of product produced Time In a reaction between 5g zinc and 50cm 3 sulfuric acid, 50cm 3 of hydrogen gas is produced in 2 minutes. What is the rate of reaction? Rate of Reaction = 50cm 3 2 minutes Rate of Reaction =25 cm 3 per minute = 25 cm 3 /min

Measuring Rates We need to be imaginative at times in the way we choose to follow the course of a reaction. If the reaction being followed generates a gas as a product, the rate can be measured using the following apparatus:

Measuring Rates

Another method used to follow a reaction which generates a gas as a product, depends upon monitoring the loss of the gas.

In the reaction between sodium thiosulfate solution and dilute hydrochloric acid, a yellow precipitate of sulfur forms, and this can be used to measure the rate of reaction. Measuring Rates If we can ‘see’ a change occurring, this can be used to measure the rate of a reaction.

The reaction is started by adding a small measured volume of dilute hydrochloric acid to a larger measured volume of sodium thiosulfate solution. The time taken for the cross to disappear is measured by a stopwatch. Measuring Rates If the reaction is carried out at different temperatures, the time taken for the cross to disappear changes. How would you control and measure the temperature of the reaction?

Time taken for cross to disappear / s Temperature / ºC x x x x x The results can be plotted and a graph like this produced... Measuring Rates

This apparatus was used to investigate the reaction of sulfuric acid with magnesium. The results from this experiment are: sulfuric acid + magnesium hydrogen + magnesium sulfate  H 2 SO 4 (aq) Mg (s) MgSO 4(aq) H 2(g)  ++

Volume of hydrogen produced/cm 3 Time /minutes Measuring Rates How long does it take to produce 80cm 3 of hydrogen? 20 minutes

Summary The rate of a chemical reaction can be measured by looking at the rate of loss of a reactant, or the rate of production of a product. The rate of a chemical reaction can be affected by the...  Amount of reactants;  Surface area of solid particles;  Temperature in the reaction vessel;  Presence of light;  Presence of a catalyst.

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