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Types of reactions.

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Presentation on theme: "Types of reactions."— Presentation transcript:

1 Types of reactions

2 What is a Chemical Reaction?
A chemical reaction is the process by which one or more substances change into one or more new substances. Reactants are the original substances in a chemical reaction. Products are the substances that are created in a chemical reaction.

3 Sample Chemical Reaction
Video 1 Click for sample reaction Video 2

4 Formula for Reaction 2Al + 3Br2  2AlBr3 Reactants Product

5 Symbols in A Chemical Reaction

6 Evidence of a Chemical Reaction

7 Chemical Reaction Versus Physical Change
chemical change – new substance forms with properties that differ from original substance density boiling point melting point physical change - changes of state evaporation condensation melting freezing

8 Types of Reactions A combustion reaction is a reaction of a carbon-based compound with oxygen. Combustion of propane: C3H8 + 5O2  3CO2 + 4H2O Combustion of ethanol: CH3CH2OH + 3O2  2CO2 + 3H2O

9 Sample Combustion Click for combustion reaction

10 Synthesis Reaction In a synthesis reaction a single compound forms from two or more reactants. Two elements form a binary compound C + O2  CO2 2C + O2 2CO Two compounds form a ternary compound CaO(s) + H2O(l)  Ca(OH)2(s) CO2(g) + H2O(l)  H2CO3(aq)

11 Example: 2H2 + O2  2H2O Example: C + O2  CO2
Synthesis reactions occur when two substances (generally elements) combine and form a compound. (Sometimes these are called combination or addition reactions.) reactant + reactant  1 product A + B → AB Example: 2H2 + O2  2H2O Example: C + O2  CO2

12 Sample Synthesis Click to see the synthesis of hydrogen and oxygen

13 Decomposition Reactions
In a decomposition reaction a single compound breaks down, often with the input of energy, into two or more elements or simpler compounds. Decomposition of water electricity 2H2O(l) O2(g) + 2H2(g)

14 1 Reactant  Product + Product In general: AB  A + B
Decomposition reactions occur when a compound breaks up into the elements or in a few to simpler compounds 1 Reactant  Product + Product In general: AB  A + B Example: H2O  2H2 + O2 Example: HgO  2Hg + O2

15 Sample Decomposition Click to see the decomposition of ammonium dichromate Click to see the continuation of the decomposition

16 Single Displacement In a displacement reaction a single element reacts with a compound and displaces another element from the compound. 2Al(s) + 3CuCl2(aq)  2AlCl3(aq) + 3Cu(s) Aluminum displaces copper.

17 A + BC  AC + B (if A is a metal) Zn(s) + HCl(aq)  ZnCl2 + H2(g)
Single Replacement Reactions occur when one element replaces another in a compound. A metal can replace a metal (+) OR a nonmetal can replace a nonmetal (-). element + compound product + product A + BC  AC + B (if A is a metal) Zn(s) HCl(aq)  ZnCl2 + H2(g)

18 Sample Single Displacement
Click to see the alkali series with water

19 Double Displacement In a double-displacement reaction two compounds in aqueous solution appear to exchange ions and form two new compounds. One of the products must be a solid precipitate, a gas, or a molecular compound, such as water. HCl(aq) + NaOH(aq)  HOH(l) + NaCl(aq)

20 Sample Double Displacement
Click to see double displacement

21 Law of Conservation of Mass
Atoms cannot be created or destroyed, in a chemical reaction, they are rearranged. The atoms that make up the reactants are still there in the products – they are just rearranged. The total mass of the reactants must equal the total mass of the products. When balancing equations, reactant atoms equal the atoms on the product side. Cannot change subscripts when balancing equations. Coefficients are used. (Number added to the front of a compound.

22 Rates of reaction Objectives
To understand that a chemical reaction involves collisions between particles To be able to describe the four factors which will affect the rate of a chemical reaction.

23 Boardworks GCSE Additional Science: Chemistry Rates of Reaction
Why are some reactions faster than others? Teacher notes This illustration contains several discussion points relating to rates of reaction, including: Red characters: these represent particles that have a large amount of kinetic energy and are therefore likely to react. Blue characters : these represent particles that have a small amount of kinetic energy and are therefore unlikely to react (hence why some are asleep). Bumper cars: the collision between two bumper cars represents the large amount of energy released during a reaction.

24 What does rate of reaction mean?
Boardworks GCSE Additional Science: Chemistry Rates of Reaction What does rate of reaction mean? The speed of different chemical reactions varies hugely. Some reactions are very fast and others are very slow. The speed of a reaction is called the rate of the reaction. What is the rate of these reactions? rusting baking explosion Photo credit: © 2007 Jupiterimages Corporation slow fast very fast

25 Rates of Reaction A chemical reaction involves a collision between particles. The particles collide and make new substances The particles which react are called the reactants The substances which are made are called the products

26 Reactions, particles and collisions
Boardworks GCSE Additional Science: Chemistry Rates of Reaction Reactions, particles and collisions Reactions take place when particles collide with a certain amount of energy. The minimum amount of energy needed for the particles to react is called the activation energy, and is different for each reaction. The rate of a reaction depends on two things: the frequency of collisions between particles the energy with which particles collide. Teacher notes See the ‘Energy Transfer’ presentation for more information on activation energy. If particles collide with less energy than the activation energy, they will not react. The particles will just bounce off each other.

27 Changing the rate of reactions
Boardworks GCSE Additional Science: Chemistry Rates of Reaction Changing the rate of reactions Anything that increases the number of successful collisions between reactant particles will speed up a reaction. What factors affect the rate of reactions? increased temperature increased concentration of dissolved reactants, and increased pressure of gaseous reactants increased surface area of solid reactants use of a catalyst.

28 Boardworks GCSE Additional Science: Chemistry Rates of Reaction
Slower and slower! Reactions do not proceed at a steady rate. They start off at a certain speed, then get slower and slower until they stop. As the reaction progresses, the concentration of reactants decreases. This reduces the frequency of collisions between particles and so the reaction slows down. 0% 25% 50% 75% 100% percentage completion of reaction reactants product

29 Boardworks GCSE Additional Science: Chemistry Rates of Reaction
Teacher notes This animated graph summarizes the qualitative information provided by the gradient of a graph that plots amount of product in a reaction against time.

30 Boardworks GCSE Additional Science: Chemistry Rates of Reaction
Teacher notes This animated graph follows-on from the graph on the previous slide, and illustrates how the change in the rate of a reaction can be explained in terms of changing amounts of reactants and product.

31 Temperature and collisions
Boardworks GCSE Additional Science: Chemistry Rates of Reaction Temperature and collisions How does temperature affect the rate of particle collision?

32 Temperature When we increase the temperature we give the particles energy This makes them move faster This means they collide with other particles more often So the reaction goes faster.

33 Temperature and particle collisions
Boardworks GCSE Additional Science: Chemistry Rates of Reaction Temperature and particle collisions Teacher notes This simulation illustrates how increasing the temperature increases the number of collisions between particles.

34 Surface area If we make the pieces of the reactants smaller we increase the number of particles on the surface which can react. This makes the reaction faster. The particles on the surface can react When cut into smaller pieces the particles on the inside can react

35 Effect of surface area on rate of reaction
Boardworks GCSE Additional Science: Chemistry Rates of Reaction Effect of surface area on rate of reaction Any reaction involving a solid can only take place at the surface of the solid. If the solid is split into several pieces, the surface area increases. What effect will this have on rate of reaction? low surface area high surface area This means that there is an increased area for the reactant particles to collide with. The smaller the pieces, the larger the surface area. This means more collisions and a greater chance of reaction.

36 Surface area and particle collisions
Boardworks GCSE Additional Science: Chemistry Rates of Reaction Surface area and particle collisions Teacher notes This simulation illustrates how increasing the surface area increases the number of collisions between particles and solid reactants

37 Concentration There are more particles in the same volume to react
If we make one reactant more concentrated (like making a drink of orange squash more concentrated) There are more particles in the same volume to react So the reaction goes faster. There are less red particles in the same volume so there is less chance of a collision There are more red particles in the same volume so there is more chance of a collision so the reaction goes faster

38 Bell Work 11/28 What does a reaction rate tell you?
What 5 factors affect reaction rates? How does a catalyst make a reaction go faster?

39 Bell Work Review 1. How fast a chemical reaction is going. 2. Temperature, concentration, surface area, stirring and catalysts (pressure can also be listed). 3. By lowering the activation energy.

40 Effect of concentration on rate of reaction
Boardworks GCSE Additional Science: Chemistry Rates of Reaction Effect of concentration on rate of reaction The higher the concentration of a dissolved reactant, the faster the rate of a reaction. Why does increased concentration increase the rate of reaction? At a higher concentration, there are more particles in the same amount of space. This means that the particles are more likely to collide and therefore more likely to react. lower concentration higher concentration

41 Concentration and particle collisions
Boardworks GCSE Additional Science: Chemistry Rates of Reaction Concentration and particle collisions Teacher notes This simulation illustrates how increasing the concentration increases the number of collisions between particles.

42 Effect of pressure on rate of reaction
Boardworks GCSE Additional Science: Chemistry Rates of Reaction Effect of pressure on rate of reaction Why does increasing the pressure of gaseous reactants increase the rate of reaction? As the pressure increases, the space in which the gas particles are moving becomes smaller. The gas particles become closer together, increasing the frequency of collisions. This means that the particles are more likely to react. lower pressure higher pressure

43 Using a catalyst A catalyst is a chemical which is added to a reaction. It makes the reaction go faster. The catalyst does not get used up in the reaction. It gives the reaction the energy to get started

44 Boardworks GCSE Additional Science: Chemistry Rates of Reaction
What are catalysts? Catalysts are substances that change the rate of a reaction without being used up in the reaction. Catalysts never produce more product – they just produce the same amount more quickly. reaction (time) energy (kJ) Ea without catalyst Different catalysts work in different ways, but most lower the reaction’s activation energy (Ea). Ea with catalyst Teacher notes See the ‘Energy Transfer’ presentation for more information on activation energy.

45 Boardworks GCSE Additional Science: Chemistry Rates of Reaction
Everyday catalysts Many catalysts are transition metals or their compounds. For example: Nickel is a catalyst in the production of margarine (hydrogenation of vegetable oils). Iron is a catalyst in the production of ammonia from nitrogen and hydrogen (the Haber process). Platinum is a catalyst in the catalytic converters of car exhausts. It catalyzes the conversion of carbon monoxide and nitrogen oxide into the less polluting carbon dioxide and nitrogen. Photo credit: © 2007 Jupiterimages Corporation Teacher notes See the ‘Reversible Reactions’ presentation for more information on the Haber process.

46 Boardworks GCSE Additional Science: Chemistry Rates of Reaction
Catalysts in industry Why are catalysts so important for industry? Products can be made more quickly, saving time and money. Catalysts reduce the need for high temperatures, saving fuel and reducing pollution. Teacher notes See the ‘Enzymes’ Biology presentation for more information on enzymes as catalysts and their use in industrial processes. Catalysts are also essential for living cells. Biological catalysts are special types of protein called enzymes.


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