Chemical Reactions Dr. (Mrs.) Ndukwe, Nelly Acha MOUNTAIN TOP UNIVERSITY CHM 101 Chemical Reactions Dr. (Mrs.) Ndukwe, Nelly Acha           Chemical Reactions by Dr. (Mrs.) Ndukwe, Nelly Acha is licensed under a Creative Commons Attribution-Non Commercial 4.0 International License

Definition of Chemical Reaction A chemical reaction is a chemical change which forms new substances. In a chemical reaction atoms/molecules comprising the reactants combine together to form products Chemical reactions are usually characterized by the formation of one or more products with properties different from the reactants. All chemical reactions involve one form of chemical transformation or another. Common examples around us include beer making where yeast turns the sugar into alcohol and rusting of iron because of exposure to air.

Breakage of Bonds during Chemical Reaction Energy plays important role in chemical reactions as bonds between atoms in the reactants must be broken. Atoms and molecules are reassembled into products by forming new bonds. Energy is absorbed to break bonds and energy is evolved as bonds are made. Chemical reactions occur at a characteristic reaction rate at a given temperature and chemical concentration. Reaction rates increase with increasing temperature because there is more thermal energy available to reach the activation energy necessary for breaking bonds between atoms

The Four Major Types of Chemical Reactions (1) Direct combination or Synthesis Reactions (2) Decomposition reactions (3) Substitution or Single Replacement Reactions (4) Metathesis or Double Displacement Reactions

The Four Major Types of Chemical Reactions       Name                             General Reaction Pattern Direct combination or synthesis                A  +  B ----> AB Decomposition                                              AB ---->  A  +  B Substitution or Single Replacement       A  +  BC ---->  B  +  AC Metathesis or Double Displacement        AB  +  CD ----AD  +  CB

Direct Combination or Synthesis Reactions In combination or synthesis reaction, two or more reactants unite to form a single product. It involves the combination of two or more chemical species combine to form a more complex product. A well-known example of a synthesis reaction is the formation of water via the fusion of hydrogen gas and oxygen gas Another example of a synthesis reaction is the formation of sodium chloride (table salt).

Other Examples of Synthesis Reactions S   +   O2 --------->  SO2 sulphur    oxygen            sulphur dioxide   2 S      +    3 O2 --------->  2 SO3    sulphur        oxygen                 sulphur trioxide 2 Fe   +   O2  --------->  2 FeO    iron        oxygen                    iron (II)  oxide  

Rusting of Iron During a Synthesis Reaction

Decomposition or Analysis Reaction A single reactant is decomposed or broken down into two or more   products. In a decomposition reaction a compound is broken into smaller chemical species                 CaCO3   ---------->    CaO     +      CO2      calcium carbonate          calcium oxide     carbon dioxide The electrolysis of water into oxygen and hydrogen gas is an example of a decomposition reaction:          2 H2O ----------->  2 H2    +     O2         water                           hydrogen        oxygen

Substitution or Single Replacement Reactions A single free element replaces or is substituted for one of the elements in a compound.  The free element is more reactive than the one its replaces.    Zn    +     2 HCl   ---------->  H2    +  ZnCl2       zinc      hydrochloric acid             hydrogen    zinc chloride    Cu   +   2 AgNO3  ----------->  2 Ag    +    Cu(NO3)2  copper        silver nitrate               silver    copper (II) nitrate   H2    +   2 AgNO3 ----------->  2 Ag   +   2 HNO3 hydrogen     silver nitrate             silver         nitric acid 2 Na   +  2 H2O  ----------->  2 NaOH    +     H2  sodium          water              sodium hydroxide     hydrogen

Metathesis or Double Displacement Reactions This reaction type can be viewed as an "exchange of partners."  For ionic compounds, the positive ion in the first compound combines with the negative ion in the second compound, And the positive ion in the second compound combines with the negative ion in the first compound.

Metathesis or Double Displacement Reactions    HCl    +     NaOH  ----------->  NaCl   +  H2O       hydrochloric        sodium          sodium chloride        water               acid               hydroxide                           BaCl2   +   2 AgNO3 ---------->  2 AgCl    +   Ba(NO3)2    barium                  silver                  silver          barium    chloride                nitrate               chloride              nitrate                                                                                       (precipitate) CaCO3   +   2 HCl   ----------->  CaCl2   +  H2CO3  calcium         hydrochloric           calcium         carbonic  carbonate             acid                 chloride            acid

Acid-Base (Neutralization) Reactions In simple terms, an acid is a substance which can lose a H+ ion (i.e. a proton) and a base is a substance which can accept a proton. When equal amounts of an acid and base react, they neutralize each other to form neutral species. For example, when hydrochloric acid and sodium hydroxide react, they form water and sodium chloride (table salt).

Combustion Reaction Combustion also known as burning is the combination of a substance with oxygen. The products are carbon dioxide, water, and possible other waste products Combustion is similar to a decomposition reaction, except that oxygen and heat are required for it to occur. If there is not enough oxygen, the reaction may not occur. Sometimes, with limited oxygen, the reaction will occur, but it produces carbon monoxide (CO) or even soot to give incomplete combustion.

Difference Between a Chemical Reaction and a Physical Change There are several differences between a physical and chemical change in matter or substances. In a physical change a substance doesn't change what the substance is. While in a chemical reaction, a new substance is formed and energy is either given off or absorbed.

Difference Between a Chemical Reaction and a Physical Change CONT. If a piece of paper is cut into small pieces it still is paper. This would be a physical change in the shape and size of the paper. If the same piece of paper is burned, it is broken up into different substances that are not paper to give a chemical reaction

Difference between a chemical reaction and a physical change Physical changes can be reversed; chemical changes cannot be reversed easily without extraordinary means, if at all. For example, a cup of water can be frozen when cooled and then can be returned to a liquid form when heated. Mixture of sugar into water to make sugar solution is physical change as the water could be left out to evaporate and the sugar crystals would remain. However, in cake making involving mixture of flour, water, sugar and other ingredients will take extraordinary means to separate the various ingredients out to their original form

Exothermic and Endothermic Reactions When heat is given off in a chemical change or reaction, It is called an exothermic reaction. When heat is absorbed in a chemical change or reaction, It is called an endothermic reaction. The speed at which chemical reactions take place depend on the temperature pressure and how concentrated the substances involved in the chemical reaction are. Sometimes substances called catalysts are used to speed up or help along a chemical reaction.           Chemical Reactions by Dr. (Mrs.) Ndukwe, Nelly Acha is licensed under a Creative Commons Attribution-Non Commercial 4.0 International License