What is a chemical reaction A chemical reaction is a process that converts one or more substances into another substance Chemical Reactions The starting substances are the reactants The resulting substances are the ID: 913966
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Slide1
Chemistry I
Chemical Reactions
Slide2What is a chemical reaction?
A
chemical reaction
is a process that converts one or more substances into another substance.
Slide3Chemical Reactions
The starting substances are the
reactants
.
The resulting substances are the
products
.
We say that the
reactants
will react to
yield
the
product
.
Example: sodium reacts with chlorine to yield sodium chloride.
Slide4Classifying Chemical Reactions
We can classify a chemical reaction by the types of substances involved and the changes they undergo or by the changes in energy.
Slide5Energy Changes
Exothermic
:
A reaction which releases energy.
Endothermic
:
A reaction which absorbs energy.
Slide6The system
In order to discuss a chemical reaction we must define our system.
A system is everything that we are studying.
If something is not a part of the system then it is part of the surroundings.
When we discuss a chemical reaction, the system generally consist of the reactants and the products.
Slide7Exothermic Reaction
The total energy of the products is less than the total energy of the reactants.
The systems energy decreases.
This energy is released to the surroundings.
An observer would notice an increase in the temperature of the surroundings.
It is often said that the reaction gives off heat.
Slide8Endothermic Reaction
The energy of the products is higher than the energy of the reactants.
The energy of the system increases.
Energy moves from the surroundings to the system.
Reactions often require heating.
The reaction may lower the temperature in the surroundings.
Slide9Classification by substances involved.
Chemical reactions can also be classified by the types of substances that make up the reactants and the products and the changes which take place.
Slide10Reaction Types
Synthesis
Decomposition
Single-Replacement
Double-Replacement
Combustion
Slide11Synthesis
Two or more elements or compounds combining to form one more complex compound.
A + B ----> C
Example: hydrogen plus oxygen yields water.
Slide12Decomposition
A complex substance is broken down into two or more simpler substances.
A ----> B + C
Slide13Single Replacement
A single element replaces another element in a compound.
A +BC -----> AC + B
AC + D ------> AD + C
Slide14Double Replacement
Elements in two compounds switch places.
AB + CD -----> AD + CB
An acid-base neutralization is an example of a double replacement.
Slide15Combustion
A hydrocarbon compound reacts with oxygen to produce water and carbon dioxide and releases energy.
HC + O
2
-------> H
2
O + CO
2
Slide16Discussion Questions
Compare and contrast physical and chemical changes.
What clues would you look for to determine whether a reaction was endothermic or exothermic.
Review: What are the indicators of a chemical reaction?
What must be true about a chemical reaction to insure that the law of conservation of mass is not violated? Explain.