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Chemical reactions occur all around us. Some require energy in order to create the reaction while others release energy when they occur.
We discussed in the previous section that chemical reactions begin with 2 or more substances, but then through the reaction process a new substance is produced.
When a new substances with new properties occur, the change can not be reversed.
REACTANTS
The substances that are there before a chemical reaction
PRODUCTS
The substances that form afterwards
What clues are there to indicate a chemical change has occurred?
Signs a chemical change has occurred:
a release of energy
temperature change
colour change
odour
Bubbling or gas production
absorption of energy
Photosynthesis - absorbs heat energy to convert the carbon dioxide and water into glucose and oxygen
cooking food - heat is absorbed into the pan to cook the food
While watching this video, respond to the questions on your handout.
To understand the energy implications of chemical reactions, it’s important to keep in mind two key ideas:
It takes energy to break bonds.
Energy is released when bonds are formed.
In the classic vinegar and baking soda volcano experiment, what compounds are used to initiate the reaction?
vinegar (acetic acid) and baking soda (sodium bicarbonate)
What happens when they are combined?
The compounds react to make molecules.
What are the products that are created?
Sodium acetate, water and carbon dioxide.
Was there a release of energy or an absorption of energy and why?
Energy is release because atoms that have an attraction for one another are brought back together.
What are chemical reactions that release energy called?
exothermic - more energy is released whent eh bonds are formed than it used to break the bonds in the reactants.
What happens when there is an exothermic reaction?
There is an increast in termperature of the reaction mixture.
What are chemical reactions that absorb energy called?
endothermic - more energy is absorbed when the bonds are broken than are released when new bods are formed in the new products.
What happens when there is an endothermic reaction?
There is a decrease in temperature of the reaction mixture.
Combustion and Neutralization
Combustion and Neutralization
What happens when you light a candle?
You light the match or push the button on a lighter.
You then ignite the wick.
You see the wick burning.
You can feel the heat and see the light being given off.
When you burn a candle, the wax burns in the presence of oxygen and it produces carbon dioxide and water.
This is why we need oxygen to allow for the chemical reaction to occur. This is called a combustion reaction.
Neutralization reactions occur when an acid is added to a base to produce a compound and water. The pH of the product is more neutral (closer to 7) than the pH of either reactant. For example: acid-base reaction between baking soda and vinegar produces carbon dioxide bubbles that make a cake being baked to rise.
This reaction can be written as follows:
Magnesium hydroxide reacts with hydrochloric acid to produce magnesium chloride and water.
A word equation can be used as well:
magnesium hydroxide + hydrochloric acid → magnesium chloride + water
Read "Did you Know?" on page 29 to learn other examples of chemical reactions.
Complete the questions for Check for understanding. You will find these on the Unit 2: Exit Slips for section 2.3.
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