Section 4.1CHEMICAL REACTIONS

• A chemical reaction occurs when 2 or more substances combine to form a new substance.

• Reactants – materials that are combined to start a reaction

• Products – new materials produced by the reaction

• A chemical reaction can be written as a word equation, with reactants on the left and products on the right.

• Label the reactants and products in the following word equation:

wood + oxygen ---- carbon dioxide + water + energy

• A chemical reaction can be written as a word equation, with reactants on the left and products on the right.

• Label the reactants and products in the following word equation:

wood + oxygen ---- carbon dioxide + water + energy

REACTANTSLeft side

PRODUCTSRight side

Remember:Indicators of Chemical Reactions are:

• color change• change in odor• formation of a solid (precipitate) • formation of a gas (bubbling)• release or absorption of energy

(turning hot or cold)

Chemical Reactions Involving Energy

• Exothermic reactions release energy to the surroundings. (heat ‘exits’)

• Give 1 example:

• Endothermic reactions absorb energy from their surroundings. (heat ‘enters’)

• Give 1 example:

Chemical Reactions Involving Oxygen

• A combustion reaction occurs when oxygen reacts with a chemical to produce a new substance and a release of energy. (burning)

• Corrosion occurs when oxygen in the air reacts with a metal, such as the rusting of iron.

• Cellular respiration takes place in body cells. It is the reaction of oxygen with glucose from food to produce water, carbon dioxide and energy.

C6H12O6(s) + O2(g) CO2(g) + H2O(l )+ 36 ATP (energy)

(glucose)

Write a word equation and then a chemical equation for the following reactions:

1. When hydrogen peroxide (H2O2) is left out in the sun it decomposes into water and oxygen gas.

Write a word equation and then a chemical equation for the following reactions:

1. When hydrogen peroxide (H2O2) is left out in the sun it decomposes into water and oxygen gas.

Hydrogen peroxide + sunlight water + oxygen

H2O2 + sunlight H2O + O2

2. Iron left exposed to the atmosphere rusts.

2. Iron left exposed to the atmosphere (oxygen) rusts.

iron + oxygen iron oxide

Fe + O2 FeO

or

Fe + O2 Fe2O3

3. Sodium and bromine react explosively to produce sodium bromide.

3. Sodium and bromine react explosively to produce sodium bromide.

sodium + bromine sodium bromide + Energy

Na + Br2 NaBr + Energy

• Do Reflection

page 162

# 4,5,8,9

Section 4.2 Conservation of Mass

• The Law of Conservation of Mass: In any chemical reaction, the total mass of the reactants is equal to the total mass of the products.

Matter is neither created nor destroyed during chemical reactions, though it may change in appearance.

• Sometimes it may appear as though this law is not upheld;

ex. when Alka-Seltzer is added to water,

fizzing occurs as the reaction proceeds • We find the final mass of the products is less then

the initial mass of the reactants.

Why is this?

What is an open system?

• The law is proven in a closed system, where additional materials are not able to enter or leave during a chemical reaction.

• If the beaker were capped, the gas would be forced to stay in the beaker.

• The final mass would then equal the initial mass.

Reactants Products

Section 4.3Factors Affecting the Rate of Reaction

How to speed up chemical reactions (and get a date). Catalysts• A catalyst helps a reaction proceed more quickly. • It is not altered during the reaction, so it is NOT a

reactant. • ex: In your intestine, many catalysts, called enzymes,

are working to speed the rate of breaking down food. • Fun fact: without these enzymes each meal would take

100 years to digest! Q: What does the enzyme catalase do? (see p.166)

• The enzyme helps the substrate it reacts with, break down.

• Note that the enzyme is not changed by the reaction.

2. Concentration

With more reactants, a reaction proceeds more quickly. There are more atoms of each reactant available to react.

3. Temperature

An increase in temperature also speeds the rate of reaction.

This is because with more heat, atoms of each reactant move faster and collide with each other more often. Products then form more quickly.

4. Surface Area

When the surface area of a reactant is increased, reaction rate speeds up. This occurs because more of the reactant is available to react.

• Review for exam:• P154 #10• P 162 #8,9

• QuestAPlus and Exambank