Recognizing and Understanding Chemical Changes

1. Recognizing and Understanding Chemical Changes

2. What is a chemical change? • Any change where a new substance is formed How do we know this has happened? 4 different things can happen: 1. change in colour 2. change in odour 3. formation of gas and/or solid 4. release or absorption of heat

3. Understanding Chemical Changes • we need to first understand Kinetic Molecular Theory (KMT) to explain chemical changes • Kinetic molecular theory (KMT): all matter is made up of particles continually moving in random motion • Temperature (T) is used to measure speed of particles because if you increase T, speed of particles increases • we use KMT and collision reaction theory to understand chemical changes

4. Collision Reaction Theory • chemical reactions involve collisions • the outcome of the collisions depends on the energy and orientation of the reactants • an effective collision has the correct amount of energy and the proper orientation to allow the valence shells of the reactants to overlap • this allows their electrons to rearrange and form bonds • very few of the collisions between molecules are effective, but to..

5. To make a reaction faster: • Increase the # of collisions (greater chance of a successful collision) • Increase the # of effective collisions (more collisions with enough energy)

6. There are 4 variables we can change to make the reaction go faster • 1. Temperature • 2. Concentration • 3. Surface Area • 4. Catalysts

7. Temperature • increase temperature, the speed of the molecules increases • more molecules collide more often and more effectively

8. Concentration • Increasing the concentration of the reactant increases the # of collisions between molecules

9. Surface Area • Increasing the surface area of the reactants, • increases the # of collisions that are effective between the molecules

10. Catalysts • Catalyst: speeds up a chemical reaction, but is not consumed in the reaction • reduce the collision energy required for the chemical reaction

11. Representing Chemical Reactions • All chemical reactions can be represented in one of 2 ways: • A word equation • A chemical equation

12. 1. Word Equations • Word equation: a representation of a chemical reaction using only the names of the reactants and products • Example:

13. 2. Chemical Equations • Chemical Equation: A representation of a chemical reaction that uses coefficients in front of the chemical formulas of reactants and products and states of matter of all compounds/elements. • States of Matter: (s): solid(g): gas (l): liquid(aq): aqueous • Example:

15. Balancing Equations • Law of Conservation of Mass: States that in a chemical reaction, the total mass of the reactants is always equal to the total mass of the products. • Since the mass of the reactants is always equal to the mass of the products, we know that the # of each kind of atom is the same before and after the reaction. • Procedure for Balancing a Chemical Equations • 1. Balance all atoms other than oxygen and hydrogen • 2. Balance hydrogen • 3. Balance oxygen • 4. Check to see if everything is balanced