Heat Energy and Chemical Equations Part 1: Changes in Matter & Energy Balancing Equations Types of Reactions

Physical vs Chemical Changes in Matter & Energy Matter is constantly undergoing changes. These changes can be identified as physical or chemical Physical Changes is any change that does not result in new substances formed. The original substance is just in a new form. For Example Phase Changes and Dissolving H 2 O (s) H 2 O (l) NaCl (s) Na +1 (aq) + Cl -1 (aq)

Changes in Matter (continued) Chemical Changes occur when substances collide and change into new substances. This occurs because the bonds of the substances you start with break and new bonds are formed resulting in new substances. When a chemical change occurs it is called a chemical reaction. For Example Burning of paper & Rusting of a metals

Chemical Equations A chemical equation can be used to show the changes that take place during a chemical or physical change. The substances that enter the reaction are written on the left side of the equation and called reactants. If the reaction involves more than one substance they will be separated by a “+” sign. The new substances that are formed from the reaction are known as products and they are written on the right side of the equation. Separating the reactants and the products is an arrow and is read as yields or produces. For Example H 2 O (s) + energy H 2 O (l) CH 4 + O 2 CO 2 + 2H 2 O + energy

Energy and Changes in Matter Energy is defined as the ability to do work and it is NOT matter Energy is measured in the unit Joules (J) See Table D Can be POTENTIAL or KINETIC

Potential Energy Energy that is stored (i.e. in a chemical bond). Something has the “potential” to do some kind of work Example: the child at the top of the slide has potential energy

Kinetic Energy Energy of motion Example: the child going down the slide now has kinetic energy

Law of Conservation of Energy Energy, like matter, is neither created nor destroyed, rather it is converted.

Endothermic and Exothermic Chemical and Physical changes always involve the loss and gain of energy. This energy is most often expressed or described as heat. Based on whether energy is absorbed or released you can classify energy changes as either endothermic or exothermic.

Understanding Heat Flow Heat is defined as the energy that transfers from one object to another. Heat will always flow from warm  cool. What will happen if the two objects are touching?

Heat Energy vs. Temperature We can measure heat flow by identifying changes in temperature. When an objects temperature goes down it is losing heat when it goes up it is absorbing heat. Temperature is a measure of the average kinetic energy or speed of the particles in matter.

Heat Energy and Changes in Matter In virtually all changes in matter, energy is released or absorbed. System vs. Surroundings The system is what ever is being observed the surroundings are everything around the system. In a chemical change we can measure changes of the surroundings to identify whether or not the reaction (the system) gained or lost energy.

Examples

Exothermic Processes (Changes) Exothermic processes RELEASE ENERGY (i.e. explosions). A good way to remember this is to associate “EXO” with “OUT”. They have a –ΔH value because heat is leaving the system. The surrounding temperature increases because the system or reaction in this loses energy Heat is a product and will be written on the right side of the equation.

Endothermic Processes (Changes) Endothermic processes ABSORB ENERGY A good way to remember this is to associate “ENDO” with “INSIDE”. Has a +ΔH value because heating is entering the system. The surrounding temperature decreases because the system or reaction absorbs heat from the surroundings make it feel cold. Heat is a reactant written on the left side of the equation.

Examples of Endothermic and Exothermic Processes in the form of equations

Balancing Equations Based on the Law of Conservation of Mass In a closed system, the mass of the reactants must equal the mass of the products. Example: formation of water H 2 + O 2  H 2 O ** mass must be conserved

H 2 + O 2  H 2 O 2g + 32g = 18g How do we get these masses to balance? 2H 2 + O 2  2H 2 O 4g + 32g = 36g

2H 2 O

Counting atoms when balancing: The number of atoms of each element must be equal on each side of the reaction. Therefore the masses will be equivalent. 2H 2 + O 2  2H 2 O 4H 2O = 4H 2O

Rules to Balance Equations: 1. Start with the element that is only found once on both sides. 2. Keep polyatomic ions together. Count as a unit if not broken up. 3. Coefficients must be smallest possible whole number.

Examples: K + Cl 2  KCl Al 2 (SO 4 ) 3 + CaCl 2  AlCl 3 + CaSO 4 H 2 O + CO 2  H 2 CO

Try These: H 2 + I 2  HI Na + Cl 2  NaCl Al + O 2  Al 2 O

Phases in equations You must write in the phase of each reactant and product. (s)SolidMg (s) magnesium metal (l)LiquidBr 2 (l) bromine liquid (g)GasCH 4 (g) methane gas (aq) aqueousNaBr (aq) sodium bromide dissolved in water

Summary Chemical equations are written to show the chemical change from reactants to products. Endothermic Reactions absorb heat. Heat is a reactant. Exothermic reactions release heat. Heat is a product. Mass of reactants = Mass of products Equations must be balanced so that the number of atoms of each element on each side of the equation are equal.