Chemical Reactions Chemical Combinations Chemical Reactions In a

Chemical Reactions Chemical Combinations

Chemical Reactions In a reaction, one or more substances reacts or changes to form new substances. Chemical reactions are written in shorthand as Ø Reactants Products “ ” is read as yields or produces

Chemical Reactions In a reaction (rxn), bonds between atoms are broken and then formed again. Atoms are NOT created or destroyed, just rearranged. Ø Part of Dalton’s Atomic Theory – chemical reactions occur when atoms are joined separated or rearranged. This satisfies the Law of Conservation of Mass which says that matter can neither be created nor destroyed.

Chemical Reaction In order for something to be classified as a reaction, there must be a CHANGE. Indicators of chemical change Ø Ø Formation of a gas/vapor – bubbles Formation of a precipitate – solid Color change (not a shade change) – blue to red Evolution or absorption of heat – gets hot or cold

Chemical Equations A Chemical Equation is the representation of a chemical reaction It shows formulas and states. States are written after the formula as a subscript. Ø Ø Solid (s) Liquid (l) Gas (g) Aqueous-dissolved in water (aq) Used for substances that are soluble in water (Solubility Rules)

Chemical Equations Catalysts are substances that speed up a reaction but do NOT participate in the reaction. Ø In other words, the catalyst does not change. Catalysts are NOT part of the chemical reaction, so they are not written as a reactant or product. Instead they are written above the yields sign. Ex. H 2 O 2 (aq) Mn. O 2> H 2 O(g) + O 2 (g)

Skeleton Equation “Bare Bones” The skeleton equation just contains the formulas and states for the substances in the reaction. It does not contain the relative amount of reactant and product. Skeleton Equations tell you QUALITATIVELY what your reactants and products are. However, they do not tell you QUANTITATIVELY how much reactant and product there is.

Writing Skeleton Equations Write the formulas and states of the reactants (what you start with) on the left of the arrow. Write the formulas and states of the products (what you end with) on the right of the arrow. If a catalyst is present, write it above the arrow. Ø Ø is used to represent heat. elec. is used to represent electricity.

Writing Skeleton Equations Hydrochloric acid reacts with calcium metal to form hydrogen gas and calcium chloride. A solution of silver(I) nitrate reacts with copper metal to produce silver metal and copper(II) nitrate. Propane, C 3 H 8, burns in the presence of oxygen to produce carbon dioxide and water vapor.

BALANCING EQUATIONS

Balanced Equations Balancing a chemical equation tells you the relative amount of reactant needed and product made in a chemical reaction. It makes the equation satisfy the Law of Conservation of Mass. Both sides of the equation MUST be Equal. Ø Example Ø H 2 O 2 (aq) H 2 O(g) + O 2 (g) becomes 2 H 2 O 2 (aq) 2 H 2 O(g) + O 2 (g)

Balanced Equations Coefficients (numbers in front) are used to balance the equation. You CANNOT change the subscripts of the substances!!!! Once a formula is written for a compound in a reaction, do NOT change it to balance the equation.

Steps for Balancing Equations Determine the correct formulas for all reactants and products. Ø Write the skeleton equation. Ø Magnesium combines with oxygen gas to produce magnesium oxide. Mg (s) + O 2 (g) Mg. O (s) Count the number of atoms of each element on each side of the yields arrow. Ø Ø Reactant – 1 Mg Product - 1 Mg 2 O 1 O

Steps for Balancing Equations cont. “Balance” elements one at a time using coefficients. Ø Ø 2 Mg (s) + O 2 (g) 2 Mg. O (s) Check for atom balance by recounting using coefficients and subscripts for each element. Ø Ø No coefficient is assumed to mean a coefficient of 1 (same as in math) Reactant. Product- 2 Mg 2 O 2 O Finally, make sure coefficients are present in the lowest whole number ratio. Ø 2: 1: 2 - cannot be reduced

CLASSIFYING REACTIONS

Types of Reactions Synthesis A + B AB Ø Ø 2 reactants, 1 product Ex: Decomposition AB A + B Ø Ø 1 reactant, 2 products Ex:

Types of Reactions Single Replacement (Displacement) Ø Ø Ø A + BC AC + B element + compound yields element + compound Ex. : Double Replacement (Displacement) Ø Ø Ø AB + CD AD + CB 2 compounds produces 2 NEW compounds Ex. :

Types of Reactions Combustion Complete CH + O 2 CO 2 + H 2 O Ø Ø Hydrocarbon + oxygen gas yields carbon dioxide + water Ex. : Incomplete CH + O 2 CO + H 2 O Ø Hydrocarbon + oxygen gas yields carbon monoxide + water

PREDICTING PRODUCTS

Predicting the Product 1. Products for simple chemical reactions can be predicted based on the reactants. Classify the reaction by type. Ø 2. Synthesis, Decomposition, Single Replacement, Double Replacement, or Combustion Use the Reference Chart to match the reaction type and predict the products. Ø Use examples to help you figure out what the product will be.

Predicting the Products Synthesis Ø Decomposition Ø Ø Single element will replace “like” element in compound Use Activity Series Double Replacement Ø Ø Separate the reactant into its parts Single Replacement Ø Combine the reactants Compounds “swap” partners Use solubility rules to determine state Combustion of a Hydrocarbon Ø Products are always CO 2 and H 2 O

Predicting the Product: Single Replacement In Single Replacement reactions, you must determine if the single element is reactive enough to actually replace the element in the compound. Ø Ø Ø Activity series is used to determine relative reactivity. The HIGHER up the element is, the more reactive it is. Only single elements that are HIGHER on the activity series can replace an element in a compound. Ex. Sodium, Na, can replace copper, Cu. Ex. Silver, Ag, cannot replace potassium, K.

Predicting the Product: Double Replacement In Double Replacement reactions, you must determine if the products are actually DIFFERENT from the reactants. Ø When an ionic compound is dissolved in water, aqueous, it separates into its component ions. Ø Ex. Na. Cl(aq) separates into Na+ and Cl- If all of the reactants and products are aqueous ionic compounds, then nothing actually changes. Ex. KCl (aq) + Na. NO 3(aq) KNO 3(aq) + Na. Cl(aq) Ø Ø All compounds are aqueous – no change In order for a change to occur, a solid, gas, or molecular compound must be formed. Ex. KCl(aq) + Ag. NO 3(aq) Ag. Cl(s) + KNO 3(aq) Ø Silver chloride is a solid precipitate - change

Predicting the Product: Double Replacement To eliminate the products that don’t change in a double replacement reaction, you can write a Net Ionic Equation. Ø Net Ionic Equations only contain the substances that actually change in a reaction. Steps Na 2 SO 4(aq) + Ba. Cl 2(aq) Na. Cl(aq) + Ba. SO 4(s) 1. Write all aqueous ionic compounds as their component ions. Na 1+(aq) + SO 42 -(aq) + Ba 2+(aq) + Cl 1 -(aq) Na 1+(aq) + Cl 1 -(aq) + Ba. SO 4(s) Ø 2. Eliminate any ion that is on both sides. Ø 3. Na 1+(aq) and Cl 1 -(aq) are the same on both sides. They are called SPECTATOR ions. Write the ions/compounds that are remaining. Ø Ba 2+(aq) + SO 42 -(aq) Ba. SO 4(s)