NIS – CHEMISTRY

1. NIS – CHEMISTRY Lecture 45 Chemical Reactions OzgurUnal

2. Chemical Equations • The process by which the atoms of one or more substances are rearranged to form different substances is called a chemical reaction  another name for chemical change Remember chemical changes? Can you provide some examples on chemical changes taking place around you everyday?

3. Evidence of Chemical Equations • Change in color • Change in temperature • Producing sound • Producing gas or bubbles • Producing light etc. How can you tell a chemical reaction has taken a place? What changes take place during a chemical reaction?

4. Representing Chemical Equations Chemists use statements called equations to represent chemical reactions. Reactants on the left  starting substances Products on the right  substances formed during reaction Arrow ( ) represents the direction of reaction You read the arrow as “react to produce” or “yield”. • Check out Table 9.1 • Symbols are used to show the physical states of the reactants and the products • Solid, liquid, gas of aqueous

5. Representing Chemical Equations Word Equations: You can use statements called word equations to indicate the reactants and products of chemical reactions. Figure 9.3 aluminum (s) + bromine (l)  aluminum bromide (s) Word equation: “Aluminum and bromine react to produce aluminum bromide.” • Example: Write the word equation for the following reactions: • carbon (s) + sulfur (s)  carbon disulfide (l)

6. Representing Chemical Equations Skeleton equation: A skeleton equation uses chemical formulas rather than words to identify the reactants adn products. Al (s) + Br2 (l)  AlBr3 (s) C (s) + S (s)  CS2 (l) • Example: Write down the skeleton equation for the following reaction. • Hydrogen and bromine gases react to yield hydrogen bromide • H2 (g) + Br2 (g)  HBr (g)

7. Representing Chemical Equations • To accurately describe a chemical reaction by an equation, the equation must show equal numbers of atoms of each reactant and each product on both sides of the arrow. • Such an equation is called a balanced chemical equation. • Achemical equation is a statement that uses chemical formulas to show the identities and relative amounts of the substances involved in a chemical reaction. • 2Al (s) + 3Br2 (l)  2AlBr3 (s) The law of conservation of mass states that the total mass of the reactants should equal the total mass of the products. Is the total mass in the reaction in Figure 9.4 conserved?

8. NIS – CHEMISTRY Lecture 46 Balancing Chemical Equations OzgurUnal

9. Balancing Chemical Equations • Acoefficient in a chemical equation is the number written in front of a reactant or product. • Coefficienct s are usually whole numbers and are not ususally written if the value is one. • The purpose of balancing a chemical equation is to conserve the mass. • Follow the steps listed • in Table 9.2

10. Balancing Chemical Equations • Steps for balancing chemical equations: • 1- Write the skeleton equation for the reaction • 2- Count the atoms of the elements in the reactants • 3- Count the atoms of the elements in the products • 4- Change the coefficients to make the number of atoms of each element equal on both sides of the equation • 5- Write the coefficients in their lowest possible ratio • 6- Check your work