Physical and Chemical Change CHAPTER 4 Physical and Chemical Change 4.3 Chemical Reactions in the Lab

Why would the presence of water on Mars be a sign that life might exist there?

Why would the presence of water on Mars be a sign that life might exist there? Many reactions, including those that sustain life, involve chemicals dissolved in water.

DRY Light blue powder No reaction

No reaction Chemical reaction DRY IN SOLUTION NaS(aq) CuSO4(aq) Light blue powder NaS(aq) CuSO4(aq) No reaction Chemical reaction A precipitate is an insoluble compound.

A solution with water as the solvent is called an aqueous solution. “Aqua” means “water.” SYMBOLS (s) indicates a solid (l) indicates a liquid (g) indicates a gas (aq) indicates a substance dissolved in water (an aqueous solution)

O H S O Cu S O 2– 2+ Cu O H A salt is an ionic compound that forms ions when dissolved in water. O H O H

Can you translate what you see into a chemical equation? What are the reactants? What are the products?

Can you translate what you see into a chemical equation? Cu(s) Zn(s) + CuSO4(aq) and Zn(s) is disappearing

Can you translate what you see into a chemical equation? Cu(s) Zn(s) + CuSO4(aq) and Zn(s) is disappearing a salt in water!

Can you translate what you see into a chemical equation? Cu2+(aq) SO42–(aq) Cu(s) Zn(s) + and Zn(s) is disappearing

Cu2+(aq) SO42-(aq) Cu(s) Zn(s) + and Zn(s) is disappearing Cu2+(aq) + 2e– Cu(s) The copper ion gains 2 electrons to form solid copper. reduction: a chemical reaction that decreases the charge of an atom or ion by accepting electrons.

Where could those electrons come from? Cu2+(aq) SO42-(aq) Cu(s) Zn(s) + and Zn(s) is disappearing Cu2+(aq) + 2e– Cu(s) The copper ion gains 2 electrons to form solid copper. Where could those electrons come from?

Cu2+(aq) + 2e– Cu(s) Zn(s) Zn2+(aq) + 2e– SO42-(aq) Cu(s) Zn(s) + and Zn(s) is disappearing Cu2+(aq) + 2e– Cu(s) Zn(s) Zn2+(aq) + 2e– Zinc gives up 2 electrons to become a zinc ion, dissolved in water.

oxidation: a chemical reaction that increases the charge of an atom or ion by giving up electrons. Zn(s) Zn2+(aq) + 2e– Zinc gives up 2 electrons to become a zinc ion, dissolved in water.

REACTION NAME Reactants Products Cu2+(aq) + 2e– Cu(s) reduction

Cu2+(aq) + 2e– Cu(s) reduction Zn(s) Zn2+(aq) + 2e– oxidation REACTION NAME Reactants Products Cu2+(aq) + 2e– Cu(s) reduction Zn(s) Zn2+(aq) + 2e– oxidation

To obtain the overall reaction add the reactants, then the products. REACTION NAME Reactants Products Cu2+(aq) + 2e– Cu(s) reduction Zn(s) Zn2+(aq) + 2e– oxidation These 2 reactions are happening at the same time. To obtain the overall reaction add the reactants, then the products.

Cu2+(aq) + Zn(s) Cu(s) + Zn2+(aq) redox REACTION NAME Reactants Products Cu2+(aq) + 2e– Cu(s) reduction Zn(s) Zn2+(aq) + 2e– oxidation Cu2+(aq) + Zn(s) Cu(s) + Zn2+(aq) redox

- + H2O OH– + H+ dissociation The double arrow indicates that the reaction goes in both ways. The heavier arrow means that the reverse reaction is favored.

Classify these items as acids or bases. Bases: bathroom cleaner, baking soda Acids: lemon juice, gastric acid

HCl(l) H+(aq) + Cl–(aq) Acids give foods like lemons their sour taste. Acids react with metal to create hydrogen gas (H2). Acids can corrode metal and burn skin. Example: H2O(l) HCl(l) H+(aq) + Cl–(aq) Hydrochloric acid acid: a chemical that dissolves in water to create more H+ ions than there are in neutral water.

NaOH(l) Na+(aq) + OH–(aq) Bases create a bitter taste. Bases have a slippery feel, like soap. Bases can neutralize acids. Example: H2O(l) NaOH(l) Na+(aq) + OH–(aq) Sodium hydroxide

NaOH(l) Na+(aq) + OH–(aq) Bases create a bitter taste. Bases have a slippery feel, like soap. Bases can neutralize acids. Example: H2O(l) NaOH(l) Na+(aq) + OH–(aq) Sodium hydroxide OH–(aq) + H+(aq) H2O(l) Result: fewer H+ ions

NaOH(l) Na+(aq) + OH–(aq) base: a chemical that dissolves in water to create fewer H+ ions (or more OH– ions)than there are in neutral water. Example: H2O(l) NaOH(l) Na+(aq) + OH–(aq) Sodium hydroxide OH–(aq) + H+(aq) H2O(l) Result: fewer H+ ions

Higher H+ concentration Lower H+ concentration The pH scale helps to determine whether a solution is acidic or basic.

Higher H+ concentration Lower H+ concentration A solution with water as the solvent is called an aqueous solution. Redox reactions (transfer of electrons) Acid-base reactions (transfer of protons, H+ ions) two types of chemical reactions that take place in an aqueous environment. Higher H+ concentration Lower H+ concentration