Acid and Base Reactions

Reactions of Acids with Metals

Try to remember back when we were talking about types of chemical reactions. Single replacement reactions were of the form A + BC  AC + B. A and B change places. Do you remember how we could determine whether a single replacement reaction would happen or not? We did that lab where we reacted a penny with HCl. Why did we have to file a notch in the penny?

Let’s look at Table J. Element A had to be below B for them to change places. Another way of saying this was that the element by itself had to be more active then the element in the compound. What element is always the cation in an acid? Hydrogen

So, which metals will acids react with; the ones above or below hydrogen on Table J? Above

Here’s the reaction that we did in the lab awhile ago:

Mg (s) + 2HCl (aq)  H2 (g) + MgCl2 (aq)

This occurred because Mg is more active than H and they could change places. Why did I have to write H2?

Here’s a list of metals and acids. In your notebooks, determine whether a reaction will happen, and if it will, write out the balanced equation.

1) Au and HF NR 2) Zn and H2SO4 Zn + H2SO4  H2 + ZnSO4 3) Li and HNO2 2Li + 2HNO2  2LiNO2 + H2

Acids and bases react to form a salt and water. This is called a Neutralization Reaction. You know that acids have a pH of 7 and bases are >7, so if similar quantities of an acid and base are mixed together, the pH will end up more neutral, in the middle of the scale.

This is what antacids are designed to do. Heartburn is a symptom that results from the stomach producing too much acid. Antacids are generally weak bases that neutralize this excess acid.

We can use the universal indicator to see evidence of this in action. Milk of Magnesia is a common antacid which contains magnesium hydroxide. What is the formula for magnesium

hydroxide? Mg(OH)2

How can you tell from the formula that this is a base? The OH What color should it turn litmus? Blue How about universal indicator? Blue Let’s remember that the indicator is green when neutral and red or orange when acidic.

I’m going to stir up this MoM using a magnetic stir plate and add some excess HCl, just like heartburn. Let’s see what happens.

The solution is initially red, but then turns back to blue when the MoM has neutralized the acid. Since its blue there’s still excess base, so we can do it again. If I added just the right amount of HCl the result would be green.

Let’s write out the reaction that’s happening here:

Mg(OH)2 + 2HCl  2HOH + MgCl2 Base Acid Water Salt

This is a double replacement reaction, so the cations of the two reactants are changing places. Because the cation of the acid is H+ and the anion of the base is OH- we get water as a product. Remember a salt does not have to be sodium chloride! The term salt just refers to an ionic substance that has a metallic cation and an anion other than hydroxide.

Let’s write the net ionic equation for this reaction: + - H (aq) + OH (aq)  H2O (l)

**Note that all neutralization reactions have the same net ionic equation

Let’s practice writing some neutralization reactions.

How about soda and lithium hydroxide; lithium hydroxide is used to purify the air is space shuttles and submarines.

What would the formula for carbonic acid be? H2CO3 What about lithium hydroxide? LiOH

Okay, let’s react these two together; what are our products?

H2CO3 + LiOH  HOH + Li2CO3

Let’s balance the reaction: H2CO3 + 2LiOH  2HOH + Li2CO3