Chemical Reactions

Chemical Reactions Chemical reactions involve a chemical change. It’s a process in which one or more reactants are changed into one or more different products. Products must have different physical and chemical properties.

Indicators that a chemical reaction is taking place. Transfer of energy (flame, heat, light, cooling, sound) Formation of a precipitate (solid from two liquids) Color change or fluorescence (not the best indicators but qualify sometimes) Energy release comes from the breaking of chemical bonds! Gas production

Endothermic Reactions vs. Exothermic Reactions

Endothermic Reactions Endothermic reactions take in energy from its surroundings. (Think about those cold packs you put on a sprained ankle. You pop a button and a chemical reaction takes place that cools the pack!)

Exothermic Reactions Exothermic reactions release energy to their surroundings. (think about those hand-warmers you use ice fishing or deer hunting!)

Representing Chemical Reactions: Chemical reactions can be written as word equations or as chemical equations.

Word equations – names of reactants are written out to the left of the arrow, and products are written out to the right. No symbols are used, amounts not given. Methane + oxygen  carbon dioxide + water Reactants Are written to the left of the arrow. Products Are written to the right of the arrow.

Examples of Word Equations: Carbon dioxide + water + energy  oxygen + sugar

Hydrogen + oxygen  water + energy

Chemical Equations – chemical symbols or formulas are written (reactants on the left, products on the right). An unbalanced chemical equation is called a “skeleton equation”. A balanced equation shows the ratios between reactants and products. • CH4 + O2CO2 + H2O Reactants Products

Qualitative Information – important information included in a chemical equation that indicate states of matter and/or special conditions required for the reaction to take place. These give pertinent information but are not always included. It depends on how much information the person writing the equation wants to convey! States of Matter: (l) = liquid state (s) = solid state (g) = gaseous state (aq) = aqueous (dissolved in water) Reaction conditions:

Quantitative Information – important information included in a chemical equation that indicates the amounts of reactants and products (like a recipe!) This information is crucial when satisfying the Law of Conservation of Matter. 2C8H18(g) + 25O2(g)16 CO2(g) + 18 H2O (l) The numbers in front of the symbols/formulas are called coefficients. These are used to “balance” the equation to satisfy the Law of Conservation of Matter. The number of each atom on the left side of the arrow equals the number of atoms on the right side of the arrows. Please write down, on a separate sheet of paper, the number of each element present on each side of the equation. Do the numbers match? Turn in this sheet when you come to class tomorrow.

There are 5 general types of reactions: Synthesis (sometimes called “combination”), Decomposition, Combustion, Single-Displacement, Double-displacement.

Synthesis To “synthesize” means to form by combining parts…. “synthesis” means to combine single parts into a single entity. Put simply, in a synthesis reaction, “smaller” reactants combine to form larger products. • To qualify as a synthesis reaction, one of the products is larger (more complex) than any of the reactants. Use the following examples to help understand how to identify a synthesis reaction….

Sodium + Chlorine  Sodium Chloride Na (s) + Cl2(g)NaCl(s) Cl • NaCl In this reaction, the element sodium combines with the element chlorine to form sodium chloride. The compound sodium chloride is a combination of two elements and, therefore, more complex than either of the reactants.

Copper + Sulfur  Copper sulfide Cu (s) + S (s) Cu2S (s) Cu S Cu2S In this reaction, the element copper combines with the element sulfur to form copper sulfide. Again, the compound copper sulfide is a combination elements and, therefore, more complex than either of the reactants.

Here we have two reactants that are compounds reacting to form three products. One of those products, glucose (with 24 atoms), is far more complex than any of the reactants. (You may notice that two of the products are simpler. Remember that the definition states that only one product needs to be larger/more complex!)

Silver reacts with sulfur in the air to make silver sulfide, the black material we call tarnish. Ag + S  Ag2S

An iron bar rusts. The iron reacts with oxygen in the air to make rust. Fe + O2 Fe2O3

Decomposition To decompose means to separate into smaller parts. Decomposition means to “break down”. A larger reactant(s) breaks down into simpler products. • All products are smaller (less complex) than any of the reactants.

Here’s a decomposition reaction where the reactant, sucrose (with 45 atoms), breaks down into two smaller, much less complex products. **Note the catalyst used (written above the yields sign). Look up the name of this catalyst and write it on a separate sheet of paper. Turn this in tomorrow. If you care to watch this unusual reaction, follow the link below. Watch for the steam being given off! Would you say this reaction is endothermic or exothermic? Explain. Put your answers on the same sheet of paper that has your answers from above. http://www.chemie.uni-regensburg.de/Organische_Chemie/Didaktik/Keusch/D-sugar_coal-e.htm

Respiration is the reverse of Photosynthesis. Because Photosynthesisis a synthesis reaction, the reverse of it is decomposition! The molecule glucose is broken down into the smaller molecules carbon dioxide and water.

NaN3(s) → N2(g) + Na(s) This reaction takes place to inflate the airbag in an automobile in the event of a collision. There’s actually a second reaction that occurs that’s not illustrated here! Sodium azide NaN3 decomposes incredibly rapidly into smaller molecules (nitrogen gas and sodium metal).

C6H12O6C2H5OH + CO2 Glucose (simple sugar) ferments to ethyl alcohol and carbon dioxide.

Combustion Combustion means the act or process of burning. Chemists describe it as “rapid oxidation” accompanied by heat and usually light (in other words… a release of energy!!). (Rusting is slow oxidation so…..rusting and combustion are similar processes.) • Many things that we burn to get energy from are hydrocarbons. Hydrocarbons are chemical molecules that contain hydrogen and carbon (go figure!). Things like gasoline, propane, butane, fuel oil, and wood are hydrocarbons. Combustion of a hydrocarbon produces water and carbon dioxide.

Combustion C20H42 + O2 CO2 + H2O This is the combustion reaction of parafin wax (candle wax). Parafin is a hydrocarbon (a molecule that contains hydrogen & carbon). First, take note that oxygen is one of the reactants. You probably already know that in order for something to burn….you need oxygen! Oxygen will always be a reactant in a combustion reaction. Second, see that carbon dioxide and water are products. This will always be true of a combustion reaction involving hydrocarbons!

Methane combines with oxygen in the air to make carbon dioxide and water vapor. CH4 + O2CO2 + H2O *another example of the combustion of a hydrocarbon.

Some metals oxidize rapidly in the presence of oxygen and can be classified as combustion reactions. Mg + O2 MgO Now this is an interesting reaction that can be classified in a couple different ways! Because the product is larger (more complex) than the reactants, you could classify this as a synthesis reaction. Because one of the reactants is combining with oxygen (oxidation) and giving off energy (light and heat) rapidly it can be classified as a combustion reaction.

The combination of hydrogen gas with oxygen gas is another example of a reaction that can be classified in a couple ways. H2 + O2 H2O

Single Displacement A single displacement reaction is characterized by a “free” element (typically a metal) replacing another element (a metal cation) in a compound.

In this example, a coiled piece of copper metal is placed in a solution of silver nitrate: Cu (s) + AgNO3(aq) Ag (s) + CuNO3(aq) Notice that the “free” element (in this case copper) changes places (displaces) the metal in the compound (silver).

Here we have an iron nail (the “free” metal) being placed in a solution of copper II nitrate. Fe(s) + Cu(NO3)2(aq) Cu(s) + Fe(NO3)2 (aq) Again, a switch takes place between the “free” metal and the metal in the compound.

Predicting whether a single displacement reaction will take place. Whether a single displacement reaction takes place or not is dependent on the reactivity of the “free” metal compared to the reactivity of the metal in the compound.

The reactivity of metals is ranked on the Activity Series. The higher a metal is on the Activity Series the more reactive it is. Notice that lithium ranks the highest on the Activity Series, and gold ranks the lowest.

Here again is our first example where a coil of copper metal is placed in a solution of silver nitrate. • Cu (s) + AgNO3(aq) Ag (s) + CuNO3(aq) The reaction takes place because copper (the “free” metal) is more reactive (higher on the activity series) than the metal in the compound (silver).

What if you placed a copper coil into a solution of iron II chloride?

In this case a reaction would not take place because the “free” metal (copper) is not reactive enough to displace the metal in the compound (iron). In other words, the “free” metal is not higher on the activity series than the metal in the compound.

Double Displacement • A double displacement reaction is characterized by cations exchanged between ionic compounds in solution. • This type of reaction involves the formation of a molecular compound, a gas, or a precipitate (solid). • Use the solubility chart to predict whether a solid will form.

2KI(aq) + Pb(NO3)2(aq)  ??? Here we have two ionic compounds in solution: potassium iodide and lead II nitrate. When these two colorless solutions are combined a solid, yellow, precipitate forms. What’s the chemical formula for the new solid product?

First let’s look at the two “possible” products. Remember that a double displacement reaction involves cations switching places. When that occurs, the two possible products are lead II iodide and potassium nitrate.

The question is, which of the two products is the solid (precipitate)?

To make that prediction, you’ll need a solubility chart. **You have a solubility chart – Table 11-1 – on the backside of the periodic table in your packet.** Lets consider the product potassium nitrate first. Look up either the cation (potassium) or the anion (nitrate) on the chart. (**The ions are listed down the left-hand side.) Finding the potassium ion (3rd one down) you’ll see that the chart tells you that all compounds made with the potassium ion are soluble. Soluble means that the substance will remain dissolved in solution. Insoluble means that the substance cannot be dissolved and therefore remains a solid (or turns into a solid). What this means is that the solid formed could not be potassium nitrate.

Just for further clarification … lets say that you looked up the anion, nitrate, instead. You’ll find that, according to the chart, compounds made with the nitrate ion (4th one down) are soluble. Either way, potassium nitrate is not the solid being formed in this reaction. Since there are only two products, and we’ve eliminated one of them, it should be obvious that the other product must be the solid. But lets check it against the chart anyway!

Again, pick either the cation or the anion to look up. Let’s start with the cation (lead). Looking down the left side you’ll see it’s not listed. That’s ok! Try finding the anion, iodide, instead. Iodide is listed (8th down). But wait!! The first part says that iodides are soluble. How can this be?!? Keep reading …. there’s exceptions! Compounds made with the iodide ion are soluble EXCEPT when combined with the silver, mercury, and LEAD ions. That means a compound made from the combination of lead and iodide will be insoluble. In other words, a solid in solution.

The final equation indicating the solid that forms: 2KI(aq) + Pb(NO3)2 (aq)  PbI2(s) + 2KNO3(aq)

So let’s take a look at exactly what’s happening during a double displacement reaction like the previous example. To start, you have two solutions. One solution contains one ionic compound (the solute) that’s dissociated in water (the solvent). The other solution contains another ionic compound (solute) dissociated in water (solvent).

In our previous example, one solution contained potassium iodide. The other solution contained lead II nitrate. When ionic compounds dissociate in water, they break into individual cations and anions. Solution of KI Solution of Pb(NO3)2

When the two solutions are combined, all four ions are now able to interact with one another. Solution of KI Solution of Pb(NO3)2

Some of these interactions have no result (they simply dissociate from one another again). However, as we determined from the solubility chart, when the lead II ion encounters an iodide ion, they come together to form an insoluble compound.

When this happens, the insoluble compound sinks to the bottom of the container in the form of a precipitate. The other ions remain dissociated in solution. These ions, that remain dissociated are called “spectator ions” (aptly named because they don’t participate in the reaction!).

Chemical Reactions