Question

Predict the outcome of the reactions represented by the following equations by using the activity series, and balance the equations. (a) $\mathrm{Cu}(s)+\mathrm{HCl}(aq)⟶$ (b) $\mathrm{Au}(s)+\mathrm{NaBr}(aq)$ (c) $\mathrm{Mg}(s)+{\mathrm{CuSO}}_4(aq)$ (d) $\mathrm{Zn}(s)+\mathrm{KBr}(aq)$

Short answer

(a) No reaction, (b) No reaction, (c) Reacts to form ${\text{MgSO}}_4+\text{Cu}$, (d) No reaction.

Step-by-step solution

Understanding the Activity Series

The activity series is a list of elements ordered by their ability to displace other elements in chemical reactions, usually in aqueous solutions. Metals at the top of the activity series are more reactive than metals lower on the list.

Predicting Reaction Outcomes

For a reaction to occur between a solid metal and a compound in aqueous solution, the solid metal must be higher in the activity series than the metal in the compound. - (a) Copper (Cu) and Hydrochloric Acid (HCl): Copper is below hydrogen in the activity series, so no reaction occurs. - (b) Gold (Au) and Sodium Bromide (NaBr): Gold is below sodium in the activity series, so no reaction occurs. - (c) Magnesium (Mg) and Copper(II) Sulfate (CuSO₄): Magnesium is above copper in the activity series, so a reaction occurs. - (d) Zinc (Zn) and Potassium Bromide (KBr): Zinc is below potassium in the activity series, so no reaction occurs.

Balancing the Equations

Once the reactions that will occur are identified, balance the equations where applicable.- (a) \ $extCu(s)+extHCl(aq)\to \text{No reaction}$- (b) \ $extAu(s)+extNaBr(aq)\to \text{No reaction}$- (c) \ $extMg(s)+ext{CuSO}_4(aq)\to {\text{MgSO}}_4(aq)+extCu(s)$ is already balanced.- (d) \ $extZn(s)+extKBr(aq)\to \text{No reaction}$

Key concepts

Predicting Reaction Outcomes

When we encounter a chemical reaction, one of the key tasks is to predict whether a reaction will actually take place. To determine this, chemists use the activity series of metals. This handy list ranks metals based on their reactivity. Essentially, a more reactive metal will displace a less reactive metal from its compound. For example, if you mix a more reactive metal with the compound of a less reactive metal, the more reactive metal will take the place of the less reactive one.

Sometimes, you find that no reaction occurs. This happens when the metal in its elemental form is less reactive compared to the metal present in the compound. Let's look at the exercise provided:

• Copper does not displace hydrogen from hydrochloric acid because copper is less reactive than hydrogen.
• Gold does not react with sodium bromide for the same reason. Gold sits below sodium on the activity series.
• However, when magnesium meets copper(II) sulfate, a reaction does occur. Magnesium, being more reactive than copper, displaces it, forming magnesium sulfate and solid copper.
• Zinc finds no reaction with potassium bromide, as potassium is higher in the activity series than zinc.

By referencing the activity series, chemists can make educated predictions about whether specific chemical reactions will take place.

Balancing Chemical Equations

Once you've figured out which reactions will happen, it's time to balance the chemical equations. Balancing is important because it reflects the Law of Conservation of Mass, ensuring that matter is neither created nor destroyed. In balancing, you adjust the coefficients (the numbers in front of each compound) so that the same number of atoms for each element is present on both sides of the equation.

Let's explore this using the exercise:

• For the reaction of magnesium and copper(II) sulfate, the balanced equation is: $$\mathrm{Mg}(s)+{\mathrm{CuSO}}_4(aq)\to {\mathrm{MgSO}}_4(aq)+\mathrm{Cu}(s)$$Here, the equation is already balanced. You can see there’s one magnesium on each side, one copper, one sulfur, and four oxygen atoms.
• The other reactions do not occur, so there's no equation to balance.

Balancing equations may sometimes require careful adjustments, but always keep track of the atom count for each element on both sides.

Metal Reactivity

Understanding metal reactivity is crucial in predicting chemical reactions. The reactivity of metals depends on how easily they lose electrons to form positive ions. More reactive metals lose electrons more easily and can cause chemical changes in compounds with less reactive metals. The activity series is a tool that ranks metals, and sometimes hydrogen, based on their reactivity.

In reacting scenarios, if a metal's reactivity is higher than another metal within a compound, the more reactive metal can replace the other one. This reactivity kicks off single replacement reactions, which are common in chemistry. Let's consider the reactions from our exercise:

• Magnesium features prominently as a highly reactive metal, readily taking the place of copper in copper(II) sulfate.
• Copper, gold, and zinc, when compared with other elements like hydrogen or sodium, do not display sufficient reactivity to cause a reaction with their respective chlorides or bromides.

Thus, knowing where a metal stands in the reactivity series helps in accurately forecasting chemical interactions, guiding practical reactions and real-world applications.