Chemistry: Physical and Chemical Changes, Reaction Rates, Reversible Reactions, and Redox

In the chemical reaction $MgO + Ba \rightarrow Mg + BaO$, how does the equation demonstrate that magnesium oxide is reduced?

Magnesium oxide is reduced because it loses oxygen to form elemental magnesium ($Mg$).

What is the percentage by mass of iron in iron(III) oxide, $Fe_2O_3$, given that $A_r(Fe) = 56$ and $A_r(O) = 16$?

$70\%$ (calculated as $\frac{2 \times 56}{(2 \times 56) + (3 \times 16)} \times 100 = \frac{112}{160} \times 100$).

What is the oxidation number of phosphorus in the phosphate ion, $PO_4^{3-}$, and how is it calculated?

The oxidation number is $+5$. Since oxygen has an oxidation state of $-2$, the calculation is $P + 4(-2) = -3$, which yields $P = +5$.

In the thermal decomposition reaction $4CuO \rightarrow 2Cu_2O + O_2$, how do the oxidation states change, and why is this classified as a redox reaction?

The oxidation number of copper decreases from $+2$ to $+1$ (reduction), while the oxidation number of oxygen increases from $-2$ to $0$ (oxidation). It is a redox reaction because oxidation and reduction happen simultaneously.

What apparatus setup can be attached to the conical flask shown below to measure the volume of hydrogen gas produced over time during the reaction of samarium with water?

A gas syringe (or a delivery tube leading to an inverted measuring cylinder filled with water).

Explain in terms of collision theory why an increase in temperature increases the rate of a chemical reaction.

Increasing the temperature increases the kinetic energy of the particles so they move faster and collide more frequently. Crucially, a much larger proportion of particles possess energy equal to or greater than the activation energy ($E_a$), leading to more successful collisions per unit time.

What is the scientific definition of activation energy?

The minimum amount of energy colliding particles must have in order to react.

Why does the total mass of the flask and contents shown below decrease as the decomposition of aqueous hydrogen peroxide progresses?

Oxygen gas ($O_2$) is formed during decomposition and escapes through the loosely fitting cotton wool plug into the surroundings.

How do metals and ionic solids differ in their electrical conductivity in solid and liquid states?

Metals conduct electricity well in both the solid and liquid states, whereas ionic solids do not conduct electricity when solid, but are good conductors when molten (liquid).

What precipitates are observed when aqueous sodium hydroxide is added separately to an iron(II) chloride solution and an iron(III) chloride solution?

Iron(II) chloride forms a green precipitate of iron(II) hydroxide, while iron(III) chloride forms a brown (or red-brown) precipitate of iron(III) hydroxide.

How do solutions of iron(II) chloride and iron(III) chloride react with potassium iodide and acidified potassium manganate(VII) solutions?

Iron(III) chloride acts as an oxidising agent, turning colourless potassium iodide solution brown; iron(II) chloride acts as a reducing agent, decolourising purple acidified potassium manganate(VII) solution.

State two defining characteristics of a dynamic chemical equilibrium established in a closed system.

1. The rate of the forward reaction equals the rate of the reverse reaction. 2. The concentrations of all reactants and products remain constant.

For the equilibrium $ICl(l) + Cl_2(g) \rightleftharpoons ICl_3(s)$, where $ICl$ is a dark brown liquid and $ICl_3$ is a yellow solid, what is observed when the pressure is decreased, and why?

The mixture becomes darker brown because decreasing the pressure shifts the equilibrium toward the side with more moles of gas (the left-hand side, having $1\,\text{mole}$ of gas compared to $0\,\text{moles}$ on the right).

The equilibrium $2NO_2(g) \rightleftharpoons N_2O_4(g)$ consists of brown $NO_2$ and pale yellow $N_2O_4$. If the mixture contains $20\%$ $NO_2$ at $25\,^{\circ}\text{C}$ and $90\%$ $NO_2$ at $100\,^{\circ}\text{C}$, is the forward reaction exothermic or endothermic?