Topic 9

Topic 9 on a page

Everything from this topic in one place, for the last read before an exam. If anything here is unfamiliar, go back to the full page.

Definitions to learn word for word

• Activation energy $E_\text{a}$: the minimum energy that colliding particles must have for a collision to be successful and lead to a reaction (Collisions and activation energy).
• Catalyst: a substance that increases the rate of a reaction by providing an alternative route with a lower activation energy, and is not used up (Catalysts and reaction profiles).
• Dynamic equilibrium: in a closed system, the rate of the forward reaction equals the rate of the backward reaction, and the concentrations of reactants and products remain constant (Dynamic equilibrium).
• Le Chatelier's principle: if a system at equilibrium is subjected to a change, the position of equilibrium shifts to oppose the change (Concentration and pressure).

Measuring a rate

• Initial rate: the gradient of the tangent at time 0.
• Rate at time t: draw a tangent at that point and find $\Delta y \div \Delta x$, using a large triangle.
• A product curve has a positive gradient and a reactant curve a negative one. Quote the rate as a positive number.

Collision theory and the factors

Reaction needs a collision with energy of at least $E_\text{a}$ and the right orientation.

Write "frequency of successful collisions", not "more collisions" alone. A gas at pressure $p$ has concentration $c = p \div RT$ (SI units).

The Maxwell–Boltzmann distribution

• Axes: number of molecules against energy. The curve starts at the origin, has no maximum energy, and never touches the axis at the high-energy end.
• Area under the curve = total number of molecules: it stays the same when the conditions change.
• Higher temperature: the peak is lower and to the right, the tail is higher, and the shaded area beyond $E_\text{a}$ is much bigger.
• Catalyst: same curve, and the $E_\text{a}$ line moves to the left.

Reaction profiles

• Reactants and products are drawn at levels that give the sign of ΔH.
• A catalysed route has two lower humps with the intermediate in the dip between them.
• A catalyst changes neither the reactants, the products, nor ΔH. It lowers $E_\text{a}$ for the forward and backward reactions equally.

Catalysts in industry

Less energy: lower $E_\text{a}$, so a lower temperature, so less fuel and less CO₂. Higher atom economy: a better route with fewer by-products. Atom economy = $M_\text{r}$ of the desired product ÷ sum of $M_\text{r}$ of all products × 100%.

Le Chatelier at a glance

Justify in four parts: the change, "shifts to oppose it", the direction, the result. Count only gas moles. Equal moles of gas on both sides means pressure has no effect on the position.

The compromise

For an exothermic reversible reaction, a low temperature gives a high yield but a slow rate, and a high temperature gives a fast rate but a low yield.

Evaluate data in four steps: describe with numbers, explain with chemistry, spot the conflict, recommend a compromise (catalyst, recycling, cost and safety).

The ten traps

1. Writing "more collisions" with no word for successful or frequent.
2. Saying that temperature works only by more frequent collisions. The main effect is the proportion with enough energy.
3. Drawing a Maxwell–Boltzmann curve that touches the x-axis, starts away from the origin, or has a different area at a different temperature.
4. Moving the curve for a catalyst, instead of the $E_\text{a}$ line.
5. Putting the intermediate at the top of a hump, or changing the product level for a catalysed route.
6. Saying that equilibrium means the reaction has stopped or that the concentrations are equal.
7. Applying a pressure argument to a reaction with equal gas moles, or counting solids and liquids.
8. Shifting to the exothermic side when the temperature is raised.
9. Saying a catalyst increases the yield or shifts the position.
10. Saying a high temperature is used in the Haber process "to raise the yield". It raises the rate at the cost of the yield.

Mixed test

1. A tangent to a graph of [A] against time at 20 s passes through (0 s, 0.80 mol dm⁻³) and (40 s, 0.20 mol dm⁻³). Find the rate of reaction at 20 s.
2. In a disappearing cross experiment the cross vanishes after 40 s at 20 °C and after 20 s at 30 °C. Calculate the rate at each temperature, and the ratio.
3. 0.50 g of calcium carbonate reacts with an excess of hydrochloric acid. Calculate the total volume of carbon dioxide at room temperature and pressure. If 60 cm³ is collected in the first 30 s, find the mean rate over that time. ($M_\text{r}$ of CaCO₃ = 100.1)
4. Explain, using collision theory, why powdered marble reacts faster than marble chips.
5. Draw and describe the Maxwell–Boltzmann distribution at two temperatures, and explain why a small rise in temperature can double the rate.
6. A catalyst is added to a reaction at constant temperature. State what happens to the Maxwell–Boltzmann curve and to $E_\text{a}$, and explain the effect on the rate.
7. State two features of a system at dynamic equilibrium.
8. For $\ce{2NO(g) + O2(g) <=> 2NO2(g)}$, ΔH = −114 kJ mol⁻¹, predict and justify the effect on the position of (a) raising the temperature, (b) raising the pressure, (c) adding a catalyst, (d) adding more O₂.
9. For the Haber process, equilibrium yield falls from 89% at 200 °C to 8% at 600 °C, while the relative rate rises from 1 to 3500. Calculate the factors and explain why about 450 °C is used.
10. Explain two ways in which a catalyst makes an industrial process more sustainable.