Rate of Chemical Reactions

Lessons

Senior 3 · Chemistry

Rate of Chemical Reactions

4 lessons

Lesson 4 of 4

Catalysts and Light

25 min

Why hydrogen peroxide comes in a brown bottle

Hydrogen peroxide, $\mathrm{H_2O_2}$ , slowly breaks down into water and oxygen even as it stands in its bottle. Pharmacies sell it in brown glass, because light speeds the breakdown up. Yet drop a pinch of black manganese(IV) oxide into the same solution and it froths with oxygen within seconds; a slice of raw potato does the same more gently. When the fizzing stops, the manganese(IV) oxide can be filtered off, dried and weighed, and its mass has not changed. A substance that speeds up a reaction and comes out unchanged is a catalyst.

Definition

Catalyst: a substance that increases the rate of a chemical reaction but is chemically unchanged, and has the same mass, at the end. A small amount is enough, because it is used again and again. Example: manganese(IV) oxide, $\mathrm{MnO_2}$ , catalyses $\mathrm{2H_2O_2(aq) \rightarrow 2H_2O(l) + O_2(g)}$ .

Enzyme: a protein made by living cells that acts as a catalyst for one particular reaction. Example: catalase, found in potato and liver, catalyses the same breakdown of hydrogen peroxide inside cells.

Measuring the effect of a catalyst

$35\,\mathrm{cm^3}$ of hydrogen peroxide solution of concentration $0.20\,\mathrm{mol\,dm^{-3}}$ is placed in a flask joined to a gas syringe, as in Figure 1.1. In one run 0.50 g of manganese(IV) oxide powder is added; in the other, nothing is added. Afterwards the manganese(IV) oxide is filtered off, washed, dried and weighed: it still has a mass of 0.50 g.

$\mathrm{2H_2O_2(aq) \xrightarrow{MnO_2} 2H_2O(l) + O_2(g)}$

Table 4.1: Volume of oxygen ( $\mathrm{cm^3}$ ) (illustrative results)

time (s)	with 0.50 g $\mathrm{MnO_2}$	no catalyst
0	0	0
10	28	–
20	48	–
30	61	0
40	70	–
50	76	–
60	80	1
70	82	–
80	83	–
90	84	1
100	84	–
110	84	–
120	84	2

Worked example

Problem. Use Figure 4.1 and Table 4.1. (a) Find the initial rate with the catalyst. (b) Find the average rate over the first 20 s. (c) Show that the final volume of oxygen is what the equation predicts. (Molar volume $24\,\mathrm{dm^3\,mol^{-1}}$ .)

Solution. (a) The tangent passes through (0, 0) and (20, 66): rate $= \frac{66}{20} = 3.3\,\mathrm{cm^3\,s^{-1}}$ .

(b) $\frac{48 - 0}{20 - 0} = 2.4\,\mathrm{cm^3\,s^{-1}}$ .

(c) $n(\mathrm{H_2O_2}) = \frac{35}{1000} \times 0.20 = 0.0070\,\mathrm{mol}$ . Two moles of $\mathrm{H_2O_2}$ give one mole of $\mathrm{O_2}$ , so $n(\mathrm{O_2}) = 0.0035\,\mathrm{mol}$ :

$V = 0.0035 \times 24\,000 = 84\,\mathrm{cm^3}$

Without the catalyst the same $84\,\mathrm{cm^3}$ would form in the end, but only over many months.

How a catalyst works

A catalyst provides a different route, or pathway, from reactants to products, with a lower activation energy (Figure 4.2). At the same temperature, many more collisions now have enough energy to react, so the number of effective collisions per second rises. The reactants and products are the same, so a catalyst does not change the energy given out, nor the amount of product: it only gets there sooner.

Catalysts matter to industry because a faster reaction at a lower temperature saves fuel and money. Living things depend on enzymes, which let reactions run quickly at body temperature.

Table 4.2: Catalysts at work

Reaction or process	Catalyst	Product or purpose
Haber process (Unit 2)	iron	ammonia, for fertilisers
Contact process (Unit 3)	vanadium(V) oxide	sulfuric acid
car catalytic converter	platinum, palladium and rhodium	turns carbon monoxide, nitrogen monoxide and unburnt fuel into carbon dioxide, nitrogen and water
fermentation of sugars, as in brewing urwagwa and baking bread	enzymes from yeast	ethanol and carbon dioxide
digestion of starch in the mouth	amylase in saliva	sugars

Worked example

Problem. For a reaction (illustrative values), the activation energy is $80\,\mathrm{kJ\,mol^{-1}}$ without a catalyst and $50\,\mathrm{kJ\,mol^{-1}}$ with one, and the reaction gives out $60\,\mathrm{kJ\,mol^{-1}}$ . (a) By how much does the catalyst lower the activation energy? (b) What is the energy given out with the catalyst? (c) Explain why the catalysed reaction is faster at the same temperature.

Solution. (a) $80 - 50 = 30\,\mathrm{kJ\,mol^{-1}}$ . (b) Still $60\,\mathrm{kJ\,mol^{-1}}$ : the reactants and products are the same, so the energy difference between them is the same. (c) Fewer kilojoules are needed to react, so a larger fraction of the collisions have at least the activation energy; there are more effective collisions per second.

Effect of light

Some reactions take their activation energy from light. A molecule absorbs light, and the energy breaks a bond and starts the reaction. The brighter the light, the faster the reaction; in the dark, some of these reactions hardly go at all.

Reaction	Equation	What light does
photosynthesis in green leaves	$\mathrm{6CO_2(g) + 6H_2O(l) \rightarrow C_6H_{12}O_6(aq) + 6O_2(g)}$	chlorophyll absorbs the light that drives the reaction
darkening of silver chloride	$\mathrm{2AgCl(s) \rightarrow 2Ag(s) + Cl_2(g)}$	white silver chloride turns grey-violet as silver forms; the basis of black-and-white photography
hydrogen and chlorine	$\mathrm{H_2(g) + Cl_2(g) \rightarrow 2HCl(g)}$	slow in the dark, explosive in bright sunlight
breakdown of hydrogen peroxide	$\mathrm{2H_2O_2(aq) \rightarrow 2H_2O(l) + O_2(g)}$	speeded up by light, so it is stored in brown bottles

A simple test. Silver chloride is precipitated by mixing silver nitrate and sodium chloride solutions, $\mathrm{AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)}$ , and spread on filter paper. A coin is placed on half the paper, which is left in sunlight for a few minutes: the uncovered silver chloride darkens, while the part under the coin stays white.

Warning

Common mistakes and safety. (1) A catalyst is not a reactant and does not appear on either side of the equation; write it over the arrow. (2) A catalyst does not give more product, only the same product sooner. (3) Enzymes are proteins: above about 40–60 °C most are destroyed (denatured), so heating an enzyme reaction too much slows it down. (4) Silver nitrate stains skin and is harmful to eyes: wear gloves and eye protection, and if any splashes onto your skin or into your eye, rinse with plenty of running water for at least 15 minutes.

Tip

Same start, same finish, lower hill. A catalyst keeps the reactants, products and energy change the same and lowers only the activation energy. Light supplies the energy to climb the hill.

FuseSchool: What Are Catalysts? (4 min). Watch for the alternative pathway with a lower activation energy, and the industrial examples.

✦

Practice

Define a catalyst. Describe how you would show that manganese(IV) oxide is not used up when it catalyses the breakdown of hydrogen peroxide.
Use an energy profile to explain how a catalyst increases the rate of a reaction.
In Figure 4.1, at what time did the catalysed reaction finish? Calculate its average rate over the whole reaction.
What volume of oxygen, at room temperature and pressure, forms when $50\,\mathrm{cm^3}$ of $0.20\,\mathrm{mol\,dm^{-3}}$ hydrogen peroxide breaks down completely?
Name the catalyst used in (a) the Haber process; (b) the Contact process; (c) a car's catalytic converter.
Explain why hydrogen peroxide is sold in brown bottles, and why the bottle should be kept in a cool place.
A student repeats the catalysed experiment with 1.0 g of manganese(IV) oxide instead of 0.50 g. Predict the effect on (a) the initial rate, (b) the final volume of oxygen. Explain.
A mixture of hydrogen and chlorine in a sealed glass bulb does not react in a dark cupboard. Explain why it may explode when carried into bright sunlight, and write the equation.

Show answersHide answers

A substance that speeds up a reaction and is chemically unchanged at the end. Weigh the manganese(IV) oxide before adding it; when the reaction is over, filter it off, wash and dry it, and weigh it again: the mass is the same. It can also be reused to catalyse a fresh portion of hydrogen peroxide.
The catalyst gives a pathway with a lower activation energy. More collisions have at least this energy, so there are more effective collisions per second.
At 90 s. Average rate $= \frac{84}{90} = 0.93\,\mathrm{cm^3\,s^{-1}}$ .
$n(\mathrm{H_2O_2}) = 0.050 \times 0.20 = 0.010\,\mathrm{mol}$ ; $n(\mathrm{O_2}) = 0.0050\,\mathrm{mol}$ ; $V = 0.0050 \times 24\,000 = 120\,\mathrm{cm^3}$ .
(a) Iron; (b) vanadium(V) oxide; (c) platinum, palladium and rhodium.
Light speeds up its breakdown into water and oxygen, and brown glass keeps out much of the light. Heat speeds it up too, so a cool place slows the breakdown further.
(a) The initial rate increases, because there is more catalyst surface on which the reaction can happen. (b) The final volume is unchanged at $84\,\mathrm{cm^3}$ : it depends only on the amount of hydrogen peroxide.
In the dark, few collisions have the activation energy. Sunlight is absorbed by chlorine molecules and breaks them into reactive atoms, which start a very fast reaction: $\mathrm{H_2(g) + Cl_2(g) \rightarrow 2HCl(g)}$ .

time (s)	with 0.50 g $\mathrm{MnO_2}$	no catalyst
0	0	0
10	28	–
20	48	–
30	61	0
40	70	–
50	76	–
60	80	1
70	82	–
80	83	–
90	84	1
100	84	–
110	84	–
120	84	2

time (s)	with 0.50 g $\mathrm{MnO_2}$	no catalyst
0	0	0
10	28	–
20	48	–
30	61	0
40	70	–
50	76	–
60	80	1
70	82	–
80	83	–
90	84	1
100	84	–
110	84	–
120	84	2

1Why hydrogen peroxide comes in a brown bottle

2Measuring the effect of a catalyst

3How a catalyst works

4Effect of light

Why hydrogen peroxide comes in a brown bottle

Measuring the effect of a catalyst

How a catalyst works

Effect of light

time (s)	with 0.50 g $\mathrm{MnO_2}$	no catalyst
0	0	0
10	28	–
20	48	–
30	61	0
40	70	–
50	76	–
60	80	1
70	82	–
80	83	–
90	84	1
100	84	–
110	84	–
120	84	2