Reaction pathway diagrams — questions

1. deduce

The diagram shows the reaction pathway diagram for burning ethanol. Which statement is correct?

Circle one answer.

• The curve rises in the middle, so the reaction is endothermic
• The products are lower, so the reaction takes in thermal energy
• The products have less energy than the reactants, so it is exothermic
• The products have more energy than the reactants, so it is endothermic

Nudge (only if you are stuck): Compare the level of the products with the level of the reactants.

2. identify

Match each letter on the reaction pathway diagram to what it shows.

Write the number of the matching option.

1. The products
2. The energy taken in from the surroundings
3. The reactants
4. The energy given out to the surroundings

• A: ______
• B: ______
• C: ______

Nudge (only if you are stuck): Which side are the reactants on? Which way does the arrow point?

3. identify / explain / determine

The diagrams show two reactions, P and Q. (a) When one of these reactions is carried out in a beaker, the thermometer in the beaker falls. Identify which reaction it is, and explain how its diagram shows this. (b) Supplement. Two other reactions, R and S, are drawn on one energy scale. For R the reactants are at 180 kJ/mol and the top of the hump is at 210 kJ/mol. For S the reactants are at 30 kJ/mol and the top of the hump is at 190 kJ/mol. Determine which reaction, R or S, needs more energy to start. Explain your answer using activation energy.

(a) The reaction that makes the thermometer fall

(b) Supplement: R or S, and why

Nudge (only if you are stuck): For (b), where does an activation energy arrow start?

4. deduce

Supplement. Burning propane has ΔH = −2220 kJ/mol. Which statement is correct?

Circle one answer.

• The chemicals lose energy, and the surroundings get colder
• The chemicals gain energy, and the surroundings get warmer
• The chemicals lose energy, and the surroundings get warmer
• The chemicals gain energy, and the surroundings get colder

Nudge (only if you are stuck): From whose point of view is ΔH written: the chemicals' or the surroundings'?

5. deduce

Supplement. Deduce from its enthalpy change whether each reaction is exothermic or endothermic. Match each one.

Write the number of the matching option.

1. Endothermic
2. Exothermic

• 1. 2Mg + O₂ → 2MgO, ΔH = −1203 kJ/mol: ______
• 2. C + H₂O → CO + H₂, ΔH = +131 kJ/mol: ______
• 3. N₂ + 3H₂ → 2NH₃, ΔH = −92 kJ/mol: ______
• 4. 2HgO → 2Hg + O₂, ΔH = +182 kJ/mol: ______

Nudge (only if you are stuck): Which part of each ΔH value tells you which way the energy goes?

6. define

Supplement. Which is the definition of the activation energy, Ea, of a reaction?

Circle one answer.

• The difference in energy between the reactants and the products
• The energy at the top of the hump, read from the bottom of the graph
• The minimum energy that colliding particles must have to react
• The energy that colliding particles give out when they react

Nudge (only if you are stuck): Picture two particles that collide and bounce apart without reacting. What did they lack?

7. identify

Supplement. Match each arrow on the reaction pathway diagram to what it shows.

Write the number of the matching option.

1. The enthalpy change, ΔH
2. The energy of the products
3. The activation energy, Ea

• Arrow X: ______
• Arrow Y: ______

Nudge (only if you are stuck): Where does each arrow start, and where does it end?

8. determine

Supplement. Use the reaction pathway diagram. (a) Determine the activation energy, Ea. (b) Determine the enthalpy change, ΔH. Include its sign.

(a) Activation energy: ______________ unit ________

(b) Enthalpy change: ______________ unit ________

Nudge (only if you are stuck): Read the three levels first: reactants, top of the hump, products.

9. describe

Supplement. Hydrogen peroxide decomposes: 2H₂O₂ → 2H₂O + O₂. ΔH = −196 kJ/mol and Ea = 75 kJ/mol. Describe its reaction pathway diagram, labelled with reactants, products, ΔH and Ea: (a) where the products' level goes; (b) the ΔH arrow and its label; (c) the Ea arrow and its label; (d) the labels on the two axes. You may draw it on paper first.

(a) The products' level

(b) The ΔH arrow

(c) The Ea arrow

(d) The axes

Nudge (only if you are stuck): Start from the sign of ΔH: is the reaction exothermic or endothermic?

10. deduce

Supplement. A reaction has ΔH = −70 kJ/mol and Ea = 90 kJ/mol. On its reaction pathway diagram, drawn on an energy scale, the reactants' level is at 170 kJ/mol. Which row describes the diagram correctly?

Circle one answer.

• A
• B
• C
• D

Nudge (only if you are stuck): Work out the two levels yourself first, then find the row that matches.

11. describe / state / explain

A camping stove burns butane gas. When the tap is opened, the gas comes out but does not catch fire. Pressing the spark button lights it. Then the flame keeps burning on its own and heats a tin of soup. (a) Describe where the products' level is on the reaction pathway diagram for burning butane, compared with the reactants' level, and explain why. (b) Supplement. State whether ΔH for burning butane is positive or negative. (c) Supplement. Explain, using activation energy, why the gas needs the spark. (d) Supplement. Explain why the flame keeps burning after the spark has stopped.

(a) The products' level

(b) Supplement: the sign of ΔH

(c) Supplement: why the spark is needed

(d) Supplement: why the flame keeps burning

Nudge (only if you are stuck): Does the stove give energy to the soup? What does the spark give the particles?