Bond energy calculations

A student investigates the temperature change when 10 g of solid magnesium chloride is dissolved in 100 cm³ of water.

Figure 2 shows the apparatus the student uses.

Name the piece of apparatus labelled X in Figure 2.

The student carries out the following steps

step 1 measure the initial temperature of the water in the beaker

step 2 add the magnesium chloride

step 3 stir the mixture

step 4 record the new temperature.

State why the mixture is stirred in step 3.

State two ways the apparatus could be changed to reduce the heat energy lost during the investigation.

The student carries out the experiment three times.

Figure 3 shows the results.

Calculate the mean temperature change for this investigation.

Explain, using the information in Figure 3, the type of reaction that occurs when magnesium chloride dissolves in water.

This question is about molecules containing two atoms.

In an experiment, hydrochloric acid is reacted with zinc.

The volume of gas produced is measured by collecting the gas in an upturned measuring cylinder over water.

Give a method of collecting and measuring the gas that would give a more accurate volume of gas.

Iodine has an atomic number of 53 and a mass number of 127.

How many electrons are there in an iodine molecule, I₂?

Iodine reacts with hydrogen to form hydrogen iodide, H I.

Explain whether or not a solution of hydrogen iodide in water conducts electricity.

Chlorine gas is bubbled through a colourless solution of potassium bromide, KBr. The mixture turns orange.

Give the name of the substance that causes the orange colour.

Write the balanced equation for this reaction.

Hydrogen iodide reacts with oxygen to form water and iodine.

The equation for this reaction is

Calculate the energy change for this reaction using the bond energies in Figure 11.

Include a sign and unit in your answer.

The relative atomic mass of argon is 40 and the relative atomic mass of potassium is 39 but potassium appears after argon in the periodic table.

State why potassium appears after argon in the periodic table.

Potassium reacts with water to form two products.

Give the formulae of both products.

The reaction of potassium with water is exothermic.

On Figure 9, draw and label the reaction profile diagram for this reaction, labelling the activation energy.

Some reactions are endothermic.

Explain, in terms of bond breaking and bond forming, why some reactions are endothermic.

Ethene reacts with hydrogen chloride.

C l H

Figure 10 shows the bond energies for the different bonds in the three molecules in the reaction.

Calculate the energy change for this reaction.

energy change =_........ kJ mol

Ammonia can be produced from the reaction of hydrogen with nitrogen.

What is the percentage by mass of nitrogen in ammonia, NH₃?

(relative atomic masses: H = 1.0, N = 14)

The reaction between hydrogen and nitrogen is exothermic.

Figure 13 shows the reaction profile of this exothermic reaction.

Which arrow represents the activation energy for the reaction?

Describe what the reaction profile shows about the energy involved in bond breaking and bond making in this reaction.

Ammonia, NH₃, and silicon dioxide, SiO₂, are both compounds that are made of two non-metallic elements.

Ammonia has a boiling point of –33 °C. Silicon dioxide has a boiling point of 2230 °C.

Explain why the boiling points of ammonia and silicon dioxide are so different.

In some chemical reactions, bonds are broken in the reactant molecules and new bonds are formed to make the product molecules.

Which row is correct about the energy changes for these processes?

Hydrogen reacts with fluorine.

Figure 12 shows the bond energies for the bonds in the three molecules in the equation.

Calculate the energy change for this reaction.

energy change =_........ kJ mol

*(b) The reaction profile for an uncatalysed exothermic reaction is shown in Figure 13.

Using some examples of catalysts you have met in chemistry, discuss what catalysts do and their effect on the activation energy of a reaction.

You can use Figure 13 to illustrate your answer.

Pentadecane, C₁₅H₃₂, is a hydrocarbon and is used as a fuel.

The incomplete combustion of pentadecane produces carbon monoxide. Carbon monoxide is a toxic gas.

Explain why the incomplete combustion of pentadecane can produce carbon monoxide as one of the products.

Explain how carbon monoxide behaves as a toxic gas.

1 mole of pentadecane can be cracked to form 1 mole of octane, C₈H₁₈, and 1 mole of propene, C₃H₆, and 2 moles of another product.

Complete the balanced equation for this reaction by adding the formula of the missing product.

Figure 11 shows the reaction of propene, C₃H₆, with water.

H H H

Figure 11

Figure 12 shows some bond energies.

Use the bond energies in Figure 12 to calculate the energy change of the reaction in Figure 11.

energy change of reaction =_........ kJ

Methane gas, CH₄, was burned using the apparatus shown in Figure 13.

Explain why water droplets form on the bottom of the beaker of cold water.

Calcium carbonate reacts with dilute hydrochloric acid to produce calcium chloride, water and carbon dioxide.

A student wanted to measure the amount of gas produced in two minutes.

The student suggested that this could be done by counting the number of bubbles formed. However, the bubbles are produced too quickly to count them.

Figure 4 shows a conical flask in which the calcium carbonate and dilute hydrochloric acid are reacting.

Complete Figure 4 to show the apparatus that could be used to measure accurately the volume of gas given off in two minutes.

The reaction between calcium carbonate and dilute hydrochloric acid is exothermic.

Explain, in terms of bond breaking and bond making, why some reactions are exothermic.

*(c) An investigation was carried out into the rate of reaction of calcium carbonate with dilute hydrochloric acid.

5.0g of small lumps of calcium carbonate were reacted with 50 cm³ of 0.50 mol dm^–3 hydrochloric acid. Another 5.0 g of the same sized lumps of calcium carbonate were reacted with 50cm³ of 1.0 mol dm^–3 hydrochloric acid. The volume of gas collected in two minutes was recorded for each experiment.

The two experiments were then repeated, each using 5.0 g of large lumps of calcium carbonate.

Figure 5 shows the results.

Explain, in terms of collision of particles, how these results show the effect of the size of the lumps of calcium carbonate and the effect of the concentration of the acid on the rate of this reaction.