Using Resources (review)

1. The Haber process is an important industrial reaction used to make ammonia (NH3 ).

a) State the number of atoms in one molecule of ammonia.

Answer: There are four atoms in one molecule of ammonia.

b) Write a word equation for the formation of ammonia from its elements.

Answer: nitrogen + hydrogen  ⇋  ammonia

c) State where the nitrogen for the reaction comes from.

Answer: The air provides the nitrogen for the Haber process.

d) Describe where the hydrogen for the reaction comes from.

Answer: Hydrogen is made from a chemical reaction between natural gas (methane) and steam.

2. Water is essential for human life. In the UK, rain water is treated, and this is the main source of potable water.

a) What does potable water mean?

Answer: water that is safe to drink

b) In the UK water treatment process, state how microbes are removed from rain water.

Answer: Microbes are removed from rainwater to make it potable by sterilising (adding chlorine or adding ozone or treating with UV).

c) Explain why waste water must be treated before it can be potable.

Answer: Industrial waste water might have harmful chemicals in it. Sewage and agricultural waste water could have microbes in it. Both types of water may contain organic matter, which could make the water unsafe to drink.

3. The table below has some information about shopping bags.

a) Life cycle assessment is a useful tool to evaluate the impact on the environment throughout its life. List the four stages of the life cycle assessment.

Answer: The life cycle assessment has four stages: extracting and processing raw materials, manufacturing and packaging, use and operation during its lifetime, disposal at the end of its useful life.

b) Use the information in the table above, the stages of life cycle assessment and your own knowledge to evaluate the use of plastic and paper shopping bags.

Answer: In the extracting and processing, plastic bags have a higher impact on the environment as the crude oil is to make plastic can cause environmental problems. However, paper bags are made from renewable trees and if the trees are farmed sustainably there will be replacement of the trees that are cut down. In the manufacture of the bags, plastic bags require a lot of energy, which can cause pollution, whereas paper bags use a lot of water, which can cause water pollution. So both have a negative impact on the environment. Plastic bags weigh less, and therefore take less energy to transport, which is better for the environment. As plastic is waterproof and more durable it is more likely to be reused more often, and at the end of its life it can be recycled indefinitely. Paper bags are more likely to be single use and there is a limit to their recycling. However, if the bags are put in landfill as paper can biodegrade this is better for the environment than plastic. Overall plastic shopping bags have the lowest impact on the environment.

c) Justify the use of thermosoftening plastic shopping bags if recycling is to occur.

Answer: Thermosoftening plastics melt when they are heated. Whereas thermosetting plastics do not melt when they are heated. It is essential that the plastic used to make the plastic bag can melt so the plastic can be recycled and remoulded into new products.

4. The Haber process can be described by the symbol equation N2(g) + 3H2(g)  ⇋  2NH3(g). The energy change for the forward reaction is -92 kJ/mol.

a) Calculate the energy change when 2 moles of ammonia are made.

Answer: -92 × 2 = -184 kJ

b) State whether the forward reaction is exothermic or endothermic. Explain why.

Answer: exothermic

c) Justify the use of a temperature of 450°C for the Haber process.

Answer: As temperature is increased, an equilibrium system will oppose the change. For the Haber process this will favour the reverse, endothermic reaction and reduce the yield. However, at low temperatures the rate of reaction is low. This is because not enough collisions are favourable and have enough energy to reach the activation energy level. So, a compromise of 450°C is used which gives an acceptable yield in an acceptable time.

d) Calculate the mass in tonnes of nitrogen needed to make 17 tonnes of ammonia.

Answer: 17 tonnes of ammonia = 17000000 g of ammonia ;
17000000 ÷ 17 = 1000000 moles of ammonia ;
Ratio is 1 nitrogen : 2 ammonia, which means that we need 1000000 ÷ 2 = 500000 moles of nitrogen ;
500000 × (14 + 14) = 14000000 g nitrogen = 14 tonnes of nitrogen