Plant Reactions
Technical & Teaching Notes
Related sheets
Plants, matter and energy; Plant nutrients; Making and testing nutrients; Homemade fertilisers; Soil culture; Water culture; Floating culture
Introduction and context
Reactions that happen in plant cells make countless numbers of compounds needed for all the plant’s life processes.They obey the same rules of chemistry as any others that happen in laboratories or industrial plants. The difference is that they happen in plant cells rather than glassware or stainless steel reactors.Students use the ideas of atom rearrangements and energy transfers to understand reactions that happen in plants.
Although not essential to do so, this sheetmight be best used after the sheet Plants, matter and energy.
Possible barriers to learning
The session allows a number of common misconceptions and other hindrances to learning associated with matter and energy to be addressed. These include:
energy is a material and has mass (like a Mars bar)
there are different forms of energy
plants breathe in oxygen at night and carbon dioxide during the day
plants photosynthesise but do not respire.
Other concepts that may be addressed are ideas about:
energy stores, energy transfers and reaction surroundings
exothermic and endothermic reactions
- makingchemical bonds is an exothermicchange; breaking chemical bonds is an endothermic change.
Links to National curriculum for science in England at key stage 3
In biologypupils should be taught aboutphotosynthesis and cellular respiration. This includes:
- the reactants in, and products of, photosynthesis, and a word summary for photosynthesis
- aerobic and anaerobic respiration in living organisms, including the breakdown of organic molecules to enable all the other chemical processes necessary for life
- a word summary for aerobic respiration
In chemistrypupils should be taught about atoms, elements and compounds, chemical reactions and energetics. This includes:
- a simple (Dalton) atomic model
- differences between atoms, elements and compounds
- chemical symbols and formulae for elements and compounds
- chemical reactions as the rearrangement of atoms
- representing chemical reactions using formulae and using equations
- exothermic and endothermic chemical reactions (qualitative).
In physicspupils should be taught about energy changes and transfers, forces, static electricity and energy in matter. This includes:
- processes that involve energy transfer: metabolism of food, burning fuels
separation of positive or negative charges when objects are rubbed together: transfer of electrons, forces between charged objects
forces as pushes or pulls, arising from the interaction between two objects
- internal energy stored in materials.
Teaching Notes
Activity 1: Ammonia
In this activity students are introduced to energy transfers that happen when bonds are broken or made:
Breaking chemicalbonds is an endothermic change
Making chemical bonds is an exothermic change.
As an alternative to drawing circle diagrams, coloured discs or counters could be used. To be consistent with the convention commonly used these are colours used to represent atoms:
Hydrogen, white; Carbon, black; Nitrogen, blue; Oxygen, red; Chlorine, green; Metals, grey
Answers
1.
2. It takes 1 nitrogen molecule and 3 hydrogen molecules to make 2 ammonia molecules.
3. N2 + 3H2 2NH3
4. 1 x N-N and 3 x H-H broken; 3 x N-H formed.
Note: The hydrogen-hydrogenbond in a hydrogenmolecule is a single covalent bond: H-H; the nitrogen-nitrogen bond in a nitrogen molecule is a triple covalent bond: N≡N. However, at this stage the nature of the bond could be introduced but it is not necessary for the activity.
5. The combined strength of the bonds in the reactants (1 nitrogen molecule and 3 hydrogen molecules) is greater than the strength of the bonds in the products (3 ammonia molecules).
Activity 2: Urea
This activity allows students to following a more complex chemical reaction that involves molecules. It’s relevance to plants is that urea is a fertiliser which is broken down in wet soil to make ammonia and carbon dioxide. Urease is an enzyme that catalyses the reaction.
Answers
1.
2. Two
3. 4 x N-H, 2 x C-N, 1 x C-O and 2 x O-H broken, 6 x N-H and 2 x C-O formed.
Some students might simplify this to 2 x C-N, C-O and 2 x O-H broken, 2 x N-H and C-O formed. However, you need to be wary. This might suggest that we know how the reaction happens, but the purpose of the circle diagrams is simply to allow some ‘atom accountancy’, in other words, keeping track of the number and types of atoms. It tells us nothing about the reaction mechanism.
4. (a) Energy is transferred to the surroundings, (b) Exothermic.
Activity3: Photosynthesis and respiration
This activity illustrates the two types of energy transfers during chemical reactions: endothermic and exothermic.
The fact that energy is not matter and does not have mass can be pointed out. However, quantities of energy that are transferred can be measured; this is important in question 8.
It provides an opportunity to introduce or reinforce the idea that plants respire all the time, not just at night, and photosynthesise during the day. Photosynthesis is faster than respiration, which means that over 24 hours there is a net intake of carbon dioxide and a net output of oxygen.
The activity may be quite demanding and students may need some guidance and support.
Answers
1. C6H12O6.
2. Students may attempt this. The starting point is:
However, it is very difficult to continue simply because everything becomes so crowded.
This is the time to introduce other ways of modelling molecules. It can also be pointed out that ball-and-stick models can show more than which and how many atoms are used to make a molecule and which atoms are bonded to which. Models can also show how the bonded atoms are arranged in space, in other words, represent the structure and shape of a molecule.
3. (a) Black = carbon, red= oxygen, white = hydrogen
(b) How the bonded atoms in a molecule are arranged in space, in other words, its shape
(c) Because it shows the bonded atoms a distance apart, whereas they are (more or less)touching.
Note: This is consistent with the convention most commonly used (see answer to question 1).
4. Reactants: In each carbon dioxide molecule 2 x C-O bonds are broken and in each water molecule 2 x O-H bonds are broken.
Products: In a glucose molecule, 5 x C-C, 7 x C-O, 7 x C-H and 5 x O-H bonds are formed and in each oxygen molecule an O-O bond is formed.
5. The total strength of bonds in the reactants (6CO2 + 6H2O) is less than the total strength of bonds in the products (C6H12O6 + 6O2).
6. (a) Exothermic
(b) It’s an energy store as well as a substance that can be converted into other important compounds needed by the plant
(c) It is transferred to reactants involved in other chemical reactions that are happening in plant cells.
7. The same. The reaction stages are not simply the same ways for photosynthesis and respiration but in reverse order (in other words, the forward and back stages are different to one another). However, the overall change is the same in both directions. So, for specific quantity of glucose, the total amount of energy transferred from the Sun in photosynthesis is the same as the amount of energy transferred to the surroundings in respiration.
8. (a) Photosynthesis: 6CO2 + 6H2O C6H12O6 + 6O2
(b) Respiration: C6H12O6 + 6O2 6CO2 + 6H2O
Notes
Students might write the names of the reactants and products rather than their chemical formulae.
It is important to stress that the energy change is the same only when the amount of glucose produced by photosynthesis is the same as the amount of glucose used in respiration.
Science & Plants for Schools:
Plant Reactions: p. 1