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A student is investigating chemical reactions that produce heat - OCR Gateway - GCSE Chemistry - Question 18 - 2020 - Paper 3
Question 18
A student is investigating chemical reactions that produce heat. (a) She adds zinc to hydrochloric acid, HCl. Zinc chloride, ZnCl2, and hydrogen gas are made. (i)... show full transcript
Worked Solution & Example Answer:A student is investigating chemical reactions that produce heat - OCR Gateway - GCSE Chemistry - Question 18 - 2020 - Paper 3
Step 1
Step 2
Step 3
Explain what is meant by the term activation energy.
Answer
Activation energy is the minimum amount of energy required to initiate a chemical reaction. It represents the energy barrier that must be overcome for reactants to be transformed into products. In the context of the reaction profile shown in Fig. 18.1, the activation energy is depicted as the peak of the energy graph, where the energy requirement is highest before the reaction proceeds to form the products.
Step 4
The reaction of methane with oxygen produces heat. Explain why.
Answer
The reaction of methane with oxygen produces heat because it involves bond breaking and bond making processes. During the reaction, bonds in the reactants (C-H and O=O) are broken, which requires energy. However, the formation of new bonds in the products (CO2 and H2O) releases more energy than is consumed in bond breaking. Therefore, the net energy change is negative, indicating that energy is released in the form of heat.
Step 5
Calculate the total energy transferred to make the bonds in the products in Fig. 18.2.
Answer
To calculate the total energy transferred to make the bonds in the products, we need to sum the bond energies of the products CO2 and H2O.
- For CO2: 2 C=O bonds =
- For H2O: 2 O-H bonds =
Total energy transferred = .
Step 6
Use your answers to parts (ii) and (iii) to calculate the energy change for the reaction in Fig. 18.2.
Answer
To calculate the energy change for the reaction, we use the formula:
Using the values from part (ii) (reactants bond energy) and part (iii) (total energy transferred to make products), we find:
Energy change = Energy of bonds broken - Energy of bonds made.
(Insert the specific numerical values here based on calculations. For example, if bonds broken are 3000 kJ/mol and made are 2538 kJ/mol, then the energy change would be: ).