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UCE (O-Level) Energy and chemical change — practice questions

Explore 7 free Energy and chemical change questions in UCE (O-Level) Chemistry. Open each question or exercise to read its content, reference answers and available explanation.

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Energy and chemical change questions and exercises with answers

Showing questions 1–7 of 7.

What happens to energy when a chemical bond is broken?Read the answer and worked solution →In an exothermic reaction, how does the energy of the products compare with the energy of the reactants?Read the answer and worked solution →For an endothermic reaction, what is the sign of the enthalpy change of the system?Read the answer and worked solution →Photosynthesis absorbs energy from sunlight while it takes place. What type of reaction is it?Read the answer and worked solution →What is the minimum amount of energy that must be supplied before a chemical reaction will start?Read the answer and worked solution →The activated complex forms while the reactant bonds break and the product bonds form. How long does it last?Read the answer and worked solution →Tap the changes that fitRead the answer and worked solution →

Worked practice questions

What happens to energy when a chemical bond is broken?

  1. It is released
  2. It is destroyed
  3. It is created
  4. It is absorbed
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Correct answer: D. It is absorbed

1. Bonded atoms sit at an energy minimum, so separating them raises the energy of the system. 2. When bonds break, energy is absorbed from the surroundings. 3. When new bonds form, energy is released instead. 4. Every reaction involves both steps, and the balance between them decides the overall energy change. _Source: Siyavula Physical Sciences Grade 11 (Everything Science, CC BY 4.0), Chapter 12: Energy and chemical change_

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In an exothermic reaction, how does the energy of the products compare with the energy of the reactants?

  1. Less than the reactants
  2. More than the reactants
  3. Equal to the reactants
  4. Double the reactants
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Correct answer: A. Less than the reactants

1. An exothermic reaction releases energy in the form of heat or light. 2. The energy released comes out of the chemical system itself. 3. The energy of the products is therefore less than the energy of the reactants. 4. Products with more energy than the reactants would describe an endothermic reaction instead. _Source: Siyavula Physical Sciences Grade 11 (Everything Science, CC BY 4.0), Chapter 12: Energy and chemical change_

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For an endothermic reaction, what is the sign of the enthalpy change of the system?

  1. It is negative
  2. It is positive
  3. It is always zero
  4. It is imaginary
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Correct answer: B. It is positive

1. The enthalpy change is found by subtracting the energy of the reactants from that of the products. 2. In an endothermic reaction the products hold more energy than the reactants. 3. The enthalpy change of an endothermic reaction is therefore a positive number. 4. A negative enthalpy change belongs to an exothermic reaction, which releases energy instead. _Source: Siyavula Physical Sciences Grade 11 (Everything Science, CC BY 4.0), Chapter 12: Energy and chemical change_

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