smc-4263-8-Indicators of chemical change

CHEMISTRY, M3 EQ-Bank 20

During a laboratory investigation, a student mixed two solutions and observed a sudden colour change, an increase in temperature, and the formation of bubbles.

Explain why these observations indicate a chemical change. (3 marks)

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Describe two types of chemical reactions that could cause at least two of these observations each. (2 marks)

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a. Colour Change:

This suggests that new chemical compounds are forming that have different properties from the original reactants.

Temperature Increase (Exothermic Reaction):

The release of heat indicates that the reaction is exothermic, where energy is released as bonds are formed in the products.

Gas Production (Bubbles):

The formation of bubbles without boiling is a sign that a gas is being produced as a result of the reaction.

b. Acid-Base Reaction:

When an acid reacts with a base, it can lead to a colour change if an indicator is present (e.g., phenolphthalein changes from colourless to pink). An exothermic reaction may also occur, causing a temperature increase.

Decomposition Reaction:

Certain decomposition reactions, such as the breakdown of hydrogen peroxide, produce gas (oxygen) and heat. The bubbling and increase in temperature can be observed in this type of reaction.

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a. Colour Change:

This suggests that new chemical compounds are forming that have different properties from the original reactants.

Temperature Increase (Exothermic Reaction):

The release of heat indicates that the reaction is exothermic, where energy is released as bonds are formed in the products.

Gas Production (Bubbles):

The formation of bubbles without boiling is a sign that a gas is being produced as a result of the reaction.

b. Acid-Base Reaction:

When an acid reacts with a base, it can lead to a colour change if an indicator is present (e.g., phenolphthalein changes from colourless to pink). An exothermic reaction may also occur, causing a temperature increase.

Decomposition Reaction:

Certain decomposition reactions, such as the breakdown of hydrogen peroxide, produce gas (oxygen) and heat. The bubbling and increase in temperature can be observed in this type of reaction.

CHEMISTRY, M3 EQ-Bank 19

Describe two common indicators of a chemical change. Provide an example of each indicator. (3 marks)

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Colour Change: A change in colour that is not just mixing of colours can indicate a chemical reaction. For example, when iron reacts with oxygen it rusts and it changes colour from metallic grey to reddish-brown.
\(\ce{2Fe(s) + O2(g) -> 2FeO(s)}\)
Gas Production: Bubbling or fizzing without heating shows that a gas is being produced. For example, adding hydrochloric acid to marble chips (calcium carbonate) produces bubbles of carbon dioxide gas.
\(\ce{CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + CO2(g) + H2O(l)}\)

Other answers could include: the formation of a precipitate, change in temperature, light being produced.

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Colour Change: A change in colour that is not just mixing of colours can indicate a chemical reaction. For example, when iron reacts with oxygen it rusts and it changes colour from metallic grey to reddish-brown.
\(\ce{2Fe(s) + O2(g) -> 2FeO(s)}\)
Gas Production: Bubbling or fizzing without heating shows that a gas is being produced. For example, adding hydrochloric acid to marble chips (calcium carbonate) produces bubbles of carbon dioxide gas.
\(\ce{CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + CO2(g) + H2O(l)}\)

Other answers could include: the formation of a precipitate, change in temperature, light being produced.

A student conducts an experiment by mixing an unknown metal powder with a solution of hydrochloric acid. The following observations are made: rapid bubbling, a slight rise in temperature, and a distinctive metallic odour.

Based on these observations, which of the following best identifies the indicators of a chemical change and explains what might have occurred?

Formation of a precipitate, indicating the metal is insoluble in acid.
Production of a gas and increase in temperature, suggesting the metal reacts to release hydrogen gas.
Emission of light and production of an odor, implying the metal is highly reactive.
Absorption of heat and production of a gas, indicating an endothermic reaction producing carbon dioxide.

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\(B\)

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The rapid bubbling indicates the release of a gas, likely hydrogen, which is common when active metals react with acids. The slight rise in temperature suggests an exothermic reaction.
The presence of a metallic odour might be due to vaporization of trace compounds.
Therefore, the correct indicators are gas production and temperature change, which signify a chemical reaction.

\(\Rightarrow B\)

CHEMISTRY, M3 EQ-Bank 7 MC

Which of the following observations indicates a chemical change has occurred?

Dissolving sugar in water
Change in temperature when two solutions are mixed
Melting of ice into water
Mixing sand with iron filings

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\(B\)

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A chemical change is one where a new substance is created and cannot be reversed.
The changing temperature of a mixed solution is the result of bonds breaking and forming new products thus indicating a chemical change.
All other options can be reversed by physical processes and therefore represent physical changes only.

\(\Rightarrow B\)

CHEMISTRY, M3 EQ-Bank 7

Sodium and zinc are each reacted with water and steam. Predict the observations that would be made about these reactions. (4 marks)

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Sodium \((\ce{Na}) \):

Sodium is an alkali metal that will react vigorously with both water and steam to produce a hydroxide and hydrogen gas according to the chemical equation
\(\ce{2Na(s) + 2H2O(l) \rightarrow 2NaOH(aq) + H2(g)} \)
The reaction will show no colour change in the aqueous solution as \(\ce{Na(aq)}\) is colourless like water.

Zinc \((\ce{Zn})\):

Zinc is less reactive and will not react with water but will form an oxide with steam (due to the higher energy present) according to the equation
\(\ce{Zn(s) + H2O(steam) \rightarrow ZnO(aq) + H2(g)} \)
While zinc metal is a silver colour, zinc oxide is white.

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Sodium \((\ce{Na}) \):

Sodium is an alkali metal that will react vigorously with both water and steam to produce a hydroxide and hydrogen gas according to the chemical equation
\(\ce{2Na(s) + 2H2O(l) \rightarrow 2NaOH(aq) + H2(g)} \)
The reaction will show no colour change in the aqueous solution as \(\ce{Na(aq)}\) is colourless like water.

Zinc \((\ce{Zn})\):

Zinc is less reactive and will not react with water but will form an oxide with steam (due to the higher energy present) according to the equation
\(\ce{Zn(s) + H2O(steam) \rightarrow ZnO(aq) + H2(g)} \)
While zinc metal is a silver colour, zinc oxide is white.

CHEMISTRY, M3 EQ-Bank 6

Three unknown metals are reacted with dilute \(\ce{HCl(aq)}\) and the following observations are made:

\begin{array} {|c|l|}
\hline
\rule{0pt}{2.5ex} \textit{Metal} \rule[-1ex]{0pt}{0pt} & \ \ \ \ \ \ \textit{Observations} \\
\hline
\rule{0pt}{2.5ex} \text{A} \rule[-1ex]{0pt}{0pt} & \text{No observable reaction} \\
\hline
\rule{0pt}{2.5ex} \text{B} \rule[-1ex]{0pt}{0pt} & \text{Slow bubbling} \\
\hline
\rule{0pt}{2.5ex} \text{C} \rule[-1ex]{0pt}{0pt} & \text{Fast, abrupt bubbling} \\
\hline
\end{array}

You are told that the metals in question are Magnesium, Platinum and Zinc.

Explain which of the above metals correspond to \(\text{A}\), \(\text{B}\) and \(\text{C}\), giving reasons for your answer. (3 marks)

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Describe the type of reaction between \(\ce{HCl}\) and metal \(\ce{C}\). (1 mark)

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Provide one balanced chemical equation between \(\ce{HCl}\) and either metal \(\text{B}\) or metal \(\text{C}\). (1 mark)

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a. Identifying metals \(\text{A}\), \(\text{B}\) and \(\text{C}\):

The metals \(\text{A}\), \(\text{B}\) and \(\text{C}\) differ by their relative reactivity.
Magnesium is the most reactive as it is an alkali earth metal (Metal \(\text{C}\)).
Zinc and Platinum are both less reactive than magnesium. Zinc however exists higher on the reactivity series of metals than Platinum which is the least reactive of the three metals.
Therefore, metal \(\text{B}\) is zinc and metal \(\text{A}\) is platinum.

b. A single displacement reaction.

c. Include one of the following:

\(\ce{Mg(s) + 2HCl(aq) \rightarrow MgCl2(aq) + H2(g) }\)

\(\ce{Zn(s) + 2HCl(aq) \rightarrow ZnCl2(aq) + H2(g) }\)

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a. Identifying metals \(\text{A}\), \(\text{B}\) and \(\text{C}\):

The metals \(\text{A}\), \(\text{B}\) and \(\text{C}\) differ by their relative reactivity.
Magnesium is the most reactive as it is an alkali earth metal (Metal \(\text{C}\)).
Zinc and Platinum are both less reactive than magnesium. Zinc however exists higher on the reactivity series of metals than Platinum which is the least reactive of the three metals.
Therefore, metal \(\text{B}\) is zinc and metal \(\text{A}\) is platinum.

b. A single displacement reaction.

c. Include one of the following:

\(\ce{Mg(s) + 2HCl(aq) \rightarrow MgCl2(aq) + H2(g) }\)

\(\ce{Zn(s) + 2HCl(aq) \rightarrow ZnCl2(aq) + H2(g) }\)